A reversible reaction takes place in a sealed container. Describe what happens to the concentrations of reactants and products from the start of the reaction until equilibrium is reached.

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

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

Model answer (4 marks)

1. The forward reaction converts reactants into products, so the concentration of reactants decreases.
2. The concentration of products increases as the forward reaction proceeds.
3. The reverse reaction also occurs, with products reacting to reform reactants.
4. At equilibrium the concentrations of reactants and products remain constant because the forward and reverse reactions occur at exactly the same rate.

Examiner tips

  • Use the word ‘decreases’ for reactants and ‘increases’ for products. Mention the reverse reaction explicitly. State that concentrations are constant at equilibrium. Keep each point short and to the point.

Common mistakes

  • Mixing up which species increases or decreases. Forgetting to mention the reverse reaction. Saying concentrations ‘change’ at equilibrium instead of ‘remain constant’.

Mark scheme (4 marks)

  1. The forward reaction converts reactants into products, so the concentration of reactants decreases
  2. The concentration of products increases (as the forward reaction proceeds)
  3. The reverse reaction also occurs / products react to re-form reactants
  4. At equilibrium the concentrations of reactants and products remain constant because the forward and reverse reactions occur at exactly the same rate

Key terms in this question

reversible reaction · equilibrium · concentration

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