A reversible reaction reaches dynamic equilibrium in a closed container. The equilibrium constant, Kc, for the reaction has a value much greater than 1. Explain what is meant by the term dynamic equilibrium and describe what the value of Kc tells us about the position of equilibrium and the relative concentrations of reactants and products at equilibrium.

WJEC A-Level Chemistry (Wales) — 3.8 Equilibrium constants · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

1. Dynamic equilibrium means that both the forward and reverse reactions are still occurring.
2. The rate of the forward reaction equals the rate of the reverse reaction.
3. Consequently the concentrations of reactants and products remain constant over time.
4. A Kc value much greater than 1 indicates that the equilibrium position lies to the right, i.e. it favours the formation of products.
5. Therefore, at equilibrium the concentration of products is greater than that of the reactants.

Examiner tips

  • Use the word "dynamic" to show you understand the reactions are still happening. Show the equality of forward and reverse rates. State that concentrations are constant. Explain that Kc≫1 favours products. Mention higher product concentration at equilibrium.

Common mistakes

  • Confusing static equilibrium with dynamic equilibrium. Forgetting to mention that rates are equal. Saying concentrations change instead of remaining constant. Not linking Kc≫1 to product‑favoured equilibrium. Using incorrect units or not specifying that Kc is dimensionless.

Mark scheme (5 marks)

  1. The forward and reverse reactions are both occurring / both reactions are still happening
  2. The rate of the forward reaction equals the rate of the reverse reaction
  3. The concentrations of reactants and products remain constant / do not change over time
  4. A Kc value much greater than 1 means the position of equilibrium lies to the right / favours the products
  5. The concentration of products is greater than the concentration of reactants at equilibrium

Key terms in this question

dynamic equilibrium · position of equilibrium

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