Energy changes and reversible reactions - AQA GCSE Chemistry Exam Questions

Practise 12 exam-style questions on Energy changes and reversible reactions 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

A reversible reaction is exothermic in the forward direction.

What can you say about the reverse reaction? Refer to the type of energy change and the amount of energy transferred.

Model answer

The reverse reaction is endothermic, and the same amount of energy is transferred as in the forward reaction, but in the opposite direction (taken in rather than given out).

Explanation

If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The energy given out in one direction exactly equals the energy taken in for the other direction.

Mark scheme

1 mark
the reverse reaction is endothermic
1 mark
the same amount of energy is transferred (as in the forward reaction)
allow the energy change has the same size but opposite sign

Question 2

3 marks

A few drops of water are added to some white anhydrous copper sulfate.

Describe what is observed, and explain why the mixture gets hot.
hydrated copper sulfate ⇌ anhydrous copper sulfate + water
The forward reaction is endothermic.

Model answer

The white powder turns blue. The water reacts with the anhydrous copper sulfate to form hydrated copper sulfate, which is the reverse reaction. Because the forward reaction is endothermic, the reverse reaction is exothermic, so energy is released and the mixture gets hot.

Explanation

This reaction is used as a test for water: water turns white anhydrous copper sulfate blue. The heat released is the same amount of energy that was taken in when the blue crystals were heated to make the white powder.

Mark scheme

1 mark
the white solid turns blue
1 mark
the reverse reaction happens (anhydrous copper sulfate reacts with water to form hydrated copper sulfate)
1 mark
the reverse reaction is exothermic, so energy is released (to the surroundings)
allow it gives out the same energy that was taken in when the copper sulfate was dehydrated

Question 3

3 marks

The diagram shows the reaction profile for the forward reaction in the Haber process:
N₂ + 3H₂ ⇌ 2NH₃
The energy change labelled B is 92 kJ/mol.

Explain what the diagram shows about the reverse reaction, and give the energy change for the reverse reaction.

Model answer

The products (ammonia) are at a lower energy than the reactants, so the forward reaction is exothermic. The reverse reaction starts at the energy of ammonia and finishes at the higher energy of nitrogen and hydrogen, so energy must be taken in and it is endothermic. Its energy change is +92 kJ/mol, the same size as B but with the opposite sign.

Explanation

Reading a reaction profile backwards gives the reverse reaction. The gap between the two energy levels is the same in both directions, so the same amount of energy is transferred.

Mark scheme

1 mark
the forward reaction is exothermic because the products are at a lower energy than the reactants
1 mark
the reverse reaction goes from the lower energy (ammonia) to the higher energy (nitrogen and hydrogen), so it is endothermic
1 mark
energy change for the reverse reaction = +92 kJ/mol
allow 92 kJ/mol taken in

Question 4

4 marks

Some homes store energy from the Sun in a salt hydrate. In summer, energy from solar panels is used to heat the salt hydrate, which drives off its water. In winter, water vapour is passed over the dry salt, and the energy released is used to heat the house.
salt hydrate ⇌ dry salt + water

Explain how this system stores energy and why the energy released in winter is equal to the energy taken in during summer.

Model answer

Heating the salt hydrate in summer drives the forward reaction, which is endothermic, so energy from the solar panels is taken in and stored in the dry salt. In winter, water vapour is added and the reverse reaction happens, which is exothermic, so energy is released to heat the house. Because the reaction is reversible, exactly the same amount of energy is released in the reverse reaction as was taken in during the forward reaction. The energy stays stored for as long as the dry salt is kept away from water.

Explanation

This applies the idea that a reversible reaction that is endothermic in one direction is exothermic by the same amount in the other. Keeping the products apart stops the reverse reaction, so the energy can be stored for months.

Mark scheme

1 mark
removing the water (forward reaction) is endothermic, so energy is taken in in summer
1 mark
adding water (reverse reaction) is exothermic, so energy is released in winter
1 mark
the reaction is reversible and the same amount of energy is transferred in each direction
1 mark
the energy stays stored as long as the dry salt is kept away from water
allow energy can be released when it is needed

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