Ammonia is an incredibly useful chemical. It is used to make nitrogen-based fertilisers. Without those, it would be impossible to grow enough food to feed all the people in the world. So how do we make it?
Ammonia has the chemical formula NH3. Both hydrogen and nitrogen are simple to get; we can obtain nitrogen from the air, and hydrogen from methane. The reaction to make ammonia is then
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g).
Remember that ⇌ means a reversible reaction; at a molecular level, we see both the making and breakup of ammonia all the time. Since the reaction is reversible, we need to pick the right temperature and pressure conditions to make ammonia.
What conditions would maximise the yield?
The forward reaction (nitrogen + hydrogen → ammonia) is exothermic, so that direction is favoured at lower temperatures. The forward reaction also reduces the volume of the gas (there are four molecules on the left hand side, and two on the right hand side), so that direction is favoured at high pressures. So the best way to convert as much hydrogen and nitrogen into ammonia as possible is to run the reaction at as high a pressure and as low a temperature as possible. Real life is a bit more complicated than this.
What conditions are actually used and why?
The Haber process is an example of an important principle, which most chemistry textbooks don’t mention;
Chemical factories are there to make profits, not chemicals
If a company can’t make a product for less money than they can sell it for, they won’t make it for long. It’s really important that chemical factories can make things cheaply.
This means that real chemical factories making ammonia use the Haber process running at about 200 atmospheres (so 200 times higher than the pressure of the atmosphere). In theory, they could increase the pressure even more, and increase the yield. The disadvantage of this is that high pressures are expensive to make. If you have ever seen a pressure cooker, you will have seen how the metal walls have to be much thicker and stronger than for a normal saucepan. Making a chemical works strong enough to withstand pressure higher than 200 atmospheres is very expensive, and not viable when making ammonia.
The temperature used for the Haber process is about 450 °C. Lower temperatures would increase the yield some more, but reduce the rate of reaction. That means that it would take more time to make a certain amount of ammonia, and for businesses, time costs money. The temperature used is a compromise between the need to have a good yield and a good rate of reaction.
When the Haber process is used in chemical factories, a catalyst (iron) is used to speed up the reaction.
Although the conditions only convert about 40 % of the reactants into ammonia, the leftover hydrogen and nitrogen are not wasted. Once the reaction has reached equilibrium, the reacting mixture is cooled down. The ammonia turns to liquid, and is stored. The remaining hydrogen and nitrogen are collected, and run through the Haber process again.
The Haber process makes an important chemical. It’s also a useful one to think about when you are learning chemistry, because it recaps ideas about reversible reactions, rates of reaction and industrial chemistry. Now you know how it works, try the questions to see how well you understand it.