The Contact Process is used industrially to manufacture sulfuric acid. One key step involves the following reversible reaction carried out in a closed system: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) The forward reaction is exothermic. Industrial chemists must choose conditions carefully to obtain a good yield of sulfur trioxide at a reasonable rate. Explain how changes in temperature and pressure each affect the position of equilibrium in this reaction, and state the direction each change would shift the equilibrium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Contact Process produces sulfur trioxide as an intermediate in the manufacture of sulfuric acid. The reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) is reversible and exothermic in the forward direction. It is carried out in a closed system so that dynamic equilibrium can be established.
Model answer (5 marks)
Increasing the temperature adds heat to the system. Because the forward reaction is exothermic, the added heat is taken up by the reverse (endothermic) reaction, so the equilibrium shifts to the left, producing more SO₂ and O₂.
Increasing the pressure favours the side with fewer moles of gas. The reactants contain 3 moles of gas (2 SO₂ + O₂) and the products contain 2 moles (2 SO₃). Therefore, a rise in pressure shifts the equilibrium to the right, increasing the yield of SO₃.
Thus, higher temperature shifts the equilibrium to the left (reverse), while higher pressure shifts it to the right (forward).
Increasing the pressure favours the side with fewer moles of gas. The reactants contain 3 moles of gas (2 SO₂ + O₂) and the products contain 2 moles (2 SO₃). Therefore, a rise in pressure shifts the equilibrium to the right, increasing the yield of SO₃.
Thus, higher temperature shifts the equilibrium to the left (reverse), while higher pressure shifts it to the right (forward).
Examiner tips
- Use Le Chatelier’s principle explicitly; link temperature to heat and pressure to moles of gas.
- State the direction of shift for each change and the resulting effect on SO₃ yield.
- Include the correct stoichiometric moles (3 vs 2) to justify the pressure effect.
Common mistakes
- Saying temperature shift is to the right (wrong direction).
- Confusing exothermic with endothermic when explaining temperature effect.
- Omitting the moles of gas or giving incorrect numbers for each side.
Mark scheme (5 marks)
- States that increasing temperature shifts the equilibrium in the direction of the endothermic (reverse/backward) reaction
- Links the temperature shift correctly to Le Chatelier's principle — the system counteracts the increase in temperature by absorbing heat via the endothermic reaction
- Correctly counts the moles of gas on each side: 3 moles on the left (reactants) and 2 moles on the right (products)
- States that increasing pressure shifts the equilibrium towards the side with fewer moles of gas — i.e. to the right, producing more SO₃
- Links the pressure shift to Le Chatelier's principle — the system counteracts the increased pressure by reducing the number of moles of gas
Key terms in this question
closed system · reversible reaction
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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