Effect of pressure

A change in pressure has a significant effect only on an equilibrium that has gaseous components. Increasing the pressure on the system (which decreases its volume) makes the system respond by shifting the equilibrium in the direction that produces fewer moles of gaseous molecules; decreasing the pressure (increasing the volume) shifts the equilibrium toward the side with more moles of gaseous molecules.

Consider ammonia synthesis, $N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)$, taking place in a cylinder fitted with a piston. Pushing the piston down raises the pressure (and lowers the volume), and the system responds by reducing the total number of gas molecules present -- i.e. it favours the formation of ammonia (4 mol of gas becoming 2 mol). Pulling the piston up lowers the pressure (and raises the volume), favouring the decomposition of ammonia back to $N_2$ and $H_2$ instead (Fig. 8.6). …