The change of pressure can be observed on the reactions which involves gaseous substances. According to Le-Chateliers’s principle, increase of pressure on a system at equilibrium will shift the equilibrium in the direction in which pressure is reduce.
By increase in pressure, the volume occupied by the system decreases. Hence the total number of moles per unit volume increases. To compensate this stress, the reaction which produces small number of moles of gaseous substance will be favored so that the increased pressure is reduced.For example:
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g)
On increasing pressure, the volume occupied by the system will decrease. As a result, there will be a large number of moles of gaseous substance per unit volume. This effect of increased pressure can be undoing if the equilibrium shifts in the direction in which there are lesser number of gaseous moles.
As in the above reaction, the numbers of moles of the products are less than those of the reactants, the equilibrium will shift towards the forward direction, and hence more ammonia will be formed.
If the numbers of moles of the products are more than those of the reactants, the equilibrium will shift towards the backward direction.
If there is no change in the number of moles of products and reactants i.e. numbers of moles of the products are equal to the number of moles of reactants then the pressure will have no effect on the equilibrium.