AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2022: Question 6

10 marks · Standard Demand difficulty · Short Answer

Answer a series of short questions about catalysts, reversible reactions, dynamic equilibrium, and the effects of pressure and temperature on the Haber process for producing ammonia.

Practise this question

Question

Question text

06 This question is about catalysts and equilibrium.

06.1 What type of substance is a catalyst in biological systems?

[1 mark]

Tick ( ) one box.

Algae

Alkene

Enzyme

Formulation

06.2 Explain how a catalyst increases the rate of a reaction.

[2 marks]

The reversible reaction for the production of ammonia is:

N2(g) + 3 H2(g) ⇌ 2 NH3(g)

06.3 What can scientists predict using Le Chatelier’s Principle?

[1 mark]

06.4 Describe how a reversible chemical reaction is able to reach equilibrium.

[2 marks]

06.5 Explain the effect of increasing the pressure on the yield of ammonia.

[2 marks]

06.6 The forward reaction to produce ammonia is exothermic.

Explain the effect of increasing the temperature on the yield of ammonia.

[2 marks]

Mark scheme

Show the mark scheme

Question 6

AO /

Question Answers Extra information Mark

Spec. Ref.

06.1 enzyme 1 AO1

5.6.1.4

AO /

Spec. Ref.

06.2 provides a different reaction 1 AO1

pathway 5.6.1.4

(which) has a lower activation

energy

AO /

Spec. Ref.

06.3 the effects of changing allow the effects of changing 1 AO1

conditions on the position of an conditions on the yield of an 5.6.2.4

equilibrium (in a closed system) equilibrium reaction (in a closed

system)

AO /

Spec. Ref.

06.4 (when) the forward and reverse 1 AO1

reactions have the same rate 5.6.2.3

in apparatus which prevents the allow in a closed system 1

escape of reactants and

products

AO /

Spec. Ref.19

06.5 yield increases 1 AO2

5.6.2.1

(because) there are more moles allow (because) there are fewer 1 5.6.2.2

(of gas) on the left hand side moles (of gas) on the right hand 5.6.2.3

side 5.6.2.4

5.6.2.7

AO /

Spec. Ref.

06.6 yield decreases 1 AO2

5.6.2.1

(because) the system shifts in 1 5.6.2.2

the endothermic direction 5.6.2.3

5.6.2.4

5.6.2.6

Total Question 6 10

How to answer it

Catalysts and Equilibrium

What this question tests

This question checks whether you can identify an enzyme as a biological catalyst, explain how catalysts work by giving an alternative pathway with lower activation energy, and apply Le Chatelier’s Principle to the Haber process. You also need to describe dynamic equilibrium in a closed system and explain how pressure and temperature affect the yield of ammonia.

Question 06.1 — What type of substance is a catalyst in biological systems? [1 mark]

✅ Correct answer

Enzyme

1 mark for ticking Enzyme only.

💡 Key knowledge

A catalyst in living things is called an enzyme.
Enzymes speed up reactions in cells without being used up.

❌ Common errors

Choosing alkene or formulation because they look similar or are chemistry words.
Writing “catalyst” instead of naming the biological type. The question asks for the specific term.

Question 06.2 — Explain how a catalyst increases the rate of a reaction. [2 marks]

✅ Correct answer

A catalyst provides a different reaction pathway that has a lower activation energy.

Two marking points: one for the idea of a different pathway, one for lower activation energy.

💡 Key knowledge

Lower activation energy means more particles have enough energy to react.
This increases the number of successful collisions per second.

🧠 Exam technique

Use the exact phrase “different reaction pathway”.
Then link it to lower activation energy.
For full marks, keep your answer short and precise.

❌ Common errors

Saying a catalyst “adds energy” or “makes particles move faster” — not correct.
Saying only “it speeds up the reaction” without explaining how can lose the second mark.
Confusing activation energy with temperature.

Question 06.3 — What can scientists predict using Le Chatelier’s Principle? [1 mark]

✅ Correct answer

The effect of changing conditions on the position of equilibrium in a closed system.

Allow: the effect of changing conditions on the yield of an equilibrium reaction in a closed system.

💡 Key knowledge

Le Chatelier’s Principle predicts how equilibrium shifts when conditions change.
Common changes: temperature, pressure, and concentration.

🧠 Exam technique

If you forget the exact wording, mention equilibrium shift and yield.
The mark scheme accepts both ideas.

Question 06.4 — Describe how a reversible chemical reaction is able to reach equilibrium. [2 marks]

✅ Correct answer

Equilibrium is reached when the forward and reverse reactions have the same rate in a closed system so reactants and products cannot escape.

2 marks available: one for equal rates, one for closed system / no escape of reactants and products.

💡 Key knowledge

At equilibrium, reactions are still happening.
The amounts of reactants and products stay constant because the rates are equal.

🧠 Exam technique

Use the phrase dynamic equilibrium if you know it.
“Closed system” is important — it stops substances escaping.

❌ Common errors

Saying the reaction stops. It does not stop.
Forgetting that equilibrium only happens in a closed system.
Saying the amounts are equal — they are usually constant, not necessarily equal.

Question 06.5 — Explain the effect of increasing the pressure on the yield of ammonia. [2 marks]

✅ Correct answer

The yield increases because there are more moles of gas on the left-hand side of the equation.

Alternative accepted: there are fewer moles of gas on the right-hand side.

💡 Key knowledge

Equation: N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g)

Left side: 4 moles of gas
Right side: 2 moles of gas
Increasing pressure favours the side with fewer gas molecules.

🧠 Exam technique

Always compare the number of moles of gas on each side.
State the result first: yield increases.
Then explain why using the side with fewer gas molecules.

❌ Common errors

Saying pressure changes the catalyst’s action — it does not.
Talking about “more particles” without linking to moles of gas.
Forgetting that the right-hand side has fewer gas moles.

Question 06.6 — Explain the effect of increasing the temperature on the yield of ammonia. [2 marks]

✅ Correct answer

The yield decreases because the system shifts in the endothermic direction.

2 marks available: one for yield decreases, one for shifting in the endothermic direction.

💡 Key knowledge

The forward reaction to make ammonia is exothermic.
If temperature increases, the equilibrium shifts to oppose the change.
So it favours the endothermic reverse reaction.

🧠 Exam technique

Write both parts: what happens to yield and why.
Use endothermic direction rather than just “the other side”.

❌ Common errors

Saying yield increases because “more heat means more reaction” — this misses equilibrium reasoning.
Confusing exothermic and endothermic directions.
Not linking temperature change to a shift in equilibrium.

Quick 10-mark overview: what the examiner wanted

🧠 What distinguished top responses

Used correct scientific terms: enzyme, activation energy, equilibrium, closed system.
Gave short, direct explanations matching the mark points.
For ammonia yield, compared the numbers of gas moles correctly.

💡 Must-know facts

Catalysts speed up reactions by lowering activation energy.
Equilibrium means forward rate = reverse rate.
Higher pressure favours the side with fewer gas molecules.
Higher temperature favours the endothermic direction.

Topics

Chemistry · C6: The Rate and Extent of Chemical Change

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Higher), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.