Le Chatelier's Principle
Get to grips with Le Chatelier's Principle for GCSE Chemistry with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. A Higher topic: if you change the conditions of a system at equilibrium, the position of equilibrium shifts to oppose that change (Le Chatelier).
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
Le Chatelier's principle states that if a system at equilibrium is subjected to a change, the equilibrium position shifts to counteract that change. This is a Higher-only topic.
For concentration: increasing the concentration of a reactant shifts the equilibrium to the right, making more product, until equilibrium is re-established. For temperature: increasing the temperature shifts the equilibrium in the endothermic direction.
For pressure, which applies only to gases: increasing the pressure shifts the equilibrium towards the side with fewer molecules of gas. Counting the molecules on each side of the equation is therefore the first step in any pressure question.
Revision notes
Changing concentration
Increasing the concentration of a reactant shifts the equilibrium to the right, producing more product.
Removing a product also shifts it right, as the system replaces what was removed. Increasing a product's concentration shifts it left.
Changing temperature
Increasing the temperature shifts the equilibrium in the endothermic direction, absorbing the extra energy.
Decreasing the temperature shifts it in the exothermic direction. Identifying which direction is endothermic is the first step.
Changing pressure
This applies to gaseous equilibria only. Increasing the pressure shifts the equilibrium towards the side with fewer molecules of gas.
Count the molecules on each side using the balancing numbers. If both sides have the same number, changing the pressure has no effect.
Key points
- A change causes the equilibrium to shift to counteract it.
- Increasing a reactant concentration shifts right.
- Increasing temperature shifts in the endothermic direction.
- Increasing pressure shifts towards fewer gas molecules.
- Pressure changes affect gases only.
- Equal molecules on both sides means no pressure effect.
Worked examples
Example 1
Explain the effect of increasing the concentration of a reactant on a system at equilibrium. [2 marks]
Model answer
The equilibrium shifts to the right (1 mark), so more product is formed until equilibrium is re-established (1 mark).
Example 2
In the reaction N₂ + 3H₂ ⇌ 2NH₃, explain the effect of increasing the pressure. [3 marks]
Model answer
There are four molecules of gas on the left and two on the right (1 mark). Increasing the pressure shifts the equilibrium towards the side with fewer gas molecules (1 mark), so the equilibrium shifts to the right and more ammonia is produced (1 mark).
Example 3
The forward reaction is exothermic. Explain the effect of increasing the temperature. [2 marks]
Model answer
Increasing the temperature shifts the equilibrium in the endothermic direction (1 mark), which is the reverse reaction, so less product is formed (1 mark).
Common mistakes
Shifting towards more gas molecules under high pressure.
The equilibrium shifts towards FEWER molecules to reduce the pressure.
Applying pressure changes to solutions.
Pressure affects gaseous equilibria only.
Getting the temperature direction wrong.
Higher temperature favours the endothermic direction.
Not counting the gas molecules.
Always count using the balancing numbers before answering a pressure question.
Exam tips
- Count gas molecules on each side before answering pressure questions.
- Identify which direction is endothermic before answering temperature questions.
- State the direction of the shift and the effect on yield.
- Remember this is a Higher-only topic.
Key terms
- Le Chatelier's principle
- The rule that equilibrium shifts to counteract a change.
- Equilibrium position
- The relative amounts of reactants and products at equilibrium.
- Shift
- A change in the equilibrium position.
- Yield
- The amount of product formed.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.