Fertilisers and the chemical industry · Grade 10
The chemical industry turns raw materials into useful products on a huge scale. The Haber process makes ammonia from nitrogen and hydrogen; most of it becomes fertilisers. Because the reaction is reversible, its conditions are a compromise between yield, rate and cost. NPK fertilisers supply nitrogen, phosphorus and potassium to help crops grow. Sulfuric acid is made by the Contact process and nitric acid by the Ostwald process.
Key facts
- Haber process: N2 + 3H2 ⇌ 2NH3 (exothermic). Nitrogen comes from the air and hydrogen from natural gas.
- Conditions: about 450 °C, about 200 atmospheres and an iron catalyst. The ammonia is cooled to a liquid and removed; unreacted gases are recycled.
- A lower temperature would give a higher yield but a slow rate; a higher pressure would give a higher yield but costs more and is more dangerous. The conditions are a compromise.
- NPK fertilisers provide nitrogen (N), phosphorus (P) and potassium (K). Ammonia + nitric acid → ammonium nitrate; ammonia + sulfuric acid → ammonium sulfate.
- Potassium chloride and potassium sulfate are mined. Phosphate rock is treated with nitric, sulfuric or phosphoric acid to make soluble phosphates.
- Contact process: 2SO2 + O2 ⇌ 2SO3 with a vanadium(V) oxide catalyst, about 450 °C and 1–2 atmospheres.
- Too much fertiliser washes into rivers and causes eutrophication: algae grow, oxygen runs out and fish die.