Ethanol can be manufactured by the hydration of ethene. The reaction is reversible and reaches a dynamic equilibrium in a closed container. C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g) ΔH = −46 kJ mol⁻¹ The equilibrium constant, Kc, for this reaction at 300 °C is 2.4 × 10⁻² mol⁻¹ dm³. Explain what the value of Kc tells us about the position of equilibrium at 300 °C, and predict and explain what happens to the value of Kc when the temperature is increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Kc = 2.4 × 10⁻² mol⁻¹ dm³, which is less than 1, so the equilibrium lies to the left and the reactants are favoured.
The concentration of ethanol (product) is therefore low compared with the concentrations of ethene and water.
Because the reaction is exothermic (ΔH = –46 kJ mol⁻¹), increasing the temperature shifts the equilibrium to the endothermic direction, i.e. to the left.
Consequently Kc decreases when the temperature is raised.
At the higher temperature the equilibrium concentration of ethanol is even lower, so the equilibrium shifts further to the left.
The concentration of ethanol (product) is therefore low compared with the concentrations of ethene and water.
Because the reaction is exothermic (ΔH = –46 kJ mol⁻¹), increasing the temperature shifts the equilibrium to the endothermic direction, i.e. to the left.
Consequently Kc decreases when the temperature is raised.
At the higher temperature the equilibrium concentration of ethanol is even lower, so the equilibrium shifts further to the left.
Examiner tips
- State that Kc<1 means the equilibrium lies to the left. Explain the effect of temperature using Le Chatelier’s principle and the sign of ΔH. Show the logical chain: exothermic + ↑T → shift left → Kc↓.
- common_mistakes
- :
- Saying Kc>1 favours products. Claiming Kc increases with temperature. Ignoring the exothermic nature of the reaction.
Mark scheme (5 marks)
- Kc less than 1 indicates the equilibrium lies to the left / favours the reactants
- The concentration of products is low compared to the concentration of reactants
- Increasing temperature causes Kc to decrease
- The reaction is exothermic, so increasing temperature favours the reverse/endothermic reaction
- This means less ethanol (product) is present at equilibrium / equilibrium shifts further to the left
Key terms in this question
equilibrium constant · Kc · dynamic equilibrium · position of equilibrium
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