The following reversible reaction reaches dynamic equilibrium in a closed container: N₂O₄(g) ⇌ 2NO₂(g). The forward reaction is endothermic. Explain what happens to the equilibrium position and the concentration of NO₂ when the temperature is increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Dinitrogen tetroxide, N₂O₄, is a colourless gas that decomposes into brown nitrogen dioxide, NO₂, in a reversible reaction. When the system reaches dynamic equilibrium in a closed container, both gases are present at constant concentrations.
Model answer (4 marks)
An increase in temperature supplies extra heat.
1. The equilibrium shifts to the endothermic side – the forward reaction.
2. The shift is to the right, towards the products.
3. Consequently the concentration of NO₂ increases.
4. At the new equilibrium the concentrations of N₂O₄ and NO₂ are again constant, but at the new, higher value for NO₂.
1. The equilibrium shifts to the endothermic side – the forward reaction.
2. The shift is to the right, towards the products.
3. Consequently the concentration of NO₂ increases.
4. At the new equilibrium the concentrations of N₂O₄ and NO₂ are again constant, but at the new, higher value for NO₂.
Examiner tips
- Use the word ‘shift’ and specify direction (right).
- Mention that the forward reaction is endothermic.
- State the effect on NO₂ concentration.
- Remember the equilibrium concentrations are constant after the shift.
Common mistakes
- Saying the equilibrium shifts to the left or to the reactants.
- Failing to mention that NO₂ concentration increases.
Mark scheme (4 marks)
- An increase in temperature shifts the equilibrium position in the direction of the endothermic reaction
- The equilibrium shifts to the right / towards the products because the forward reaction is endothermic and absorbs the extra heat energy
- The concentration of NO₂ increases
- At the new equilibrium the concentrations of reactants and products remain constant again (but at different values)
Key terms in this question
dynamic equilibrium · reversible reaction · equilibrium position
Related
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