AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The effect of changing conditions on equilibrium (HT only): The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.
A reversible reaction in a closed system reaches dynamic equilibrium when forward and reverse rates are equal, so concentrations stay constant.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A reversible reaction in a closed system reaches dynamic equilibrium when forward and reverse rates are equal, so concentrations stay constant. Assessed at Higher Tier only, Le Chatelier’s Principle predicts how a system responds to a change: the equilibrium shifts in the direction that opposes the change. If temperature rises, the endothermic direction is favoured. If reactant concentration increases, the forward reaction is favoured. If pressure increases, the side with fewer gas molecules is favoured. A catalyst does not shift equilibrium; it only speeds up its attainment.
For example: In N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure favours the forward reaction because four gas molecules form two.
What earns the marks
- State that equilibrium is dynamic: forward and reverse reactions continue at equal rates in a closed system.
- Identify the change imposed on the system, such as temperature, pressure or concentration.
- Apply Le Chatelier’s Principle by stating that the equilibrium shifts to oppose the change.
- For temperature, link the shift to the endothermic direction when temperature increases and the exothermic direction when temperature decreases.
- For pressure, compare the number of gas molecules on each side and favour the side with fewer when pressure increases.
- For concentration, state that increasing a reactant favours the forward reaction and increasing a product favours the reverse reaction.
- Explain that a catalyst increases the rate of both forward and reverse reactions equally and does not change the equilibrium position.
Common mistakes and how to put them right
Saying that increasing temperature always favours the forward reaction.
Put it right by identifying which direction is endothermic and favouring that direction when temperature rises.
Confusing rate with position of equilibrium.
Put it right by explaining that a catalyst changes how quickly equilibrium is reached, not where it lies.
Ignoring the state symbols when applying pressure changes.
Put it right by counting only gas molecules and comparing the two sides of the equation.
Exam tips
- Write the balanced equation first and label each side with the number of gas molecules before predicting pressure effects.
- Use the phrase ‘the equilibrium shifts to oppose the change’ and then name the favoured direction explicitly.
- Check whether the reaction is exothermic or endothermic in the direction stated before deciding the effect of temperature.
- As this is a Higher Tier only topic, ensure you can fully explain the opposing shift rather than just stating it.
Try the lesson
The start of the The effect of changing conditions on equilibrium (HT only): The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle. lesson, made from checked teaching notes for this specification point.
Have a go first
A reversible reaction reaches equilibrium in a closed container. According to Le Chatelier’s Principle, what happens to the position of equilibrium if the temperature is increased?
Listen
A reversible reaction in a closed system reaches dynamic equilibrium when forward and reverse rates are equal, so concentrations stay constant. Assessed at Higher Tier only, Le Chatelier’s Principle predicts how a system responds to a change: the equilibrium shifts in the direction that opposes the change. If temperature rises, the endothermic direction is favoured. If reactant concentration increases, the forward reaction is favoured. If pressure increases, the side with fewer gas molecules is favoured. A catalyst does not shift equilibrium; it only speeds up its attainment. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure favours the forward reaction because four gas molecules form two. A strong answer earns marks for these points. State that equilibrium is dynamic: forward and reverse reactions continue at equal rates in a closed system. Identify the change imposed on the system, such as temperature, pressure or concentration. Apply Le Chatelier’s Principle by stating that the equilibrium shifts to oppose the change. For temperature, link the shift to the endothermic direction when temperature increases and the exothermic direction when temperature decreases. A common slip: Saying that increasing temperature always favours the forward reaction. Instead, identifying which direction is endothermic and favouring that direction when temperature rises. Exam tip: Write the balanced equation first and label each side with the number of gas molecules before predicting pressure effects.
Key words
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Dynamic equilibrium
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Le Chatelier’s Principle
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Effect of temperature on equilibrium
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Effect of pressure and concentration on equilibrium
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See every definition
- Dynamic equilibrium
- A reversible reaction in a closed system where forward and reverse reactions continue at equal rates, so concentrations stay constant.
- Le Chatelier’s Principle
- A principle that predicts how a system at equilibrium responds to a change: the equilibrium shifts in the direction that opposes the change.
- Effect of temperature on equilibrium
- If temperature rises, the endothermic direction is favoured; when temperature decreases, the exothermic direction is favoured.
- Effect of pressure and concentration on equilibrium
- If pressure increases, the side with fewer gas molecules is favoured; increasing a reactant favours the forward reaction and increasing a product favours the reverse reaction.
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