# A chemist is investigating the production of ethanol by the reversible reaction between ethene and steam: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g). The forward reaction is exothermic. The reaction is carried out in a closed system using a catalyst. Explain how changing the temperature and pressure would each affect the position of equilibrium in this reaction, and state one advantage of using a catalyst.

> OCR A-Level Chemistry A (H432) — 5.3 Transition elements · Explain · 5 marks

> The industrial production of ethanol uses the reversible reaction: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g). The forward reaction is exothermic. The reaction reaches dynamic equilibrium in a closed system.

## Mark scheme (5 marks)

1. Decreasing temperature shifts the equilibrium position in the direction of the exothermic reaction (forward reaction), producing more ethanol.
2. Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction (backward reaction), producing less ethanol.
3. There are 2 moles of gas on the left and 1 mole of gas on the right, so increasing pressure shifts the equilibrium towards the side with fewer moles of gas (right/forward direction), producing more ethanol.
4. Decreasing pressure shifts the equilibrium towards the side with the greater number of moles of gas (left/backward direction), producing less ethanol.
5. A catalyst increases the rate of reaction by providing a different reaction pathway with a lower activation energy, but does not change the position of equilibrium.

## Key terms

- [closed system](https://www.gradenine.co.uk/glossary/closed-system)
- [catalyst](https://www.gradenine.co.uk/glossary/catalyst)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-chemist-is-investigating-the-production-b7e3729b) · Published by Druglandscape Ltd.