The Haber process is the reaction by which ammonia is produced using nitrogen gas and hydrogen gas with the equation;
N2 (g) + 3H2 (g) ↔ 2NH3 (g)
The reaction is reversible as it is in equilibrium. Equilibrium is a condition in which the forward and reverse reactions are occurring at equal rates. The position of the equilibrium can be changed to produce either more product or more reactant to obtain maximum yield by using le Chatelier’s principle. Le Chatelier’s principle states that any change to the system will result in a shift of the reaction to the side which opposes the change.
Increasing the pressure will cause the equilibrium to try to decrease the pressure. The higher pressure on one side means that there are more moles on that side. This shifts the equilibrium to the right to increase the yield because the pressure is lower on the right so it will try to even the pressure out.
This forward reaction is exothermic and increasing the temperature will cause the system to try and decrease the temperature. If we increase the temperature, the yield will decrease because the system opposes the change.
However the rate of reaction is affected by temperature. Therefore a compromise between rate and yield is needed.
Using the graph, we can see that the yield of ammonia is highest at 350 ºC and at 400 atm pressures, however this can be dangerous so pressures of 200 atm and a temperature between 450 °C and 500 °C is most commonly used. A catalyst is also used, which in this case is iron, as this does not affect the position of the equilibrium but does affect the rate that the reaction occurs.