A chemical company produces ethanol by the reversible reaction of ethene with steam: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g). The forward reaction is exothermic. The company wants to maximise the yield of ethanol. Explain how changing the temperature and pressure would each affect the position of equilibrium in this reaction, and state the conditions the company should choose to maximise the yield of ethanol.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The reaction C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g) is reversible and exothermic in the forward direction. There are 2 moles of gas on the left-hand side and 1 mole of gas on the right-hand side.
Model answer (5 marks)
Decreasing the temperature favours the exothermic forward reaction, so the equilibrium shifts towards ethanol.
Increasing the pressure favours the side with fewer gas moles; the products side has 1 mole vs 2 moles on the reactants side, so the equilibrium shifts towards ethanol.
To maximise yield the company should operate at low temperature and high pressure.
Increasing the pressure favours the side with fewer gas moles; the products side has 1 mole vs 2 moles on the reactants side, so the equilibrium shifts towards ethanol.
To maximise yield the company should operate at low temperature and high pressure.
Examiner tips
- Use Le Chatelier’s principle to justify temperature and pressure effects
- Mention the exothermic nature and gas‑mole difference
- State the optimal conditions clearly
Common mistakes
- Confusing the direction of the temperature shift (writing that lower temperature favours the endothermic side)
- Ignoring the gas‑mole difference and saying pressure has no effect
- Giving the wrong optimal conditions (high temperature or low pressure)
Mark scheme (5 marks)
- Decreasing temperature shifts the equilibrium position in the direction of the exothermic reaction (forward direction), increasing ethanol yield
- Reference to Le Chatelier's principle: a decrease in temperature counteracts the change by releasing heat (favouring the exothermic forward reaction)
- There are fewer moles of gas on the right-hand side (products side) — 1 mole compared to 2 moles on the left
- Increasing pressure shifts the equilibrium position towards the side with the smaller number of moles of gas, i.e. towards the products, increasing ethanol yield
- To maximise yield, the company should use a low temperature and a high pressure
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
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