Le Chatelier’s Principle is one of the most important concepts in equilibrium chemistry. It appears everywhere in IB Chemistry—Paper 1 multiple-choice, Paper 2 explanation questions, and even IA investigations involving reversible reactions. Yet many students only memorize the rule without fully understanding how to apply it. This guide breaks down exactly how Le Chatelier’s Principle works and shows you how to explain equilibrium shifts in clear, IB-friendly language.
Quick Start Checklist
Le Chatelier’s Principle states:
“When a system at equilibrium is disturbed, it shifts to oppose the change.”
Changes that affect equilibrium include:
- concentration
- pressure (for gases)
- temperature
- volume (for gases)
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Understanding the Principle
A reaction at equilibrium has:
- equal forward and reverse reaction rates
- constant concentrations of reactants and products
When a stress is applied, the equilibrium position shifts to minimize that stress.
This helps predict changes in concentration, pressure, or temperature.
How Le Chatelier’s Principle Works (IB Breakdown)
1. Changing concentration
If you add more reactant:
- equilibrium shifts right
- more products form
If you remove reactant:
- equilibrium shifts left
- more reactant forms
The system wants to reduce the concentration change.
2. Changing pressure (gaseous systems only)
Higher pressure favors the side with fewer moles of gas.
Lower pressure favors the side with more moles of gas.
Example:
N₂ + 3H₂ ⇌ 2NH₃
Left side: 4 moles of gas
Right side: 2 moles of gas
Increasing pressure → shifts equilibrium toward NH₃.
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3. Changing temperature
Temperature changes favor either the forward or reverse reaction depending on enthalpy:
- Endothermic forward reaction (ΔH > 0):
increasing temperature shifts right - Exothermic forward reaction (ΔH < 0):
increasing temperature shifts left
IB examiners expect you to mention enthalpy explicitly in your explanation.
4. Catalysts
Catalysts do not shift equilibrium.
They increase the rate of both forward and reverse reactions equally.
They help the system reach equilibrium faster, but do not change the equilibrium position.
How IB Tests Le Chatelier’s Principle
Common question types:
- “Predict and explain the shift when …”
- “State the effect on yield when …”
- “Use Le Chatelier’s Principle to explain …”
- Application to the Haber process
- Graph-based equilibrium questions
You’ll also see combined conditions—e.g., pressure + temperature + catalysts in one question.
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Common Misconceptions
- “Catalysts shift equilibrium.”
They don’t. They only speed up the approach. - “Equilibrium shifts toward the side that’s ‘stronger’.”
Incorrect—equilibrium shifts to counteract the change. - “Increasing concentration always increases rate equally.”
Only the disturbed direction changes initially.
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Frequently Asked Questions
1. Does equilibrium ever stop shifting?
Yes. After the system counteracts the disturbance, it reaches a new equilibrium where forward and reverse rates are equal again. Concentrations may be different, but equilibrium is re-established.
2. Can Le Chatelier’s Principle predict how much equilibrium shifts?
No. It predicts the direction, not the magnitude.
Quantitative predictions require the equilibrium constant, Kc.
3. Why doesn’t adding a catalyst change equilibrium yield?
Because a catalyst lowers activation energy for both directions equally.
The ratio of forward to reverse rate constants remains unchanged.
Conclusion
Le Chatelier’s Principle explains how a system at equilibrium responds to changes in concentration, temperature, pressure, or volume. It shifts in the direction that counteracts the disturbance and restores balance. Mastering this principle is essential for equilibrium, energetics, and industrial chemistry topics in IB Chemistry. With RevisionDojo’s chemistry-specific learning tools, these explanations become much easier to understand and apply.
