Question. Methanol is made industrially from carbon monoxide and hydrogen:CO(g) + 2H2(g) ⇌ CH3OH(g) ΔH = −91 kJ mol−1 (a) State and explain the effect of increasing the pressure on the equilibrium yield of methanol. (2)
(b) State and explain the effect of increasing the temperature on the equilibrium yield of methanol. (2)
(c) The process uses a copper-based catalyst. State its effect on the yield. (1)
(d) Suggest why a temperature of about 250 °C is used. (2)Model answer.(a) The yield increases. There are 3 gas molecules on the left and 1 on the right, so the equilibrium shifts to the side with fewer gas molecules to oppose the increase in pressure.(b) The yield decreases. The forward reaction is exothermic, so increasing the temperature shifts the equilibrium in the endothermic (backward) direction, to the left.(c) No effect on the yield. The catalyst increases the rates of the forward and backward reactions equally, so equilibrium is reached faster but its position is unchanged.(d) A lower temperature would give a higher yield, but the rate would be too slow. 250 °C is a compromise that gives an acceptable yield at an acceptable rate.Examiner's comments. For the 'explain' marks you must link the change to the equation: count the gas molecules on each side for pressure, and use the sign of ΔH for temperature. Saying only 'the equilibrium shifts right' earns the first mark but not the second.
(b) State and explain the effect of increasing the temperature on the equilibrium yield of methanol. (2)
(c) The process uses a copper-based catalyst. State its effect on the yield. (1)
(d) Suggest why a temperature of about 250 °C is used. (2)Model answer.(a) The yield increases. There are 3 gas molecules on the left and 1 on the right, so the equilibrium shifts to the side with fewer gas molecules to oppose the increase in pressure.(b) The yield decreases. The forward reaction is exothermic, so increasing the temperature shifts the equilibrium in the endothermic (backward) direction, to the left.(c) No effect on the yield. The catalyst increases the rates of the forward and backward reactions equally, so equilibrium is reached faster but its position is unchanged.(d) A lower temperature would give a higher yield, but the rate would be too slow. 250 °C is a compromise that gives an acceptable yield at an acceptable rate.Examiner's comments. For the 'explain' marks you must link the change to the equation: count the gas molecules on each side for pressure, and use the sign of ΔH for temperature. Saying only 'the equilibrium shifts right' earns the first mark but not the second.