The Effect of Temperature Changes on Equilibrium

Higher tier only

How does temperature affect equilibrium?

Le Chatelier's Principle can be used to predict the effect of changes in temperature on systems in equilibrium
To make this prediction it is necessary to know whether the reaction is exothermic or endothermic
If the temperature of the reaction increases:
- The equilibrium will shift in the direction of the endothermic reaction
If the temperature of a reaction decreases:
- The equilibrium will shift in the direction of the exothermic reaction

Worked example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride and the forward reaction is exothermic:

ICl (l) + Cl₂ (g) ⇌ ICl₃(s)

dark brown yellow

What colour will the mixture turn when heated? Explain your answer.

Answer:

The system will oppose the increase in temperature
- Increasing the temperature of an equilibrium reaction favours the endothermic reaction
If the forward reaction is exothermic, then the backward reaction must be endothermic
Therefore, the equilibrium will move to the left and produce more reactants
This means that the colour of the mixture will become increasingly brown as the temperature increases

Examiner Tip

Remember all reversible reactions are exothermic in one direction and endothermic in the other.

Mark schemes don't usually give you a mark for stating that the system opposes the change but for the explanation of which reaction is favoured and why.

The Effect of Temperature Changes on Equilibrium (AQA GCSE Chemistry: Combined Science)

Revision Note