# 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.

> OCR A-Level Chemistry B: Salters (H433) — 5.1 Rates, equilibrium and pH · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Decreasing temperature shifts the equilibrium position in the direction of the exothermic reaction (forward direction), increasing ethanol yield
2. Reference to Le Chatelier's principle: a decrease in temperature counteracts the change by releasing heat (favouring the exothermic forward reaction)
3. There are fewer moles of gas on the right-hand side (products side) — 1 mole compared to 2 moles on the left
4. 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
5. To maximise yield, the company should use a low temperature and a high pressure

## Key terms

- [reversible reaction](https://www.gradenine.co.uk/glossary/reversible-reaction)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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