Effect of pressure on an equilibrium

For the Higher Tier, you also need to know what happens to the amount of product [product: A product is a substance formed in a chemical reaction. ] in an equilibrium mixture of gases if the pressure is changed.

Count the molecules in the equation

Changing the pressure has little effect on an equilibrium mixture without gases - but can have a big effect on an equilibrium mixture containing gases. If the pressure is increased, the position of equilibrium moves in the direction of the fewest molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ].

Look again at the Haber process, which makes ammonia (NH₃):

N₂(g) + 3H₂(g) 2NH₃(g)

On the left, there are 1 + 3 = 4 molecules of gas. On the right, there are two molecules of gas. If the pressure is increased, the position of equilibrium will move to the right and more ammonia will be made. If the pressure is reduced, the position of equilibrium will move to the left and less ammonia will be made.

Look at the graph. You can see that for any given temperature the yield of ammonia increases as the pressure increases. You can also see that, for any given pressure, the yield goes down as the temperature increases. This is because the forward reaction is exothermic [exothermic: Heat energy is released in an exothermic reaction. We know this because the surroundings get warm. ].