A student says: 'Once a reversible reaction reaches equilibrium, the reaction has stopped.' Explain why the student is wrong.

AQA GCSE Chemistry (8462) — 4.6.2 Reversible reactions and dynamic equilibrium · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

At equilibrium the forward reaction is still occurring, and the reverse reaction is also still occurring. The rates of the forward and reverse reactions are equal, so the concentrations of reactants and products remain constant. Therefore the reaction has not stopped, only the net change is zero.

Examiner tips

Use the term ‘dynamic equilibrium’ to show you understand the reaction continues. Mention that rates are equal, not that the reaction stops. Show the consequence: concentrations remain constant.
Avoid saying the reaction has stopped or that no reaction occurs at all.

Common mistakes

Saying the reaction has stopped or that no reaction occurs. Confusing equilibrium with completion of the reaction. Failing to mention that rates are equal or that concentrations remain constant.

Mark scheme (4 marks)

At equilibrium, the forward reaction is still occurring / has not stopped
At equilibrium, the reverse reaction is also still occurring / has not stopped
The forward and reverse reactions occur at exactly the same rate
So the concentrations / amounts of reactants and products remain constant

Key terms in this question

reversible reaction · equilibrium