The production of hydrogen iodide from hydrogen and iodine reaches a dynamic equilibrium in a sealed container according to the equation: H₂(g) + I₂(g) ⇌ 2HI(g). Explain what is meant by a dynamic equilibrium, and predict and explain what would happen to the yield of hydrogen iodide if the temperature is increased, given that the forward reaction is exothermic.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The production of hydrogen iodide from hydrogen and iodine reaches a dynamic equilibrium in a sealed container according to the equation: H₂(g) + I₂(g) ⇌ 2HI(g). The forward reaction is exothermic.
Model answer (5 marks)
A dynamic equilibrium is a state in which the forward and reverse reactions are still occurring, but the rates of the two reactions are equal, so the concentrations of reactants and products remain constant.
Because the forward reaction H₂ + I₂ → 2HI is exothermic, increasing the temperature adds heat to the system. According to Le Chatelier’s principle, the equilibrium will shift to absorb the added heat, i.e. towards the endothermic reverse reaction.
Thus the equilibrium will move to the left, decreasing the concentration of HI. The yield of hydrogen iodide will therefore decrease.
Because the forward reaction H₂ + I₂ → 2HI is exothermic, increasing the temperature adds heat to the system. According to Le Chatelier’s principle, the equilibrium will shift to absorb the added heat, i.e. towards the endothermic reverse reaction.
Thus the equilibrium will move to the left, decreasing the concentration of HI. The yield of hydrogen iodide will therefore decrease.
Examiner tips
- Define dynamic equilibrium clearly (both reactions occur, rates equal).
- State that heating favours the endothermic direction.
- Explain that the shift reduces HI concentration.
- Use correct chemical equation and terminology.
Common mistakes
- Confusing dynamic equilibrium with static equilibrium.
- Saying the temperature change has no effect.
Mark scheme (5 marks)
- At dynamic equilibrium, the forward reaction and reverse reaction are both still occurring
- The rate of the forward reaction equals the rate of the reverse reaction
- Increasing temperature causes the equilibrium to shift in the direction of the endothermic reaction
- The equilibrium shifts towards the reverse / left-hand side reaction
- The yield of hydrogen iodide decreases
Key terms in this question
dynamic equilibrium · exothermic · yield
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
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- Decode the mark scheme abbreviations →
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