Ethanol can be produced by the hydration of ethene in a reversible reaction: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g) ΔH = −46 kJ/mol. The reaction is carried out at 300 °C and high pressure with a catalyst. Explain how changing the temperature and changing the pressure each affect the position of equilibrium and the yield of ethanol.

Increasing the temperature supplies heat to the system. Because the forward reaction is exothermic (ΔH = −46 kJ mol⁻¹), the system responds by shifting the equilibrium to the endothermic reverse direction, i.e. to the left. This reduces the concentration of ethanol and therefore the yield of product.

Increasing the pressure favours the side of the reaction with fewer gas molecules. The forward reaction consumes two moles of gas (C₂H₄ + H₂O) and produces one mole of gas (C₂H₅OH). Thus a rise in pressure shifts the equilibrium to the right, increasing the concentration of ethanol and the yield of product.