Changing Pressure

Changing the pressure only affects the position of an equilibrium involving gases, and only when there are different numbers of moles of gas on each side of the equation.By Le Chatelier's principle, if the pressure is increased the position of equilibrium shifts towards the side with fewer moles of gas. This reduces the number of gas molecules and so opposes the increase in pressure. If the pressure is decreased, the equilibrium shifts towards the side with more moles of gas.For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g) there are 4 moles of gas on the left and 2 on the right. Increasing the pressure shifts the equilibrium to the right and increases the yield of ammonia.In 2NO₂(g) ⇌ N₂O₄(g), increasing the pressure shifts the equilibrium to the right, so after first darkening on compression, the brown colour of NO₂ fades. In H₂(g) + I₂(g) ⇌ 2HI(g) there are 2 moles of gas on each side, so pressure has no effect on the position of equilibrium.Increasing the pressure also increases the rate of a gaseous reaction, because the molecules are closer together and collide more often. However, high pressures are expensive to produce and need strong equipment, which limits their use in industry.