Le Chatelier's Principle
Factors Affecting Chemical Equilibrium | Class 11
1. The Principle
2. Effect of Change in Concentration
- Addition of Reactant: Equilibrium shifts Forward (to consume added reactant).
- Removal of Product: Equilibrium shifts Forward (to produce more product).
- Addition of Product: Equilibrium shifts Backward.
3. Effect of Change in Pressure
Relevant only for gaseous reactions where moles of gaseous reactants $\neq$ moles of gaseous products ($\Delta n_g \neq 0$).
- Increase Pressure (Decrease Volume): Equilibrium shifts towards the side with fewer gaseous moles.
- Decrease Pressure (Increase Volume): Equilibrium shifts towards the side with more gaseous moles.
Example: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ ($\Delta n_g = -2$).
High Pressure favors forward reaction (fewer moles, $2 < 4$).
4. Effect of Change in Temperature
This is the only factor that changes the value of the Equilibrium Constant ($K$).
| Reaction Type | Increase Temp | Decrease Temp |
|---|---|---|
| Exothermic ($\Delta H < 0$) | Shift Backward ($K \downarrow$) | Shift Forward ($K \uparrow$) |
| Endothermic ($\Delta H > 0$) | Shift Forward ($K \uparrow$) | Shift Backward ($K \downarrow$) |
Hint: Treat Heat as a product in Exothermic and reactant in Endothermic.
5. Addition of Inert Gas
A. At Constant Volume
Adding an inert gas (He, Ne, Ar) at constant volume does not change the partial pressures of reacting species.
Result: No effect on equilibrium.
B. At Constant Pressure
Adding inert gas increases the total volume, decreasing the partial pressures of reactants/products.
Result: Equilibrium shifts towards the side with more gaseous moles (to restore pressure).
6. Effect of Catalyst
A catalyst increases the rate of both forward and backward reactions equally. It does not shift the equilibrium position but helps attain equilibrium faster.
Practice Quiz
Test your ability to predict shifts.
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