Le Chatelier's Principle and Changing Conditions on Equilibrium — AQA GCSE Chemistry (Higher Tier)
Le Chatelier's principle predicts how the position of an equilibrium shifts when conditions change.
Le Chatelier's principle
If a system at equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to oppose (counteract) the change.
The effect of changing conditions
Concentration:
- Increasing the concentration of a reactant shifts the equilibrium towards the products (using up the added reactant).
- Removing a product also shifts it towards the products.
Temperature:
- Increasing the temperature shifts the equilibrium in the endothermic direction.
- Decreasing the temperature shifts it in the exothermic direction.
Pressure (reactions involving gases):
- Increasing the pressure shifts the equilibrium towards the side with the fewer gas molecules.
- Decreasing the pressure shifts it towards the side with more gas molecules.
Why it matters
Industrial chemists choose conditions to maximise the yield of a desired product (while balancing cost and rate) — for example, in the Haber process for making ammonia.
Exam tips
- The equilibrium always shifts to oppose the change made.
- More reactant → shift to products.
- Higher temperature → shift in the endothermic direction.
- Higher pressure → shift to the side with fewer gas molecules.
- State the change, then the direction of shift, then the reason.