Ammonia, an important ingredient in nitrogen-based fertilisers, is manufactured through the Haber process. This process involves a reversible reaction between nitrogen and hydrogen:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
The Haber process consists of the following steps:
1. Nitrogen and hydrogen are pumped into a compressor through pipes.
2. The gases are compressed to around 200 atmospheres within the compressor.
3. The pressurised gases are pumped into a tank and passed over an iron catalyst at 450°C. In this tank, some of the nitrogen and hydrogen react to form ammonia.
4. The ammonia is cooled, causing it to liquefy, and then it is removed.
5. Unreacted nitrogen and hydrogen gases are recycled and passed back over the catalyst, which forms more ammonia.
To increase the yield of ammonia, the reaction conditions can be adjusted to favour the forward reaction. This allows more ammonia to be produced at a lower cost.
The forward reaction in the Haber process is exothermic. Lowering the temperature would shift the position of equilibrium to the right, favouring the production of ammonia. However, a lower temperature also decreases the rate of reaction. Therefore, there’s a trade-off: while lower temperatures favour ammonia production, they also slow down the reaction.
At 450°C, a balance is achieved with a relatively fast rate of reaction and a high yield of ammonia.
A high pressure shifts the position of equilibrium to the right. However, working with very high pressures can be expensive. To maintain a cost-effective production process, a pressure of 200 atmospheres is used.
An iron catalyst increases the rate of the reaction, without being used up in the process. This contributes to a more efficient production of ammonia.