The Haber process

The Haber process makes ammonia, which is used to make fertilisers.

nitrogen + hydrogen ⇌ ammonia

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The raw materials are nitrogen from the air and hydrogen from natural gas (methane).

1. The purified gases are passed over an iron catalyst at a high temperature (about 450 °C) and a high pressure (about 200 atmospheres).
2. Because the reaction is reversible, only some of the gases react to form ammonia.
3. The mixture is cooled. The ammonia liquefies and is removed.
4. The unreacted nitrogen and hydrogen are recycled back into the reactor.

The forward reaction is exothermic, and there are fewer gas molecules on the right (2) than on the left (4).

Temperature — 450 °C is a compromise. A lower temperature gives a higher yield, because the equilibrium shifts towards the exothermic direction. But the rate would be too slow. 450 °C gives a reasonable yield at a reasonable rate.

Pressure — 200 atmospheres is a compromise. A higher pressure gives a higher yield (fewer molecules on the right) and a faster rate. But very high pressures need expensive equipment and lots of energy, and are more dangerous.

Catalyst: iron speeds up the reaction so equilibrium is reached faster, but it doesn't change the yield.