GCSE Combined Science revision · Chemistry: reactions and energy

Higher: equilibrium and changing conditions

Higher only

The idea

If a system at equilibrium is changed, the position of equilibrium shifts to counteract the change (Le Chatelier's principle). Increasing a reactant's concentration shifts the equilibrium to the right, making more products. Increasing temperature shifts it in the endothermic direction; decreasing temperature shifts it in the exothermic direction.

Identify the change.
Decide which direction counteracts it.
State the effect on the amount of product.

Watch out Raising the temperature favours the endothermic direction, whichever way that is.

See it worked

Worked example

In a reaction the forward reaction is exothermic. What happens to the yield of product if the temperature is increased?

Increasing temperature favours the endothermic direction.
Here that is the reverse reaction, so the yield of product falls.

Answer The yield falls

Your turn

Question 1Calculator allowed1 mark

If the concentration of a reactant is increased, the equilibrium shifts:

Question 2Calculator allowed1 mark

A forward reaction is endothermic. Decreasing the temperature will:

Question 3Calculator allowed3 marks

State Le Chatelier's principle.

Write your answer, then see a model answer

Change in conditions.
Shift to counteract.

Answer If a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts to counteract the change.

In past papers

14 official questions in 14 papers since 2018. The latest:

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