Nitrogen monoxide, NO, is produced as a by-product during combustion reactions in car engines. It can be removed by the following reversible reaction in a catalytic converter: 2CO(g) + 2NO(g) ⇌ 2CO₂(g) + N₂(g). Explain what is meant by dynamic equilibrium and describe the conditions under which this reversible reaction would reach dynamic equilibrium in a catalytic converter.

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).

Catalytic converters in cars use reversible reactions to reduce harmful exhaust emissions. One such reaction involves carbon monoxide reacting with nitrogen monoxide.

Model answer (4 marks)

Dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction.
The concentrations of CO, NO, CO₂ and N₂ remain constant because the forward and reverse rates are equal.
The system must be closed or sealed so that no reactants or products can escape.
Both the forward and reverse reactions continue to occur simultaneously at dynamic equilibrium.

Examiner tips

  • Use the exact phrase ‘rate of the forward reaction equals the rate of the reverse reaction’ for full credit.
  • Mention that concentrations remain constant and that the system is closed.
  • Show that both reactions continue to occur simultaneously.
  • Keep the answer concise and to the point.

Common mistakes

  • Confusing dynamic equilibrium with static equilibrium or equilibrium of a single reaction direction.
  • Failing to state that the system must be closed or that concentrations remain constant.
  • Using vague wording such as ‘the reaction stops’ or ‘the reaction is balanced’ instead of the required rate comparison.

Mark scheme (4 marks)

  1. Dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the backward/reverse reaction
  2. The concentrations of reactants (CO and NO) and products (CO₂ and N₂) remain constant at dynamic equilibrium
  3. The system must be closed/sealed so that no reactants or products can escape
  4. Both the forward reaction and backward/reverse reaction continue to occur simultaneously at dynamic equilibrium

Key terms in this question

dynamic equilibrium · reversible reaction

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