A student sets up a closed container in which the following reversible reaction reaches dynamic equilibrium: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. Explain what is meant by dynamic equilibrium and predict the effect on the equilibrium position of increasing the temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
At dynamic equilibrium the forward and reverse reactions occur at the same rate, so the concentrations of N₂, H₂ and NH₃ remain constant although the reactions do not stop.
Because the forward reaction is exothermic, increasing the temperature supplies heat, which is taken up by the endothermic reverse reaction. Therefore the equilibrium shifts to the left, towards the reactants, and the yield of NH₃ decreases.
Because the forward reaction is exothermic, increasing the temperature supplies heat, which is taken up by the endothermic reverse reaction. Therefore the equilibrium shifts to the left, towards the reactants, and the yield of NH₃ decreases.
Examiner tips
- Define dynamic equilibrium in terms of equal forward and reverse rates and constant concentrations.
- Explain that heat is a reactant for the endothermic reverse reaction.
- State that the equilibrium shifts to the left, reducing NH₃ yield.
Common mistakes
- Saying the reactions stop at equilibrium.
- Confusing the direction of shift with the sign of ΔH.
- Not mentioning that concentrations stay constant at equilibrium.
Mark scheme (4 marks)
- At dynamic equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction.
- The concentrations of reactants and products remain constant (not that reactions have stopped).
- Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction.
- The equilibrium position shifts to the left / towards the reactants, so the yield of NH₃ decreases.
Key terms in this question
dynamic equilibrium · reversible reaction · equilibrium position
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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