How is Le Chatelier's Principle used in the Haber process?

It helps us predict the yield of ammonia, when the pressure or the temperature is changed.

According to Le Châtelier's principle, if the conditions of a closed system at equilibrium change, the equilibrium position will move to the direction that will oppose this change. In this way, either the forward or the backward reaction will be favoured, thus producing a higher or a lower yield of product.

If the pressure is increased, the equilibrium position will move to the right, as to oppose the increase in pressure (there are fewer moles of gas on the right side (2 moles) of the equation than the left side (4 moles)). The forward reaction is favoured and therefore a higher yield of ammonia is produced.

If the temperature is increased, the equilibrium position will move to the left, as to oppose the increase in temperature (the forward reaction is exothermic). The backward reaction is favoured and therefore a lower yield of ammonia is produced.