Chemguide: Core Chemistry 14 - 16

The effect of pressure on the rates of chemical reactions involving gases

This very short page explains why changing the pressure changes reaction rates involving gases. Pressure changes only have an effect on reacting gases.

The overall effect

An increase of pressure on a reaction mixture involving gases increases the rate of reaction.

Explanation

If you increase the pressure, you are either forcing the same number of particles in the gas into a smaller volume, or increasing the number of particles in a given volume.

Either way, the particles are now closer together. The number of collisions taking place between particles in a given time is increased, and so the reaction speeds up.