The position of equilibrium
3 marks · NormalEthene (3) reacts reversibly with steam to form ethanol (85):
The forward reaction is exothermic.
Explain, with reasons, the effect of decreasing both temperature and pressure on the yield of ethanol at equilibrium.
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Common mistakes: Mixing up effect of temperature and pressure; Not relating yield change to equilibrium position
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Marking points
M1 · 1 mark · States that decreasing temperature favours the exothermic (forward) reaction.
M2 · 1 mark · States that decreasing pressure favours side with more gas molecules (left/reactants).
A1 · 1 mark · Links both effects to yield: yield of ethanol increases with lower temperature but decreases with lower pressure.
Worked solution
Decreasing temperature favours the exothermic reaction (formation of ethanol), so the yield of ethanol increases.
However, decreasing pressure favours the side with more gas molecules (left, reactants), so equilibrium shifts left and the yield of ethanol decreases.
Overall, the effects oppose each other: lower temperature favours ethanol, lower pressure reduces its yield.