Le Châtelier’s Principle
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.
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.
Test Yourself
Types of Stresses and Their Effects
For the example case column. The example relates to ONE of the reactions below. They match in color to what relates:
Gas phase: $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g) + \text{heat}$
Aqueous phase: $\text{Fe}^{3+}(aq) + \text{SCN}^-(aq) \rightleftharpoons \text{FeSCN}^{2+}(aq)$
| Type of Stress | Effect | Equilibrium Shift | Example Case |
|---|---|---|---|
| The category of change being made to the system. | The specific action taken within that category. | The direction the reaction moves to restore balance. | Real-world application (Blue = Gas, Red = Aqueous). |
| Change in Concentration | Adding a reactant | Shifts right (toward products) | Add N₂ → Shift Right |
| Adding a product | Shifts left (toward reactants) | Add NH₃ → Shift Left | |
| Removing a reactant | Shifts left (toward reactants) | Remove H₂ → Shift Left | |
| Removing a product | Shifts right (toward products) | Remove NH₃ → Shift Right | |
| Adding water (Dilution) | Shifts toward side with more aqueous particles | Add H₂O to aqueous system → Shift Left | |
| Removing water (Evaporation) | Shifts toward side with fewer aqueous particles | Evaporate H₂O → Shift Right | |
| Change in Pressure | Increase pressure | Shifts toward fewer gas molecules | System compressed → Shift Right (2 mol vs 4) |
| Decrease pressure | Shifts toward more gas molecules | Volume increased → Shift Left (4 mol vs 2) | |
| No change in pressure (equal gas molecules) | No shift | H₂ + Cl₂ ⇌ 2HCl → No Shift | |
| Change in Temperature | Increase temperature | Shifts toward endothermic direction (absorbs heat) | Heating exothermic system → Shift Left |
| Decrease temperature | Shifts toward exothermic direction (releases heat) | Cooling exothermic system → Shift Right | |
| Adding a Reactive Species | Adding a substance that precipitates or reacts with an ion | Shifts to replace the consumed ion | Add OH⁻ to react with Fe³⁺ → Shift Left |
| Addition of a Catalyst | Speeds up both forward and reverse reactions equally | No shift | Add Platinum powder → No Shift |