As the reaction is reversible, the reaction will reach a dynamic equilibrium. A dynamic equilibrium is where the concentrations of all of the substances remains constant; the number of forward reactions will be the same as the number of backwards reactions.
Ammonia is a very useful product. It is used to create fertilisers, cleaning fluids and explosives.
Ammonia is created by using a compromise of the ideal conditions for the position of a reversible reaction:
The pressure is around 200 atm.
An increase in pressure will move the dynamic equilibrium to the side that has fewer molecules. This is because the reversible reaction will want to move its pressure back to what it was before the increase in pressure, and having fewer molecules reduces the pressure in the container. There are fewer molecules on the ammonia side (right) compared to the reactants side (left); there are 2 molecules on the right and 4 molecules on the left. Therefore, increasing the pressure will move the dynamic equilibrium to the right resulting in an equilibrium with more of ammonia. The pressure that the reaction takes place in is 200 atm. We could carry out the reaction at a greater pressure than 200 atm, e.g. 500 atm or 600 atm. However, building a vessel that is able to hold a high pressure is very expensive. Therefore, a compromise between cost and optimal conditions is found, which is why a pressure of 200 atm is used. At 200 atm more ammonia is produced than carrying the reaction out at air pressure and it is not too expensive to build a container that can withstand 200 atm.
The reaction is carried out at a temperature of 450°C.
The forwards reaction is exothermic, and the backwards reaction is endothermic. We want to have as much ammonia as possible, which we can achieve by decreasing the temperature. Decreasing the temperature will mean that the reversible reaction will want to reverse the change by increasing the temperature. The reversible reaction will do this by increasing the number of exothermic reactions, which will mean that we would have more of the forward reactions and less of the backwards reaction, thus causing the equilibrium to move towards the right with more ammonia, and less nitrogen and hydrogen. However, a low temperature (such a room temperature) would mean that the nitrogen and hydrogen particles would have little kinetic energy. This means that they will move around slowly and are less likely to collide with one another. This would result in a very slow rate of reaction. So, a low temperature will push the dynamic equilibrium towards the right (more ammonia), but at a low temperature, it will take a very longer time to reach this dynamic equilibrium. Therefore, in industry, the Haber process is carried out at 450°C because it is a compromise between a good dynamic equilibrium and time taken to achieve this equilibrium. For example, it is better to obtain a 10% yield after 20 seconds, than to wait 60 seconds to obtain a 20% yield.
The reaction is carried out with an iron catalyst.
The iron catalyst speeds up the rate of reaction meaning that ammonia is produced quicker. The presence of the iron catalyst does not alter the position of the dynamic equilibrium, it just means that the dynamic equilibrium is reached in a shorter period of time.
The diagram shows what the reaction vessel for the Haber process looks like.
Nitrogen and hydrogen gas enter the chamber at the top. The nitrogen for the reaction is taken from air (air is about 78% nitrogen and is in the form of N2) and the hydrogen is obtained from hydrocarbons (natural gas or crude oil). The gases pass through a compressor, which increases the pressure of the gases to 200 atm. The vessel contains trays of iron catalyst, which speeds up the rate of reaction of nitrogen and hydrogen. The ammonia produced is a gas. At the bottom of the reaction vessel, there is a cooling tank. The cooling tanks causes ammonia to condense from a gas to a liquid. The liquid ammonia is removed into storage vessels. The unreacted nitrogen and hydrogen in the cooling tank remain as gasses because they have lower boiling points than ammonia. The nitrogen and hydrogen gas are recycled and travels back to the top of the reaction vessel to go through the reaction vessel again. A large percentage of the nitrogen and hydrogen are recycled; only about 15% of the nitrogen and hydrogen react to produce ammonia when the gases pass through the reaction chamber.