Nitrogen dioxide, NO₂, and dinitrogen tetroxide, N₂O₄, exist in a reversible reaction in a sealed container: 2NO₂(g) ⇌ N₂O₄(g). The forward reaction is exothermic. Explain what happens to the position of equilibrium when the temperature of the sealed container is increased.

OCR GCSE Chemistry A: Gateway Science (J248) — C5.3 Equilibria · Explain · 4 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 and dinitrogen tetroxide (N₂O₄) is a colourless gas. They interconvert in a reversible reaction inside a sealed container, eventually reaching dynamic equilibrium.

Model answer (4 marks)

The equilibrium shifts to the left, towards the reactants (NO₂). Increasing the temperature favours the endothermic direction. The system acts to counteract the change (Le Chatelier’s principle). Consequently the concentration of NO₂ increases and that of N₂O₄ decreases, until a new equilibrium is established.

Examiner tips

  • Use the word ‘shifts to the left’ to show direction. Mention that the forward reaction is exothermic, so heating favours the reverse. Show the link to Le Chatelier’s principle. State the effect on concentrations of both gases.

Common mistakes

  • Saying the equilibrium shifts to the right or towards N₂O₄. Forgetting to explain that the forward reaction is exothermic. Not linking the temperature change to Le Chatelier’s principle. Leaving out the concentration change of NO₂ and N₂O₄.

Mark scheme (4 marks)

  1. The equilibrium position shifts to the left / towards the reactants / towards NO₂
  2. Because increasing temperature favours the endothermic reaction / direction
  3. This is because the system acts to counteract / oppose the change (Le Chatelier's principle)
  4. The concentration of NO₂ increases and the concentration of N₂O₄ decreases (but both remain constant once the new equilibrium is reached)

Key terms in this question

reversible reaction

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