Equilibrium - AQA GCSE Chemistry Exam Questions

Practise 12 exam-style questions on Equilibrium 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.

12 questions
1–5 marks each
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12 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

State the condition needed for a reversible reaction to reach equilibrium.

Explain why this condition is needed.

Model answer

The reaction must take place in a closed system, where none of the reactants or products can escape. This is needed so that the products stay in the mixture and can react to re-form the reactants, so the reverse reaction can take place at the same time as the forward reaction.

Explanation

Equilibrium is reached when the forward and reverse reactions occur at exactly the same rate. If a product escapes (for example a gas leaving an open flask), the reverse reaction cannot keep up, so equilibrium is never reached.

Mark scheme

1 mark
a closed system / apparatus that prevents the escape of reactants and products
allow sealed container; ignore references to temperature or catalyst
1 mark
so the products stay in the mixture and can react to re-form the reactants (the reverse reaction can take place)
allow so that the reverse reaction can happen at the same rate as the forward reaction

Question 2

2 marks

Sulfur dioxide reacts with oxygen in a reversible reaction:
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
In an equilibrium mixture of these gases, the concentration of sulfur trioxide (SO₃) is much greater than the concentration of sulfur dioxide (SO₂). A student concludes that, at equilibrium, the forward reaction must be faster than the reverse reaction.

Explain why the student is wrong.

Model answer

At equilibrium the forward and reverse reactions always happen at the same rate. The concentrations of reactants and products do not have to be equal; they just stay constant. A higher concentration of SO₃ only shows that the equilibrium mixture contains more products than reactants.

Explanation

Students often confuse equal rates with equal concentrations. Equal rates are what define equilibrium; the relative amounts of reactants and products at equilibrium can be anything, depending on the reaction and the conditions.

Mark scheme

1 mark
at equilibrium the rates of the forward and reverse reactions are (always) equal
1 mark
the concentrations do not need to be equal, only constant / a high concentration of SO₃ just means the mixture contains more products than reactants
allow the position of equilibrium lies to the right

Question 3

3 marks

A few drops of liquid bromine are placed in a flask, which is then sealed with a stopper. The flask fills with orange-brown bromine vapour.
Br₂(l) ⇌ Br₂(g)
After a few minutes the colour of the vapour stops getting darker, but some liquid bromine is still left in the flask.

Explain why the colour of the vapour stays the same.

Model answer

The flask is sealed, so an equilibrium is reached. Bromine is still evaporating and bromine vapour is still condensing, but both happen at the same rate. So the amount of bromine vapour in the flask stays constant and the colour does not change.

Explanation

Equilibrium applies to physical changes as well as chemical reactions. In a sealed flask, the vapour cannot escape, so condensation speeds up until it matches evaporation. If the stopper were removed, the vapour would escape and all the liquid would eventually evaporate.

Mark scheme

1 mark
the rate of evaporation equals the rate of condensation
allow rate of forward process = rate of reverse process
1 mark
so the amount / concentration of bromine vapour stays constant
1 mark
evaporation and condensation are both still happening / the equilibrium is dynamic
do not accept evaporation has stopped

Question 4

4 marks

A mixture of nitrogen and hydrogen is sealed in a container with a catalyst at a constant temperature. The gases react to form ammonia:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Describe how the rates of the forward and reverse reactions change from the moment the gases are mixed until equilibrium is reached. Explain why the rates change.

Model answer

When the gases are first mixed there is no ammonia, so the forward reaction is at its fastest and the reverse rate is zero. As nitrogen and hydrogen are used up their concentrations fall, so the forward reaction slows down. As ammonia builds up its concentration rises, so the reverse reaction speeds up. Eventually the two rates become equal; equilibrium has been reached and both rates then stay constant.

Explanation

The rate of each reaction depends on the concentration of the substances reacting in that direction. Because one rate falls while the other rises, they must meet - that point is equilibrium. Both reactions keep going after that, at the same rate.

Mark scheme

1 mark
at the start the forward rate is at its highest and the reverse rate is zero because there is no ammonia
allow reverse reaction cannot happen at first
1 mark
the forward rate decreases because the concentrations of nitrogen and hydrogen decrease (as they are used up)
1 mark
the reverse rate increases because the concentration of ammonia increases
1 mark
until the two rates are equal (equilibrium), after which both rates stay constant
do not accept both reactions stop

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