The Effect of Pressure Changes on Equilibrium Simplified Revision Notes for GCSE AQA Chemistry Combined Science

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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.

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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.

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Example: In the production of ammonia from nitrogen and hydrogen:

N2​(g)+3H2​(g)⇌2NH3​(g)

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.

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Increasing pressure favours the side with fewer moles of gas, while decreasing pressure favours the side with more moles of gas.

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