The Contact process is used to manufacture sulfuric acid. One stage involves the reaction of sulfur dioxide with oxygen to form sulfur trioxide. This reaction reaches a dynamic equilibrium. Explain what is meant by the term 'dynamic equilibrium' and describe how the equilibrium position can be altered to increase the yield of sulfur trioxide.

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).

In the Contact process, sulfur dioxide and oxygen react reversibly to form sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic.

Model answer (4 marks)

A dynamic equilibrium is a state where the forward and reverse reactions are both occurring simultaneously and the rates of the two reactions are equal, so the concentrations of reactants and products remain constant.

To increase the yield of SO₃:
1. Lower the temperature – the reaction is exothermic, so decreasing T shifts the equilibrium to the right, favouring SO₃.
2. Increase the pressure – the forward reaction produces fewer moles of gas (3 → 2), so higher pressure shifts the equilibrium to the right, also increasing SO₃.

Examiner tips

  • Use the definition of dynamic equilibrium – mention both directions and equal rates. Show the effect of temperature on an exothermic reaction. Show the effect of pressure using the change in moles of gas.

Common mistakes

  • Saying the equilibrium is static or that reactions stop. Ignoring the temperature effect or stating the wrong direction. Forgetting that the forward reaction has fewer gas moles.

Mark scheme (4 marks)

  1. The forward and reverse reactions are both occurring / the reaction proceeds in both directions simultaneously
  2. The rate of the forward reaction equals the rate of the reverse reaction
  3. Decreasing the temperature shifts the equilibrium position to the right / towards the exothermic (forward) reaction, increasing the yield of sulfur trioxide
  4. Increasing the pressure shifts the equilibrium position to the right / towards fewer moles of gas, increasing the yield of sulfur trioxide

Key terms in this question

dynamic equilibrium · equilibrium position · yield

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