Vanadium(V) oxide is used as a catalyst in the Contact process, which produces sulfur trioxide via the reversible reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic. Explain how the use of vanadium(V) oxide as a catalyst increases the rate of this reaction, and explain why increasing the temperature, whilst speeding up the reaction, is not favoured in terms of the equilibrium yield of sulfur trioxide.

OCR A-Level Chemistry B: Salters (H433) — 5.3 Transition elements · Explain · 5 marks · View as Markdown

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

The Contact process is a key stage in the industrial manufacture of sulfuric acid. A compromise temperature of around 450 °C is chosen for the reaction, balancing rate of reaction against yield of sulfur trioxide.

Model answer (5 marks)

A catalyst provides an alternative reaction pathway with a lower activation energy. Consequently, a greater proportion of the colliding SO₂ and O₂ molecules possess the energy required to react, so the rate of formation of SO₃ increases.

The forward reaction is exothermic. Raising the temperature increases the rate of both forward and reverse reactions, but according to Le Chatelier’s principle the equilibrium shifts towards the endothermic direction – the reverse reaction. Therefore, at higher temperatures the equilibrium concentration of SO₃ is lower, reducing the overall yield of sulfur trioxide.

Examiner tips

  • Use the word ‘catalyst’ and ‘alternative pathway’ to show understanding of the role of the catalyst.
  • Mention ‘lower activation energy’ and ‘greater proportion of molecules with sufficient energy’ to justify the rate increase.
  • Explain that the forward reaction is exothermic and that increasing temperature shifts equilibrium to the endothermic side, lowering SO₃ yield.
  • Keep the answer concise – 5 marks can be achieved with two short paragraphs.

Mark scheme (5 marks)

  1. The catalyst provides a different (alternative) reaction pathway
  2. This alternative pathway has a lower activation energy
  3. A greater proportion of colliding particles now have at least the activation energy, so the rate of reaction increases
  4. Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction (the backward reaction), because the forward reaction is exothermic
  5. This means less sulfur trioxide (product) is produced / the yield of SO₃ decreases at higher temperatures

Key terms in this question

catalyst · reversible reaction

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