Le Chatelier's Principle
Equilibrium response to external stress
Principle
If a system at equilibrium is disturbed, it shifts to counteract the disturbance
Three Types of Stress
1. Concentration Change
Add reactant → shifts right (toward products)
Add product → shifts left (toward reactants)
Remove species → shifts to replace it
2. Pressure/Volume Change (gases)
Increase pressure (decrease V) → shifts to side with fewer gas molecules
Decrease pressure (increase V) → shifts to side with more gas molecules
3. Temperature Change
Raise T → shifts in endothermic direction (absorbs heat)
Lower T → shifts in exothermic direction (releases heat)
Note: This changes K value
Example: N₂ + 3H₂ ⇌ 2NH₃ (ΔH < 0, exothermic)
Add N₂: Shifts right → more NH₃ produced
Increase pressure: Shifts right (4 mol gas → 2 mol gas)
Increase temperature: Shifts left (endothermic direction)
Remove NH₃: Shifts right → produces more NH₃
Important Notes
- Catalyst does NOT shift equilibrium; only speeds up reaching equilibrium
- Concentration and pressure changes don't change K; temperature does
- Inert gas addition at constant V has no effect
- Only gases count for pressure/volume effects