Le Châtelier’s Principle
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Topic Summary & Highlights
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Core Concept
When a system at equilibrium is disturbed by an external stress (change in concentration, pressure, or temperature), the system shifts in a direction that minimizes the disturbance and re-establishes equilibrium.
Key Idea: The system responds by partially counteracting the applied stress.
Practice Tips
Le Châtelier’s Principle does not affect the value of the equilibrium constant (K) unless the temperature changes.
Changes in pressure only affect systems involving gases.
Adding inert gases at constant volume does not change equilibrium because it doesn’t affect the partial pressures of the reactants or products.
Memorize the Effects: Know how concentration, pressure, and temperature changes affect equilibrium.
Types of Stresses and Their Effects
| Type of Stress | Effect | Equilibrium Shift |
|---|---|---|
| Change in Concentration | Adding a reactant | Shifts right (toward products) |
| Adding a product | Shifts left (toward reactants) | |
| Removing a reactant | Shifts left (toward reactants) | |
| Removing a product | Shifts right (toward products) | |
| Change in Pressure | Increase pressure | Shifts toward fewer gas molecules |
| Decrease pressure | Shifts toward more gas molecules | |
| No change in pressure (equal gas molecules) | No shift | |
| Change in Temperature | Increase temperature | Shifts toward endothermic direction (absorbs heat) |
| Decrease temperature | Shifts toward exothermic direction (releases heat) | |
| Addition of a Catalyst | Speeds up both forward and reverse reactions equally | No shift |