# Nitrogen and hydrogen react together in a closed container to form ammonia according to the equation: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). At a certain temperature the equilibrium constant, Kc, has a value of 6.2 × 10⁻² mol⁻² dm⁶. The temperature is then lowered and the value of Kc increases significantly. Explain what the change in Kc tells us about the position of equilibrium, and what this indicates about whether the forward reaction is exothermic or endothermic.

> WJEC A-Level Chemistry (Wales) — 3.8 Equilibrium constants · Explain · 5 marks

## Mark scheme (5 marks)

1. A larger value of Kc means the concentration of products is greater relative to reactants at equilibrium
2. The position of equilibrium has shifted to the right / towards the products
3. Lowering temperature favours the exothermic direction (Le Chatelier's principle / the system opposes the change by releasing heat)
4. Because lowering temperature shifts equilibrium to the right and Kc increases, the forward reaction (N₂ + H₂ → NH₃) must be exothermic
5. Kc only changes with temperature (not with changes in concentration or pressure)

## Key terms

- [position of equilibrium](https://www.gradenine.co.uk/glossary/position-of-equilibrium)
- [exothermic](https://www.gradenine.co.uk/glossary/exothermic)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](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/nitrogen-and-hydrogen-react-together-in-ee5c2802) · Published by Druglandscape Ltd.