# Nitrogen dioxide, NO₂, is a brown gas. At high temperatures it decomposes to form colourless nitrogen monoxide, NO, and oxygen according to the equation below. The system reaches dynamic equilibrium in a closed container.

2NO₂(g) ⇌ 2NO(g) + O₂(g)

The equilibrium constant expression for this reaction is:

Kc = [NO]²[O₂] / [NO₂]²

When the temperature is increased, the mixture becomes darker brown. Explain what this observation tells us about the value of Kc and the direction of the equilibrium, and explain what would happen to the value of Kc if the pressure were increased instead.

> Eduqas A-Level Chemistry — 3.8 Equilibrium constants · Explain · 5 marks

> Nitrogen dioxide, NO₂, is a brown gas that exists in equilibrium with colourless nitrogen monoxide and oxygen in a closed container. The equilibrium constant, Kc, for this reaction is temperature-dependent.

## Mark scheme (5 marks)

1. Increasing temperature shifts the equilibrium to the left / towards NO₂
2. The darker colour shows the concentration of NO₂ has increased
3. The value of Kc decreases when temperature is increased
4. The forward reaction is endothermic / the reverse reaction is exothermic (consistent with shift left on heating)
5. Increasing pressure does NOT change the value of Kc (Kc only changes with temperature)

## Key terms

- [equilibrium constant](https://www.gradenine.co.uk/glossary/equilibrium-constant)
- [Kc](https://www.gradenine.co.uk/glossary/kc)
- [dynamic equilibrium](https://www.gradenine.co.uk/glossary/dynamic-equilibrium)

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- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/nitrogen-dioxide-no-is-a-brown-f1b41fe6) · Published by Druglandscape Ltd.