AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The effect of temperature changes on equilibrium (HT only): the relative amount of products at equilibrium decreases for an exothermic reaction.
According to Le Chatelier's principle, if a change is made to the conditions of a system at dynamic equilibrium, the system responds to counteract the change.
Study notes
From the checked teaching notes for this specification point.
The idea
According to Le Chatelier's principle, if a change is made to the conditions of a system at dynamic equilibrium, the system responds to counteract the change. For a reversible reaction where the forward direction is exothermic, the reverse path is inherently endothermic. Increasing the temperature of such a system forces the equilibrium to shift in the endothermic direction to absorb the excess thermal energy. Consequently, the equilibrium shifts to the left, and the relative amount of products decreases. For example, in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic. Heating the reaction mixture shifts the equilibrium to the left, lowering the yield of ammonia while increasing the amounts of nitrogen and hydrogen.
What earns the marks
- Identify that the forward reaction is exothermic and the reverse reaction is endothermic.
- State that increasing the temperature causes the system to favour the endothermic reaction to absorb heat.
- Conclude that the equilibrium position shifts to the left (towards the reactants).
- Deduce that the relative amount of products decreases, lowering the overall yield.
Common mistakes and how to put them right
Assuming a faster reaction rate means a higher yield.
Put it right: Rate and yield are distinct; heating increases the rate of reaching equilibrium but decreases the yield of an exothermic reaction.
Treating exothermic reactions as favoured by heat.
Put it right: Heating always favours the endothermic direction to oppose the temperature increase.
Confusing equilibrium shift with reaction speed.
Put it right: A shift to the left alters the relative amounts of reactants and products, not just how fast they form.
Exam tips
- Always identify which direction is exothermic and which is endothermic before predicting the effect of a temperature change.
- Explicitly state 'shifts to the left' or 'shifts to the reactants' and clearly conclude that the 'amount of product decreases'.
Try the lesson
The start of the The effect of temperature changes on equilibrium (HT only): the relative amount of products at equilibrium decreases for an exothermic reaction. lesson, made from checked teaching notes for this specification point.
Have a go first
For an exothermic reaction at equilibrium, what happens to the relative amount of products when the temperature is increased?
Listen
According to Le Chatelier's principle, if a change is made to the conditions of a system at dynamic equilibrium, the system responds to counteract the change. For a reversible reaction where the forward direction is exothermic, the reverse path is inherently endothermic. Increasing the temperature of such a system forces the equilibrium to shift in the endothermic direction to absorb the excess thermal energy. Consequently, the equilibrium shifts to the left, and the relative amount of products decreases. For example, in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic. Heating the reaction mixture shifts the equilibrium to the left, lowering the yield of ammonia while increasing the amounts of nitrogen and hydrogen. A strong answer earns marks for these points. Identify that the forward reaction is exothermic and the reverse reaction is endothermic. State that increasing the temperature causes the system to favour the endothermic reaction to absorb heat. Conclude that the equilibrium position shifts to the left (towards the reactants). Deduce that the relative amount of products decreases, lowering the overall yield. Exam tip: Always identify which direction is exothermic and which is endothermic before predicting the effect of a temperature change.
See it
Put it in order
Put these steps in the order they happen.
- 1. System responds to counteract the change
- 2. Equilibrium shifts in the endothermic direction
- 3. Equilibrium shifts to the left, towards the reactants
- 4. Relative amount of products decreases
- 5. Temperature increases
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
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