Reversible reactions, ammonia and the Haber process [Chemistry only]
Many reactions are irreversible. But in a reversible reaction, the products can react to produce the original reactants. At equilibrium, the concentrations of reactants and products do not change. Ammonia is widely used in fertilisers and is manufactured using the Haber process.
Due to the Haber process being a reversible reaction, the yield of ammonia can be changed by changing the pressure or temperature of the reaction.
Increasing the pressure of the reaction increases the yield of ammonia. However, if the pressure is made too high, the equipment needed to safely contain the reaction becomes very expensive.
Increasing the temperature of the reaction actually decreases the yield of ammonia in the reaction. This means that we could get a bigger yield of ammonia with a lower temperature. However, if the temperature is too low, the rate of the reaction would be so slow that it would take too long to make the ammonia.
When choosing the conditions for the Haber process, it is important to consider all the factors that affect the yield of the ammonia along with the rate of reaction and the overall cost of production. You must be able to explain why the following reaction conditions were chosen.
A pressure of 200 atmospheres – chosen to give a decent yield and increased rate of reaction. If the pressure were any higher than this, it would increase equipment costs significantly as the equipment would have to be a lot stronger to contain the higher pressure. Higher pressure also increases the risk of explosions occurring.
A temperature of 450°C – chosen to give a decent yield and keep the rate of reaction high. This, combined with the use of the hot iron catalyst, means that a good yield of ammonia is produced constantly. Since the hydrogen and nitrogen is recycled, very little of the reactants are wasted.
These reaction conditions are not the best in terms of the yield but balance a decent yield with a decent rate of reaction and a reasonable production cost. These are called compromise conditions as they are chosen to give a good compromise between yield, rate and cost.
The iron catalyst that is used to increase the rate of reaction will eventually stop working effectively. It is said that the catalyst is ‘poisoned’ and it will need to be replaced regularly.