A change in pressure will only have an effect on equilibria involving gaseous molecules. An increase in pressure results in more molecules per area within the system. The effect of this upon the position of equilibrium is dependent on the gas molecules on either side of equilibrium. The system reacts to an increase in pressure by favouring a reduction in pressure. Since we have four gas molecules on the left and two on the right of our reaction, an increase in pressure will shift the position of equilibrium to the right, favouring the forward reaction and increasing the overall yield of ammonia produced.
