Extend of Chemical Change GCSE

GCSE workbook Extent of Chemical Change

1

5.6.2 Reversible reactions and dynamic equilibrium 5.6.2.1 Reversible reactions Content

Key opportunities for skills development

In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions and are represented: A +B

C + D

The direction of reversible reactions can be changed by changing the conditions.

For example:

5.6.2.2 Energy changes and reversible reactions Content

Key opportunities for skills development

If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred in each case. For example:

5.6.2.3 Equilibrium Content

Key opportunities for skills development

When a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.

WS 1.2

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2

GCSE Combined Science: Trilogy 8464. GCSE exams June 2018 onwards. Version 1.0 22 April 2016

5.6.2.4 The effect of changing conditions on equilibrium (HT only) Content

Key opportunities for skills development

The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.

If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.

The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.

Students should be able to make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information.

5.6.2.5 The effect of changing concentration (HT only) Content

Key opportunities for skills development

If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.

If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.

If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.

Students should be able to interpret appropriate given data to predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium.

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5.6.2.6 The effect of temperature changes on equilibrium (HT only) Content

Key opportunities for skills development

If the temperature of a system at equilibrium is increased: • the relative amount of products at equilibrium increases for an endothermic reaction • the relative amount of products at equilibrium decreases for an exothermic reaction.

If the temperature of a system at equilibrium is decreased: • the relative amount of products at equilibrium decreases for an endothermic reaction • the relative amount of products at equilibrium increases for an exothermic reaction.

Students should be able to interpret appropriate given data to predict the effect of a change in temperature on given reactions at equilibrium.

5.6.2.7 The effect of pressure changes on equilibrium (HT only) Content

Key opportunities for skills development

For gaseous reactions at equilibrium: • an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction • a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.

Students should be able to interpret appropriate given data to predict the effect of pressure changes on given reactions at equilibrium.

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4

Helping yourself Read the section on Atomic structure and the Periodic table in your textbook Read the relevant sections of these websites bbc.co.uk/schools/gcsebitesize/chemistry gcsescience.com docbrown.info revisioncentre.co.uk youtube for video tutorials other teachers!

5

Reversible Reactions When you heat the crystals of blue copper sulfate, they break down into a while powder. !"#!! . 5!! ! ! → !"#!! ! + 5!! !(!)

When you add water to the white powder, it turns back to blue crystals. !"#!! ! + 5!! ! ! → !"#!! . 5!! !(!)

This is called a .................... Reaction as it can go both forward and backwards.

We use the symbol ⇌ to show that a reaction is reversible. !"#!! . 5!! ! ! ⇌ !"#!! ! + 5!! !(!)

Many chemical reactions are reversible e.g . The reaction between chlorine and iodine. When Chlorine is passed over Iodine it forms a brown liquid iodine monochloride, ICl If the chlorine keeps flowing, yellow crystals will form on the side of tube. This is iodine trichloride !"!!

The reaction between Iodine mono and trichloride is reversible. To get more yellow crystals all you need to do is add more chlorine.

Iodine Monochloride

Iodine Trichloride

!"# ! + !!! ! ⇌ !"!! (!) e.g. The thermal decomposition of ammonium chloride. When you heat white solid ammonium chloride it breaks down into ammonia and hydrogen chloride gas. The gases then recombine at the top of the tube to form a white solid again where it cools. !!! !" ! ⇌ !!! ! + !"#(!)

Sublimes and reforms on cooling

Solid Ammonium Chloride

6

Dynamic Equilibrium Imagine you are looking through a window at a swimming pool during a busy session. You count there are 30 people in the pool. You come back after 30 minutes and you see that there are still thirty people in the pool. However you can see that some people are arriving and diving into the pool while others are getting out and leaving. The same situation happens over the next two hours – the number of swimmers is constant even though people are entering and leaving the pool.

There is a balance between those who are entering and those who are leaving the pool. This is described as being in a state of ...............................

However the situation is constantly changing as people come and leave. We describe it as ............................ (opposite of static).

The pool is described as a system and is the part of the universe we are studying in isolation from the rest of the universe. Systems either be in a state of change or a state of equilibrium.

It also either be closed or open. A .......................... system is one where no substances are added to the system or allowed to escape.

Dynamic Equilibrium is like a man running up a downwards escalator at equal rates. It looks like he’s not moving but he’s trying to go up and the escalator goes down.

Or Dynamic equilibrium is like water running into a bucket at the same rate at which it is leaving from a tap at the bottom. The water level remains the same even though the water is constantly moving.

Dynamic Equilibrium occurs when the forward and backward reaction occur at the same rate so that there is no overall change in concentrations.

Reversible reactions in Closed Systems

7

Imagine a substance that can exist in two forms, blue and red, the reaction is reversible.

Suppose we start off with a sample that is entirely blue. The reaction starts off with a high concentration of blue squares, at the beginning of the reaction, the rate at which they will turn into red squares will be relatively high. But this will fall as the number of red squares increases and they will start to turn back to blue squares with increasing speed.

A stage will be reached at which the rate the blue squares turn to red squares will equal the rate at which red squares are turning to blue squares. When this point we reached the number of red and blue squares will remain constant even though the two reactions are still going one. Dynamic Equilibrium has been reached.

Reaction starts

Dynamic Equilibrium has been reached – constant numbers of different coloured squares.

If we were to plot this on a graph, it would look like this: Rate of blue squares turning red starts high and gradually falls to a constant level.

Rate of red squares turning blue starts at zero and gradually increases to a constant level.

Rate of forward and backward reactions equal.

Constant number of red and blue squares.

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Le Chatelier’s Principle Once …………………. equilibrium has been established, the concentration of reactants or products remains constant and the reaction never goes to completion.

But we are able to manipulate the proportions of either the reactants or products in the equilibrium by changing a number of factors. a) The temperature b) The pressure c) The concentration of a substance Changing the above shifts the …………………………. of equilibrium of the system so that we may get a higher proportion of the product that we want.

Le Chatelier noticed that if a dynamic equilibrium is disturbed by changing the conditions, the reaction moves to …………………… the change.

Let’s take the example of the reaction which makes ammonia from nitrogen and hydrogen.

This is a reversible reaction !! ! + 3!! (!) ⇌ 2!!! !

This reaction tells us that three molecules of hydrogen react with one of nitrogen to form two molecules of ammonia. But only a small amount of ammonia is formed because dynamic equilibrium is reached.

We want lots of ammonia so we need to shift the position of equilibrium to the right.

1. Decrease the Temperature In this system the forward reaction is exothermic – surroundings get hotter !! ! + 3!! → 2!!! !

∆! =Exothermic

The back reaction is therefore endothermic - surroundings get colder.

2!!! ! → !! ! + 3!! ∆! = Endothermic If you cool the mixture, the system will act to oppose the change and move in the exothermic direction and produce more ammonia.

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!! ! + 3!! ⇌ 2!!! ! ∆! = Exothermic Shift right If we heat up the mixture the position of equilibrium will shift to the left in the endothermic (cold) direction to oppose the change.

!! ! + 3!! ⇌ 2!!! ! ∆! = Exothermic Shift left Remember:

If we want to shift equilibrium in the exothermic direction – .......... it down If we want to shift equilibrium in the endothermic direction – ............. it up

Therefore we will need a low temperature so we have a higher proportion of ammonia.

But low temperatures slows down the rate of reaction so it will take longer to reach equilibrium 2. Increasing the Pressure Pressure is caused by collisions between gas molecules and the wall of the container, so the fewer molecules present, the lower the pressure.

If you apply more pressure the equilibrium mixture will act to oppose the change.

More

ammonia

will

form

to

reduce the number of ..................

There are fewer molecules of gas on the right hand side. !! ! + 3!! ⇌ 2!!! !

4 molecules

2 molecules

So high pressure is will shift equilibrium to the right. Unfortunately the higher the pressure the greater the expense and danger of the process.

High pressure !! ! + 3!! ⇌ 2!!! !

10

Low pressure 3. Removing the ammonia The equilibrium mixture is a balance between the levels of nitrogen, hydrogen and ammonia present.

If we cool the mixture, the ammonia will condense first and so it can be run off. This will cause more ammonia to be produced as the nitrogen and hydrogen will reach to counter the change in concentration of ammonia.

Equilibrium shifts to the right.

4. Adding a catalyst This ........................... change the position of equilibrium by simply speeds up the rate of both the forward and backward reaction. But it saves time and money as we reach dynamic equilibrium much more quickly.

This reaction uses an Iron catalyst.

Choosing an optimum conditions We can manipulate a system in dynamic equilibrium to our advantage by shifting the position of equilibrium to gain the highest yield of product.

Unfortunately issues of rates of reaction, expense and feasibility mean that often we have to make a compromise. In this reaction usually 450ËšC and 200atm is the compromise chosen.

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Reversible Reaction Problems 1)

What would happen to the amount of products formed in the following equilibrium reactions if the conditions were changed The forward reaction is endothermic.

CH4(g) +

H2O(g)

⇌

CO(g)

+ 3 H2(g)

a) What would happen, and why, if the temperature was increased?

[3]

b) What would happen, and why, if the pressure was increased?

[3]

c) What would happen, and why, if more steam was added?

[3]

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2)

Yellow chromate ions are in equilibrium with orange chromate ions.

2 CrO42-(aq) + 2 H+(aq)

⇌

Cr2O72-(aq) + H2O(l)

yellow

orange

a) What would happen if acid was added to a yellow solution of chromate ions and why?

[3]

b) What would happen and why if alkali was added to a yellow solution of chromate ions and why?

3)

[3]

Ethanoic acid is a weak acid. Some ethanoic acid and universal indicator were placed in a flask.

CH3COOH(aq) ⇌

CH3COO-(aq) + H+(aq)

colourless

colourless

Acidic

Alkali

a) What colour would the universal indicator be?

[1]

b) What would happen to the colour and why if some sodium ethanoate was added?

[3]

13

Le Chatelier’s Principle Problems 1. The following equilibrium is set up between iodine and chlorine !"# ! + !!! ! ⇌ !"!! ! ∆! = !"#$ℎ!"#$% Brown

Yellow

What would be seen when: a) More Chlorine is added?

b) The reaction is equilibrium mixture is cooled down?

c) The equilibrium mixture is heated up?

2. The following equilibrium is set up between hydrogen and bromine. !! ! + !!! ! ⇌ 2!"#(!)

What would be the result of: a) Adding more bromine?

b) Adding more hydrogen?

c) Removing the product?

14

d) Increasing the pressure?

3. The dichromate and chromate ion exist in an equilibrium !!! !!!! !" + !! ! ! ⇌ 2!"!!!! !" + 2! ! (!")

Orange

Yellow

What would be the effect of: a) Adding more acid?

b) Adding more water?

c) Adding some alkali?

15

EQUILIBRIA 1 1

A mixture of chemicals A, B and C are present in a closed system at dynamic equilibrium.

A+B ⇌ C

a Describe what is happening when the mixture is at dynamic equilibrium. ………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………

b What is a closed system? ……………………….………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………

c Under certain conditions, the position of this equilibrium lies to the right. Explain what this means. ………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………

2

Complete the table to show what would happen to the position of the following gaseous equilibria if the following changes were made. Tick (ü) the correct column in each case.

Equilibrium

Energy change (forward reaction)

A(g) + 2 B(g) ⇌ X(g) + Z(g)

exothermic

P(g) + Q(g) ⇌ 2 X(g)

endothermic

A2(g) ⇌ X(g) + Z(g)

exothermic

2 P(g)

3

⇌ 2 C(g) + D(g)

Increase temperature moves left

no move

move right

Increase pressure moves left

no move

move right

endothermic

Methanol (CH3OH) can be made by reaction of hydrogen with carbon dioxide in a dynamic equilibrium.

CO2(g) + 3H2(g) ⇌ CH3OH(g) + H2O(g) a If the steam is removed from the equilibrium mixture, what happens to the equilibrium yield of methanol? ………..…………………………………………………………………………………………………………………………………………………

b Explain your reasoning. ………………………………………………………………………..…………………………………………… ………..………………………………………………………………………………………………………………………………………………… ………..………………………………………………………………………………………………………………………………………………… ………..………………………………………………………………………………………………………………………………………………… ………..…………………………………………………………………………………………………………………………………………………

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Chemsheets GCSE 1185

4

The hydrogen used in the Haber process is made in the reaction shown below, which is an equilibrium.

CH4(g) + H2O(g) ⇌ CO(g) + 3 H2(g) a i

Energy change = +206 kJ/mol

If the temperature of this equilibrium was increased, what would happen to the equilibrium yield of hydrogen? ……………………………………………………………………………………………………………………………………………………

ii

Explain your reasoning. ……………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………

b i

If the pressure of this equilibrium was increased, what would happen to the equilibrium yield of hydrogen? ……………………………………………………………………………………………………………………………………………………

ii

Explain your reasoning. ……………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………

5

Hydrogen can also be made in the reaction shown below.

CO(g) + H2O(g) ⇌ CO2(g) + H2(g) a i

Energy change = –42 kJ/mol

If the temperature of this equilibrium was increased, what would happen to the equilibrium yield of hydrogen? ……………………………………………………………………………………………………………………………………………………

ii

Explain your reasoning. ……………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………

b i

If the pressure of this equilibrium was increased, what would happen to the equilibrium yield of hydrogen? ……………………………………………………………………………………………………………………………………………………

ii

Explain your reasoning. ……………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………

Area

Strength To develop Area

Strength To develop Area

Strength To develop

Done with care and thoroughness

Understands closed system

Deduce how P affects position

Good SPG

Understands idea of position

Deduce how reagents affect position

Understands dynamic equilibrium

Deduce how T affects position

Explain why position moves

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23-May-2018

Chemsheets GCSE 1185

EQUILIBRIA 2 1

Sulfur trioxide is made by the following reaction. The forward reaction is exothermic.

2 SO2(g) + O2(g) ⇌ 2 SO3(g) a This reaction reaches a state of dynamic equilibrium in a closed system. i

What is a closed system? ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………………………………………………

ii

Describe what is happening when the mixture is at dynamic equilibrium. …………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………

b i

If the temperature of this equilibrium was increased, what would happen to the equilibrium yield of sulfur trioxide? ……………………………………………………………………………………………………………………………………………………

ii

Explain your reasoning. ……………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………

c i

If the pressure of this equilibrium was increased, what would happen to the equilibrium yield of sulfur trioxide? ……………………………………………………………………………………………………………………………………………………

ii

Explain your reasoning. ……………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………

2

Complete the table to show what would happen to the position of the following gas phase equilibria if the following changes were made. Tick (ü) the correct column in each case.

Equilibrium

Energy change (forward reaction)

P(g) + Q(g) ⇌ R(g)

endothermic

P(g) + Q(g) ⇌ R(g) + S(g)

exothermic

P(g) ⇌ R(g) + S(g)

exothermic

2 P(g) + Q(g) ⇌ 2 R(g)

endothermic

2 P(g) ⇌ 2 R(g) + S(g)

exothermic

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23-May-2018

Increase temperature movesl eft

no move

move right

Increase pressure movesl eft

no move

move right

Chemsheets GCSE 1185

3

Iodine trichloride breaks down to form iodine monochloride and chlorine in an equilibrium reaction. The forward reaction is endothermic.

ICl3(g) ⇌ ICl(g) + Cl2(g) a i

If the temperature of this equilibrium was increased, what would happen to the equilibrium yield of chlorine? ……………………………………………………………………………………………………………………………………………………

ii

Explain your reasoning. ……………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………

b i

If the pressure of this equilibrium was increased, what would happen to the equilibrium yield of chlorine? ……………………………………………………………………………………………………………………………………………………

ii

Explain your reasoning. ……………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………

2–

4

2–

Yellow chromate ions (CrO4 ) react with hydrogen ions in acid to form orange dichromate ions (Cr2O7 ).

CrO4

2–

+

+ 2H ⇌ Cr2O7

2–

+ H 2O

a If acid is added to an equilibrium mixture, will it become more yellow or more orange. Explain your answer. …………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………

b If alkali is added to an equilibrium mixture, will it become more yellow or more orange. Explain your answer. …………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………

Area

Strength To develop Area

Strength To develop Area

Strength To develop

Done with care and thoroughness

Understands closed system

Deduce how reagents affect position

Good SPG

Deduce how T affects position

Explain why position moves

Understands dynamic equilibrium

Deduce how P affects position

© www.CHEMSHEETS.co.uk

23-May-2018

Chemsheets GCSE 1185

1.

Ammonia, NH3, is made industrially by the Haber process. This is an equilibrium reaction.

N2(g) + 3H2(g) 2NH3(g) ΔH = –92 kJ mol–1 (a)

State the pressure and temperature that are used in the Haber process. pressure .......................................................................................................... temperature .....................................................................................................

[2]

(b)

Describe and explain why these conditions are a compromise between rate and equilibrium. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[9] [Total 11 marks]

2.

Ethene and steam can be converted into ethanol.

The equilibrium is shown below.

C2H4(g) + H2O(g) C2H5OH(g)

ΔH = – 46 kJ mol–1

le Chatelier’s principle can be used to predict the effect of changing conditions on the position of equilibrium.

16

(i)

Name the catalyst used in this reaction. .........................................................................................................................

[1]

(ii)

State le Chatelier’s principle. ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[1]

(iii)

Using le Chatelier’s principle, predict and explain the conditions that would give the maximum equilibrium yield of ethanol from ethene and steam. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[3]

(iv)

The actual conditions used are 60 atmospheres pressure at 300 °C in the presence of a catalyst. Compare these conditions with your answer to (iii) and comment on why these conditions are used. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[3] [Total 8 marks]

3.

Many industrial processes, used to manufacture important chemicals, involve equilibrium reactions. Chemists use their understanding of rates of reaction and of yields at equilibrium to find the most economic conditions for the reactions.

17

Chemists were investigating the production of a chemical, X2Y, that could be formed from X2 and Y2 as shown in equilibrium 3.1 below.

2X2(g) + Y2(g) 2X2Y(g) (a)

equilibrium 3.1

State le Chatelier’s principle. ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[2]

(b)

State and explain the effect on equilibrium 3.1 of a decrease in pressure on: (i)

the equilibrium position of the reaction, ................................................................................................................ ................................................................................................................ ................................................................................................................

[2]

(ii)

the rate of the reaction. ................................................................................................................ ................................................................................................................ ................................................................................................................ ................................................................................................................

[2]

(c)

The chemists measured the percentage conversion of X2 at various temperatures. The results are shown in the graph below.

(i)

Use the graph to predict the percentage conversion at 350 °C. answer = .....................................................% [1]

(ii)

The forward reaction in equilibrium 3.1 is exothermic. Explain how the 18

graph supports this statement. ................................................................................................................ ................................................................................................................ ................................................................................................................ ................................................................................................................ ................................................................................................................

[2]

(d)

The chemists decided to use a catalyst in the process.

State, and explain, the effect of using a catalyst on: (i)

the rate of conversion of X2 and Y2 into X2Y, ................................................................................................................ ................................................................................................................

[2]

(ii)

the percentage conversion at equilibrium of X2 and Y2 into X2Y. ................................................................................................................ ................................................................................................................ ................................................................................................................

[2] [Total 13 marks]

4.

Methanol can be used as a fuel or as the feedstock for a variety of organic compounds.

It is manufactured from carbon monoxide and hydrogen.

CO(g) + 2H2(g) CH3OH(g) (i)

ΔH = –129 kJ mol–1

Describe and explain how the composition of the equilibrium mixture is affected by increasing the temperature ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... increasing the pressure in the reaction. .........................................................................................................................

19

......................................................................................................................... .........................................................................................................................

[4]

(ii)

Describe and explain the effect of increasing the pressure on the rate of reaction. ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[2]

(iii)

The reaction is carried out by passing gaseous reactants over a transition metal catalyst.

Name this type of catalysis. .........................................................................................................................

[1]

(iv)

Suggest and explain the effect of a catalyst on the equilibrium position. ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[2] [Total 9 marks]

5.

Nitrogen dioxide, NO2, is a brown gas whilst dinitrogen tetroxide, N2O4, is a colourless gas. The following equilibrium between these two gases was set up.

2NO2(g) N2O4(g)

ΔH = –58 kJ mol–1

Describe, and explain, what you would see after the following changes have been made and the system allowed to reach equilibrium again. (i)

The temperature is increased. ......................................................................................................................... .........................................................................................................................

20

Think creatively

1. Create a colourful poster explaining the basics of collision theory and what factors affect the rate of reaction and why.

2. Come up with a creative way of explaining Le Chatelier’s principle and a memorable way to remember which way equilibrium will shift.

3. Research how a heterogeneous catalysts works and write a beginners guide to catalysts.

21

1.

Ammonia, NH3, is made industrially by the Haber process. This is an equilibrium reaction.

N2(g) + 3H2(g) 2NH3(g) ΔH = –92 kJ mol–1 (a)

State the pressure and temperature that are used in the Haber process. pressure .......................................................................................................... temperature .....................................................................................................

[2]

(b)

Describe and explain why these conditions are a compromise between rate and equilibrium. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[9] [Total 11 marks]

2.

Ethene and steam can be converted into ethanol.

The equilibrium is shown below.

C2H4(g) + H2O(g) C2H5OH(g)

ΔH = – 46 kJ mol–1

le Chatelier’s principle can be used to predict the effect of changing conditions on the position of equilibrium.

22

(i)

Name the catalyst used in this reaction. .........................................................................................................................

[1]

(ii)

State le Chatelier’s principle. ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[1]

(iii)

Using le Chatelier’s principle, predict and explain the conditions that would give the maximum equilibrium yield of ethanol from ethene and steam. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[3]

(iv)

The actual conditions used are 60 atmospheres pressure at 300 °C in the presence of a catalyst. Compare these conditions with your answer to (iii) and comment on why these conditions are used. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[3] [Total 8 marks]

3.

Many industrial processes, used to manufacture important chemicals, involve equilibrium reactions. Chemists use their understanding of rates of reaction and of yields at equilibrium to find the most economic conditions for the reactions.

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Chemists were investigating the production of a chemical, X2Y, that could be formed from X2 and Y2 as shown in equilibrium 3.1 below.

2X2(g) + Y2(g) 2X2Y(g) (a)

equilibrium 3.1

State le Chatelier’s principle. ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

[2]

(b)

State and explain the effect on equilibrium 3.1 of a decrease in pressure on: (i)

the equilibrium position of the reaction, ................................................................................................................ ................................................................................................................ ................................................................................................................

[2]

(ii)

the rate of the reaction. ................................................................................................................ ................................................................................................................ ................................................................................................................ ................................................................................................................

[2]

(c)

The chemists measured the percentage conversion of X2 at various temperatures. The results are shown in the graph below.

(i)

Use the graph to predict the percentage conversion at 350 °C. answer = .....................................................% [1]

(ii)

The forward reaction in equilibrium 3.1 is exothermic. Explain how the 24

graph supports this statement. ................................................................................................................ ................................................................................................................ ................................................................................................................ ................................................................................................................ ................................................................................................................

[2]

(d)

The chemists decided to use a catalyst in the process.

State, and explain, the effect of using a catalyst on: (i)

the rate of conversion of X2 and Y2 into X2Y, ................................................................................................................ ................................................................................................................

[2]

(ii)

the percentage conversion at equilibrium of X2 and Y2 into X2Y. ................................................................................................................ ................................................................................................................ ................................................................................................................

[2] [Total 13 marks]

4.

Methanol can be used as a fuel or as the feedstock for a variety of organic compounds.

It is manufactured from carbon monoxide and hydrogen.

CO(g) + 2H2(g) CH3OH(g) (i)

ΔH = –129 kJ mol–1

Describe and explain how the composition of the equilibrium mixture is affected by increasing the temperature ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... increasing the pressure in the reaction. .........................................................................................................................

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Looking at some equilibrium reactions

1. Reaction of iodine and chlorine

The U-tube above is set up containing a small amount of iodine crystals.

In the fume cupboard, pass chlorine gas through the U-tube and over the iodine. When no more reaction is observed, dismantle the chlorine supply from the U-tube and allow the tube to stand.

Now pass chlorine gas through the U-tube again. Shut down the chlorine supply again and stopper the U-tube at both ends with bungs.

Q.1 What happens when the chlorine is passed over the iodine?

__________________________________________________________________

________________________________________________________________[3]

Q.2 Write two equations to show what has happened when iodine has reacted with chlorine.

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________________________________________________________________[2]

Q.3 Draw a dot-cross diagram to show the bonding in iodine(I)chloride or iodine monochloride.

[1]

Q.4 Draw dot-cross diagram to show the bonding in iodine(III)chloride or iodine trichloride.

[1]

Q.5 Which of the two reactions reaches a state of dynamic equilibrium?

________________________________________________________________[1]

Q.6 Give two characteristics of a dynamic equilibrium that this reaction shows.

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________________________________________________________________[2]

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2. Reaction of bromine water and alkali

!!! + !! !à !"!! + !! ! + 2!!

(a) Take a test tube and add a little bromine water to it. Now add a little 2M sodium hydroxide solution until no further chemical change occurs. Shake. Keep this mixture for part (b).

Q.7 What happens when the alkali is added to the bromine water?

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Q.8 Explain why this happens using the equation above and Le Chatelier’s principle.

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(b) Now add a little 2M sulphuric acid to the mixture from part (a). Shake.

Q.9 What happens when an acid is added?

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Q.10 Explain why this happens using the equation above and Le Chatelier’s principle.

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________________________________________________________________[3]

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3. Chromate(VI)/Dichromate(VI) reaction

2!"!!!! + 2!! → !!! !!!! + !! ! chromate(VI) dichromate(VI)

(a) Take a test tube and place a little potassium chromate(VI) solution into it. Now add a little 2M sulphuric acid to it. Shake. Keep this mixture for part (b).

Q.11 What colour are chromate(VI) ions? ________________________[1]

Q.12 What is the oxidation state of the chromium in this ion? Explain your answer.

__________________________________________________________________[2]

Q.13 What happens when an acid is added?

__________________________________________________________________[1]

Q.14 What ions are causing this new colour? _________________________[1]

Q.15 What is the oxidation state of the chromium in this ion? Explain your answer.

__________________________________________________________________[2]

Q.16 Explain why this happens using the equation above and Le Chatelier’s principle.

___________________________________________________________________

___________________________________________________________________

__________________________________________________________________[3]

(b) Now add a little 2M sodium hydroxide solution to the mixture from part (a) and shake.

Q.17 What happens?

________________________________________________________________[1]

Q.18 Explain why this happens using the equation above and Le Chatelier’s principle.

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__________________________________________________________________

________________________________________________________________[3]

4. Cobalt(II) ions

[!"(!! !)! ]!! + 4!" ! → [!"!!! ]!! + 6!! !

∆! = +!"

hexaaquacobalt(II) tetrachlorocobaltate(II)

(a) Dissolve a little cobalt(II) sulphate solid in deionised water so that you make about one half of a boiling tube of the solution.

Q.19 What is the colour of this solution? What ions of cobalt are causing this colour?

________________________________________________________________[2]

(b) Add a little concentrated hydrochloric acid to the solution and shake until it all becomes purple. Divide up the solution into four equal portions in four test tubes.

Q.20 Why does the mixture go purple?

___________________________________________________________________[2]

(c) Add a little concentrated hydrochloric acid to portion 1. Shake.

Q.20 What happens? ____________________________________________[1]

Q.21 Explain why this happens using the equation above and Le Chatelier’s principle.

__________________________________________________________________

__________________________________________________________________

________________________________________________________________[3]

(d) Now add water to portion 2. Shake.

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Q.22 What happens? ____________________________________________[1]

Q.23 Explain why this happens using the equation above and Le Chatelier’s principle.

_________________________________________________________________

_________________________________________________________________

________________________________________________________________[3]

(e) Now put the tube with portion 3 into a beaker of hot water. Leave it to stand in the water for several minutes.

Q.24 What happens? ____________________________________________[1]

Q.25 Explain why this happens using the equation above and Le Chatelier’s principle.

__________________________________________________________________

_________________________________________________________________

________________________________________________________________[3]

(f) Finally put the tube containing portion 4 into a beaker of icy-cold water.

Q.26 What happens? ____________________________________________[1]

Q.27 Explain why this happens using the equation above and Le Chatelier’s principle.

__________________________________________________________________

__________________________________________________________________

________________________________________________________________[3]

Q.28 Is the forward reaction exothermic or endothermic?

________________________________________________________________[1]

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