Reversible reactions - AQA GCSE Chemistry Exam Questions

Practise 14 exam-style questions on 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.

14 questions
1–4 marks each
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14 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

1 mark

The combustion of methane is not a reversible reaction.

What does this mean?

Model answer

It means that the products, carbon dioxide and water, cannot react together to re-form methane and oxygen.

Explanation

Most reactions you meet, such as combustion and neutralisation, are treated as going in one direction only. Reversible reactions are a special case where the products can turn back into reactants.

Mark scheme

1 mark
the products (carbon dioxide and water) cannot react to re-form methane and oxygen
allow the reaction only goes in one direction

Question 2

2 marks

Ethanol can be made by reacting ethene with steam:
C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g)

Name the reactant and the products of the reverse reaction.

Model answer

In the reverse reaction, ethanol is the reactant, and ethene and steam (water) are the products:
ethanol → ethene + water

Explanation

In a reversible reaction the substances on the right of the equation are the reactants of the reverse reaction. Here ethanol breaks down into ethene and water.

Mark scheme

1 mark
reactant of the reverse reaction: ethanol
allow C₂H₅OH
1 mark
products of the reverse reaction: ethene and water / steam
both needed

Question 3

3 marks

Iodine monochloride (ICl) is a brown liquid. It reacts with chlorine, a yellow-green gas, to form iodine trichloride (ICl₃), a yellow solid:
ICl(l) + Cl₂(g) ⇌ ICl₃(s)

When chlorine is passed over the brown liquid, yellow crystals form. When the supply of chlorine is removed, the yellow crystals slowly turn back into a brown liquid.

Explain these observations.

Model answer

When chlorine is added, the forward reaction happens: iodine monochloride reacts with chlorine to form yellow iodine trichloride. When the chlorine is removed, the reverse reaction happens: iodine trichloride breaks down into brown iodine monochloride and chlorine. The reaction is reversible, and changing the amount of chlorine present changes its direction.

Explanation

This is an unfamiliar example of the same idea as heating ammonium chloride. Here it is the amount of one substance (chlorine), rather than the temperature, that decides which direction the reaction goes.

Mark scheme

1 mark
adding chlorine causes the forward reaction, forming (yellow) ICl₃
1 mark
removing chlorine causes the reverse reaction: ICl₃ decomposes back into (brown) ICl and chlorine
1 mark
the reaction is reversible and its direction is changed by changing the conditions / amount (concentration) of chlorine

Question 4

4 marks

A student heats a small amount of solid ammonium chloride at the bottom of a test tube. The top of the test tube stays cool.

Describe what the student would observe and explain the observations.

Model answer

The white solid at the bottom of the tube disappears, because when it is heated ammonium chloride decomposes into ammonia gas and hydrogen chloride gas. A white solid then appears near the cool top of the tube, because the two gases react together to re-form ammonium chloride when they cool down. This shows the reaction is reversible:
ammonium chloride ⇌ ammonia + hydrogen chloride

Explanation

This is the classic example of a reversible reaction whose direction is changed by changing the temperature. Heating drives the forward reaction and cooling drives the reverse reaction.

Mark scheme

1 mark
the white solid at the bottom of the tube disappears
1 mark
because ammonium chloride decomposes into ammonia and hydrogen chloride (gases)
1 mark
a white solid forms near the (cool) top of the tube
1 mark
because the gases react to re-form ammonium chloride when they are cooled (reverse reaction)
allow the reaction is reversible

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