What are Reversible Reactions?
A reversible reaction is one that can occur in both directions. They are given the symbol...
For example Ammonia is made by a reversible reaction. This is summarised by the equation below...
As you can see from the equation above - ammonia is made by reacting Nitrogen and Hydrogen together. It terms of chemistry it takes 1 molecule of Nitrogen and 3 molecules of hydrogen to make 2 molecules of Ammonia.
Reversible reactions occur in both directions, that it Nitrogen and Hydrogen make ammonia and Ammonia breaks down to form Nitrogen and Hydrogen. Each reversible reaction has an equilibrium - that is where an equal amount of reactants and products are being made so it would appear that there is no net reaction (no overall reaction).
Controlling a reversible reaction
We can control a reversible reaction in two main ways...
2. Temperature - If we increase the temperature the reaction will favour the endothermic reaction (the one that takes in heat). In this case the reverse reaction (making nitrogen and hydrogen) is endothermic and would increase.
So therefore, to increase the yield of ammonia we can increase the pressure and decrease the temperature.
The Haber Process
The Haber process is the industrial way of making ammonia. It is a very important process as from ammonia we make fertilisers which are used around the world to provide enough food to keep the world's population alive. The picture below shows a diagram of how ammonia is produced.
As ammonia production is a reversible reaction, chemists have to use their knowledge of how to speed up the reaction along with understanding how to increase the rates of reaction. They could for example keep the temperature really low and the pressure really high to get a high yield of ammonia - but this would prove expensive as the high pressure would need specialist equipment that could withstand the pressure and the low temperature would require massive cooling and would slow down the speed the particles move at (slowing down the reaction). Therefore ammonia is produced at a moderately high pressure (200 atm) and a fairly high temperature (450 Celcius). They also use an iron catalyst to speed up the reaction.