Pressure only affects the equilibrium of reactions involving gases
One mole of gas of any substance takes up the same amount of space, if measured under the same conditions of temperature and pressure.
Therefore, if one side of the reaction contains more moles of gas than the other, this is the side that will be favoured under low pressure
Example: Ammonia is produced by the following reaction:\(N_2 + 3H_2 \rightleftharpoons 2NH_3 \)
- LHS has four moles of gas, RHS has two.
- Therefore, under high pressure, the rate of reaction in the forwards direction will be greater than that in the backwards direction because the RHS takes up less space

Le Chatelier's Principle