What Is Le Chatelier’s Principle? IB Chemistry Explained

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.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.