(HT only) Students should be able to: interpret graphs of reaction…

The idea

Raising temperature increases rate but shifts the exothermic forward reaction backwards, lowering yield, so about 450°C is a compromise. Raising pressure increases rate and shifts equilibrium towards fewer gas molecules, favouring ammonia, but very high pressure is expensive and hazardous, so about 200 atmospheres is used. The iron catalyst raises rate without changing equilibrium position. Graphs of rate against temperature or pressure show rate rising, while yield graphs show the equilibrium trade-off. Commercial choices also depend on the cost and availability of raw materials and energy, so the conditions balance yield, speed and expense.

What earns the marks

• Increasing temperature increases rate but shifts the exothermic equilibrium backwards, reducing ammonia yield.
• Increasing pressure increases rate and shifts equilibrium towards the side with fewer gas molecules, increasing ammonia yield.
• A catalyst increases the rate of both forward and reverse reactions equally and does not change the position of equilibrium.
• The chosen conditions are a compromise between rate of production, equilibrium yield, safety and cost.
• Graphs of rate against temperature or pressure show rate increasing as the variable increases.
• Graphs of yield against temperature show yield decreasing as temperature rises for this exothermic reaction.
• Raw material and energy costs influence the commercial conditions chosen.
• The Haber process uses about 450°C and about 200 atmospheres with an iron catalyst.

Common mistakes and how to put them right

• Claiming a catalyst increases yield.

  Put it right: A catalyst speeds up both directions equally and leaves the equilibrium position unchanged.

• Saying higher temperature always gives more ammonia.

  Put it right: Higher temperature lowers ammonia yield at equilibrium.

• Stating that pressure has no effect on equilibrium.

  Put it right: Increasing pressure favours the side with fewer gas molecules, which is the ammonia side.

Exam tips

• Use Le Chatelier reasoning explicitly: state the change, then the direction the equilibrium shifts, then the effect on yield.
• When interpreting a graph, read the axes first and quote a trend, for example rate rises with temperature while yield falls.
• Link each commercial condition to a reason involving rate, equilibrium, cost or safety to reach the higher tariff.