Pressure mainly affects equilibrium when the reaction involves gases and the number of gas molecules is different on the two sides of the reaction.
- If pressure is increased:
The system shifts towards the side with fewer gas molecules (to reduce pressure). - If pressure is decreased:
The system shifts towards the side with more gas molecules (to increase pressure). - If both sides have the same number of gas molecules:
Changing pressure has no effect on the equilibrium position.
In short: Pressure changes push the equilibrium toward the side that balances the number of gas particles.