10.10 The Haber process

Making ammonia

Ammonia is used to make fertilisers, as well as nitric acid and explosives.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

• Nitrogen comes from the air.
• Hydrogen comes from natural gas (methane) reacting with steam.

The reaction is reversible, and the forward reaction is exothermic.

The process

1. The purified gases are compressed to about 200 atmospheres.
2. They pass over an iron catalyst at about 450 °C.
3. Only some of the gases react. The mixture is cooled, so the ammonia liquefies and is removed.
4. The unreacted nitrogen and hydrogen are recycled back into the reactor, so none is wasted.

Choosing the conditions (Higher)

Temperature:

• The forward reaction is exothermic, so a lower temperature gives a higher yield of ammonia.
• But a lower temperature makes the reaction too slow.
• 450 °C is a compromise between a reasonable yield and a fast enough rate.

Pressure:

• There are 4 molecules of gas on the left and 2 on the right. A higher pressure shifts the equilibrium to the right, giving a higher yield, and also increases the rate.
• But very high pressures need expensive, strong equipment and more energy, and are more dangerous.
• 200 atmospheres is a compromise between yield and cost and safety.

Catalyst:

• Iron speeds up both the forward and backward reactions, so equilibrium is reached faster.
• It does not change the yield.