The reaction between sulfur dioxide and oxygen is a reversible reaction, as shown: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic. A chemical engineer suggests increasing the temperature of the reaction vessel once dynamic equilibrium has been reached. Explain the effect this change has on the position of equilibrium and on the concentrations of SO₂ and SO₃.

Pearson Edexcel International GCSE Chemistry (4CH1) — 3.3 Reversible reactions and equilibria · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Increasing the temperature shifts the equilibrium to the endothermic side. The forward reaction is exothermic, so the reverse reaction is endothermic. Therefore the equilibrium moves to the left, producing more SO₂ and less SO₃.

- Temperature ↑ → equilibrium shifts left (reverse direction)
- [SO₂] ↑, [SO₃] ↓

Examiner tips

  • Use the Le Chatelier principle: state that increasing T favours the endothermic direction.
  • Mention that the forward reaction is exothermic, so the reverse is endothermic.
  • Show the direction of shift (left) and the resulting concentration changes.
  • Use correct chemical symbols and units (e.g. [SO₂] and [SO₃]).

Common mistakes

  • Confusing the direction of shift (saying it moves right instead of left).
  • Failing to identify which reaction is endothermic.
  • Using vague terms like ‘more’ or ‘less’ without specifying the species.

Mark scheme (4 marks)

  1. An increase in temperature shifts the equilibrium position in the direction of the endothermic reaction
  2. The backward/reverse reaction is the endothermic reaction, so equilibrium shifts to the left / in the reverse direction
  3. The concentration of SO₂ increases
  4. The concentration of SO₃ decreases

Key terms in this question

reversible reaction · dynamic equilibrium

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