A student is investigating the contact process, used to manufacture sulfuric acid. One step involves the reversible reaction between sulfur dioxide and oxygen to form sulfur trioxide. The reaction reaches dynamic equilibrium in a sealed container. Explain what is meant by dynamic equilibrium and describe how increasing the pressure affects the position of equilibrium in this reaction, which produces a greater volume of gas on the left-hand side than on the right-hand side.

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).

The reversible reaction in the contact process: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). There are 3 moles of gas on the left-hand side and 2 moles of gas on the right-hand side.

Model answer (4 marks)

At dynamic equilibrium the forward and reverse reaction rates are equal, so the concentrations of reactants and products remain constant.

Increasing the pressure favours the side with fewer moles of gas. Here the products (2 SO₃) contain fewer moles than the reactants (3 SO₂ + O₂), so the equilibrium shifts to the right, producing more SO₃ and reducing the pressure.

Examiner tips

  • Use the exact phrase "dynamic equilibrium" and state both rate equality and constant concentrations. Mention the 3 → 2 moles difference to justify the pressure shift.
  • common_mistakes
  • :
  • Saying the equilibrium shifts to the left because pressure increases. Failing to mention that concentrations remain constant at equilibrium.

Mark scheme (4 marks)

  1. At dynamic equilibrium, the rate of the forward reaction equals the rate of the backward reaction.
  2. At dynamic equilibrium, the concentrations of reactants and products remain constant.
  3. Increasing the pressure shifts the position of equilibrium to the right / towards the products.
  4. Because the equilibrium shifts to the side with fewer moles of gas, which reduces the pressure.

Key terms in this question

dynamic equilibrium · reversible reaction · position of equilibrium

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