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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (5 marks)

Increasing the temperature shifts the equilibrium to the left, towards the formation of NO₂. The darker brown colour indicates a higher concentration of NO₂.
The equilibrium constant Kc therefore decreases when the temperature is increased, because the forward reaction is endothermic and the reverse reaction is exothermic.
If the pressure is increased, the equilibrium position does not change, but the value of Kc remains unchanged, as Kc is independent of pressure.

Examiner tips

  • Use the word ‘shifts’ and specify left/right; link colour change to NO₂ concentration; state Kc decreases with heating; mention endothermic forward reaction; note Kc is pressure‑independent.

Common mistakes

  • Saying the equilibrium shifts to the right; confusing Kc with Q; claiming Kc increases with temperature; ignoring that Kc is pressure‑independent.

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 in this question

equilibrium constant · Kc · dynamic equilibrium

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