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GCSE Chemistry · GCSEChapter 14 · Reversible reactions, equilibrium and the Haber process
GCSE · Chapter 14

Reversible reactions, equilibrium and the Haber process

By the end of this chapter you will be able to

  • Explain what dynamic equilibrium is, and what stays constant
  • [H] Predict the direction of equilibrium shift from a change in conditions
  • Explain why the Haber process uses high pressure, a moderate temperature and an iron catalyst
  • Explain the compromise between rate and yield in the Haber process
  • [H] Calculate a percentage yield

You should already know

  • Reaction energy and exothermic reactions (Chapter 12)
  • Rates and catalysts (Chapter 13)
  • Percentage yield (Chapter 8)

Where this leads

  • Alkanes, alkenes and crude oil (Chapter 15)
  • Fertilisers and the Earth’s resources (Chapter 19)

GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe reversible reactions, using the reversible symbol, and dynamic equilibrium in a closed system, [H] Predict the effect of changing temperature, pressure and concentration on the position of equilibrium (Le Chatelier), Describe the Haber process: conditions, catalyst and the compromise between rate and yield, Describe the contact process (idea) and its use in making sulfuric acid, [H] Calculate a percentage yield from data.

Exercise P  Do you remember?

1
FWhat does a catalyst do to the activation energy? Write the word.
2
FIs a reaction exothermic if the products have less energy than the reactants? Write yes or no.
3
FWhat is 34 / 2? Write the number.
4
FWhat is the Mr of NH3? Write the number.
5
FWhat is 25.5 / 34 x 100? Write the number.
14.1

Reversible reactions and dynamic equilibrium

GCSE
Describe reversible reactions, using the reversible symbol, and dynamic equilibrium in a closed system
Video lesson: Reversible reactions and dynamic equilibriumComing soon
Context. A pupil says, “When a reaction reaches equilibrium, it stops.” The teacher disagrees. “The reaction is still going, in both directions, at the same rate.” This is the idea of dynamic equilibrium.

A reversible reaction can go in both directions. The symbol ⇌ shows this: N2 + 3H2 ⇌ 2NH3. In a closed system, where no substance can escape, the reaction reaches dynamic equilibrium: the forward and backward reactions happen at the same rate. The concentrations of reactants and products then stay constant, although the reactions are still happening.

closed containerN<sub>2</sub> + 3H<sub>2</sub>⇌2NH<sub>3</sub>forward ratebackward rate
Dynamic equilibrium in a closed container: forward and backward rates are equal

Key ideas

  • Reversible reaction: the products can react back to the reactants.
  • Dynamic equilibrium: forward and backward rates are equal.
  • In a closed system, the concentrations are constant at equilibrium.
  • A reaction that is exothermic in one direction is endothermic in the other, by the same energy.
Worked example 14.1.1
In a closed system at equilibrium, are the forward and backward rates equal? Write yes or no.

Method. Recall what equilibrium means.

  1. Recall. At dynamic equilibrium, the rates in both directions are the same.
    same rate
  2. Decide. So the answer is yes.
    yes
  3. Write the answer.
    yes

Check. The concentrations are also constant. ✓

Worked example 14.1.2
In a closed system at equilibrium, do the concentrations change? Write yes or no.

Method. Think about the constant amounts.

  1. Recall. The concentrations remain constant at equilibrium.
    constant
  2. Decide. So they do not change.
    no
  3. Write the answer.
    no

Check. The reactions are still happening. ✓

Worked example 14.1.3
What symbol shows a reversible reaction? Write the symbol.

Method. Recall the symbol.

  1. Recall. A reversible reaction uses a pair of arrows, one above the other.
    pair of arrows
  2. Decide. The symbol is ⇌.
    ⇌
  3. Write the answer.
    ⇌

Check. It means the reaction can go both ways. ✓

Watch out

Watch out: equilibrium is not the same as equal amounts. At equilibrium, the concentrations are constant, but they are not necessarily equal.

Exam tip

Exam tip. Use the words “same rate” and “constant concentrations” for equilibrium.

Exercise 14A  Reversible reactions and dynamic equilibrium

1
FWhat is a reversible reaction? Write the words.
2
FWhat is a closed system? Write the words.
3
FAt dynamic equilibrium, are the forward and backward rates equal? Write yes or no.
4
FAt equilibrium, are the concentrations of reactants and products constant? Write yes or no.
5
FWhat is the word for the equilibrium where both reactions still happen? Write the word.
6
FIn the Haber reaction, which symbol shows the reaction is reversible? Write the symbol.
7
FIf the forward reaction is exothermic, what is true about the backward reaction? Write the word.
8
PSDoes the amount of product at equilibrium always equal the amount of reactant? Write yes or no.
9
RA pupil says, “At equilibrium, nothing is happening.” Explain why this is wrong, in one sentence. Accept any sentence that says both reactions are still happening at equal rates.
10
RExplain why the concentrations stay constant at equilibrium, in one sentence. Accept any sentence that says products form at the same rate as they break down.
14.2

Le Chatelier’s principle [H]

GCSE
[H] Predict the effect of changing temperature, pressure and concentration on the position of equilibrium (Le Chatelier)
Video lesson: Le Chatelier’s principle [H]Coming soon
Context. A chemist wants more ammonia from the same reaction. “What can I change?” she asks. Le Chatelier’s principle predicts what happens when a condition changes: the system moves to oppose the change.

[H] Higher tier only. Le Chatelier’s principle: if a change is made to a system at equilibrium, the position of equilibrium moves to reduce the effect of the change. Increasing the temperature favours the endothermic direction; increasing the pressure favours the side with fewer gas molecules; increasing the concentration of a reactant moves the equilibrium to use it up. A catalyst does not change the position of equilibrium.

ChangeEquilibrium shifts towardsWhy
increase temperature (exothermic forward)the reactantsthe reaction absorbs the extra energy
increase pressure (fewer gas molecules on the right)the productsthe side with fewer gas molecules reduces the pressure
add more nitrogenthe productsthe system uses up the added reactant
remove ammoniathe productsthe system makes more ammonia to replace it
Predicting the shift of equilibrium in the Haber process

Key ideas

  • Increasing temperature moves equilibrium in the endothermic direction.
  • Increasing pressure moves equilibrium towards the side with fewer gas molecules.
  • Adding a reactant moves equilibrium towards the products.
  • A catalyst does not change the position of equilibrium; it only speeds up both reactions.
Worked example 14.2.1
[H] For N2 + 3H2 ⇌ 2NH3 (forward exothermic), what happens to the yield of NH3 when the temperature is increased? Write the word: higher or lower.

Method. The forward reaction releases heat.

  1. Look. Raising the temperature favours the endothermic reverse reaction.
    reverse
  2. Decide. So the equilibrium moves to the reactants and the yield falls.
    lower
  3. Write the answer.
    lower

Check. A lower temperature gives a higher yield, but a slower rate. ✓

Worked example 14.2.2
[H] In the same reaction, what happens to the yield of NH3 when the pressure is increased? Write the word: higher or lower.

Method. Count the gas molecules on each side.

  1. Count. There are 4 gas molecules on the left and 2 on the right.
    4 and 2
  2. Decide. Increasing pressure favours the side with fewer molecules: the products.
    products
  3. Answer. So the yield of NH3 is higher.
    higher
  4. Write the answer.
    higher

Check. A high pressure is used in the Haber process. ✓

Worked example 14.2.3
[H] Does adding a catalyst change the yield of NH3 at equilibrium? Write yes or no.

Method. Recall what a catalyst does.

  1. Recall. A catalyst speeds up both directions equally.
    both directions
  2. Decide. So it does not change the position of equilibrium.
    no
  3. Write the answer.
    no

Check. The catalyst only helps reach equilibrium sooner. ✓

Watch out

Watch out: Le Chatelier’s principle predicts the position of equilibrium, not the rate. A change that lowers the yield may still be needed for a faster rate.

Exam tip

Exam tip. For pressure, count the gas molecules (not the atoms) on each side of the equation.

Exercise 14B  Le Chatelier’s principle [H]

1
F[H] Which way does equilibrium move when the temperature of an exothermic forward reaction is increased? Write the word: forward or backward.
2
F[H] Which way does equilibrium move when the pressure is increased in N2 + 3H2 ⇌ 2NH3? Write the word: forward or backward.
3
F[H] Which way does equilibrium move when more nitrogen is added? Write the word: forward or backward.
4
F[H] Does removing ammonia as it forms move equilibrium forward or backward? Write the word.
5
F[H] Does a catalyst change the position of equilibrium? Write yes or no.
6
PS[H] Predict whether the yield of an exothermic reaction is higher at a high or low temperature. Write the word: high or low.
7
PS[H] In a reaction with 3 gas molecules on the left and 1 on the right, does high pressure favour the products? Write yes or no.
8
R[H] Explain, in one sentence, why a catalyst does not change the yield of a reversible reaction. Accept any sentence that says it speeds up both the forward and backward reactions equally.
9
R[H] Explain why increasing the pressure increases the yield of ammonia, in one sentence. Accept any sentence that says the forward reaction produces fewer gas molecules, which reduces the pressure.
14.3

The Haber process

GCSE
Describe the Haber process: conditions, catalyst and the compromise between rate and yield
Video lesson: The Haber processComing soon
Context. Ammonia is needed to make fertilisers, and a farm depends on it. “How is ammonia made from nitrogen and hydrogen?” asks a student. The answer is the Haber process, which uses an iron catalyst, a high pressure and a moderate temperature.

In the Haber process, nitrogen (from the air) and hydrogen (from natural gas) react to form ammonia: N2 + 3H2 ⇌ 2NH3. The forward reaction is exothermic. The reaction is run at about 450 degrees C, at about 200 atmospheres pressure, with an iron catalyst. Unreacted gases are recycled, and the ammonia is cooled and removed as a liquid.

N<sub>2</sub> + H<sub>2</sub>compressor200 atmreactor: iron catalyst, about 450 degCcool: NH<sub>3</sub> liquefiesunreacted N<sub>2</sub> and H<sub>2</sub> recycled
The Haber process: compress, react over an iron catalyst, cool to remove ammonia, and recycle the unreacted gases
ConditionTypical valueEffect on yieldEffect on rate
temperatureabout 450 degrees Clower temperature gives a higher yield (forward reaction is exothermic)higher temperature gives a faster rate
pressureabout 200 atmosphereshigher pressure gives a higher yield (4 gas molecules become 2)higher pressure gives a faster rate
catalystironno effect on the yieldspeeds up the rate, so equilibrium is reached sooner
Conditions in the Haber process, and their effects

Key ideas

  • Haber: N2 + 3H2 ⇌ 2NH3.
  • Temperature about 450 degrees C; pressure about 200 atm; iron catalyst.
  • A high pressure increases the yield; a catalyst increases the rate without changing the yield.
  • Ammonia is removed by cooling, and unreacted gases are recycled.
Worked example 14.3.1
In the Haber process, why is an iron catalyst used? Write the words.

Method. Think about the rate.

  1. Look. The reaction is slow without a catalyst.
    slow
  2. Conclude. The iron catalyst speeds up the reaction, so equilibrium is reached sooner.
    sooner
  3. Write the answer.
    it speeds up the reaction so equilibrium is reached sooner

Check. The catalyst does not change the yield. ✓

Worked example 14.3.2
In the Haber process, what happens to the unreacted nitrogen and hydrogen? Write the words.

Method. Recall the process.

  1. Recall. Unreacted gases are separated from the ammonia.
    separated
  2. Decide. They are recycled back into the reactor.
    recycled
  3. Write the answer.
    they are recycled back into the reactor

Check. Recycling makes the process more efficient. ✓

Worked example 14.3.3
What is the approximate temperature used in the Haber process? Write the number in degrees C.

Method. Recall the typical condition.

  1. Recall. The Haber process uses about 450 degrees C.
    450
  2. Check. This is a moderate temperature, chosen as a compromise.
    moderate
  3. Write the answer.
    450

Check. This is a compromise between rate and yield. ✓

Watch out

Watch out: the Haber process does not use the highest pressure or the lowest temperature. Those give the highest yield, but the rate and cost make them impractical; the conditions are a compromise.

Exam tip

Exam tip. Give the conditions with their numbers and an explanation of the compromise.

Exercise 14C  The Haber process

1
FWhat are the two reactants in the Haber process? Write the names.
2
FWhat is the product of the Haber process? Write the name.
3
FWhat catalyst is used in the Haber process? Write the name.
4
FWhat is the approximate pressure used in the Haber process, in atmospheres? Write the number.
5
FIs the forward reaction of the Haber process exothermic or endothermic? Write the word.
6
PSIn the Haber process, why is ammonia cooled and removed? Write the words.
7
FIn the Haber process, what gas is taken from the air? Write the name.
8
RExplain why a high pressure is used in the Haber process, in one sentence. Accept any sentence that says high pressure gives a higher yield, because it favours the side with fewer gas molecules.
9
RExplain why the Haber process uses a moderate temperature of about 450 degrees C, in one sentence. Accept any sentence that says a lower temperature gives a higher yield but a slower rate.
10
RExplain why unreacted gases are recycled in the Haber process, in one sentence. Accept any sentence that says the yield per pass is low, so recycling uses the reactants more efficiently.
14.4

Choosing conditions: the compromise

GCSE
Describe the Haber process: conditions, catalyst and the compromise between rate and yield
Video lesson: Choosing conditions: the compromiseComing soon
Context. A manager asks the chemist to run the Haber process at the highest possible yield. “Can we use a very low temperature and a huge pressure?” she asks. The answer is no: the rate and cost must also be considered, so a compromise is chosen.

The conditions are chosen as a compromise. A low temperature gives a high yield but a slow rate. A high pressure gives a high yield but needs expensive equipment and more energy. So the process uses a moderate temperature, a high pressure and a catalyst, which together give a reasonable rate and yield at a sensible cost.

ReasonExplanation
temperature 450 degrees Ca compromise: a lower temperature would give a better yield but a very slow rate
pressure 200 atma compromise: higher pressure gives a better yield, but costs more to build and run
recyclingthe yield per pass is low, so unreacted nitrogen and hydrogen are recycled
The compromise in the Haber process

Key ideas

  • Rate and yield are both important, and the conditions are a compromise.
  • Low temperature: higher yield, slower rate.
  • High pressure: higher yield, higher cost.
  • A catalyst makes a moderate temperature give a reasonable rate.
Worked example 14.4.1
Why is a very high pressure not always used in the Haber process? Write the words.

Method. Think about the cost.

  1. Look. A higher pressure needs stronger equipment and more energy.
    stronger equipment
  2. Conclude. So the cost is too high for a small gain in yield.
    cost
  3. Write the answer.
    the cost of the equipment and energy is too high

Check. A compromise is needed. ✓

Worked example 14.4.2
Why is a catalyst used at a moderate temperature? Write the words.

Method. Think about the rate.

  1. Look. A moderate temperature gives a slow reaction without a catalyst.
    slow
  2. Conclude. The catalyst speeds it up, so the rate is reasonable.
    reasonable rate
  3. Write the answer.
    the catalyst gives a reasonable rate at a moderate temperature

Check. The yield is not changed by the catalyst. ✓

Watch out

Watch out: a catalyst does not increase the yield. It helps the reaction reach equilibrium faster, so a moderate temperature still gives a usable rate.

Exam tip

Exam tip. When you explain a compromise, give one benefit of each choice and one drawback.

Exercise 14D  Choosing conditions: the compromise

1
FWhat is the drawback of a very low temperature in the Haber process? Write the words.
2
FWhat is the drawback of a very high pressure in the Haber process? Write the words.
3
FWhat is the benefit of a catalyst in the Haber process? Write the words.
4
FWhich condition gives a higher yield: a low or a high temperature, for the forward exothermic reaction? Write the word.
5
FWhich condition is used to give a reasonable rate of reaction: a moderate or a very low temperature? Write the word.
6
FWhich condition does the Haber process use at about 200 atm? Write the word: pressure or temperature.
7
RGive one benefit and one drawback of a high pressure in the Haber process. Accept benefit: higher yield; drawback: higher cost or more energy needed.
8
RExplain why the yield of the Haber process is not 100% at equilibrium, in one sentence. Accept any sentence that says the reaction is reversible and reaches equilibrium before all the reactants are used.
9
R[H] Explain how a manager could use a moderate temperature, a high pressure and a catalyst together, in one or two sentences. Accept any answer that gives a reasonable rate and yield at a sensible cost.
10
R[H] What is meant by a compromise in the Haber process, in one sentence? Accept any sentence that says the conditions balance the rate, the yield and the cost.
14.5

The contact process and percentage yield [H]

GCSE
Describe the contact process (idea) and its use in making sulfuric acid, [H] Calculate a percentage yield from data
Video lesson: The contact process and percentage yield [H]Coming soon
Context. A factory makes sulfuric acid, which is used to make fertilisers and car batteries. “How is it made from sulfur dioxide?” asks a student. The contact process turns sulfur dioxide into sulfur trioxide over a catalyst, which is then dissolved to make sulfuric acid.

In the contact process, sulfur dioxide and oxygen are passed over a vanadium(V) oxide catalyst to form sulfur trioxide: 2SO2 + O2 ⇌ 2SO3. The sulfur trioxide is absorbed into concentrated sulfuric acid, and then diluted to make the product. The conditions are chosen as a compromise, as in the Haber process.

The percentage yield = actual yield / theoretical yield x 100, as in Chapter 8.

SO<sub>2</sub> + aircatalyst (vanadium oxide)SO<sub>3</sub> absorbed in acidsulfuric acid is made from the SO<sub>3</sub>
The contact process (idea): SO2 and O2 over a catalyst make SO3, which is absorbed to make sulfuric acid

Key ideas

  • Contact process: 2SO2 + O2 ⇌ 2SO3 over a vanadium(V) oxide catalyst.
  • Sulfur trioxide is absorbed to make sulfuric acid.
  • The conditions are a compromise between rate and yield.
  • [H] Percentage yield = actual / theoretical x 100.
Worked example 14.5.1
[H] A reaction should give 34 g of ammonia, but 25.5 g is made. What is the percentage yield? Write the number.

Method. Actual divided by theoretical, times 100.

  1. Divide. 25.5 / 34 = 0.75.
    0.75
  2. Percentage. 0.75 x 100 = 75%.
    75
  3. Write the answer.
    75

Check. 75% of 34 is 25.5. ✓

Worked example 14.5.2
In the contact process, what catalyst is used? Write the name.

Method. Recall the catalyst.

  1. Recall. The catalyst is vanadium(V) oxide.
    vanadium(V) oxide
  2. Check. It is a solid, so the gases pass over it.
    solid
  3. Write the answer.
    vanadium(V) oxide

Check. The contact process uses a solid catalyst. ✓

Worked example 14.5.3
In the contact process, what is absorbed to make sulfuric acid? Write the name.

Method. Recall the step.

  1. Recall. Sulfur trioxide is absorbed into concentrated sulfuric acid.
    sulfur trioxide
  2. Check. Water is then added to dilute the product to the final acid.
    dilute
  3. Write the answer.
    sulfur trioxide

Check. Dilution with water gives the final acid. ✓

Watch out

Watch out: in a percentage yield, the actual yield is always less than or equal to the theoretical yield in a sensible experiment. Do not put the numbers the wrong way round.

Exam tip

Exam tip. Write the actual yield on top and the theoretical yield below, then multiply by 100.

Exercise 14E  The contact process and percentage yield [H]

1
FWhat is the formula of sulfur dioxide? Write the formula.
2
FWhat is the formula of sulfur trioxide? Write the formula.
3
FIn the contact process, what is the name of the catalyst? Write the name.
4
F[H] Calculate the percentage yield if 20 g is expected and 15 g is obtained. Write the number.
5
PS[H] Calculate the percentage yield if 40 g is expected and 30 g is obtained. Write the number.
6
PS[H] A reaction should give 50 g of product. The actual yield is 40 g. What is the percentage yield? Write the number.
7
PS[H] The actual yield is 12 g from a theoretical yield of 16 g. What is the percentage yield? Write the number.
8
R[H] In the Haber process, the theoretical yield is 34 g of ammonia and the actual is 25.5 g. Explain one reason the actual yield is lower, in one sentence. Accept any reason: the reaction is reversible, or some product is lost in the process.
9
R[H] Explain why the contact process uses a catalyst, in one sentence. Accept any sentence that says the catalyst increases the rate so that the process is economic.

Chapter summary

SkillMethod
Reversible reactionCan go both ways; shown with ⇌.
Dynamic equilibriumClosed system; forward and backward rates equal; concentrations constant.
[H] Le ChatelierChanges shift equilibrium to oppose them: temperature, pressure, concentration.
Haber processN2 + 3H2 ⇌ 2NH3; about 450 degrees C, about 200 atm, iron catalyst; recycle.
CompromiseRate versus yield versus cost; a catalyst helps the rate without changing the yield.
Contact process2SO2 + O2 ⇌ 2SO3; vanadium(V) oxide catalyst; makes sulfuric acid.

Can you…?

  • I can describe a reversible reaction and dynamic equilibrium in a closed system
  • [H] I can predict the effect of temperature, pressure and concentration using Le Chatelier
  • I can describe the Haber process, its conditions and the compromise
  • I can describe the contact process and the use of the sulfuric acid it makes
  • [H] I can calculate a percentage yield

Key terms

reversible reaction
a reaction that can go in both directions
dynamic equilibrium
the state where the forward and backward reactions occur at the same rate in a closed system
closed system
a system in which no substances can escape
Le Chatelier’s principle
[H] if a change is made to a system at equilibrium, the position moves to oppose the change
compromise
a set of conditions that balances the rate, the yield and the cost

Exercise 14Z  Check-up

1
What symbol shows that a reaction is reversible? Write the symbol.
2
At dynamic equilibrium, are the forward and backward rates equal? Write yes or no.
3
What is the name of the iron-based catalyst used in the Haber process? Write the name.
4
What is the approximate temperature of the Haber process, in degrees C? Write the number.
5
[H] For N2 + 3H2 ⇌ 2NH3, which side has more gas molecules? Write the word: left or right.
6
[H] In the Haber reaction, which direction is favoured by a higher pressure? Write the word: left or right.
7
Give the product of the contact process that is absorbed to make sulfuric acid. Write the formula.
8
[H] Calculate the percentage yield if 20 g is expected and 16 g is obtained. Write the number.
9
Explain why the Haber process uses a catalyst, in one sentence. Accept any sentence that says the catalyst speeds up the rate without changing the yield.

Total 23 marks. Allow about 23 minutes.

Exercise 14X  Exam-style questions

1
RDescribe what happens at dynamic equilibrium in a closed system, in one or two sentences. Accept any answer that says the forward and backward reactions occur at the same rate, so the concentrations stay constant.[2 marks]
2
FName the two gases that react in the Haber process, and the product. Write the three names.[2 marks]
3
R[H] Explain why a low temperature gives a higher yield of ammonia but is not used, in one or two sentences. Accept any answer that says the yield is higher, but the rate is too slow.[3 marks]
4
R[H] Explain why increasing the pressure increases the yield in the Haber process, in one sentence. Accept any sentence that says the forward reaction has fewer gas molecules.[2 marks]
5
FGive the conditions of the Haber process: temperature, pressure and catalyst. Write the three values.[3 marks]
6
RExplain the compromise in the conditions of the Haber process, in two sentences. Accept any answer that says a higher pressure or lower temperature would improve the yield but increase cost or slow the rate.[3 marks]
7
PS[H] A reaction should give 40 g of product, and the actual yield is 32 g. Calculate the percentage yield. Write the number.[2 marks]
8
FName the catalyst used in the contact process and the substance absorbed to make sulfuric acid. Write two names.[2 marks]
9
R[H] Explain why the yield of a reversible reaction is always less than 100%, in one sentence. Accept any sentence that says equilibrium is reached before all the reactants are used.[2 marks]
10
RExplain, in one sentence, why a catalyst does not change the position of equilibrium. Accept any sentence that says it speeds up both the forward and backward reactions equally.[2 marks]
← Chapter 13: The rate of chemical reactions and catalystsChapter 15: Crude oil, alkanes and alkenes →

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