Effect of Pressure

For reactions involving gases, increasing the pressure increases the rate of reaction.Pressure is usually increased by reducing the volume of the container at constant temperature. The same number of gas molecules is then packed into a smaller volume, so there are more molecules per unit volume. Increasing the pressure of a gas is therefore the same as increasing its concentration.With the molecules closer together, they collide more frequently, so there are more successful collisions per unit time and the rate increases. As with concentration, the energy of the molecules does not change, so the proportion of collisions with energy greater than or equal to the activation energy stays the same.Worked example. A gas mixture is compressed from 2.0 dm³ to 1.0 dm³ at constant temperature. The volume halves, so the pressure doubles and the concentration of every gas doubles:c = n ÷ VIf 0.10 mol of a gas was present, its concentration rises from 0.050 mol dm⁻³ to 0.10 mol dm⁻³.Pressure has almost no effect on reactions that involve only solids, liquids or solutions, because these are almost incompressible and their concentrations do not change when the pressure changes.Many industrial gas-phase processes are run at raised pressure partly to increase the rate, although higher pressure costs more because of the energy for compression and the need for strong, expensive equipment.