The manufacture of ethanol by the fermentation of glucose is a reversible reaction. Industrial chemists also produce ethanol by the direct hydration of ethene using steam, shown by the equation: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g). This reaction is exothermic in the forward direction. Explain how changing the temperature and pressure would each affect the position of equilibrium in this reaction, and state the direction in which the equilibrium would shift in each case.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In industry, ethanol can be produced by reacting ethene gas with steam at high temperatures and pressures in the presence of a catalyst. The forward reaction is exothermic. The equation for the reaction is: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g). Chemists must carefully control the conditions to maximise the yield of ethanol.
Model answer (5 marks)
Increasing the temperature adds heat to the system. Because the forward reaction is exothermic, the reverse reaction is endothermic. Le Chatelier’s principle therefore predicts a shift to the endothermic side – the equilibrium moves to the left, reducing the yield of ethanol.
Decreasing the temperature removes heat. The exothermic forward reaction is favoured, so the equilibrium shifts to the right, giving a higher ethanol yield.
Increasing the pressure favours the side with fewer gas moles. The reactants contain 2 mol gas (C₂H₄ + H₂O) and the product 1 mol gas (C₂H₅OH). Hence the equilibrium shifts to the right, increasing ethanol production.
Thus: higher T → left; lower T → right; higher P → right.
Decreasing the temperature removes heat. The exothermic forward reaction is favoured, so the equilibrium shifts to the right, giving a higher ethanol yield.
Increasing the pressure favours the side with fewer gas moles. The reactants contain 2 mol gas (C₂H₄ + H₂O) and the product 1 mol gas (C₂H₅OH). Hence the equilibrium shifts to the right, increasing ethanol production.
Thus: higher T → left; lower T → right; higher P → right.
Examiner tips
- Use Le Chatelier’s principle explicitly; mention heat as a reactant or product. State the direction of shift for each change. Show the gas‑mole comparison for pressure effect. Use correct chemical symbols and stoichiometry.
Common mistakes
- Confusing exothermic with endothermic direction. Ignoring the gas‑mole difference when discussing pressure. Failing to state the direction of shift for each condition.
Mark scheme (5 marks)
- Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction (the reverse/backward reaction)
- This is because Le Chatelier's principle states the equilibrium moves to counteract the change (i.e. to absorb the extra heat energy by favouring the endothermic direction)
- Decreasing temperature shifts the equilibrium in the direction of the exothermic reaction (the forward reaction), increasing the yield of ethanol
- Increasing pressure shifts the equilibrium towards the side with the smaller number of moles of gas
- Therefore, increasing pressure shifts equilibrium to the right (forward direction), increasing the yield of ethanol, because there is 1 mole of gas on the product side compared to 2 moles on the reactant side
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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