What Is Le Chatelier's Principle?
Le Chatelier's principle states that when a system at equilibrium is disturbed, it shifts to counteract the change and restore equilibrium. This applies to changes in concentration, temperature, and pressure.
A system at equilibrium responds to stress (concentration, temperature, or pressure changes) by shifting to oppose the change: if pressure rises, equilibrium shifts to the side with fewer moles; if temperature rises, it shifts endothermic.
- 1↓System in equilibriumForward and reverse rates equal
- 2↓Stress appliedConcentration, temperature, or pressure changes
- 3↓System respondsShifts to oppose the change
- 4New equilibriumRates balance again
Step-by-step worked examples
The equilibrium N₂ + 3H₂ ⇌ 2NH₃ is disturbed by adding more N₂. Which way does it shift?
Adding N₂ increases reactant concentration. The system shifts RIGHT (forward) to consume excess N₂ and restore equilibrium.
A system H₂ + I₂ ⇌ 2HI is at equilibrium at 200 K. Temperature is raised to 400 K. The forward reaction is endothermic. Which way does it shift?
Raising temperature favors the endothermic direction (forward). The equilibrium shifts RIGHT.
The reaction 2NO₂(g) ⇌ N₂O₄(g) is at equilibrium in a sealed container. Pressure is increased by reducing volume. Which side has fewer moles?
Left: 2 moles of NO₂; Right: 1 mole of N₂O₄. Increasing pressure shifts equilibrium RIGHT (fewer moles).
Flashcards
Quick quiz
Q1.What does Le Chatelier's principle predict?
Q2.Removing a product from equilibrium shifts?
Q3.For CO₂(g) ⇌ CO(g) + ½O₂(g), which stress favors products?
Q4.Does a catalyst shift equilibrium?
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Common mistakes
Adding a catalyst shifts equilibrium. — Correct: Catalysts do not shift equilibrium; they reach it faster.
Decreasing pressure always shifts right. — Correct: Pressure shift depends on the side with fewer moles.
Increasing temperature always favors products. — Correct: It depends on whether the forward reaction is endothermic or exothermic.
Le Chatelier shifts change the equilibrium constant K. — Correct: K is fixed at a given temperature; only temperature changes K.
FAQ
What is Le Chatelier's principle?
A system at equilibrium responds to applied stress (change in concentration, temperature, or pressure) by shifting to oppose the change.
How does adding a reactant affect equilibrium?
Adding a reactant shifts equilibrium to the right (forward), consuming excess reactant.
Does pressure always affect equilibrium the same way?
No — pressure shifts equilibrium toward the side with fewer moles of gas.
Can temperature shift equilibrium?
Yes. Increasing temperature favors the endothermic direction; decreasing favors the exothermic.