Explain what is meant by dynamic equilibrium in a reversible reaction, and describe what happens to the concentrations of reactants and products once dynamic equilibrium has been reached.

Edexcel GCSE Chemistry (1CH0) — 4.2 Reversible reactions and equilibria · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A reversible reaction is one in which the products can react together to reform the reactants.
At dynamic equilibrium the rate of the forward reaction equals the rate of the backward reaction.
Both forward and backward reactions are still occurring – the equilibrium is dynamic.
The concentrations of reactants and products therefore remain constant once dynamic equilibrium is reached.

Examiner tips

  • Use the term "dynamic equilibrium" and mention that both directions continue. Show that rates are equal and concentrations are constant. Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

  • Confusing dynamic equilibrium with static equilibrium. Saying the reactions stop instead of continuing. Omitting that concentrations remain constant.

Mark scheme (4 marks)

  1. A reversible reaction is one in which the products can react together to reform the reactants.
  2. At dynamic equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction.
  3. Both the forward and backward reactions are still occurring (the equilibrium is dynamic/active).
  4. The concentrations of reactants and products remain constant once dynamic equilibrium is reached.

Key terms in this question

dynamic equilibrium · reversible reaction · concentration

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