The Haber Process and NPK Fertilisers — AQA GCSE Chemistry (Separate)
The Haber process makes ammonia, which is used to produce fertilisers that increase crop yields.
The Haber process
The Haber process makes ammonia (NH₃) from nitrogen and hydrogen: $$N_2 + 3H_2 \rightleftharpoons 2NH_3$$
- Nitrogen comes from the air; hydrogen comes from natural gas.
- It is a reversible reaction that reaches equilibrium.
The conditions
The conditions are a compromise chosen to give a reasonable yield at a reasonable rate and cost:
- Temperature: about 450 °C. (A lower temperature would give a higher yield, since the forward reaction is exothermic, but the rate would be too slow.)
- Pressure: about 200 atmospheres. (Higher pressure favours the side with fewer gas molecules — the ammonia — but very high pressures are expensive and dangerous.)
- Catalyst: iron, which speeds up the reaction.
The ammonia is removed by cooling and liquefying it, and unreacted N₂ and H₂ are recycled.
NPK fertilisers
Plants need nitrogen, phosphorus and potassium to grow well. NPK fertilisers are formulations containing compounds of these three elements. Ammonia is used to make nitrogen-containing fertilisers (e.g. ammonium nitrate, ammonium sulfate).
Exam tips
- Learn the equation and the source of each reactant (N₂ from air, H₂ from natural gas).
- Conditions are a compromise: 450 °C, 200 atm, iron catalyst.
- Explain the compromise using Le Chatelier (yield) vs rate and cost.
- NPK fertilisers provide nitrogen, phosphorus and potassium.