The Effect of Pressure Changes on Equilibrium (AQA GCSE Chemistry Combined Science): Revision Notes
6.2.7 The Effect of Pressure Changes on Equilibrium
Pressure changes affect the equilibrium position in reactions that involve gases. According to Le Chatelier's Principle, if the pressure of a system is changed, the equilibrium will shift to counteract the change, depending on the number of gas molecules on each side of the reaction.
Increasing the pressure: The equilibrium shifts towards the side with fewer moles of gas. This reduces the pressure by decreasing the number of gas molecules in the system.
Decreasing the pressure: The equilibrium shifts towards the side with more moles of gas. This increases the number of gas molecules, raising the pressure.
Note: If the reaction involves the same number of moles of gas on both sides, changing the pressure will have no effect on the position of equilibrium.
Example: In the production of ammonia from nitrogen and hydrogen:
- There are 4 moles of gas on the left (1 mole of nitrogen and 3 moles of hydrogen) and 2 moles of gas on the right (2 moles of ammonia).
- Increasing the pressure: The equilibrium shifts to the right (towards fewer moles of gas), increasing ammonia production.
- Decreasing the pressure: The equilibrium shifts to the left (towards more moles of gas), reducing ammonia production.
Increasing pressure favours the side with fewer moles of gas, while decreasing pressure favours the side with more moles of gas.