Molecular models

Reading and listening 

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

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

The opening pages of each unit

4Chemical reactions: foundations

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, responsible for 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/natural treatment for fighting

Reading and listening

When the miracle became science The earliest evidence of the production and consumption of wine is 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 first explained 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? mildew

I recommend that you an alternative to fungicides such as

Writing The following is a paragraph taken from an article that tells how powdery mildew attacked the vineyards and the repercussions that the use of fungicides had in order to save the yearly production. 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 122 Writing LANGUAGE BANKLANGUAGE BANKLANGUAGE BANKLANGUAGE BANKLANGUAGE BANKLANGUAGE BANKLANGUAGE BANK LANGUAGE BANKLANGUAGE BANK LANGUAGE BANKLANGUAGE BANK LANGUAGE BANK 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 Now, find a similar problem and write a paragraph explaining the issue. 123 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.

Speaking

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

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

content development

Two types of activities: Understand, think, search…

Activities to strengthen learning and relate knowledge.

Working with pictures. Develop your capacity for observation and interpretation.

1

CHEMICAL CHANGES

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.

Energy exchange. Physical evidence of a chemical change

Colour change. State change.

Working with pictures

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

2 NO

O2

124 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:

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

2 NO2 The drawing on the left shows the oxidation of nitrogen monoxide to form nitrogen dioxide. We can see that the atoms are in a different order, and that mass is conserved throughout the process.

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

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

Unit 4

ä 1.3 Writing a chemical reaction: the chemical

equation

We use chemical equations to describe chemical reaction quantitatively. Focus on English

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

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’. 5 ENERGY IN CHEMICAL ä 5.2 Energy diagrams and catalysts With a classmate, discuss whether you agree with the following phrase by We will use the example of the reaction between ammonia and molecular oxygen. The products of this reaction are molecular nitrogen and water. 1 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 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 We 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 orientation or sufficient energy. It is possible for the transition state (the state between reactants and the present, while our equations are for eternity’.We 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: products) of a chemical reaction to have more energy than either of these 2.1 stages. This point in the reaction is called the First, we adjust the coefficients of the non-elemental substances: activated complex (see 2 NH3 + O2 8 N2 + 3 H2O figure below). 2.2 Regardless of whether a reaction is endothermic or exothermic, every Understand, think, search…Then, we do the same for the elemental substances. If necessary, we chemical reaction passes through this energy state. 1 1-2-4. Draw a table, like the correct the coefficients we chose before. We can use fractions. 2 NH3 + 3/2 O2 8 N2 + 3 H2O one in the example, for the following Activation energy. Catalysts reaction:

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

Uncatalysed reaction We can also multiply by two the chemical equation so that there are no Activation energyfractional coefficients: without catalyst, Ea2 NH3 + 3/2 O2 8 N2 + 3 H2O Activation energy with catalyst, Ea '

4 3 2 6

3 REACTANTS Catalysed Finally we check, element by element, that the number of atoms among the reactionreactants is the same as among the products:

Elements

H atoms

N atoms

O atoms Reactants Products PRODUCTS

4 NH3 (4 · 3 = 12)

6 H2O (6 · 2 = 12) REACTION PROGRESS

4 NH3 (4 · 1 = 4) 2 N2 (2 · 2 = 4)

3 O2 (3 · 2 = 6) 6 H2O (6 · 1 = 6)

CH The energy difference between the reactants 4 + 2 O2 8 CO2 + 2 H2O and the transition energy is called the Which bonds are broken and which activation energy. new bonds form? How many bonds

This energy is a barrier that will only be are broken and how many new bonds form? surpassed by reactant molecules with 2 sufficient energy to produce effective Explain what the stoichiometric collisions. coefficients mean in the exercise above. When this energy is very high and the 3 reactants cannot surpass it, we can use catalysts. These substances lower the Let’s check. Balance the activation energy and create a more following chemical reactions a) favourable energetic pathway. This helps CO2 + H2O 8 H2CO3 b) convert the reactants into the products.NO + O2 8 NO2 c) Cr2O3 + Al 8 Al2O3 + Cr d) NO2 + H2O 8 HNO3 + NO e) C2H2 + O2 8 CO2 + H2O

Understand, think, search…

28 If you touch a beaker containing an endothermic reaction, what will you feel?

29 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. ä 5.3 Energy exchange. Heat of reaction

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

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.

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

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. • 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: ( ) ( ) ( ) ()g g g l Q2 13 4 5 288CH O CO H O –r4 10 2 2 2+ + = 77 kJ 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.

Unit 4

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

Qr (CH4) = –890 kJ/mol; Qr (C4H10) = –2877 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.

First, we must know the molar masses, which we will calculate from the molecular formulas: MCH4 12 1 1 $ $= + 4 16= / gmol MCH4 10 12 4 1 $ $= + 10 = 58 / gmol We calculate the amount of each fuel:

n

CH4 1000

g = = 16 / gmol .62 5 mol

nCH4 10 1000 g

58 / gmol .17 2 mol

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

Q ( ) CH c 4 = . . mol mol kJ62 5 1 890 556 104 $ $ - kJ

( ) CHQc 4 10 = . . mol mol kJ17 2 1 2877 495 104 $ $ - kJ

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

135

PROJECT KEYS

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.