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

Edexcel GCSE Chemistry (1CH0) — 5.3 Dynamic equilibria, calculations involving volumes of gases (Chem only) · Explain · 4 marks · View as Markdown

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

In the Haber process, nitrogen and hydrogen react to form ammonia. This is a reversible reaction that can reach dynamic equilibrium in a closed system.

Model answer (4 marks)

A reversible reaction reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the backward reaction.
Both the forward and backward reactions continue to occur; the reactions do not stop.
At dynamic equilibrium the concentrations of the reactants remain constant.
At dynamic equilibrium the concentrations of the products remain constant.

Examiner tips

  • Use the exact phrase "rate of the forward reaction equals the rate of the backward reaction" to gain full marks. Show that both reactions continue by mentioning they do not stop. State that concentrations of both reactants and products remain constant. Keep the answer concise and avoid unnecessary words.
  • common_mistakes
  • :
  • Saying the reaction stops at equilibrium. Forgetting to mention that both forward and backward reactions still occur. Using vague terms like "steady state" instead of "dynamic equilibrium".

Mark scheme (4 marks)

  1. A reversible reaction reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the backward reaction.
  2. Both the forward and backward reactions continue to occur (the reactions do not stop).
  3. The concentrations of the reactants remain constant at dynamic equilibrium.
  4. The concentrations of the products remain constant at dynamic equilibrium.

Key terms in this question

dynamic equilibrium · reversible reaction · concentration

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