Physics and Chemistry 4 + De Cerca Andalucía Student Book Sample

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We read about and listen to the topic of the unit, learning about key concepts and developments in physics and chemistry.

Chemical reactions: foundations

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of and consumption of the production (in The earliest evidence Zagros mountains vase found in the 5400 BCE. wine is in a ceramic is dated around and Iraq) which what is now Iran its production process, and even more so Traditionally wine, divine. In fact, both and us to the mysterio and has been related had a god of wine later the Romans ely. the Greeks and s and Bacchus, respectiv lly vineyards: Dionysu ed and eventua changed, develop meant These beliefs and curiosity. This the help of logic that wine resulted disappeared with Ages it was thought while in the juice), that in the Middle (grape osition of matter the result of a from the decomp ed that wine was discover was it 19th Century : fermentation. tion, chemical reaction explained fermenta Louis Pasteur first of The French chemist for the production a scientific basis of and thereby provided the transformation l change involves which wine. This chemica carbon dioxide, ethyl alcohol and , sugar (glucose) into (fungi) such as Candida yeasts of activity Saccharomyces occurs due to the ces or the popular Pichia, Zygosaccharomy science that specialises in the study is now a it’s called? cerevisiae. There Do you know what process. l of these chemica of wine? god Greek the 1 Who was on ble for the producti l reaction is responsi 2 What chemica of wine? tion discovered?

3 When was fermenta tion process? what in the fermenta 4 What turns into fermentation? for ble responsi organisms 5 What are the main

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Writing tells how from an article that a paragraph taken sions that The following is s and the repercus on. attacked the vineyard powdery mildew save the yearly producti es had in order to the use of fungicid

missing verbs. very few In pairs, fill in the over the year with many challenges especially for The weather ________ much wetter year, lightly. It ______ a warmer regions getting off rains _________ with ns for and generous spring the ideal conditio coastal regions, usual. This ________ but ultimately temperatures than work in the vineyard intense and mildew which ________ Finally, drier weather on yield than quality. retreat. this impacted more tures _____ mildew usual higher summer tempera place earlier than took ment vine develop 14 days earlier _______ Every stage of generally up to the _____ of dates stages last and harvest favoured the A few days of rain g the harvest. to a typical year. without disruptin for many regions rain _____ ripening period del Duero where this ______ Ribera s over whether One exception to difficult decision in the harvest and frequent breaks see, to pick or wait. times), bring, provide, compare, is (three Verbs: coincide, cause, require ishfoodwine/ mspain.com/span ww.foodswinesfro in-2020 -wineSource: https://w ture-de tail/spa ew/feat ures/fea global/ whats-n tml the g harvest.h a paragraph explainin problem and write Now, find a similar 123 issue.

SDG. Reflection on and analysis of SDGs, such as gender equality, climate action, reducing inequalities, etc.

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Writing 122

The audios of each unit’s content are available at www.anayaeducacion.es

We speak about the different issues relating to the unit and the UN Sustainable Development Goals.

Speaking a, s of Galicia, Cataloni s year for the vineyard fungus 2020 was a disastrou to a plague of the and Andalusia, due which Castille-La Mancha powdery mildew, responsible for es to save the harvest.

Plasmopara viticola, treatment with fungicid generally requires tion for farmers that: prepare a presenta to going they are how You to the issue and that are relevant • Explains the SDGs fighting can help the farmers al treatment for one alternative/natur • Explain at least

We write a variety of different styles and types of text relating to the unit topic, sometimes relating them to the UN Sustainable Development Goals. By practising different styles of writing, we improve our writing skills.

content development Two types of activities: Understand, think, search…

Activities to strengthen learning and relate knowledge.

Unit

ä 1.1 Basic concepts In a chemical reaction, we begin with substances called reactants, which are there before the chemical change. Then, new substances appear called products, which are a result of the chemical change.

The reactants must be in contact for a chemical change to occur. Physical changes occur during the process too, which make the change visible. You can see some examples below.

Physical evidence of a chemical change

4

ä 1.3 Writing a chemical reaction: the chemical equation We use chemical equations to describe chemical reaction quantitatively.

Chemical equations use stoichiometric coefficients to express the relationship between the reactants and products in a chemical reaction and the proportions of their elemental units.

Sometimes, they also give information about other factors, such as the state of matter of substances, their temperature or their pressure.

Balancing a chemical equation involves choosing the values of the coefficients, in order to make sure that the number of atoms of each element is the same among the reactants as among the products.

Writing and balancing chemical equations

F ocus on English You probably already know the word equation from maths, as well as from chemistry. Have you ever wondered where it comes from? Equation comes from the Latin word aequatio, which means, an equal distribution. It can be used figuratively: ‘If we take Peter out of the equation, we will find a solution to the situation’. In can also be used in a scientific context, meaning the act of making equal or balanced: ‘To him, the equation was simple: time equals money’.

With a classmate, discuss whether you We will use the example of the reaction between ammonia and molecular catalysts agree with theand following phrase by 5 ENERGY IN CHEMICAL oxygen. The products of this reaction are molecular nitrogen and water. Energy diagrams

ä 5.2

REACTIONS

Albert Einstein: ‘One must divide one’s In the energy diagrams shown on the previous page, only the initial time between politics and equations. and final energy states of the chemical reactions are shown. However, an arrow showing the direction of the reaction: But our equations are much more the evolution of the chemical reaction is more complex than this. Not NH3 + O2 8 N2 + H2O important to me, because politics is for all collisions between molecules are effective since some lack the right the present, while our equations are for orientation or sufficient energy.

2 We choose values for the stoichiometric coefficients to make the number 1 We write the formulas of the reactants and products, separating them with

Energy exchange.

Colour change.

State change.

From the information in the illustration, work out which bonds have broken and which have formed in the chemical change.

of atoms of each element the same among the reactants and the for the transition state (the state between reactants and It isamong possible products. To do so:

2.1 First, we adjust the coefficients of the non-elemental substances: stages. This point in the reaction is called the activated complex (see 2 NH3 + O2 8 N2 + 3 H2O figure below).

During a chemical change, mass is conserved, which means that the total mass of the reactants must equal the total mass of the products.

This statement is known as the law of conservation of mass.

Atomic theory and conservation of mass The drawing on the left shows the oxidation of nitrogen monoxide to form nitrogen dioxide.

+ 2 NO

+

We can see that the atoms are in a different order, and that mass is conserved throughout the process.

O2

2 NO2

At anayaeducacion.es there is a resource about the evidence of chemical 124

eternity’.

ä 1.2 Atomic theory of chemical reactions The appearance of new substances in a chemical reaction is based on a reordering of the reactant atoms. For this to occur, the elemental units of the reactants must collide. These collisions break bonds and form new bonds, making new substances.

Since this is a reordering of atoms, the total mass of the substances before and after the chemical reaction does not change, so we can state that:

Working with pictures

res. with pictu Working city a p your ca Develop and n o rvati for obse . n tio interpreta

1 CHEMICAL CHANGES

changes.

Unit

• The state of matter of the reactants and products, since a state change implies an energy change.

• The amount of substance in the reaction, so we must note the values of the stoichiometric coefficients used.

Understand, think, search…

2.2 Then, we do the same for the elemental substances. IfRegardless necessary, we of whether a reaction is endothermic or exothermic, every correct the coefficients we chose before. We can use fractions.

For example, the following thermochemical equation tells us that the combustion of one mole of C4H10, liberates 2 877 kJ if the products are CO2 in gas state and liquid water:

2 NH3 + 3/2 O2 8

1 through 1-2-4.this Draw a table, chemical reaction passes energy state. like the

N2 + 3 H2O

one in the example, for the following reaction:

Activation energy. Catalysts

CH4 + 2 O2 8 CO2 + 2 H2O The energy difference between the reactants Which bonds are transition broken and whichis called the and the energy new bonds form? energy.

How many bonds activation are broken how many bonds Thisand energy is a new barrier that will only be form? surpassed by reactant molecules with

Note that the stoichiometric coefficient of molecular E is 1, therefore it is not written. nitrogen

Uncatalysed reaction We can also multiply by two the chemical equation so that there are no fractional coefficients: Activation energy without catalyst, 2 NH3 E+a 3/2 O2 8 N2 + 3 H2O Activation energy with catalyst, Ea' 4 3 2 6 Catalysed 3 Finally we check,REACTANTS element by element, that the number of atoms among the reaction reactants is the same as among the products: Elements

Reactants

H atoms

4 NH3 (4 · 3 = 12)

6 H2O (6 · 2 = 12)

Products PRODUCTS

N atoms

4 NH3 (4 · 1 = 4)

2 N2 (2 · 2 = 4)

O atoms

3 O2 (3 · 2 = 6)

6 H2O (6 · 1 = 6)

REACTION PROGRESS

Understand, think, search…

energy to produce effective 2 Explain sufficient what the stoichiometric

3

collisions. coefficients mean in the exercise above. When this energy is very high and the reactants cannot surpass it, we can use Let’s check.These Balance the lower the catalysts. substances followingactivation chemical reactions energy and create a more a) CO2 +favourable H2O 8 H2CO energetic pathway. This helps 3 the reactants into the products. b) NO +convert O 8 NO 2

2

c) Cr2O3 + Al 8 Al2O3 + Cr d) NO2 + H2O 8 HNO3 + NO e) C2H2 + O2 8 CO2 + H2O

ä 5.3 Energy exchange. Heat of reaction

28 If you touch a beaker containing an

Heat or work can cause an energy exchange between a system and its 125 surroundings or between two systems. When a chemical reaction occurs, there is a variation of the energy in the system which reacts.

29

To measure it, we introduce the concept of the energy of reaction, or heat of reaction, also known as enthalpy:

Search for examples of exothermic and endothermic reactions and their everyday uses.

30 What can you say about a chemical reaction if Qr = –287.9 kJ/mol? And if it has the opposite sign?

31

Write around. Does the presence of a catalyst change the value of Qr?

32 Draw

a diagram representing the exchange of energy in both endothermic and exothermic reactions.

4

ä 5.4 Thermochemical equations Thermochemical equations are equations that tell us not only what substances participate in a chemical reaction but also their state of matter and how much energy they have. The value of the heat of reaction in a thermochemical equation depends on:

products) of a chemical reaction to have more energy than either of these

endothermic reaction, what will you feel?

The icons included with some activities indicate the keys to the project.

The problem solved let you learn reasoning processes and problem-solving strategies.

13 C4H10 (g ) + O2 (g ) 8 4 CO2 (g ) + 5 H2O (l ) Qr = –2 877 kJ 2 For combustion reactions, such as the example, the heat of reaction is called the heat of combustion and is related to a mole of fuel.

Sometimes, we can find the heat of reaction on its own in a chemical equation. It will be attached to the reactants in an endothermic reaction and to the products in an exothermic reaction.

Research project Once you have studied this unit, you will be ready to continue with the research project, completing activity 4.

En el apartado «Itinerarios Académicos y Profesionales» de tu banco de recursos encontrarás información sobre los grados superiores relacionados con el desarrollo de energías renovables.

Problem solved 7 Calculate the amount of energy liberated when

We calculate the amount of each fuel:

a kilogram of butane reacts and when a kilogram of methane reacts:

Qr (CH4) = –890 kJ/mol; Qr (C4H10) = –2 877 kJ/mol; M (C) = 12 g/mol, M (H) = 1 g/mol.

The heats of combustion refer to a mole of fuel. So, we must calculate the amount of each substance by adding the reactants.

nCH4 =

1 000 g = 62.5 mol 16 g/mol

nC4H10 =

1 000 g - 17.2 mol 58 g/mol

Finally, the heat released by the fuel’s combustion:

First, we must know the molar masses, which we will calculate from the molecular formulas:

Qc (CH4) = 62.5 mol $

MCH4 = 12 $ 1 + 1 $ 4 = 16 g/mol MC4H10 = 12 $ 4 + 1 $ 10 = 58 g/mol

Qc (C4H10) = 17.2 mol $

890 kJ - 5.56 $ 104 kJ 1 mol 2 877 kJ - 4.95 $ 104 kJ 1 mol

The heat of reaction, Qr , is the energy exchanged in heat form between a system containing a chemical reaction and its surroundings.

The heat of reaction allows us to distinguish an endothermic reaction from an exothermic one. To do so, we define a sign convention: • Endothermic reactions. In this type of reaction, the energy in the system grows. The exchange of energy via heat goes from the surroundings toward the system so the heat of reaction DE is positive.

• Exothermic reactions. In this type of reaction, the energy in the system falls. The exchange of energy via heat goes from the system towards its surroundings so the heat of reaction is negative.

Understand, think, search…

33

The heat of reaction in a thermochemical equation is positive and attached to the reactants. Is the reaction endothermic or exothermic?

34 Calculate the energy necessary to form 75 g of nitric oxide in this reaction:

35

Round table. The heat of combustion of benzoic acid is –26.42 kJ/g: a) Is this an endothermic or exothermic reaction? b) How much energy is released as heat during the combustion of 3 mol of benzoic acid (C7H6O2)?

N2 (g) + O2 (g) + 181 kJ 8 2 NO (g)

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KEYS

PROJECT

SDG SDG Commitment Discover the Sustainable Development Goals and be an active part of our commitment to make a more equal and liveable world.

Developing thinking Work on strategies for thinking: reflect on the content you are learning, generate ideas, organise them, debate them, explain them…

Cooperative learning Get involved in your learning and participate in the group’s learning; you will find that cooperating improves performance and harmony in the class.

Emotional education Get to know yourself; identify the situations that bring up complicated emotions and manage them with constructive, self-affirming experiences.

ICT skills

Problem-solving guidelines ving

Problem-sol

INTERPRETING CHEMICAL EQUATIONS On this first page, we show you two tools that will help you practice balancing chemical reactions and understand the proportion of the reacting substances

How to solve a stoichiometry problem

guidelines

BALANCING CHEMICAL EQUATIONS

EXAMPLE 1

In every unit, there are problem-solving guidelines which show the steps taken to solve a problem and explain the results gained.

2

3

STEPS

SOLUTION

SOLUTION

UNDERSTAND THE PROBLEM STATEMENT AND EXTRACT DATA Search for the meaning of any relevant terms that you do not understand. On the second reading, extract the data given and identify what the problem asks for. Remember to write quantities with their symbols and use subscripts for different substances.

• The statement describes a chemical reaction.

• We distinguish the given data from what we must calculate: – Given substance: hydrochloric acid: VHCl = 150 mL; 5 M = 5 mol/L

• The problem describes an exothermic reaction and asks us to find the energy released as heat.

BALANCING THE CHEMICAL REACTION This is a stoichiometry problem since we are given substances that react. We must write the balanced chemical equation together with the data and the molar masses of the substances.

The balanced chemical equation is: mAl

VH2 , c.n.

In this case we must follow this sequence:

In this case we must follow this sequence:

1. Calculate the amount of substance:

1. Calculate the amount of substance: m (g) n (mol) = M 2. Calculate the energy released as heat from the thermochemical equation.

n (mol) M= V (L) • Definition of the heat of reaction.

OBTAIN A RESULT AND CHECK IT • Perform the calculations necessary to complete your problem-solving strategy and remember to use consistent units.

• Analyse your result and discuss it if necessary.

3. Use the molar mass and ideal gas equation to calculate the mass and volume of the unknown substances, respectively.

We calculate the quantity of C3H8, dividing by the molar mass (M), which we can write as: 440 g nC3H8 = = 10 mol 44 g/mol

Finally, we calculate the mass of Al and volume of H2: mAl = 0.25 mol $ 27 g/mol = 6.75 g atm $ L 0.375 mol $ 0.082 $ 273 K K $ mol VH2 = - 8.39 L 1 atm

We write the result with a negative sign to specify that it is energy released from the system.

Download and execute it with Java.

RATE EXPERIMENTS Here the application lets you change the number of reactant molecules. There are options for controlling the experiment on the right: • Select a reaction. We can use one of the given reactions or design our own. The energy values of the reactants and products are editable in the energy diagram, so we may have endothermic or exothermic reactions. The activation energy is also editable, so we can simulate a catalyst.

• Number of molecules at the start. Let us change the number of reactant and product molecules at the start of the reaction.

• Initial temperature.

• Chart options. It is useful to select the bar chart to have an idea of the amounts of reactant and product and the strip chart to see how the reaction progresses.

1 Do various single collision experiments, changing the

2 Do a rate experiment with the first pre-set chemical

1 Explain why the the coefficient of the simple or

reacts with enough oxygen? If the reaction occurs in standard conditions, how much CO2 forms? What about Qc =worked –875.4 kJ/mol.

Once water? we have with the application, we

elemental substances should be adjusted last.

The effect of the energy of collision is visible by making shots at greater or lesser energy from the launcher. Note that if the shot energy is less than the activation energy you will not get an effective collision. To visualise the energy diagram of the reaction you must activate the Energy View option.

2 In the same way that when we have four slices of bread and just one of cheese, the cheese ‘limits’ how many sandwiches we can prepare (just one). In stoichiometry we use the concept of the ‘limiting reactant’, which we will study in more depth in future years. If we take the example of the formation of ammonia and we have two mole of nitrogen (N2) and three of (H2), which is the limiting reactant?

Data: (C) a= step 12 g/mol; M (O) = 16 g/mol; must M take beyond and see what happens M (H) = 1where g/mol.the fundamental units of the in situations reactants areiron notmixes in stoichiometric proportions.

41 A sample of with 200 mL of sulfuric acid,

138

energy and launch angle. Draw conclusions about the probability of an effective collision and the factors on which it depends.

Understand, think, search…

40 How much energy is released when 330 g of C2H4O2

H2SO4, of concentration 3.5 M. The reaction forms hydrogen gas and iron (II) sulfate. What was the mass of the sample? What volume of hydrogen does the reaction release at 30 °C and 0.95 atm?

We calculate the quantity of HCl: mol nHCl = 5 $ 0.15 L = 0.75 mol L From that value we find the quantity of Al and H2: 2 mol Al nAl = 0.75 mol HCl $ = 0.25 mol 6 mol HCl 3 mol H2 nH2 = 0.75 mol HCl $ = 0.375 mol 6 mol HCl

https://phet.colorado.edu/es/simulation/legacy/ reactions-and-rates

Understand, think, search…

d) What volume of hydrogen, measured at 300 K and 1.2 atm, releases from the water in the reaction?

Data: M (Ca) = 40 g/mol; M (O) = 16 g/mol; M (H) = 1 g/mol.

IDENTIFY THE RELEVANT LAWS

2. Use the stoichiometric ratio to calculate the amount of each unknown substance.

SINGLE COLLISION In this part of the simulator, you can see the effects of the orientation of the two reactant molecules and the collision energy. To see how the relative orientation makes a collision effective or not, you must select Angled shot from the Launcher Options (top right corner) and launch the reactant molecule with enough energy.

c) Calculate the mass of calcium hydroxide formed with the same amount of water.

Which means that the combustion of a mole of propane releases 2 218.8 kJ.

n (mol) = M · V (L)

It begins with an example of 2:1 stoichiometry: making a sandwich requires 2 slices of bread and 1 slice of cheese. It seems simple but this is the idea we will use to predict which reactant will be leftover.

Now, we will use an application available at:

b) Calculate the amount of calcium hydride that would react with 3.6 mol of water.

MC3 H8 = 44 g/mol

5M

The simulation has three parts: Single collision, Many Collisions, and Rate Experiments. In all three cases the reactions are reversible, that is, once the products form, they interact and produce the original reactants. There are many such chemical reactions. We say that they have reached chemical equilibrium.

We will now use an application that complements the previous one. You can download it at https://phet.colourado.edu/sims/html/reactantsproducts-and-leftovers/latest/reactants-productsand-leftovers_en.html.

• The application lets you balance chemical Understand, think, search… equations in a visual and intuitive way. It 36 Calculate the mass of aluminium chloride formedof continually updates the number of atoms in each the reaction Example 1. Datum: M (Cl) = element in in the reactants and products.

To = 35.5 g/mol. begin, use the Introduction part: activate the tool in the rightofthat lets you released visualise the 37 Calculate theupper volume hydrogen in balance1 ofif elements.

Example the temperature is 25 °C. Will it be

C3H8 + 5 O2 8 3 CO2 + 4 H2O Qr = –2 218.8 kJ

So, 2 mol of Al combine with 6 mol of HCl to produce 2 mol of AlCl3 and 3 mol of H2.

REACTANTS, PRODUCTS AND LEFTOVERS

Play to raisemiddle. the level of difficulty of the 39 • UsePencils in the Calcium hydride reacts with water according to the following equations to balance.

If you do notchemical succeed reaction: with a problem, the application will give you the option guess and make2a(s)new attempt, as CaHto + H2why 0 (l ) 8 Ca(OH) + H 2 (s) 2 (g) thechemical picture: equation. a) shown Balanceinthe

– Heat of reaction: Qr = –2 218.8 kJ/mol

The balanced chemical equation is:

V = 150 mL

MAl = 27 g/mol

We will use a chemical reaction simulator to see the effects of temperature, reactant concentration, and a catalyst.

molecules, or fundamental units, must a whole numbertoin 38 Calculate the amount of be oxygen necessary this application. combust all the propane in Example 2.

–Datum given: propane, C3H8: mC3H8 = 440 g

2 Al (s) + 6 HCl (aq) 8 2 AlCl3 (aq) + 3 H2 (g)

In this unit we have studied the factors that affect the rate of chemical reactions.

given the amount of products and reactants at the end of the reaction. In future years, this will help us understand the concept of the limiting reactant.

4

4

SIMULATING REACTION RATES

more or less in the example?

• Note thatthanthe number of

• Data we can extract:

• Equation for molar mass, amount of substance, and mass: m (g) n (mol) = M • Ideal gas law, P · V = n · R · T, if there are gas reactants or products.

• Definition of molarity:

4

Calculate the energy released as heat when 440 g of propane reacts with enough oxygen.

Datum: Qr = –2 218.8 kJ/mol.

– Unknowns: mass of Al and volume of H2 in standard conditions.

Unit

We will use an application available on the Anaya website. The application has two parts: Introduction and Play.

EXAMPLE 2

A sample of pure aluminium reacts with 150 mL of hydrochloric acid (HCl) concentrated at 5 M. Aluminium chloride (AlCl3) and hydrogen gas (H2) form. What was the mass of the aluminium sample? What is the volume of hydrogen released, assuming standard conditions? Data: M (Al) = 27g/mol.

1

Unit

Simulating chemical reactions

ICT SKILLS

reaction but changing the number of molecules of both reactants.

Activate the stopwatch and the strip chart to measure the time the reaction takes. Put your results in a table and draw conclusions about the importance of the concentration of reactants. Why do we call chemical equilibrium a dynamic equilibrium?

3 Do the same experiment but this time vary the temperature. Draw conclusions.

Data: M (Fe) = 55.8 g/mol; M (S) = 32 g/mol; M (O) = 16 g/mol; M (H) = 1 g/mol.

139

42 One of the ways to get hydrogen is to convert methane:

CH4 + H2O 8 CO + H2

We calculate the energy released: (–2 218.8 kJ) Energy = 10 mol C3H8 $ = –22 188 kJ 1mol C3H8

a) Balance the chemical equation. b) Calculate the volume of methane, measured at 100 °C and 1 atm, needed to produce 150 m3 of hydrogen measured in the same conditions. c) How many mols of CO are produced?

ICT skills and programmes to support your acquisition of physics and chemistry knowledge.

137

136

A research project in each unit to implement the application of the scientific method and encourage the acquisition of skills and attitudes necessary for scientific work.

Review Science workshop

to choose Remember . for your portfolio

resources from

this unit

Unit

Organising my ideas

THE OXIDATION REACTION OF IRON

Unit

Research project

PRACTICAL WORK

INTRODUCTION One of the chemical reactions that is widely used as an example is the oxidation of iron. In neutral pH conditions a precipitate of hydrated iron(III) oxide (rust) with a striking reddish-brown colour is formed. This means that you can easily observe the chemical change through the colour change.

A series of experiments will be conducted to observe qualitatively how the chemical conditions (pH and presence of salts) affect the oxidation of the iron nails.

To compare the results, we need a control experiment, or blank, which we will carry out using water that has been boiled to remove part of the oxygen dissolved in it.

PREPARATION OF THE VESSELS • Vessel 1: one nail + boiled water.

• Vessel 2: one nail + a solution of sodium chloride.

• Vessel 3: one nail + diluted hydrochloric acid.

• Vessel 4: one nail + vinegar.

• Vessel 5: one nail + water and oil.

NaCl solution (Marine environment)

These experiments approximately reproduce the chemical conditions in coastal areas, in acidic pH situations, and in drier areas.

Diluted HCl or vinegar (acidic pH)

OBSERVATION AFTER 24 HOURS The oxidation in vessel 2 results in the formation of iron(III) oxide, which is suspended in the water in the form of a colloidal precipitate. Can you seen this reddish-brown precipitate in the other vessels? If not, this does not mean that the iron has not been oxidised, but instead that the iron(III) oxide has been dissolved, as in an acidic medium it will not form a precipitate.

Water and oil (Isolated environment)

Factors that affect LA the reaction

2 Fill in the missing information at points A, B, C, D and E in the systemic concept map below, using the knowledge you have learnt in this unit.

To see if oxidation has occurred, add a few drops of sodium hydroxide solution. If the precipitate forms this means that the oxidation had occurred before.

Now that you have completed this activity, give your opinion on cleaning iron with hydrochloric acid.

is made up of

6

Using collision theory, explain how the concentration of reactants affects the reaction rate.

7 The following data was obtained by measuring the concentration of a substance in a chemical reaction:

is shown as C

PROPOSED PROBLEM products A Analyse the effect of the degree of sub-division in a solid reactant, the concentration, and the presence of catalysts on the reaction rate.

which tells us about

whose bonds

PROCEDURE Safety measures

To do so, we suggest you perform the following experiments with these two chemical reactions:

Both reactions release gases. Use safety glasses to avoid being splashed.

Reaction 2: Zn (s) +H2SO4 (aq) 8 ZnSO4 + H2 (g)

Experiment 1.

Degree of sub-division of a solid reactant • Place a known mass of limestone powder in one test tube and the same mass of marble pieces in another. Add enough water to both tubes to suspend the limestone powder and cover the marble pieces.

• Put the same amount of hydrochloric acid in two other test tubes.

EXPERIMENT

Boiled water (BLANK)

4

A chemical reaction

An experiment is proposed in which several of the factors affecting the rate of the oxidation reaction are changed. To carry it out you will need: iron nails, diluted hydrochloric acid (sold in drugstores in Spain under the name salfumán), water, sodium chloride (table salt), vinegar, sodium hydroxide, and four containers with a large surface area and a shallow depth.

Rusted nails.

Reaction rate

Systemic concept map

1 Explain, with your own drawings, the factors which affect the reaction speed and the difference between a endothermic and exothermic reaction.

SHOP SCIENCE WORK

4

REVIEW

• Carefully add the hydrochloric acid to the test tubes with care. Write down what happens.

break

Reaction 1: CaCO3 (s) + 2 HCl (aq) 8 CaCl2 + CO2 (g) + H2O

B

YOUR PROPOSAL Design one experiment to check qualitatively how reactant concentration and degree of sub-division of a solid reactant affect the reaction rate. Design another to test the effect of a catalyst.

Reactant concentration

O O

5

Fe C

• Put the bits of zinc in a test tube and cover them with the solution of diluted sulfuric acid. Observe and write down what happens.

• In another test tube, combine bits of zinc and copper and add the same amount of diluted sulfuric acid.

Observe and write down what happens.

1 16

1.72

800

1.3

1 200

0.98

1 600

0.73

2 000

0.54

2 400

0.39

2 800

0.28

14 Calculate the molar masses of these substances from the average atomic mass data in the periodic table.

Recall that you must write the formula for each: a) Calcium chloride.

Explain the advantages of catalysts in reactions that would otherwise require extreme temperature or pressure conditions.

Amount of substance

d) C3H6O + O2 8 CO2 + H2O

10 Specify whether the following statements are true or false:

1-2-4. From the information in the images below, specify which bonds break and which form in these chemical changes:

a) A mole of water has the same number of molecules as a mole of butane. b) A mole of water has the same number of hydrogen atoms as a mole of butane. c) A mole of water has one mole of oxygen atoms and two mole of hydrogen atoms.

24

CH4

CO2

2 O2

1 Prepare a table with the conditions of each experiment,

together with your observations and draw conclusions.

142 2 From experiment 3, do another experiment with only

N2

2 H2O

+

+

Understand, think, search...

e) A mole of water has 1.8066 · 1024 molecules.

+

+

+

3 H2

2 NH3

Solutions to the numeric activities are on anayaeducacion.es.

18

400

d) A mole of water has 1.8066 · 10 atoms.

• Repeat experiment 1, with just the marble in distilled water. Repeat the experiment three times: once with the original hydrochloric acid concentration and twice with the diluted concentration.

Experiment 3. Presence of a catalyst

9

Molar mass (g/mol)

O

c) MgI2 + Mn(SO3)2 8 MgSO3 + MnI4 e) SeCl6 + O2 8 SeO2 + Cl2

4 Check that the previous reactions follow the law of conservation of mass given the masses of the atoms in the substances present.

+

• Prepare two more test tubes of hydrochloric acid diluted to half the original concentration.

Experiment 3.

b) P4 + O2 8 P2O3

Cu

Experiment 2.

Let’s check. Balance these chemical equations:

a) MgCl2 + Li2CO3 8 MgCO3 + LiCl

Mass (g)

H

2.32

8 Explain how temperature affects reaction rates according to the kinetic theory of matter.

3

Atoms or molecules

20

Concentration (mole/L)

0

b) Show the concentration data as a function of time. The slope of the graph at each point is a measurement of the reaction rate. Does the reaction rate rise or fall as the reaction progresses?

Explain why.

negative

Amount of substance (mol) H2O

Time(s)

a) Is it a reactant or a product?

energy of the reaction

E

OUR PROPOSAL Both reactions release a gas. So the speed of the gas bubbles could Chemical changes give an idea of the reaction rate. Only change one variable per part of the experiment.

1 Ideas pool. List five chemical changes you are YOU WILL NEED aware of in your everyday life.

• Powdered limestone • Pieces of marble • Hydrochloric acid solution 2 Write the chemical equations for these chemical • Copper and zinc bits • Diluted sulfuric acid solution • Test tubes • Tube changes at the molecular level: rack • Volumetric flask • Distilled water Cu

D

state of matter of the substances

13 Complete the following table with the quantities in 20 mol of water:

11 How many atoms are there in a 360 g sample of copper?

12 Calculate the number of hydrogen atoms in a gram of ammonia, hydrogen chloride, and calcium hydroxide.

What is the hydrogen mass in each sample?

e) Chlorine.

b) Silver.

f) Aluminium thrichloride.

c) Diiron trioxide.

g) Hydrogen sulfide.

d) Ozone.

h) Sulfuric acid.

15 Calculate the molarity of the following solutions from the average atomic mass data in the periodic table.

Recall that you must write the formula for each. a) 200 g of sodium hydroxide in a total volume of 500 mL. b) 20 g of sodium nitrate in water in a total volume of 250 mL. c) A solution of nitric acid in water with density 1.12 g/cm3 and percentage of composition by mass of 80 %. d) 200 mL of 2 M hydrochloric acid in water with a total volume of 500 mL.

16 Calculate the amount of substance of hydrochloric acid in the solution in part d) of the previous problem and how much there was before dilution. What do you find?

17 The following data was obtained by measuring the concentration of a solution: Time(s)

Concentration (mol/L)

1

0.012

2

0.015

3

0.017

4

0.020

5

0.023

6

0.014

Write the average value of the concentration, specifying the absolute error.

anayaeducacion.es Check “Let’s Study” and “Learn by playing” sections in the resource bank.

143

the catalyst, the copper, and the acid. What do you see?

3 Would we get the same results as in experiment 3 if we used a compound of copper, such as copper sulfate, instead of copper? Could we say that the copper sulfate catalysed the reaction? Justify your answer.

140

Visual organizer of the content, questions about the key aspects of the unit and activities to reinforce study using different learning techniques.

141

Enterprising culture Trust in your skills and knowledge, develop creativity, adapt to changing situations and have a proactive and responsible attitude.

Academic and professional

ICT

orientation

Assessment

Linguistic Plan

Learn how to obtain information, select it and apply it; to plan, manage and work on projects; to collaborate online in an ethical and safe manner.

Evaluate your personal skills, discover and awaken your calling, train yourself to make decisions and learn to choose between different options.

Discover different strategies to analyse what you have learnt and how you learnt it; train yourself to take responsibility or overcome difficulties.

Use your communication skills in the different types of text that you will see.

Language is always present, communicate!

RESOURCE

BANK

Register at www.anayaeducacion.es to access your resource bank or download your digital book. You just need an email address, the code from the inside cover of this book and permission from your parent or legal guardian.

www.anayaeducacion.es

DIGITAL BOOK RESOURCE BANK

A digital version of your book to be used online or offline. It offers access to your digital resources which are grouped by type or linked to unit content.

A space with resources, techniques and activities, designed to strengthen your knowledge.

More about the keys

Resources related to THE PROJECT KEYS

SDG

SDG Commitment with short videos that will help you understand the targets for reaching the Sustainable Development Goals worked on in this project.

Linguistic Plan with infographics that will give you models to work with the four linguistic skills, using different text types (descriptive, narrative, explanatory, etc.).

Cooperative learning Preparing for the task In small groups, of four or five members: 1 All the members of the team will review how the assigned task can be accomplished.

2 To do this, the steps can be shared out to each team member who then in turn will explain how each part of the process can be done to the others. The others listen and participate if they think they can contribute something.

Authorship / adaptation : Variant of the Educational Innovation Laboratory of the colegio Ártica - David and Roger Johnson.

Developing thinking with explanations on how to apply the different thinking techniques proposed in the project.

3 Once everyone is in agreement on how to do each part, you will all complete the tasks and, finally, verify, among everyone, that you have solved it correctly.

Cooperative learning which includes the descriptions of the cooperative learning techniques proposed in the project.

Thinking techniques

Emotional education with resources to help you overcome any worries that may arise in different situations at school (beginning of the school year, taking a test, etc.).

Logic Wheel This thinking technique will help you to establish phases when analysing specific content that you have to study.

Identify What is it? What is it like? Are there different types?

By following a logical sequence (the logic wheel), and by asking yourself a series of questions in each phase, you can:

1

• Identify content by asking yourself: What is it? What is it like?

Are there different types?

• Compare the content by formulating questions such as: In what way is it similar to ...? In what way is it different from ...?

• Establish cause-effect relationships by asking yourself questions such as: Why? What impact does it have ...?

• Argue, assess and ask yourself questions such as: What conclusions can be drawn after the analysis? What can be assessed or scored about it?

Doing a data dump of these questions into a graphic organiser will help you.

Compare

Argue, assess What can we conclude?

4

Logic wheel

2

In what way is it similar to ...?

In what way is it different from ...?

3 Establish cause-effect relationships Why? What impact does it have ...?

Authorship: Hernández, P., and García, L. A.; adapted by Escamilla, A.

ICT resources to help you use information and communication technology in a healthy, correct and safe way.

Academic and professional orientation with information on different professions linked to the subject content.

0:40/1:33

Assessment which includes resources for your portfolio, as well as rubrics and targets that will help with your self-assessment.

resources

SUBJECT KEY CONCEPTS What you should know To study Learn by playing Virtual laboratories and simulations Self-assessments Solutions

Resources classified by unit

All the resources are classified by unit so that you can find them more easily.

Course Contents Scientific activity

Page 8

What is scientific study.......................................................... 8 1. Scientific research................................................................ 10 2. Physical quantities and units............................................ 16 3. Measuring physical quantities. Errors........................... 20 4. Analysing experimental data............................................ 24 Problem-solving guidelines Solving problems....................................................................... 26

3

Compounds of carbon

Page 96

Convenient carbon................................................................... 96 1. The carbon atom................................................................... 98 2. Allotropes of carbon............................................................ 100 3. Formulas and molecular models..................................... 102 4. Hydrocarbons......................................................................... 106 5. Compounds of carbon with oxygen and nitrogen... 109 6. Molecules of special interest............................................ 112

Science workshop Research project........................................................................ 28 Thought experiments............................................................... 34 Can you talk about ‘the density’ of modelling clay...... 35

ICT skills. ICT helps you consolidate knowledge........... 114

Review............................................................................................ 36

Review............................................................................................ 118

1

The atom and the periodic table

Page 38

The complexity of an atom................................................... 38 Remember that........................................................................... 40 1. The first atomic models...................................................... 42 2. Atomic spectra and the Bohr model............................. 44 3. Quantum model of the atom............................................ 46 4. The periodic table of the chemical elements............. 49 5. Atomic masses....................................................................... 52 Problem-solving guidelines How to solve an isotopic abundance problem............... 53 ICT skills. Atomic models......................................................... 54 Science workshop The petites Curie........................................................................ 56 Cathode rays and discharge tubes..................................... 57

Science workshop Molecular models....................................................................... 116 The solubility of polystyrene................................................. 117

4

Chemical reactions: fundamentals

Page 122

When the miracle became science.................................... 122 1. Chemical changes................................................................. 124 2. Reaction rate.......................................................................... 126 3. Amount of substance.......................................................... 128 4. Stoichiometric calculations .............................................. 130 5. Energy in chemical reactions........................................... 133 Problem-solving guidelines How to solve a stoichiometry problem............................. 136 ICT skills. Simulating chemical reactions.......................... 138

Review............................................................................................ 58

Science workshop The oxidation of iron................................................................. 140 Factors that affect the reaction........................................... 141

Apendix. Formula writing and nomenclature................. 62

Review............................................................................................ 142

2

Chemical bonds and intermolecular forces

Page 72

Chemical bonds......................................................................... 72 1. The chemical bond............................................................... 74 2. Ionic bonds.............................................................................. 76 3. Covalent bonds...................................................................... 78 4. Intermolecular forces.......................................................... 82 5. Metallic bonds........................................................................ 85 6. Summary of the properties of chemical compounds............................................................................. 86

5

Some important chemical reactions

Page 146

A chemical reaction, two positions and the birth of modern chemistry............................................................... 146 1. Acids and bases.................................................................... 148 2. Combustion reactions......................................................... 152 3. Importance of combustion reactions............................ 153 4. Synthesis reactions.............................................................. 156

Problem-solving guidelines How to solve a chemical bond problem........................... 87

Problem-solving guidelines Applied chemical reaction exercises.................................. 158

ICT skills. Spreadsheets and web resources.................... 88

ICT skills. Graphic representations...................................... 160

Science workshop Types of substances.................................................................. 90 Determining the type of bond.............................................. 91

Science workshop Homemade pH indicators....................................................... 162 Identifying CO2 in combustion.............................................. 163

Review............................................................................................ 92

Review............................................................................................ 164

6

Kinematics

Page 168

Real free fall................................................................................ 168 1. Frame of reference............................................................... 170 2. Quantities of motion............................................................ 172 3. Types of motion..................................................................... 179 4. Linear motion......................................................................... 180 5. Circular motion...................................................................... 186 6. Interpreting graphs.............................................................. 190 Problem-solving guidelines Solving kinematics problems................................................ 192 ICT skills. Spreadsheets for studying motion.................. 194 Science workshop Time as a factor of force......................................................... 196 Is it uniformly accelerated motion?..................................... 197 Review............................................................................................ 198

9

Forces in fluids. Pressure

Bíbilis: evidence of an understanding of pressure..... 256 1. Pressure.................................................................................... 258 2. Hydrostatic law...................................................................... 260 3. Archimedes’ principle......................................................... 264 4. Pascal’s law............................................................................. 267 5. Atmospheric pressure......................................................... 268 6. Meteorological concepts.................................................... 272 Problem-solving guidelines Problems relating to pressure and buoyancy................. 274 ICT skills. Online interactive application.............................. 276 Science workshop Atmospheric pressure.............................................................. 278 Pascal’s barrel.............................................................................. 279 Review............................................................................................ 280

10 7

Newton’s laws of motion

Page 202

The weight of an apple........................................................... 202 1. Forces........................................................................................ 204 2. Everyday forces..................................................................... 2 08 3. Newton’s laws of motion.................................................... 210 4. Newton’s laws in everyday motion................................. 214 Problem-solving guidelines Newton’s laws of motion exercises..................................... 217 ICT skills. Physical phenomena simulators....................... 220 Science workshop Friction........................................................................................... 222 Sliding friction coefficient ..................................................... 223 Review............................................................................................ 224

8

Forces in the universe

Page 228

The universe, the great unknown....................................... 228 1. Historical evolution of the study of the universe...... 230 2. Gravitational forces.............................................................. 235 3. Applications of the universal law of gravity............... 238 4. Artificial satellites in orbit.................................................. 243

Page 256

Mechanical energy and work

Page 284

From vis viva to current energy theories........................ 284 1. Energy....................................................................................... 286 2. Work.......................................................................................... 290 3. Power......................................................................................... 295 4. Kinetic energy........................................................................ 296 5. Potential energy.................................................................... 299 6. Conservation of mechanical energy.............................. 302 7. Energy transport via mechanical waves........................ 304 Problem-solving guidelines How to solve energy problems............................................. 306 ICT skills. Working with spreadsheets............................... 308 Science workshop The Sun as a renewable energy source............................. 310 Conservation of mechanical energy................................... 311 Review............................................................................................ 312

11

Thermal energy and heat

Page 316

Heat: an invisible material or an energy transfer?...... 316 1. Thermal energy. Temperature.......................................... 318 2. Thermal equilibrium. Heat and heat transfer............. 322 3. Effects of heat........................................................................ 326 4. Heat engines........................................................................... 330 5. Energy loss.............................................................................. 332 Problem-solving guidelines Solving heating problems....................................................... 335 ICT skills. Geogebra................................................................... 336

ICT skills. Stellarium.................................................................. 248

Science workshop Constructing a spectroscopy................................................ 340 Water’s latent heat of fusion................................................. 341 Review .......................................................................................... 342

Science workshop Simulation of Einstein’s theory of gravity......................... 250 Finding celestial objects.......................................................... 251

Annex

Review............................................................................................ 252

• Glossary............................................................................................. 346

Problem-solving guidelines How to solve a gravitational problem................................ 246

4

Chemical reactions: foundations Reading and listening

When the miracle became science The earliest evidence of the production and consumption of wine is found in a ceramic vase found in the Zagros mountains (in what is now Iran and Iraq) which is dated around 5400 BCE. Traditionally wine, and even more so its production process, has been related to the mysterious and divine. In fact, both the Greeks and later the Romans had a god of wine and vineyards: Dionysus and Bacchus, respectively.

These beliefs changed, developed and eventually disappeared with the help of logic and curiosity. This meant that in the Middle Ages it was thought that wine resulted from the decomposition of matter (grape juice), while in the 19th Century it was discovered that wine was the result of a chemical reaction: fermentation.

The French chemist Louis Pasteur was the first person to explain fermentation, and thereby provided a scientific basis for the production of wine. This chemical change involves the transformation of sugar (glucose) into ethyl alcohol and carbon dioxide, which occurs due to the activity of yeasts (fungi) such as Candida, Pichia, Zygosaccharomyces or the popular Saccharomyces cerevisiae. There is now a science that specialises in the study of these chemical process. Do you know what it’s called?

1 Who was the Greek god of wine?

2 What chemical reaction is responsible for the production of wine?

3 When was fermentation discovered?

4 What turns into what in the fermentation process?

5 What are the main organisms responsible for fermentation?

122

Speaking 2020 was a disastrous year for the vineyards of Galicia, Catalonia, Castille-La Mancha and Andalusia, due to a plague of the fungus Plasmopara viticola. This causes powdery mildew, which generally requires treatment with fungicides to save the harvest.

You are going to prepare a presentation for farmers that:

• Explains the SDGs that are relevant to the issue and how they can help the farmers • Explain at least one alternative treatment for fighting mildew I recommend that you try an alternative to fungicides such as

Writing The following is a paragraph taken from an article that tells us about the causes and repercussions of the powdery mildew.

In pairs, fill in the missing verbs.

The weather ________ many challenges over the year with very few regions getting off lightly. It ______ a much wetter year, especially for coastal regions, and generous spring rains _________ with warmer temperatures than usual. This ________ the ideal conditions for mildew which ________ intense work in the vineyard but ultimately this impacted more on yield than quality. Finally, drier weather and higher summer temperatures _____ mildew retreat.

Every stage of vine development took place earlier than usual and harvest dates _____ generally up to 14 days earlier _______ to a typical year. A few days of rain favoured the last stages of the ripening period for many regions without disrupting the harvest.

One exception to this ______ Ribera del Duero where rain _____ frequent breaks in the harvest and difficult decisions over whether to pick or wait.

Verbs: coincide, compare, is (three times), bring, provide, see, cause, require Source: https://www.foodswinesfromspain.com/spanishfoodwine/ global/whats-new/features/feature-detail/spain-2020-wineharvest.html

NK E BANK A B E G A ANGU LANGUAG L ANK ANK GE BANK B B E E G G A A U U GUA K LANG ANG N L A L AN GE BANK 123 ANK GE BANK B B E E G G A A U U G A LAN LANG LANGUA LANGU

1

ä 1.1 Basic concepts

CHEMICAL CHANGES

In a chemical reaction, we begin with substances called reactants, which are there before the chemical change. Then, new substances appear called products, which are a result of the chemical change.

The reactants must be in contact for a chemical change to occur. Physical changes occur during the process too, which make the change visible. You can see some examples below.

Physical evidence of a chemical change

Energy exchange.

Colour change.

State change.

ä 1.2 Atomic theory of chemical reactions The appearance of new substances in a chemical reaction is based on a reordering of the reactant atoms. For this to occur, the elemental units of the reactants must collide. These collisions break bonds and form new bonds, making new substances.

Working with pictures

Since this is a reordering of atoms, the total mass of the substances before and after the chemical reaction does not change, so we can state that:

From the information in the illustration, work out which bonds have broken and which have formed in the chemical change.

Atomic theory and conservation of mass The drawing on the left shows the oxidation of nitrogen monoxide to form nitrogen dioxide.

+ 2 NO

124

During a chemical change, mass is conserved, which means that the total mass of the reactants must equal the total mass of the products.

This statement is known as the law of conservation of mass.

+

We can see that the atoms are in a different order, and that mass is conserved throughout the process.

O2

2 NO2

At anayaeducacion.es there is a resource about the evidence of chemical changes.

Unit

4

ä 1.3 Writing a chemical reaction: the chemical equation

We use chemical equations to describe chemical reaction quantitatively.

Chemical equations use stoichiometric coefficients to express the relationship between the reactants and products in a chemical reaction and the proportions of their elemental units.

Sometimes, they also give information about other factors, such as the state of matter of substances, their temperature or their pressure.

Balancing a chemical equation involves choosing the values of the coefficients, in order to make sure that the number of atoms of each element is the same among the reactants as among the products.

Writing and balancing chemical equations We will use the example of the reaction between ammonia and molecular oxygen. The products of this reaction are molecular nitrogen and water.

1 W e write the formulas of the reactants and products, separating them with

an arrow showing the direction of the reaction:

NH3 + O2 8 N2 + H2O 2 W e choose values for the stoichiometric coefficients to make the number

of atoms of each element the same among the reactants and among the products. To do so:

F ocu s on Eng lish You probably already know the word equation from maths, as well as from chemistry. Have you ever wondered where it comes from? Equation comes from the Latin word aequatio, which means, an equal distribution. It can be used figuratively: ‘If we take Peter out of the equation, we will find a solution to the situation’. In can also be used in a scientific context, meaning the act of making equal or balanced: ‘To him, the equation was simple: time equals money’.

With a classmate, discuss whether you agree with the following phrase by Albert Einstein: ‘One must divide one’s time between politics and equations.

But our equations are much more important to me, because politics is for the present, while our equations are for eternity’.

2.1 First, we adjust the coefficients of the non-elemental substances: 2 NH3 + O2 8 N2 + 3 H2O 2.2 T hen, we do the same for the elemental substances. If necessary, we correct the coefficients we chose before. We can use fractions.

2 NH3 + 3/2 O2 8

N2 + 3 H2O

Understand, think, search…

1

CH4 + 2 O2 8 CO2 + 2 H2O

Note that the stoichiometric coefficient of molecular nitrogen is 1, therefore it is not written.

Which bonds are broken and which new bonds form? How many bonds are broken and how many new bonds form?

We can also multiply by two the chemical equation so that there are no fractional coefficients: 2 NH3 + 3/2 O2 8

4

1-2-4. Draw a table, like the one in the example, for the following reaction:

N2 + 3 H2O

2 Explain

what the stoichiometric coefficients mean in the exercise above.

3 2 6

3 F inally we check, element by element, that the number of atoms among the

reactants is the same as among the products: Elements

Reactants

Products

H atoms

4 NH3 (4 · 3 = 12)

6 H2O (6 · 2 = 12)

N atoms

4 NH3 (4 · 1 = 4)

2 N2 (2 · 2 = 4)

O atoms

3 O2 (3 · 2 = 6)

6 H2O (6 · 1 = 6)

3

Let’s check. Balance following chemical reactions

the

a) CO2 + H2O 8 H2CO3 b) NO + O2 8 NO2 c) Cr2O3 + Al 8 Al2O3 + Cr d) NO2 + H2O 8 HNO3 + NO e) C2H2 + O2 8 CO2 + H2O

125

2 REACTION RATE

The reaction rate of a chemical reaction is defined in terms of the time it takes to make a reactant disappear or for a product to appear.

Sometimes, it is possible to determine the reaction rate from a change in another property where the chemical reaction occurs.

ä 2.1 Collision theory As we have seen, chemical reactions break and form bonds as a result of collisions between elemental units of the reactants.

Not all collisions cause the breaking or forming of bonds; not all collisions are effective. Only collisions with sufficient energy and whose molecules have the right orientation may break or form bonds.

ä 2.2 Factors that affect reaction rates Many factors may change the rate of a reaction. Their importance depends on the state of matter of the reactants. They are: Temperature In reactions involving liquid or gas, raising the temperature accelerates the velocity of the elemental units in the reactant. This causes more collisions which have sufficient energy to contribute to the reaction.

Concentration of reactants In reactions that are in aqueous solutions, increasing the concentration raises the probability of collisions, which accelerate the reaction rate.

State of sub-division of reactants

Working with pictures

In cases where one of the reactants is in a solid state, the size of the reactant determines the reaction rate. You can see an example of this in the figure below.

126

Imagine that one side of the cube is 1 cm. Calculate the available surface in all three cases. Work out a general rule as a function of the number of divisions.

Effect of sub-divisions The figure explains why the reaction rate is a function of the size of the solid reactant.

Note that as the number of subdivisions grows, the surface area in contact with the other reactant (which may be a gas or liquid) grows. The number of effective collisions between molecules of both reactants will rise and so will the reaction rate.

Unit

4

ä 2.3 Catalysts In many cases, when modifying a reaction rate, we must change certain factors to a point that it is no longer worth it. In that case, we tend to use catalysts.

Catalysts are chemical substances that modify the reaction rate without changing the nature of the reactants or the products.

This property, of not changing the reactants or products in a reaction, is the most important since it determines whether or not a catalyst may be used. Catalysts have two other properties: • They are extremely specific. Generally, for each chemical reaction there is a specific catalyst. While many reactions might be possible with the same reactants, the presence of a catalyst initiates a specific chemical reaction.

F ocu s on Eng lish Tend - to often behave or act in a certain way or to have certain characteristic.

• They take part in the chemical reaction without undergoing permanent changes. Most catalysts reappear after a reaction, so they are not included in the chemical equation as reactants or products.

Enzymes as catalysts Formed by E N

Properties

Z

Amino acids (combinations of H, C, O, N, S)

Which determine their

Structure and function

• Change their reaction rate • Are very specific • Act at specific pH and T

Y

• Digestive system • Degradation of

M E

Biological

S

molecules

• Celular respiration • Transport of

energy and signals

Applications

Industrial

• Pharmaceuticals • Food production • Development of

Enzymes are molecules that act as catalysts inside of organisms. Their action is very specific; there is practically one enzyme for each chemical reaction that occurs inside each living thing.

In the case of humans, their presence is vital since if their catalytic effect does not occur, thermoregulation, arterial pressure, and DNA formation may be harmed.

The figure shows the threedimensional structure of an enzyme. Enzymes can come in many sizes and shapes.

biofuels

Understand, think, search…

4 If we mix a solution of sulfuric acid with water, we must

6

Write around. What do you think would happen if the chemical reactions in our bodies took place without catalytic enzymes?

Given that these two substances react violently, explain why we take this precaution. With which physical factor related to reaction rates would you associate it?

7

Search for pictures of different enzymes in our bodies. Draw them in your notebook and explain the differences and similarities in their shapes and sizes. What do you think explains these differences and similarities?

always add the acid drop by drop to the water container and not the other way round.

5

Explain why ground meat spoils faster than a similar-sized cut of unground meat. With which physical factor would you associate this?

8 Search for information about lactase and the reason

why some people cannot digest milk. Do you know any other type of enzyme whose absence may also prevent digestion? How might we fix this?

127

3

ä 3.1 Amount of substance

AMOUNT OF SUBSTANCE

In a chemical reaction it helps to know how many elementary units, N, there are of each substance. To do so, we use the quantity called the amount of substance (also called a chemical amount).

Analysis of 1 mole of H 2 O 1 mole of H2O

contains

6,022 · 1023 molecules

has a mass of

each with a mass of

18 g

18 u

of which

of which

16 g are O

2g are H

16 u are O

2u are H

The amount of substance, n, is a measurement of the number of elementary units contained in a specific amount of substance. The mole is one of the base units of the SI, and has the unit symbol mol.

Since the masses of atoms are of order 10–27 kg, the number of elementary units in 1 kg of any substance is very large. Using the mass of the carbon-12 isotope as a reference, we can establish that a mole of a substance contains 6.022 · 1023 elementary units. This value is a constant known as Avogadro’s number, NA. A mole of any substance contains Avogadro’s number of elementary units (NA = 6,022 · 1023 elementary units/mol). It is defined as the amount of substance of a system that contains as many elementary units as there are atoms in 0.012 kg of carbon-12.

So if a portion of a substance contains N elementary units, the amount of the substance, n, is given by the formula: N n= NA Molar mass and volume The amount of substance is quantified in indirect form by measuring the mass or volume of a substance and establishing links.

• The molar mass, M, of a substance is the mass, m, in grams, of one mole of the substance. Its unit is g · mol–1, and its numeric value equals the molecular mass or the mass of the formula of the substance. m (g) n (mol) = M (g $ mol–1) • The molar volume is the volume occupied by a mole at a certain pressure and temperature. In standard conditions (273 K and 1 atm) the molar volume of any gas is about 22.4 L/mol.

Problem solved 1 Obtain, as explained, the value of Avogadro’s number, NA. The text has defined the mole as the amount of substance in a system that contains as many elementary units as there are atoms in 0.012 kg of carbon-12. This number of elementary units is the same as Avogadro’s number: NA = 6.022 · 1023 elementary units/mol Given that the C-12 isotope has a mass of 12 u, that is, 1.9932 · 10–26 kg (1 u = 1.661 · 10–27 kg), the quantity of C-12 isotopes in our 0.012 kg sample will be: 0.012 kg N= 8 N - 6.022 $ 1023 units = NA –26 1.9932 $ 10 kg/isotope of C-12

128

There is a resource on the amount of substance and it’s unit symbol mol at anayaeducacion.es.

Unit

4

ä 3.2 Molar concentration or molarity Many chemical reactions occur in an aqueous solution, so it is useful to be able to write the concentration of the reactants and products as a molar concentration, or molarity.

The molarity, M, of a solution is the amount of substance in a litre of solution. Its SI unit is mol/L. Molarity lets us make quick calculations of the reactant substances. This means that knowing this parameter makes it useful for directly relating the volume of a solution with the amount of substance it contains. So the mathematical formula is: nsubstance (mol) M= Vsolution (L)

Problem solved 2 Calculate the number of molecules and atoms in a

77 g sample of carbon dioxide. What volume does it occupy in standard conditions? Data: M (C) = 12 g/mol; M (O) = 16 g/mol.

First, we calculate the molar mass of CO2: MCO2 = 12 $ 1 + 16 $ 2 = 44 g $ mol

–1

Then, we work out the amount of substance in the 77 g of CO2: 77 g m n= = = 1.75 mol M 44 g/mol Therefore, the number of molecules of CO2 is: 6.022 $ 1023 molecules 1 mol 24 NCO2 - 1.054 $ 10 molecules

NCO2 = 1.75 mol $

Each CO2 molecule has two atoms of oxygen and one of carbon, so:

3 Calculate the molarity of a solution of sodium

hydroxide given that 500 mL contains 120 g of NaOH. Data: M (Na) = 23 g/mol; M (O) = 16 g/mol; M (H) = 1 g/mol.

In the first place, we calculate the molar mass of NaOH: MNaOH = 23 $ 1 + 16 $ 1 + 1 $ 1 = 40 g $ mol–1 From the molar mass we calculate the amount of substance present: 120 g m n= = = 3 mol M 40 g $ mol–1 We write the volume in the correct units: Vsolution = 500 mL = 0.5 L Therefore, we can calculate the molarity of the solution of sodium hydroxide as: M=

24

NC = NCO2 = 1.054 $ 10 atoms of carbon NO = 2 $ NCO2 = 2.108 $ 1024 atoms of oxygen And since one mole of any gas in standard conditions occupies 22.4 L: 22.4 L Vm = 1.75 mol $ = 39.2 L 1 mol

3 mol =6M 0.5 L

Note that we write the molarity as its value followed by the letter M.

Understand, think, search…

9 Calculate the molar masses of ammonia, hydrogen chloride, and calcium hydroxide.

10

Think and share with a partner. Which of the substances in the previous exercise will have the greatest amount of substance in a kilogram of mass?

11

Search for information about Avogadro’s number and prepare a short report.

12 Calculate the amount of substance in 205 mL of a 2 M

NaOH solution. Will the result change if the solution is of Ca (OH)2?

129

4 STOICHIOMETRIC CALCULATIONS

To perform stoichiometric calculations on a chemical equation we must first balance it. To do so, we use the stoichiometric coefficients to indicate the amount of substance of the reactants and products in the reaction.

Note that if we multiply the coefficients by NA, we can find the number of elemental units in the substances involved.

ä 4.1 Mass to mass stoichiometry We can calculate the mass of an unknown substance in a reaction using the mass of another given substance. Look at the steps in the following example:

Problem solved 4 Potassium chlorate, KClO3, decomposes into potassium chloride

and oxygen. Calculate the mass of oxygen obtained from the decomposition of 86.8 g of potassium chlorate due to heat: M (K) = 39.1 g/mol; M (Cl) = 35.5 g/mol; M (O) = 16.0 g/mol.

1 To relate the amount of each substance in the reaction, we must first

balance the stoichiometric coefficients:

2 KClO3 (s) 8 2 KCl (s) + 3 O2 (g) 2 We identify the given substance and the unknown substance and

write the information as a statement:

F ocu s on Eng lish Stoichiometric comes from the Greek word stoikheion which means element + -metry (the process of measuring).

Did you know that you can find out how to pronounce difficult words like this in English, by looking them up on YouTube?

Type into your search engine ‘Emma saying stoichiometric’.

2 KClO3 (s) 8 2 KCl (s) + 3 O2 (g)

mKClO3 = 86.8 g

3 We calculate the molar masses of the given and unknown substances:

MKClO3 = 122.6 g/mol ; MO2 = 32.0 g/mol 4 W e calculate the amount of the given substance based on its molar

mass: nKClO3 =

86.8 g 122.6 g $ mol–1

- 0.708 mol

5 We calculate the amount of the unknown substance using the

stoichiometric coefficients:

nO2 = 0.708 mol KClO3 $

3 mol O2 8 nO2 = 1.062 mol O2 2 mol KClO3

6 Finally, we find the mass of the unknown substance from its molar mass:

mO2 = 1.062 mol $ 32.0 g $ mol–1 = 33.984 g

Understand, think, search…

13 Calculate the mass of potassium chloride obtained from the reaction in Problem solved 4.

14 Calculate the amount of substance of PCl3 obtained from 426 g of Cl2 in this reaction:

Cl2 + P4 8 PCl3

15 Calculate the amount of substance of P4 necessary in the previous exercise.

130

16

Let’s check. Calcium carbonate and hydrochloric acid undergo the following reaction.

CaCO3 + 2 HCl 8 CaCl2 + H2O + CO2 Calculate the mass of calcium chloride obtained from 350 g of calcium carbonate.

17 What volume of hydrochloric acid is necessary in the previous reaction under standard conditions?

Unit

4

ä 4.2 Calculations with reactants in solution As we have said, many chemical reactions take place in an aqueous solution in which water acts as a solvent.

The substances in such reactions, both reactants and products, are indicated with the abbreviation (aq).

However, water sometimes participates in a reaction as a reactant or product. Look at the example below to help you understand.

Problem solved 5 Calculate the volume of a 0.2 M solution of

hydrochloric acid necessary to completely react with a 250 mL 0.1 M solution of calcium hydroxide.

The products of this chemical reaction are calcium chloride and water.

Just as in the Problem solved on the previous page, we will follow these steps: 1 We start with the balanced chemical equation and

identify the given substance and the unknown substance: 2 HCl (aq) + Ca (OH) 2 (aq) 8 2 H2O + CaCl2 (aq)

Note that water is, in addition to the solvent, a product of the reaction.

2 We write the data from the statement with the

chemical equation:

2 HCl (aq) + Ca (OH) 2 (aq) 8 2 H2O + CaCl2 (aq) VHCl? ;

VCa (OH) 2 = 250 mL = 0.250 L

3 We calculate the amount of substance using the

definition of molarity: M=

n (mol) V (L)

8 n (mol) = M $ V (L)

In this case: nCa (OH) 2 = 0.1 M $ 0.250 L = 0.025 mol 4 Then, we calculate the corresponding unknown

substance from the stoichiometric coefficients: 2 HCl 1 mol Ca (OH) 2 nHCl = 0.050 molHCl

nHCl = 0.025 mol Ca (OH) 2 $

5 Finally, we calculate the volume of the unknown

solution from the definition of molarity: M=

n (mol) V (L)

8 V (L) =

n (mol) M

In this case the data we have is: nHCl = 0.050 mol

;

M = 0.2 mol/L

So the volume of hydrochloric acid necessary to completely react with the 250 mL of solution in the statement is: V (L) =

0.050 (mol) 0.2 mol $ L–1

= 0.250 L = 250 mL

Understand, think, search…

18 Calculate the volume of 2.5 M solution of hydrochloric acid necessary to react with 0.2 mol of zinc per this reaction: 2 HCl + Zn 8 ZnCl2 + H2

19

Find out other ways of writing the concentration of a solution. What is the advantage of molarity for performing stoichiometric calculations?

20 What volume of a 2 M solution of substance A is

necessary to react completely with 3 mL of a 2 M solution of substance B if we know that their stoichiometric coefficients are equal?

21 Calculate the volume of the solution of hydrochloric

acid in the Problem solved on this page if the concentration of the reactant is 0.1 M. What is the relationship between the two results?

131

4 STOICHIOMETRIC CALCULATIONS

ä 4.3 Calculating gas reactions The relationship between the amount of substance of a gas and the volume it occupies depends on its pressure and temperature. The equation relating the quantities for an ideal gas is: p $V =n $R $T

where p is pressure; V, volume; n, the amount of substance; T, temperature, and R, the ideal gas constant, whose value depends on the units in use.

We usually use this value: atm $ L R = 0.082 K $ mol According to Avogadro’s law, two gases at the same pressure and temperature occupy the same volume. So: If the pressure and temperature in a gas reaction stay the same, the ratios of the stoichiometric coefficient will be the same as the ratios between the volumes of gases.

Problem solved 6 Show that the relationship between the volumes of

the gases in this reaction is the same as that between the stoichiometric coefficients, at the same pressure and temperature conditions.

2 N2 (g) + 3 H2 (g) 8 2 NH3 (g) The ratio between the stoichiometric coefficients is: 2 mol of N2 react with 3 mol of H2 to obtain 2 mol of NH3. We assume pressure and temperature are p and T.

The volumes are given by this equation: n $R $T V= p Using the stoichiometric ratio we have: R $T R $T R $T VN2 = 2 $ p ; VH2 = 3 $ p ; VNH3 = 2 $ p Since R · T/p does not change we can conclude that 2 L of N2 react with 3 L of H2 to produce 2 L of NH3, which is the same ratio as the stoichiometric coefficients.

Understand, think, search…

22 Calculate the quantity of methane gas in a 3 litre container if p = 1.2 atm and T = 27 °C.

26 Calculate the volume of CO2 released by burning: a) 5 mol of propane

If we cool it to 10 °C, what would the new pressure be?

b) 3 mol of butane

23 Calculate the volume of hydrogen (H2), measured at

25 °C and 0.98 atm, that escapes when 41.4 g of sodium reacts with water: 2 Na (s) + H2O 8 2 NaOH (aq) + H2 (g)

24

Check that the value of the ideal gas constant is 8.31 J · K–1 · mol–1 in SI units.

25 Check that a mole of any gas measured in standard

conditions (273 K and 1 atm) has an approximate volume of 22.4 L.

c) 2 mol of methane

27

Intuition and deduction. From the combustion reactions of the first four alkanes, find the volume and mass of CO2 released by burning one litre of each, in standard conditions.

CH4 +2 O2 8 CO2 + 2 H2O C2H6 +

C3H8 + 5 O2 8 3 CO2 + 4 H2O C4H10 +

132

7 O 8 2 CO2 + 3 H4O 2 4 13 O2 8 4 CO2 + 5 H2O 2

5

Unit

4

ä 5.1 Endothermic and exothermic reactions

ENERGY IN CHEMICAL REACTIONS

Just like other processes, chemical reactions involve energy exchanges between a system and its surroundings.

When energy is transformed in a chemical reaction it is called chemical energy.

Chemical energy is the energy associated with chemical bonds and intermolecular attraction.

As bonds break and form new bonds, creating new products, the chemical energy of the reacting substances changes.

The changes in chemical energy may imply one of two situations. In the first case, an exothermic reaction may have taken place, which means that the reactants have more energy than the products.

Or, the opposite may be true and an endothermic reaction has taken place, meaning that the products have more energy than the reactants.

Using the difference in energy between the reactants and the products, we can quantify the chemical energy of a chemical change.

F ocu s on Eng lish

DE = Eproducts – Ereactants

Exo- and endo- are prefixes that are used to create compound words and have opposite meanings. Exo- means outside or external, whereas endo- means within.

With a classmate, use an online dictionary to find words beginning with both prefixes and make a list of the most common ones.

The sign of the energy change in an endothermic reaction is positive and for an exothermic reaction, it is negative.

There are very exothermic chemical reactions, such as combustion. Those reactions release chemical energy via heat.

In contrast, endothermic chemical reactions absorb energy from the environment, cooling it.

Types of reactions Exothermic reactions

Endothermic reactions

E

E

PRODUCTS

REACTANTS

PRODUCTS REACTION PROGRESS

In an exothermic reaction, the energy of the reactants is greater than that of the products, and has a negative sign.

REACTANTS REACTION PROGRESS

In an endothermic reaction, the energy of the reactants is less than that of the products, so the energy change has a positive sign.

133

ä 5.2 Energy diagrams and catalysts

5 ENERGY IN CHEMICAL REACTIONS

In the energy diagrams shown on the previous page, only the initial and final energy states of the chemical reactions are shown. However, the evolution of the chemical reaction is more complex than this. Not all collisions between molecules are effective since some lack the right orientation or sufficient energy.

It is possible for the transition state (the state between reactants and products) of a chemical reaction to have more energy than either of these stages. This point in the reaction is called the activated complex (see figure below).

Regardless of whether a reaction is endothermic or exothermic, every chemical reaction passes through this energy state.

Activation energy. Catalysts E

Uncatalysed reaction Activation energy without catalyst, Ea

REACTANTS

The energy difference between the reactants and the transition energy is called the activation energy.

Activation energy with catalyst, Ea'

This energy is a barrier that will only be surpassed by reactant molecules with sufficient energy to produce effective collisions.

PRODUCTS

When this energy is very high and the reactants cannot surpass it, we can use catalysts. These substances lower the activation energy and create a more favourable energetic pathway. This helps convert the reactants into the products.

Catalysed reaction

REACTION PROGRESS

Understand, think, search…

ä 5.3 Energy exchange. Heat of reaction

28 If you touch a beaker containing an

Heat or work can cause an energy exchange between a system and its surroundings or between two systems. When a chemical reaction occurs, there is a variation of the energy in the system which reacts.

29

To measure it, we introduce the concept of the energy of reaction, or heat of reaction, also known as enthalpy:

endothermic reaction, what will you feel?

Search for examples of exothermic and endothermic reactions and their everyday uses.

30 What can you say about a chemical reaction if Qr = –287.9 kJ/mol? And if it has the opposite sign?

31

Write around. Does the presence of a catalyst change the value of Qr?

32 Draw

a diagram representing the exchange of energy in both endothermic and exothermic reactions.

134

The heat of reaction, Qr , is the energy exchanged in heat form between a system containing a chemical reaction and its surroundings.

The heat of reaction allows us to distinguish an endothermic reaction from an exothermic one. To do so, we define a sign convention: • Endothermic reactions. In this type of reaction, the energy in the system grows. The exchange of energy via heat goes from the surroundings toward the system so the heat of reaction DE is positive.

• Exothermic reactions. In this type of reaction, the energy in the system falls. The exchange of energy via heat goes from the system towards its surroundings so the heat of reaction is negative.

Unit

4

ä 5.4 Thermochemical equations Thermochemical equations are equations that tell us not only what substances participate in a chemical reaction but also their state of matter and how much energy they have. The value of the heat of reaction in a thermochemical equation depends on: • The state of matter of the reactants and products, since a state change implies an energy change.

Research project Once you have studied this unit, you will be ready to continue with the research project, completing activity 4.

• The amount of substance in the reaction, so we must note the values of the stoichiometric coefficients used.

For example, the following thermochemical equation tells us that the combustion of one mole of C4H10, liberates 2 877 kJ if the products are CO2 in gas state and liquid water: 13 O2 (g ) 8 4 CO2 (g ) + 5 H2O (l ) Qr = –2 877 kJ 2 For combustion reactions, such as the example, the heat of reaction is called the heat of combustion and is related to a mole of fuel.

C4H10 (g ) +

Sometimes, we can find the heat of reaction on its own in a chemical equation. It will be attached to the reactants in an endothermic reaction and to the products in an exothermic reaction.

In the section ‘Academic and Professional Orientation’ in your resource bank, you will find information about education and the development of renewable energy.

Problem solved 7 Calculate the amount of energy liberated when

We calculate the amount of each fuel:

a kilogram of butane reacts and when a kilogram of methane reacts:

Qr (CH4) = –890 kJ/mol; Qr (C4H10) = –2 877 kJ/mol; M (C) = 12 g/mol, M (H) = 1 g/mol.

The heat of combustion refer to a mole of fuel. So, we must calculate the amount of each substance by adding the reactants.

nCH4 =

1 000 g = 62.5 mol 16 g/mol

nC4H10 =

1 000 g - 17.2 mol 58 g/mol

Finally, the heat released by the fuel’s combustion:

First, we must know the molar masses, which we will calculate from the molecular formulas:

Qc (CH4) = 62.5 mol $

MCH4 = 12 $ 1 + 1 $ 4 = 16 g/mol MC4H10 = 12 $ 4 + 1 $ 10 = 58 g/mol

Qc (C4H10) = 17.2 mol $

890 kJ - 5.56 $ 104 kJ 1 mol 2 877 kJ - 4.95 $ 104 kJ 1 mol

Understand, think, search…

33

The heat of reaction in a thermochemical equation is positive and attached to the reactants. Is the reaction endothermic or exothermic?

34 Calculate the energy necessary to form 75 g of nitric oxide in this reaction:

35

Round table. The heat of combustion of benzoic acid is –26.42 kJ/g: a) Is this an endothermic or exothermic reaction? b) How much energy is released as heat during the combustion of 3 mol of benzoic acid (C7H6O2)?

N2 (g) + O2 (g) + 181 kJ 8 2 NO (g)

135

olving

Problem-s

How to solve a stoichiometry problem

guidelines

EXAMPLE 1 A sample of pure aluminium reacts with 150 mL of hydrochloric acid (HCl) concentrated at 5 M. Aluminium chloride (AlCl3) and hydrogen gas (H2) form. What was the mass of the aluminium sample? What is the volume of hydrogen released, assuming standard conditions? Data: M (Al) = 27 g/mol.

1

2

3

4

136

STEPS

SOLUTION

UNDERSTAND THE PROBLEM STATEMENT AND EXTRACT DATA Search for the meaning of any relevant terms that you do not understand. On the second reading, extract the data given and identify what the problem asks for. Remember to write quantities with their symbols and use subscripts for different substances.

• The statement describes a chemical reaction.

• We distinguish the given data from what we must calculate: – Given substance: hydrochloric acid: VHCl = 150 mL; 5 M = 5 mol/L

BALANCING THE CHEMICAL REACTION This is a stoichiometry problem since we are given substances that react. We must write the balanced chemical equation together with the data and the molar masses of the substances.

The balanced chemical equation is:

– Unknowns: mass of Al and volume of H2 in standard conditions.

2 Al (s) + 6 HCl (aq) 8 2 AlCl3 (aq) + 3 H2 (g) mAl V = 150 mL MAl = 27 g/mol

VH2 , c.n.

5 M

So, 2 mol of Al combine with 6 mol of HCl to produce 2 mol of AlCl3 and 3 mol of H2.

IDENTIFY THE RELEVANT LAWS • Equation for molar mass, amount of substance, and mass: m (g) n (mol) = M • Ideal gas law, P · V = n · R · T, if there are gas reactants or products.

• Definition of molarity: n (mol) M= V (L) • Definition of the heat of reaction.

In this case we must follow this sequence:

OBTAIN A RESULT AND CHECK IT • Perform the calculations necessary to complete your problem-solving strategy and remember to use consistent units.

• Analyse your result and discuss it if necessary.

We calculate the quantity of HCl: mol nHCl = 5 $ 0.15 L = 0.75 mol L From that value we find the quantity of Al and H2: 2 mol Al nAl = 0.75 mol HCl $ = 0.25 mol 6 mol HCl 3 mol H2 nH2 = 0.75 mol HCl $ = 0.375 mol 6 mol HCl Finally, we calculate the mass of Al and volume of H2: mAl = 0.25 mol $ 27 g/mol = 6.75 g atm $ L 0.375 mol $ 0.082 $ 273 K K $ mol VH2 = - 8.39 L 1 atm

1. Calculate the amount of substance: n (mol) = M · V (L) 2.

Use the stoichiometric ratio to calculate the amount of each unknown substance.

3. Use the molar mass and ideal gas equation to calculate the mass and volume of the unknown substances, respectively.

Unit

4

EXAMPLE 2 Calculate the energy released as heat when 440 g of propane reacts with enough oxygen.

Datum: Qr = –2 218.8 kJ/mol.

Understand, think, search…

36 Calculate the mass of aluminium chloride formed in the reaction in Example 1.

SOLUTION • The problem describes an exothermic reaction and asks us to find the energy released as heat.

• Data we can extract: – Datum given: propane, C3H8: mC3H8 = 440 g – Heat of reaction: Qr = –2 218.8 kJ/mol

Datum: M (Cl) = 35.5 g/mol.

37 Calculate the volume of hydrogen released in

Example 1 if the temperature is 25 °C. Will it be more or less than in the example?

38 Calculate the amount of oxygen necessary to combust all the propane in Example 2.

39

Pencils in the middle. Calcium hydride reacts with water according to the following chemical reaction: CaH2 (s) + H20 (l ) 8 Ca(OH)2 (s) + H2 (g)

The balanced chemical equation is: C3H8 + 5 O2 8 3 CO2 + 4 H2O Qr = –2 218.8 kJ MC3 H8 = 44 g/mol Which means that the combustion of a mole of propane releases 2 218.8 kJ.

In this case we must follow this sequence: 1. Calculate the amount of substance: m (g) n (mol) = M 2. Calculate the energy released as heat from the thermochemical equation.

a) Balance the chemical equation. b) Calculate the amount of calcium hydride that would react with 3.6 mol of water. c) Calculate the mass of calcium hydroxide formed with the same amount of water. d) What volume of hydrogen, measured at 300 K and 1.2 atm, releases from the water in the reaction?

Data: M (Ca) = 40 g/mol; M (O) = 16 g/mol; M (H) = 1 g/mol.

40 How much energy is released when 330 g of C2H4O2

reacts with enough oxygen? If the reaction occurs in standard conditions, how much CO2 forms? What about water? Qc = –875.4 kJ/mol.

Data: M (C) = 12 g/mol; M (O) = 16 g/mol; M (H) = 1 g/mol.

41 A sample of iron mixes with 200 mL of sulfuric acid,

We calculate the quantity of C3H8, dividing by the molar mass (M), which we can write as: 440 g nC3H8 = = 10 mol 44 g/mol We calculate the energy released: (–2 218.8 kJ) Energy = 10 mol C3H8 $ = –22 188 kJ 1mol C3H8 We write the result with a negative sign to specify that it is energy released from the system.

H2SO4, of concentration 3.5 M. The reaction forms hydrogen gas and iron (II) sulfate. What was the mass of the sample? What volume of hydrogen does the reaction release at 30 °C and 0.95 atm?

Data: M (Fe) = 55.8 g/mol; M (S) = 32 g/mol; M (O) = 16 g/mol; M (H) = 1 g/mol.

42 One of the ways to get hydrogen is to convert methane:

CH4 + H2O 8 CO + H2 a) Balance the chemical equation. b) Calculate the volume of methane, measured at 100 °C and 1 atm, needed to produce 150 m3 of hydrogen measured in the same conditions. c) How many mols of CO are produced?

137

ICT SKILLS

Simulating chemical reactions INTERPRETING CHEMICAL EQUATIONS

On this first page, we show you two tools that will help you practice balancing chemical reactions and understand the proportion of the reacting substances

BALANCING CHEMICAL EQUATIONS We will use an application available on the Anaya website. The application has two parts: Introduction and Play.

• The application lets you balance chemical equations in a visual and intuitive way. It continually updates the number of atoms of each element in the reactants and products. To begin, use the Introduction part: activate the tool in the upper right that lets you visualise the balance of elements.

given the amount of products and reactants at the end of the reaction. In future years, this will help us understand the concept of the limiting reactant.

REACTANTS, PRODUCTS AND LEFTOVERS We will now use an application that complements the previous one. You can download it at https://phet.colorado.edu/en/simulation/ reactants-products-and-leftovers.

It begins with an example of 2:1 stoichiometry: making a sandwich requires 2 slices of bread and 1 slice of cheese. It seems simple but this is the idea we will use to predict which reactant will be leftover.

• Note that the number of molecules, or fundamental units, must be a whole number in this application.

• Use Play to raise the level of difficulty of the equations to balance. If you do not succeed with a problem, the application will give you the option to guess why and make a new attempt, as shown in the picture:

Understand, think, search…

1 Explain why the the coefficient of the simple or elemental substances should be adjusted last.

Once we have worked with the application, we must take a step beyond and see what happens in situations where the fundamental units of the reactants are not in stoichiometric proportions.

138

2 In the same way that when we have four slices of bread and just one of cheese, the cheese ‘limits’ how many sandwiches we can prepare (just one). In stoichiometry we use the concept of the ‘limiting reactant’, which we will study in more depth in future years. If we take the example of the formation of ammonia and we have two mole of nitrogen (N2) and three of (H2), which is the limiting reactant?

Unit

4

SIMULATING REACTION RATES In this unit we have studied the factors that affect the rate of chemical reactions.

We will use a chemical reaction simulator to see the effects of temperature, reactant concentration, and a catalyst.

The simulation has three parts: Single collision, Many Collisions, and Rate Experiments. In all three cases the reactions are reversible, that is, once the products form, they interact and produce the original reactants. There are many such chemical reactions. We say that they have reached chemical equilibrium.

SINGLE COLLISION In this part of the simulator, you can see the effects of the orientation of the two reactant molecules and the collision energy. To see how the relative orientation makes a collision effective or not, you must select Angled shot from the Launcher Options (top right corner) and launch the reactant molecule with enough energy.

Now, we will use an application available at: https://phet.colorado.edu/es/simulation/legacy/ reactions-and-rates Download and execute it with Java.

RATE EXPERIMENTS Here the application lets you change the number of reactant molecules. There are options for controlling the experiment on the right: • Select a reaction. We can use one of the given reactions or design our own. The energy values of the reactants and products are editable in the energy diagram, so we may have endothermic or exothermic reactions. The activation energy is also editable, so we can simulate a catalyst.

• Number of molecules at the start. Let us change the number of reactant and product molecules at the start of the reaction.

• Initial temperature.

• Chart options. It is useful to select the bar chart to have an idea of the amounts of reactant and product and the strip chart to see how the reaction progresses.

Understand, think, search…

1 Do various single collision experiments, changing the

energy and launch angle. Draw conclusions about the probability of an effective collision and the factors on which it depends.

2 Do a rate experiment with the first pre-set chemical The effect of the energy of collision is visible by making shots at greater or lesser energy from the launcher. Note that if the shot energy is less than the activation energy you will not get an effective collision. To visualise the energy diagram of the reaction you must activate the Energy View option.

reaction but changing the number of molecules of both reactants.

Activate the stopwatch and the strip chart to measure the time the reaction takes. Put your results in a table and draw conclusions about the importance of the concentration of reactants. Why do we call chemical equilibrium a dynamic equilibrium?

3 Do the same experiment but this time vary the temperature. Draw conclusions.

139

SHOP SCIENCE WORK THE OXIDATION REACTION OF IRON

Research project INTRODUCTION The oxidation of iron is widely used as an example of a chemical reaction.

In neutral pH conditions, a precipitate of hydrated iron(III) oxide (rust) with a striking reddish-brown colour is formed. This means that you can easily observe the chemical change through the colour change.

In this experiment, you will change several of the factors affecting the rate of the oxidation reaction. To carry it out you will need: iron nails, diluted hydrochloric acid (sold in drugstores in Spain under the name salfumán), water, sodium chloride (table salt), vinegar, sodium hydroxide, and four containers with a large surface area and a shallow depth.

A series of experiments will be conducted to observe qualitatively how the chemical conditions (pH and presence of salts) affect the oxidation of the iron nails.

To compare the results, we need a control experiment, or blank. We will carry it out using water that has been boiled to remove part of the oxygen dissolved in it.

Rusted nails.

EXPERIMENT

PREPARATION OF THE VESSELS • Vessel 1: one nail + boiled water.

Boiled water (BLANK)

• Vessel 2: one nail + a solution of sodium chloride.

• Vessel 3: one nail + diluted hydrochloric acid.

• Vessel 4: one nail + vinegar.

• Vessel 5: one nail + water and oil.

NaCl solution (Marine environment)

These experiments approximately reproduce the chemical conditions in coastal areas, in areas with an acidic pH and in drier areas.

Diluted HCl or vinegar (acidic pH)

OBSERVATION AFTER 24 HOURS The oxidation in vessel 2 results in the formation of iron(III) oxide. It forms a colloidal precipitate which is suspended in the water. Can you see this reddish-brown precipitate in the other vessels? If not, this does not mean that the iron has not been oxidised, but instead that the iron(III) oxide has been dissolved. When an iron (III) oxide is in an acidic medium it will not form a precipitate.

Water and oil (Isolated environment)

To see if oxidation has occurred, add a few drops of sodium hydroxide solution. If it forms a precipitate this means that the oxidation had occurred before.

Now that you have completed this activity, give your opinion on cleaning iron with hydrochloric acid.

140

Unit

4

PRACTICAL WORK Factors that affect LA the reaction rate PROCEDURE Safety measures Both reactions release gases. Use safety glasses to avoid being splashed.

Experiment 1: Degree of sub-division of a solid reactant • Place a known mass of limestone powder in one test tube and the same mass of marble pieces in another. Add enough water to both tubes to suspend the limestone powder and cover the marble pieces.

• In two different test tubes, measure out the same amount of hydrochloric acid.

• Carefully add the hydrochloric acid to the original test tubes with care. Write down what happens.

PROPOSED PROBLEM Analyse the effect of the degree of sub-division in a solid reactant, the concentration, and the presence of catalysts on the reaction rate.

To do so, we suggest you do the following experiments with these two chemical reactions: Reaction 1: CaCO3 (s) + 2 HCl (aq) 8 CaCl2 + CO2 (g) + H2O Reaction 2: Zn (s) +H2SO4 (aq) 8 ZnSO4 + H2 (g) YOUR PROPOSAL Design one experiment to check qualitatively how reactant concentration and degree of sub-division of a solid reactant affect the reaction rate. Design another to test the effect of a catalyst.

OUR PROPOSAL Both reactions release a gas. So the speed of the gas bubbles could give an idea of the reaction rate. Only change one variable per experiment.

YOU WILL NEED • Powdered limestone • Marble pieces • Hydrochloric acid solution • Copper and zinc pieces • Diluted sulfuric acid solution • Test tubes • Tube rack • Volumetric flask • Distilled water

Experiment 2: Reactant concentration • Prepare two more test tubes of hydrochloric acid diluted to half the original concentration.

• Repeat experiment 1, with just the marble in distilled water. Repeat the experiment three times: once with the original hydrochloric acid concentration and twice with the diluted concentration.

Experiment 3: Presence of a catalyst

Understand, think, search...

• Put the pieces of zinc in a test tube and cover them with the diluted sulfuric acid solution. Observe and write down what happens.

1 Prepare a table with the conditions of each experiment,

• In another test tube, combine the zinc and copper pieces and add the same amount of diluted sulfuric acid. Observe and write down what happens.

the catalyst, the copper, and the acid. What do you see?

together with your observations and draw conclusions.

2 From experiment 3, do another experiment with only

3 Would we get the same results as in experiment 3 if we used a compound of copper, such as copper sulfate, instead of copper? Could we say that the copper sulfate catalysed the reaction? Justify your answer.

141

to choose Remember folio. rt po ur yo for

m this unit resources fro

REVIEW

Organising my ideas

Systemic concept map

1 Using diagrams, explain the factors which affect the reaction speed and the difference between an endothermic and exothermic reaction.

2 Fill in the missing information at points A, B, C, D and E in the systemic concept map below, using the knowledge you have learnt in this unit.

A chemical reaction

is made up of

is shown as C

products

A

which tells us about whose bonds break

B

D

state of matter of the substances

energy of the reaction

E

Chemical changes 1

3

negative

Let’s check. Balance these chemical equations:

a) MgCl2 + Li2CO3 8 MgCO3 + LiCl

Ideas pool. List five chemical changes you are

b) P4 + O2 8 P2O3

aware of in your everyday life.

c) MgI2 + Mn(SO3)2 8 MgSO3 + MnI4

2 Write the chemical equations for these chemical changes at the molecular level:

d) C3H6O + O2 8 CO2 + H2O e) SeCl6 + O2 8 SeO2 + Cl2

Cu

+

Cu

O O

4 Check that the previous reactions follow the law of conservation of mass given the masses of the atoms in the substances present.

5

Fe C

1-2-4. From the information in the images below, specify which bonds break and which form in these chemical changes:

+

+

+ CH4

CO2

2 O2

+

+ N2

2 H2O

3 H2

2 NH3

Solutions to the numerical activities are on 142

anayaeducacion.es.

Unit

Reaction rate 6

how the concentration of reactants affects the reaction rate.

Using

collision

theory,

explain

7 The following data was obtained by measuring the concentration of a substance in a chemical reaction:

13 Complete the following table with the quantities in 20 mol of water: Amount of substance (mol) H2O

Atoms or molecules

Mass (g)

Molar mass (g/mol)

20

18

Time(s)

Concentration (mole/L)

H

1

0

2.32

O

16

400

1.72

800

1.3

1 200

0.98

1 600

0.73

a) Calcium chloride.

e) Chlorine.

2 000

0.54

b) Silver.

f) Aluminium thrichloride.

2 400

0.39

c) Diiron trioxide.

g) Hydrogen sulfide.

2 800

0.28

d) Ozone.

h) Sulfuric acid.

a) Is it a reactant or a product? b) Show the concentration data as a function of time. The slope of the graph at each point is a measurement of the reaction rate. Does the reaction rate rise or fall as the reaction progresses?

Explain why.

8 Explain how temperature affects reaction rates according to the kinetic theory of matter.

9

4

Explain the advantages of catalysts in reactions

that would otherwise require extreme temperature or pressure conditions.

Amount of substance 10 Specify whether the following statements are true or false: a) A mole of water has the same number of molecules as a mole of butane. b) A mole of water has the same number of hydrogen atoms as a mole of butane. c) A mole of water has one mole of oxygen atoms and two mole of hydrogen atoms. d) A mole of water has 1.8066 · 1024 atoms. e) A mole of water has 1.8066 · 1024 molecules.

11 How many atoms are there in a 360 g sample of copper?

12 Calculate the number of hydrogen atoms in a gram of ammonia, hydrogen chloride, and calcium hydroxide.

What is the hydrogen mass in each sample? anayaeducacion.es Check “Let’s Study” and “Learn by playing” sections in the resource bank.

14 Calculate the molar masses of these substances from the average atomic mass data in the periodic table.

Recall that you must write the formula for each:

15 Calculate the molarity of the following solutions from the average atomic mass data in the periodic table.

Recall that you must write the formula for each. a) 200 g of sodium hydroxide in a total volume of 500 mL. b) 20 g of sodium nitrate in water in a total volume of 250 mL. c) A solution of nitric acid in water with density 1.12 g/cm3 and percentage of composition by mass of 80 %. d) 200 mL of 2 M hydrochloric acid in water with a total volume of 500 mL.

16 Calculate the amount of substance of hydrochloric acid in the solution in part d) of the previous problem and how much there was before dilution. What do you find?

17 The following data was obtained by measuring the concentration of a solution: Time(s)

Concentration (mol/L)

1

0.012

2

0.015

3

0.017

4

0.020

5

0.023

6

0.014

Write the average value of the concentration, specifying the absolute error.

143

REVIEW 18 Calculate the molarity of a 70 g solution of 70 g of ammonium chloride in 500 mL of water. How many H atoms are there?

24 Metals react with acids, releasing hydrogen. a) Write the reaction between zinc and hydrochloric acid. b) Calculate the volume of a 3 M solution of hydrochloric acid necessary to fully react with 60 g of zinc.

Stoichiometric calculations 19 Calcium carbonate decomposes due to heat, forming calcium oxide and carbon dioxide. a) Write the balanced chemical equation. b) Calculate how much carbon dioxide forms if 150 g of calcium carbonate reacts.

20 Lithium hydroxide reacts with hydrogen bromide to form lithium bromide and water. a) Write the balanced chemical equation. b) Calculate the amount of hydrogen bromide necessary for 5 mol of lithium hydroxide to react. c) What would the mass be in the previous problem? d) Calculate the mass of the products obtained from the 5 mol of lithium hydroxide. e) Check, with the data in the problem, that mass is conserved.

c) Calculate the volume of hydrogen released if the reaction takes place at 1 atm and 298 K. d) Calculate the masses of the all the substances in this reaction and check that mass is conserved.

Energy in chemical reactions 25

following statements are true or false and why: a) The energy associated with chemical bonds and intermolecular forces is kinetic energy. b) The change in energy in an endothermic reaction is positive. c) The activation energy is the difference in energy between the reactants and the products of a chemical reaction.

21 We have 250 mL of a 3 M solution of sulfuric acid.

Calculate the volume of a solution of 1.5 M sodium hydroxide necessary to complete a full reaction with the sulfuric acid.

22 Lithopone or barium sulfate (ZnS + BaSO4) is a pigment used in watercolours prepared by precipitating salts from solution. This reaction is shown in the following chemical equation:

What makes you say that? Say whether the

d) The heat of reaction is always positive. e) All reactions must overcome an energy barrier to begin. f) The activated complex energy is greater than the energy of the reactants and that of the products.

26

BaS (aq) + ZnSO4 (aq) 8 ZnS (s) + BaSO4 (s) a) Name all the compounds in this chemical reaction. Which are the reactants and which are the products?

Shared interpretation. Indicate on the diagram which is the catalysed reaction. Is it an endothermic or exothermic reaction?

Transition state

Free energy

b) Calculate the mass of lithopone obtained from a reaction of 300 mL of 0.2 M zinc sulfate with a 0.3 M barium sulfide solution. c) Calculate the volume of the necessary barium sulfide. d) Search for information about why we cannot use this pigment in oil paintings.

23 Calculate the volume of ammonia obtained from a reaction of 27.4 L of nitrogen with hydrogen at 68 °C and 704 mmHg:

144

Reaction progress

27

Think and share with a partner. Write the heat of reaction given this information:

N2 (g) + 3 H2 (g) 8 2 NH3 (l )

a) N2 (g) + O2 (g) + 181 kJ 8 2 NO (g)

How much hydrogen is needed to react with all the nitrogen?

b) C (s) + O2 (g) 8 CO2 (g) + 393,5 kJ Are they endothermic or exothermic reactions?

Unit

28 We measured the heat of combustion of a fuel and obtained the following data:

30 From the data in the previous problem, calculate the heat released when 71 g of HCl (g) reacts as in part d).

31 The heat of combustion of some saturated hydrocarbons is given in the table:

Measurement

Heat of combustion (kJ/mol)

Measurement

Heat of combustion (kJ/mol)

1

–2 190.3

4

–2 144.6

2

–2 202.7

5

–2 210.1

Methane

879

3

–2 180.5

6

–2 165.2

Propane

2 192

Butane

2 850

Octane

5 438

Hexadecane

10 663

PC/(kJ/mol)

Calculate the value of the heat of combustion and its absolute error.

29 Consider these thermochemical equations: a) CH3OH (g) 8 CH2O (g) + H2 (g) Qr = 54.5 kJ b) C2H6 (g) + 7/2 O2 (g) 8 2 CO2 + 3 H2O (l) Qr = –3 119.6 kJ c) CO (g) + H2 (g) 8 CH2O (g) Qr = 21.3 kJ/mol d) HCl (g) + NH3 (g) 8 NH4Cl (g) Qr = –175.9 kJ Specify which are endothermic and which are exothermic.

4

Write and balance the combustion reactions, knowing that carbon dioxide and water are formed in each case. a) Calculate the amount of carbon dioxide released by the combustion of one mole of each hydrocarbon. b) Calculate the energy released as heat by the combustion of one mole of each hydrocarbon.

Draw a graph comparing the amount of carbon dioxide emitted with the energy released. What do you notice?

Everyday Chemistry: Portable heat These days, eating or drinking, anytime and anywhere is routine. We are used to having products available to consume almost instantly. We can even heat or cool them before consuming them, like in the case of selfcooling or self-heating drinks.

Lid Drink CHOCOLATE

In a self-heating drink, the container has a section separate from the drink where an exothermic chemical reaction takes place.

The key to the design is to be able to activate the reaction when you want your drink. An exchange between the heat compartment and the drink then needs to take place for the drink to reach the perfect temperature.

Before you start drinking, you must break the barrier between the reactants in the heat compartment. That initiates the exothermic reaction that will warm your drink.

CaO Aluminium sheet Water Bowl and piston

• Search for information about the design of selfheating drink containers. Draw a diagram and show which compartment contains the drink and which contains the reactants for the exothermic reaction.

• Could we use an endothermic reaction to get instant cold? Search for information about the reaction between ammonium nitrate and water.

Activity • Organise a poster presentation in groups in which you tell the rest of the class about the fundamentals of instant cooling and heating in products, such as cooling packs and self-heating drinks.

145

e d

a c r ce

física y química

4 ESO

Building

Blocks

Índice

LA ACTIVIDAD CIENTÍFICA

16. CINEMÁTICA

Investigación científica • Magnitudes físicas y unidades • Medida de magnitudes físicas. Errores

Sistema de referenciay magnitudes del movimiento • Movimientos rectilíneos • Movimientos circulares

11. EL ÁTOMO Y EL SISTEMA PERIÓDICO

17. LEYES DE NEWTON

Los primeros modelos atómicos • Los espectros atómicos y el modelo de Bohr • Modelo cuántico del átomo • El sistema periódico de los elementos químicos

Fuerzas • Fuerzas cotidianas • Leyes de Newton

18. FUERZAS EN EL UNIVERSO

12. ENLACE QUÍMICO Y FUERZAS INTERMOLECULARES

Evolución histórica del estudio del universo • Las fuerzas gravitatorias • Aplicaciones de la ley de gravitación universal • Satélites artificiales en órbita

El enlace químico • El enlace iónico • El enlace covalente • Fuerzas intermoleculares • El enlace metálico • Resumen de las propiedades de los compuestos químicos

19. FUERZAS EN FLUIDOS. PRESIÓN

13. LOS COMPUESTOS DE CARBONO

Fuerzas en fluidos • Presión hidrostática • Ley de Pascal • Presión atmosférica

El átomo de carbono • Formas alotrópicas del carbono • Formulas y modelos moleculares • Hidrocarburos • Compuestos de carbono oxigenados y nitrogenados • Moléculas de especial interés

10. ENERGÍA MECÁNICA Y TRABAJO

14. REACCIONES QUÍMICAS: FUNDAMENTOS

11. ENERGÍA TÉRMICA Y CALOR

Cambios químicos • Cantidad de sustancia • Cálculos estequiométricos

15. ALGUNAS REACCIONES QUÍMICAS DE INTERÉS Ácidos y bases • Reacciones de interés

Energía • Trabajo y potencia • Energía cinética • Energía potencial • Conservación de la energía mecánica • Transporte de energía mediante ondas mecánicas

Energía térmica y temperatura • Equilibrio térmico. Calor y propagación • Efectos del calor • Motor térmico • Degradación de la energía

REACCIONES QUÍMICAS. FUNDAMENTOS

4

1 CAMBIOS QUÍMICOS 1.1 Conceptos básicos

En una reacción química se parte de unas sustancias denominadas reactivos, presentes antes del cambio, y se obtienen otras nuevas del cambio, denominadas productos de la reacción. Para que se produzca el cambio químico, los reactivos han de estar en contacto entre sí. Durante el proceso también se producen cambios físicos que los evidencian.

Evidencias físicas de un cambio químico: ejemplos

Intercambio de energía.

Cambio de color.

Cambio de estado.

1.2 Teoría atómica de las reacciones químicas Para que tenga lugar una reacción, los reactivos deben colisionar entre sí, rompiendo enlaces y formando otros nuevos. Esta reordenación de átomos genera nuevas sustancias, pero la masa total se conserva. Esto se conoce como ley de conservación de la masa: En un cambio químico, la masa se conserva: la suma de las masas de los reactivos es igual a la suma de las masas de los productos.

Teoría atómica y conservación de la masa

Comprende, piensa, investiga...

+ 2 NO

+

1 A partir de la información de la ilusO2

2 NO2

Reacción de oxidación del monóxido de nitrógeno para dar dióxido de nitrógeno. Se produce una reordenación de los átomos y la masa se conserva durante el proceso.

152

tración sobre la teoría atómica y la conservación de la masa deduce qué enlaces se han roto y cuáles se han formado en el cambio químico que se representa.

Unidad 4 1.3 Ecuaciones químicas Para expresar cuantitativamente los cambios en una reacción química se utilizan los denominados coeficientes estequiométricos.

Las ecuaciones químicas expresan la relación entre reactivos y productos durante una reacción química, y la proporción que guardan entre sí sus unidades elementales mediante coeficientes estequiométricos.

En ocasiones, también informan de los estados de agregación de las sustancias, de la temperatura o de la presión a la que tienen lugar. Ajustar una reacción consiste en elegir los valores de los coeficientes de modo que el número de cada elemento químico sea el mismo en los reactivos que en los productos.

Ejercicios resueltos 1 Escribe y ajusta la ecuación química correspondiente a la reacción entre el amoníaco y el oxígeno molecular, que tiene como productos el nitrógeno molecular y el agua.

scribimos las fórmulas de los reactivos y de los productos, E separándolos por una flecha que marca el sentido de la reacción: NH3 + O2 8 N2 + H2O Elegimos los valores de los coeficientes estequiométricos para hacer que el número de átomos de cada elemento sea el mismo en los reactivos y en los productos. Para ello: 1. A justamos, primero, los coeficientes de las sustancias no elementales: 2 NH3 + O2 8 N2 + 3 H2O 2. A continuación, los de las sustancias elementales. Si es preciso, corre­giremos los coeficientes ya elegidos, tanteando. Es posible utilizar números fraccionarios: 2 NH3 + 3/2 O2 8

N2 + 3 H2O

Observa que el coeficiente estequiométrico del nitrógeno molecular es 1, por lo que no se escribe.

También, podemos multiplicar por dos la ecuación química para que no haya coeficientes fraccionarios: 2 NH3 + 3/2 O2 8

4

N2 + 3 H2O

3 2 6

Comprende, piensa, investiga...

2 Indica qué significan los coeficien-

C omprobamos, elemento a elemento, que el número de átomos que aparece en los reactivos es el mismo que el de los productos: Elementos

Reactivos

Productos

átomos de H

4 NH3 (4 · 3 = 12)

6 H2O (6 · 2 = 12)

átomos de N

4 NH3 (4 · 1 = 4)

2 N2 (2 · 2 = 4)

átomos de O

3 O2 (3 · 2 = 6)

6 H2O (6 · 1 = 6)

tes estequiométricos en la reacción del ejercicio anterior.

3 Ajusta las siguientes reacciones químicas:

CO2 + H2O 8 H2CO3 NO + O2 8 NO2 C2H2 + O2 8 CO2 + H2O

153

1. CAMBIOS QUÍMICOS

1.4 Velocidad de reacción La velocidad a la que transcurre una reacción es el tiempo que tarda en desaparecer un reactivo o aparecer un producto. Los factores que influyen en la velocidad de reacción son la temperatura (aumenta la velocidad de los átomos en los reactivos), la concentración (mayor probabilidad de colisiones), y el grado de división de los reactivos (el tamaño de las porciones).

Comprende, piensa, investiga...

1 Al mezclar una disolución de ácido

sulfúrico concentrado con agua, siempre se ha de añadir, gota a gota, el ácido sobre el agua, y no al revés. Dado que entre ellos reaccionan violentamente, explica por qué tomamos esta precaución. ¿Con qué factor físico que afecta a la velocidad de reacción lo relacionarías?

2

Según aumenta el grado de división del reactivo sólido, aumenta la superficie de contacto con el otro reactivo y, por tanto, aumentará la velocidad de reacción.

Existen sustancias químicas, los catalizadores, que consiguen modificar esta velocidad sin alterar la naturaleza ni de reactivos ni productos.

Busca información sobre las enzimas como catalizadores y responde: a) ¿Qué crees que ocurriría si las reacciones químicas que se producen en el interior de nuestro cuerpo no se llevaran a cabo con enzimas catalíticas? b) Busca imágenes de enzimas de nuestro organismo. Dibújalas y explica las diferencias y semejanzas en sus formas y tamaños. ¿A qué crees que se deben?

1.5 La energía en las reacciones químicas En las reacciones químicas se intercambia energía química entre el sistema que sufre el cambio y sus alrededores. Esto se debe a que se rompen unos enlaces y se forman otros nuevos. Si a los reactivos les corresponde mayor energía que a los productos, la reacción se denomina exotérmica, y endotérmica en caso contrario. Un ejemplo de reacción exotérmica son las combustiones, donde se libera energía química mediante calor.

Para medir el intercambio de energía entre un sistema y sus alrededores utilizamos el llamado calor de reacción, Q. Las ecuaciones químicas que incluyen el calor de reacción se denominan ecuaciones termoquímicas.

Tipos de reacciones Reacciones exotérmicas

Reacciones endotérmicas E

E

PRODUCTOS

REACTIVOS

PRODUCTOS AVANCE DE LA REACCIÓN

154

REACTIVOS AVANCE DE LA REACCIÓN

Unidad 4

2 CANTIDAD DE SUSTANCIA 2.1 Cantidad de sustancia

Para conocer cuántas entidades de cada sustancia intervienen en una reacción química se utiliza la magnitud cantidad de sustancia, n, que se define como el número de entidades elementales que contiene una determinada porción de esa sustancia dividido entre un valor constante denominado constante de Avogadro, NA. n=

N NA

La unidad de cantidad de sustancia en el SI es el mol. Se define un mol como la cantidad de sustancia que contiene 6,022 · 1023 entidades elementales, siendo este el número de Avogadro, NA. NA = 6,022 · 1023 unidades elementales/mol Se cuantifica de forma indirecta midiendo la masa o el volumen de una sustancia.

Para ello, se define la masa molar, M, de una sustancia como la masa, m, de un mol de dicha sustancia: n (mol) =

m (g) M (g $ mol–1)

Por otra parte, el volumen molar, VM, es el volumen que ocupa un mol en condiciones normales (273 K y 1 atm), siendo el de cualquier gas aproximadamente 22,4 L/mol.

2.2 Molaridad Definimos la molaridad, M, de una disolución como la cantidad de sustancia de soluto que contiene un litro de disolución. Su unidad es mol/L. M=

nsoluto (mol) Vdisolución (L)

Ejercicios resueltos 1 Calcula la molaridad de una disolución de hidróxido de sodio sabiendo que 500 mL contienen 120 g de NaOH. Datos: M (Na) = 23 g/mol; M (O) = 16 g/mol; M (H) = 1 g/mol.

En primer lugar, determinamos cuál es la masa molar del NaOH: MNaOH = 23 $ 1 + 16 $ 1 + 1 $ 1 = 40 g $ mol–1 A partir de la masa molar, calculamos la cantidad de sustancia presente: n=

Expresamos el volumen de la disolución en las unidades adecuadas: Vdisolución = 500 mL = 0, 5 L Así, podemos calcular la molaridad de la disolución de hidróxido de sodio como: M=

3 mol =6M 0, 5 L

Observa que expresamos la molaridad como su valor seguido de la letra M.

120 g m = = 3 mol M 40 g $ mol–1

155

3 CÁLCULOS ESTEQUIOMÉTRICOS

Los cálculos estequiométricos son el conjunto de operaciones que nos permiten obtener información cuantitativa sobre las sustancias que reaccionan o que se forman en una reacción química. Es fundamental y necesario ajustar una reacción química para poder realizar cálculos estequiométricos.

3.1 Cálculos estequiométricos masa-masa Podemos realizar cálculos de masa-masa para calcular la masa de una sustancia a partir de la masa conocida de otra (ejercicio resuelto 3).

3.2 Cálculos con reactivos en disolución Por otra parte, muchas reacciones químicas tienen lugar en disolución acuosa en las que el agua actúa como disolvente. Pero, en ocasiones, el agua participa en la reacción como reactivo o como producto. Veámoslo en el ejercicio resuelto 4.

Ejercicios resueltos 3 El clorato de potasio, KClO3, se descompone en

4 Calcula el volumen de una disolución de ácido clor-

Datos: M (K) = 39,1 g/mol; M (Cl) = 35,5 g/mol; M (O) = 16,0 g/mol.

1 Partiremos de la ecuación química ajustada e iden-

cloruro de potasio y oxígeno. Calcula la masa de oxígeno obtenida al descomponerse 86,8 g de clorato de potasio por la acción del calor.

1 Para relacionar la cantidad de sustancia que reac

ciona y que se forma de cada especie, ajustamos estequiométricamente la reacción:

2 KClO3 (s) 8 2 KCl (s) + 3 O2 (g) 2 Identificamos la sustancia dato y la sustancia incóg nita: 2 KClO3 (s) 8 2 KCl (s) + 3 O2 (g)

mKClO3 = 86, 8 g

3 Calculamos las masas molares de las sustancias

dato e incógnita: MKClO3 = 122, 6 g/mol ; MO2 = 32, 0 g/mol

4 C alculamos la cantidad de la sustancia dato con su

masa molar: nKClO3 =

86, 8 g 122, 6 g $ mol–1

hídrico 0,2 M necesario para que reaccionen completamente 250 mL de una disolución de hidróxido de calcio 0,1 M. Los productos de esta reacción química son cloruro de calcio y agua. tificamos la sustancia dato y la sustancia incógnita: 2 HCl (aq) + Ca (OH) 2 (aq) 8 2 H2O + CaCl2 (aq) 2 Escribimos los datos y la ecuación química:

2 HCl (aq) + Ca (OH) 2 (aq) 8 2 H2O + CaCl2 (aq)

¿VHCl? ;

3 Calculamos la cantidad de sustancia dato a partir

de la definición de molaridad:

n (mol) 8 n (mol) = M $ V (L) V (L) nCa (OH) 2 = 0, 1 M $ 0, 250 L = 0, 025 mol

M=

4 Calculamos la cantidad de sustancia incógnita a

partir de los coeficientes estequiométricos: - 0, 708 mol

2 HCl 1 mol Ca (OH) 2 nHCl = 0, 050 molHCl

nHCl = 0, 025 mol Ca (OH) 2 $

5 Calculamos la cantidad de sustancia incógnita con

los coeficientes estequiométricos: 3 mol O2 nO2 = 0, 708 mol KClO3 $ = 1, 062 mol O2 2 mol KClO3 6 Hallamos la masa de la sustancia incógnita con su

masa molar: –1

mO2 = 1, 062 mol $ 32, 0 g $ mol = 33, 984 g

156

VCa (OH) 2 = 250 mL = 0, 250 L

5 Por último, calculamos el volumen de la disolución:

M= V (L) =

n (mol) V (L)

8 V (L) =

0, 050 (mol) –1

0, 2 mol $ L

n (mol) M

= 0, 250 L = 250 mL

Unidad 4 3.3 Cálculos de reacciones entre gases Cuando en la reacción intervienen sustancias en estado gaseoso, tendremos en cuenta que las relaciones entre los coeficientes estequiométricos son las mismas que entre los volúmenes de gas, medidos en las mismas condiciones de presión y temperatura. Para relacionar las magnitudes utilizaremos la ecuación de los gases ideales, p $ V = n $ R $ T, con el valor de atm $ L R = 0, 082 . Veamos un ejemplo: K $ mol

Ejercicios resueltos 5 Demuestra que la relación entre los volúmenes de los gases de

esta reacción química es la misma que la que obtenemos entre los coeficientes estequiométricos, si se miden en las mismas condiciones de presión y temperatura.

2 N2 (g) + 3 H2 (g) 8 2 NH3 (g) La relación entre los coeficientes estequiométricos es: 2 mol de N2 reaccionan con 3 mol de H2 para obtener 2 mol de NH3. Supongamos que las condiciones de presión y temperatura son p y T. Los volúmenes se calcularían con la ecuación: n $R $T V= p Utilizando la relación estequiométrica tenemos: R $T R $T R $T VN2 = 2 $ p ; VH2 = 3 $ p ; VNH3 = 2 $ p Como (R · T/p) no varía, se puede concluir que 2 L de N2 reaccionan con 3 L de H2 para producir 2 L de NH3, obser­vándose la misma relación que entre los coeficientes estequiométricos.

Comprende, piensa, investiga...

1 Calcula la cantidad de sustancia de PCl3 que se obtiene si reaccionan 426 g de Cl2 según la reacción: Cl2 + P4 8 PCl3

2 Calcula la cantidad de sustancia de P4 que es necesaria en la reacción de la actividad anterior.

3 Calcula el volumen necesario de una disolución de

ácido clorhídrico 2,5 M para que reaccionen 0,2 mol de cinc según esta reacción: 2 HCl + Zn 8 ZnCl2 + H2

4 Calcula el volumen de hidrógeno (H2), medido a 25 °C

y 0,98 atm, que se desprende cuando una muestra de 41,4 g de sodio reacciona con agua, como se representa en la ecuación química siguiente: 2 Na (s) + 2 H2O 8 2 NaOH (aq) + H2 (g)

5 Se hace reaccionar carbonato de calcio con ácido clorhídrico según la siguiente reacción.

CaCO3 + 2 HCl 8 CaCl2 + H2O + CO2

6

Busca información sobre otras formas de expresar la concentración de una disolución. ¿Qué ventaja tiene usar la molaridad frente a otras para realizar cálculos estequiométricos?

7 ¿Qué volumen de una disolución 2 M de una sustancia

A será necesario para hacer reaccionar completamente 3 mL de otra disolución 2 M de una sustancia B si los coeficientes estequiométricos de A y B son iguales?

8 A partir de las reacciones de combustión de los cua-

tro primeros alcanos, indica el volumen y la masa de CO2 que se desprende al quemarse un litro de cada uno de ellos, medido en c. n.

CH4 +2 O2 8 CO2 + 2 H2O C2H6 +

7 O 8 2 CO2 + 3 H4O 2 4

C3H8 + 5 O2 8 3 CO2 + 4 H2O C4H10 +

13 O2 8 4 CO2 + 5 H2O 2

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