The effect of pressure changes on equilibrium (HT only) - AQA GCSE Chemistry Exam Questions

Practise 19 exam-style questions on The effect of pressure changes on equilibrium (HT only) for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

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1–6 marks each
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19 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Carbon monoxide reacts with chlorine to form phosgene (COCl₂):
CO(g) + Cl₂(g) ⇌ COCl₂(g)

State the number of gas molecules on each side of the equation.

Predict the effect of increasing the pressure on the yield of COCl₂ at equilibrium.

Model answer

There are 2 molecules of gas on the left and 1 molecule on the right. Increasing the pressure shifts the equilibrium towards the side with fewer molecules, so the yield of COCl₂ increases.

Explanation

Count the molecules using the balancing numbers: CO + Cl₂ is 2 molecules, COCl₂ is 1. An increase in pressure favours the side with fewer molecules, here the product side.

Mark scheme

1 mark
2 molecules on the left and 1 molecule on the right
both numbers needed
1 mark
the yield of COCl₂ increases
allow the equilibrium shifts to the right

Question 2

2 marks

Phosphorus(V) chloride decomposes in a reversible reaction:
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)

Predict the effect of decreasing the pressure on the amount of chlorine at equilibrium. Explain your answer.

Model answer

The amount of chlorine increases. There is 1 molecule of gas on the left and 2 on the right, so decreasing the pressure shifts the equilibrium towards the side with more molecules, which is the right.

Explanation

When the pressure is decreased, the system counteracts the change by shifting towards the side with more gas molecules, because more molecules exert a greater pressure.

Mark scheme

1 mark
the amount of chlorine increases
1 mark
there are more molecules on the right (2) than on the left (1), so decreasing the pressure shifts the equilibrium to the right

Question 3

3 marks

Use ideas about gas molecules to explain why increasing the pressure on a gaseous equilibrium mixture shifts the position of equilibrium towards the side of the equation with fewer molecules.

Model answer

Gas pressure is caused by molecules colliding with the walls of the container, so fewer gas molecules exert a lower pressure. When the pressure is increased, the system responds to counteract the change. Shifting the equilibrium towards the side with fewer molecules reduces the total number of gas molecules, which lowers the pressure.

Explanation

This links Le Chatelier's Principle to particle ideas. The system cannot remove the extra pressure completely, but converting more molecules into fewer molecules partly reduces it.

Mark scheme

1 mark
gas pressure is caused by molecules colliding with the walls of the container, so fewer molecules exert a lower pressure
allow fewer molecules means fewer collisions with the walls
1 mark
the system responds to counteract the increase in pressure
allow reference to Le Chatelier's Principle
1 mark
shifting towards the side with fewer molecules reduces the number of molecules and so reduces the pressure

Question 4

5 marks

A sealed gas syringe contains an equilibrium mixture of brown nitrogen dioxide and colourless dinitrogen tetroxide:
2NO₂(g) ⇌ N₂O₄(g)
The plunger is pushed in quickly and held in place, at a constant temperature. The mixture first becomes darker and then slowly becomes paler.

Explain these observations.

Compare the final colour with the colour before the plunger was pushed in. Give a reason for your answer.

Model answer

When the plunger is pushed in, the same amount of brown NO₂ is squashed into a smaller volume, so its concentration increases and the mixture looks darker. Pushing in the plunger also increases the pressure. There are 2 molecules on the left and 1 on the right, so the equilibrium shifts to the right, towards the side with fewer molecules. Some brown NO₂ is converted into colourless N₂O₄, so the mixture slowly becomes paler.
The final colour is still darker than before the plunger was pushed in. The system only partly counteracts the increase in pressure, so the concentration of NO₂ at the new equilibrium is still higher than at the start.

Explanation

Two effects happen one after the other: an immediate concentration change from the smaller volume, then the slower shift in equilibrium. Le Chatelier's Principle says the change is counteracted, not cancelled, so the final mixture does not return to its original colour.

Mark scheme

1 mark
it first becomes darker because the same amount of NO₂ is squashed into a smaller volume / the concentration of NO₂ increases
1 mark
pushing in the plunger increases the pressure
1 mark
the equilibrium shifts towards the side with fewer molecules, which is the right (1 molecule compared with 2)
1 mark
so some brown NO₂ is converted into colourless N₂O₄, making the mixture paler
1 mark
the final colour is still darker than the original colour, because the change is only partly counteracted (the concentration of NO₂ stays higher than at the start)

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