Predict equilibrium shifts with Le Chatelier's principle
Use Le Chatelier's principle to predict the direction of a shift after concentration, pressure or temperature changes.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Equilibrium constants apply at a fixed temperature; include only gaseous or aqueous species in the relevant expressions. For acid–base work, distinguish strength from concentration. Written practice is self-checked
Before you begin
- Balance equations, interpret reversible arrows and use concentration in mol dm⁻³.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does Le Chatelier's principle predict after an equilibrium disturbance?
What you will learn
- State Le Chatelier's principle
- Predict qualitative effects of concentration, pressure and temperature changes
- Explain why a catalyst does not shift equilibrium
A shift tends to minimise the imposed change
Le Chatelier's principle: if a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise that change.
Adding a reactant or removing a product generally shifts toward products; adding product or removing reactant generally shifts toward reactants. This changes the equilibrium composition, not the balanced equation.
Use gas moles and heat to reason about pressure and temperature
For a gaseous equilibrium, higher pressure favours the side with fewer moles of gas; lower pressure favours the side with more moles. If gas mole totals are equal, pressure change has no position effect.
Treat heat as a reactant in an endothermic direction and a product in an exothermic direction. Increasing temperature favours the endothermic direction. A catalyst speeds both forward and reverse reactions but does not change the equilibrium position.
Pause and explain
For N₂ + 3H₂ ⇌ 2NH₃, which side is favoured by increased pressure?
Worked example
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is exothermic in the forward direction. Predict the shift when pressure increases and when temperature increases.
Show the worked solution
- There are four moles of gas on the left and two on the right; higher pressure favours fewer gas moles.
- Higher pressure shifts right toward NH₃.
- The forward reaction is exothermic, so higher temperature favours the reverse, endothermic direction.
Answer Higher pressure shifts right; higher temperature shifts left.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For H₂(g) + I₂(g) ⇌ 2HI(g), predict the effect of increasing pressure on the equilibrium position.
Give me a hint
Count gaseous stoichiometric coefficients on both sides.
Compare my reasoning
- Left side has two gas moles by coefficient.
- Right side also has two.
- There is no side with fewer gas moles, so pressure does not shift the position.
There is no equilibrium-position shift because both sides contain two moles of gas.
Look for these in your work
- Gas mole totals counted
- Equal totals recognised
- No shift concluded
For an endothermic forward reaction A + heat ⇌ B, predict the effect of increasing temperature and adding a catalyst.
Give me a hint
Treat heat as a reactant and separate rate effects from position effects.
Compare my reasoning
- Adding heat favours the direction that consumes heat.
- The endothermic forward reaction is favoured.
- A catalyst speeds both directions but does not shift the equilibrium position.
Increasing temperature shifts right toward B. A catalyst changes the rate of reaching equilibrium, not its position.
Look for these in your work
- Temperature shift correct
- Endothermic direction identified
- Catalyst effect distinguished
A manager proposes increasing pressure to improve yield for a gaseous reaction with equal total gas coefficients on each side. Assess the proposal.
Give me a hint
Pressure changes matter only if the two sides have different gas mole totals.
Compare my reasoning
- Count gas coefficients on each side.
- They are equal, so neither side has a pressure advantage.
- Pressure may affect rate or equipment choices, but it does not shift this equilibrium position by itself.
The proposal will not improve equilibrium yield by shifting position because both sides have equal gas mole totals. Other process considerations would need separate evidence.
Look for these in your work
- Coefficient totals compared
- No equilibrium shift identified
- Conclusion scoped to yield shift
Common mistakes
- Using the number of molecules rather than gaseous coefficients
- Claiming a catalyst shifts the equilibrium position
- Saying high temperature always favours products
- Forgetting to use the stated reaction enthalpy for temperature changes
What can you explain now?
What does Le Chatelier's principle predict after an equilibrium disturbance?
Compare with the explanation
The position shifts to minimise the imposed change
The system responds in a direction that opposes that change.
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Make it stick
Which disturbance does concentration, pressure or heat represent, and how does the equilibrium respond to minimise it?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.