A reversible reaction is carried out in a closed container. The reaction reaches equilibrium. Explain what the term 'reversible reaction' means and describe the conditions needed for equilibrium to be reached.

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).

Ammonium chloride decomposes when heated to form ammonia and hydrogen chloride. When cooled, ammonia and hydrogen chloride react to re-form ammonium chloride.

Model answer (4 marks)

A reversible reaction is one where the products can react together to re‑form the original reactants.
The reaction must occur in a closed or sealed container so that neither reactants nor products can escape.
Equilibrium is reached when the forward reaction and the reverse reaction occur at the same rate.
At equilibrium the concentrations of reactants and products remain constant.

Examiner tips

  • Use the exact definition of a reversible reaction. Mention a closed system. State that forward and reverse rates are equal. Show that concentrations are constant at equilibrium.

Common mistakes

  • Confusing irreversible with reversible. Forgetting to mention the closed system. Saying equilibrium is when the reaction stops instead of rates being equal.

Mark scheme (4 marks)

  1. A reversible reaction is one where the products can react together to re-form the original reactants
  2. The reaction must occur in a closed/sealed container (so neither reactants nor products can escape)
  3. Equilibrium is reached when the forward reaction and the reverse reaction occur at the same rate
  4. The concentrations of reactants and products remain constant at equilibrium

Key terms in this question

reversible reaction · equilibrium

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