The Contact Process is used in industry to produce sulfur trioxide, SO₃, which is needed to make sulfuric acid. One step in the Contact Process involves the following reversible reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic. Explain how increasing the temperature affects the position of equilibrium and the yield of SO₃ in this reaction.

OCR GCSE Chemistry A: Gateway Science (J248) — C5.3 Equilibria · Explain · 4 marks · View as Markdown

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

The Contact Process is a major industrial process used worldwide to manufacture sulfuric acid. A key reversible reaction in this process reaches dynamic equilibrium under carefully controlled conditions.

Model answer (4 marks)

Increasing the temperature shifts the equilibrium toward the endothermic direction. The forward reaction 2SO₂+O₂→2SO₃ is exothermic, so the reverse reaction is endothermic. By Le Chatelier’s principle the system responds to the added heat by favouring the endothermic reverse reaction, moving the equilibrium back toward the reactants. Consequently the concentration of SO₃ at the new equilibrium is lower, so the yield of SO₃ decreases.

Examiner tips

  • Use the term ‘Le Chatelier’s principle’ explicitly.
  • State the direction of the shift (reverse) and link it to the exothermic forward reaction.
  • Mention that the yield of SO₃ falls because less product is present at equilibrium.
  • Keep the answer concise and to the point to fit the 4‑mark limit.

Common mistakes

  • Saying the equilibrium shifts to the forward direction when temperature rises. Failing to identify that the forward reaction is exothermic. Using vague phrases like ‘the system changes’ without explaining the endothermic shift.

Mark scheme (4 marks)

  1. Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction
  2. The equilibrium shifts in the reverse/backward direction (because the forward reaction is exothermic, so the reverse is endothermic)
  3. This is because the system acts to oppose / counteract the change (Le Chatelier's principle)
  4. The yield of SO₃ decreases because less product is present at the new equilibrium position

Key terms in this question

reversible reaction

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