Nitrogen monoxide and oxygen react together to form nitrogen dioxide. The reaction reaches a dynamic equilibrium in a closed container. Explain what is meant by the term 'dynamic equilibrium' and describe the effect on the position of equilibrium when the concentration of oxygen is increased.

Edexcel GCSE Chemistry (1CH0) — 9.3 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).

The reaction between nitrogen monoxide and oxygen is shown below: 2NO(g) + O₂(g) ⇌ 2NO₂(g)

Model answer (4 marks)

At dynamic equilibrium the forward reaction and the reverse reaction are both still occurring, but the rate of the forward reaction equals the rate of the reverse reaction.

If the concentration of O₂ is increased, the rate of the forward reaction increases. Consequently the position of equilibrium shifts to the right, producing more NO₂.

Examiner tips

  • Use the definition of dynamic equilibrium – both directions occur and rates are equal. Show the effect of increasing O₂ on the forward rate and state the shift to the right. Keep the answer concise, using the exact terminology from the mark scheme.

Common mistakes

  • Saying the reaction stops or that the reverse reaction stops. Not mentioning the shift to the right or using the wrong direction. Using vague terms like ‘more gas’ instead of ‘more NO₂’.

Mark scheme (4 marks)

  1. At dynamic equilibrium, the forward reaction and the reverse reaction are both still occurring
  2. The rate of the forward reaction equals the rate of the reverse reaction
  3. Increasing the concentration of oxygen increases the rate of the forward reaction
  4. The position of equilibrium shifts to the right, producing more nitrogen dioxide

Key terms in this question

dynamic equilibrium · position of equilibrium · concentration

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