The Haber process for making ammonia, the compromise conditions of temperature and pressure, the use of an iron catalyst and recycling, and the use and importance of NPK fertilisers.

A six-mark Level of Response question. Reward describing the process: nitrogen is obtained from the air and hydrogen from natural gas (methane); they are reacted together in the reversible reaction $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$ over an iron catalyst at about $450\ \degree\text{C}$ and about $200$ atmospheres pressure; the unreacted nitrogen and hydrogen are recycled. Then explain the compromise: the forward reaction is exothermic, so a lower temperature would increase the yield of ammonia, but it would make the reaction too slow, so a moderate temperature of about $450\ \degree\text{C}$ is a compromise that gives a reasonable yield at a reasonable rate. A higher pressure increases the yield (the equilibrium shifts to the side with fewer gas moles, the ammonia side) and also increases the rate, but very high pressures are expensive and dangerous to build and maintain, so about $200$ atm is a compromise. Markers reward the raw materials, the equation and catalyst, the conditions, the recycling, and the two compromise explanations (temperature, pressure). A common error is to suggest a low temperature without noting the rate problem.

A C6.1 application question. Reward: fertilisers replace the elements (nitrogen, phosphorus and potassium) that plants take out of the soil as they grow, so the soil does not become depleted. They provide the elements plants need for healthy growth, which increases crop yields and so increases food production, helping to feed a growing population. One environmental problem from overuse is eutrophication: excess fertiliser is washed (leached) into rivers and lakes, where it causes rapid growth of algae (an algal bloom); when the algae and plants die they are decomposed by bacteria, which use up the dissolved oxygen in the water, so fish and other organisms die. Markers credit the idea that fertilisers replace lost nutrients and raise crop yields to feed more people, and a valid environmental problem such as eutrophication (leaching, algal bloom, oxygen depletion). A common error is to say fertilisers simply make plants grow without explaining the elements or the yield.