Production of AmmoniaOxford AQA IGCSE Chemistry: Revision notes

The Haber process manufactures ammonia (NH3) from two raw materials:

• Nitrogen — obtained from the air (air is about 78% nitrogen)
• Hydrogen — obtained from natural gas (or other hydrocarbon sources)

These gases are purified, mixed in the correct ratio and compressed before being fed into the reactor.

Ammonia is made by reacting nitrogen with hydrogen in a reversible reaction:

N2(g) + 3H2(g) ⇌ 2NH3(g)

The ⇌ symbol shows that the reaction can go forwards (making ammonia) or backwards (ammonia breaking back down into nitrogen and hydrogen) at the same time. This means the reaction never goes to completion — it reaches a state of equilibrium.

The Haber process uses a compromise between rate and yield:

The iron catalyst speeds up the rate at which equilibrium is reached without being used up, and without affecting the position of equilibrium. The temperature and pressure are chosen as a compromise: they do not give the maximum possible yield of ammonia, but they give a fast enough rate of reaction and an economically reasonable yield at an acceptable cost.

As the gas mixture leaves the reactor, it is cooled. Ammonia has a higher boiling point than nitrogen and hydrogen, so it liquefies and can be removed from the mixture. The unreacted nitrogen and hydrogen gases remain as gases and are recycled back into the reactor, so that no raw material is wasted.

Ammonia is the starting material for making ammonium salts (such as ammonium nitrate) which are used as fertilisers. As the global population grows, more food must be produced, and fertilisers made from ammonia are essential to increase crop yields to meet this demand.