The contact process is used to manufacture sulfuric acid. One step involves the reversible reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic. A chemical engineer considers changing the temperature and pressure conditions to increase the yield of SO₃. Explain how increasing the temperature and increasing the pressure would each affect the position of equilibrium in this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The contact process is a key industrial process. The reversible reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) reaches dynamic equilibrium when carried out in a closed system. The forward reaction is exothermic.
Model answer (5 marks)
Increasing the temperature adds heat to the system. Because the forward reaction is exothermic, the reverse reaction is endothermic. By Le Chatelier’s principle the equilibrium shifts to the endothermic side – the reactants – so the yield of SO₃ decreases.
Increasing the pressure favours the side with fewer gas molecules. The left side has 3 mol g ⁻¹ and the right side 2 mol g ⁻¹. Therefore the equilibrium shifts to the right, increasing the yield of SO₃.
Increasing the pressure favours the side with fewer gas molecules. The left side has 3 mol g ⁻¹ and the right side 2 mol g ⁻¹. Therefore the equilibrium shifts to the right, increasing the yield of SO₃.
Examiner tips
- State the effect of temperature first, then pressure, using the correct direction of shift.
- Use the terms ‘exothermic’, ‘endothermic’, ‘Le Chatelier’s principle’ and ‘moles of gas’ – these are the key phrases the mark scheme looks for.
- Show the numerical comparison of moles (3 vs 2) to justify the pressure effect.
- Keep the answer concise – 5 points can be covered in 3–4 short sentences.
Common mistakes
- Confusing the direction of the temperature shift – writing that higher temperature increases SO₃ yield.
- Forgetting to mention the number of moles of gas when explaining the pressure effect.
- Using vague language such as ‘more pressure makes it go right’ without the 3 mol vs 2 mol justification.
Mark scheme (5 marks)
- Increasing the temperature shifts the equilibrium position in the direction of the endothermic reaction
- Therefore increasing the temperature shifts the equilibrium to the left / towards the reactants, decreasing the yield of SO₃
- This is because increasing temperature counteracts the change, according to Le Chatelier's principle / the system opposes the increase in temperature by absorbing heat via the endothermic (reverse) reaction
- Increasing the pressure shifts the equilibrium towards the side with the smaller number of moles of gas
- There are 3 moles of gas on the left and 2 moles of gas on the right, so increasing pressure shifts the equilibrium to the right / towards the products, increasing the yield of SO₃
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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