A reversible reaction reaches dynamic equilibrium in a closed container. Explain what is meant by the term 'dynamic equilibrium'.

AQA A-Level Chemistry (7405) — 3.1.10 Equilibrium constant Kp (A-Level only) · Explain · 5 marks · View as Markdown

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

A chemist studies the reversible reaction between nitrogen and hydrogen to form ammonia. The reaction is carried out in a sealed vessel until equilibrium is reached.

Model answer (5 marks)

At dynamic equilibrium the forward and reverse reactions are still occurring, but the rate of the forward reaction equals the rate of the reverse reaction. Consequently the concentrations (or amounts) of reactants and products remain constant. The system must be closed so that no substances can enter or leave the container. The term ‘dynamic’ is used because particles are continually reacting, even though the overall composition is unchanged.

Examiner tips

  • Use the phrase ‘dynamic equilibrium’ to show understanding of ongoing reactions
  • Mention both rate equality and constant concentrations
  • State that the system is closed
  • Explain why the word ‘dynamic’ is used

Common mistakes

  • Saying the reaction has stopped or that no reactions occur
  • Forgetting to mention that the system is closed
  • Using ‘static’ instead of ‘dynamic’

Mark scheme (5 marks)

  1. The forward and reverse reactions are both still occurring / taking place
  2. The rate of the forward reaction equals the rate of the reverse reaction
  3. The concentrations / amounts of reactants and products remain constant
  4. The system must be closed / no substances can enter or leave the container
  5. The equilibrium is described as 'dynamic' because particles are continually reacting (not static)

Key terms in this question

dynamic equilibrium · reversible reaction

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