Qualitative Predictions via Le Chatelier's Principle
Le Chatelier's principle enables qualitative prediction of shifts in equilibrium position in response to temperature changes, pressure variations, and concentration adjustments.
The Principle
Formal Statement of Le Chatelier's Principle
Le Chatelier's Principle: If a stress (change in concentration, pressure, or temperature) is applied to a chemical system at dynamic equilibrium, the equilibrium position shifts in the direction that minimizes or partially opposes that change.
Effect of Concentration
Consider: \( \text{A} + \text{B} \rightleftharpoons \text{C} + \text{D} \)
| Change | Equilibrium Shifts | Effect on K |
|---|---|---|
| Increase [reactant] | Right (towards products) to consume the excess | No change |
| Decrease [reactant] | Left (towards reactants) to replace it | No change |
| Increase [product] | Left (towards reactants) to consume the excess | No change |
| Remove product | Right (towards products) to replace it | No change |
Concentration Shifts Leave K Unchanged
Changing concentration shifts the equilibrium position until the reaction quotient \(Q\) once again equals \(K\). The equilibrium constant \(K\) itself does not change.
Effect of Pressure (Gases Only)
Effect of Pressure Changes on Gas Equilibria
Increasing total pressure (by decreasing volume) shifts the equilibrium toward the side with fewer moles of gas. Decreasing pressure shifts the equilibrium toward the side with more moles of gas.
Worked Example: The Haber Process
Predicting Shifts for Ammonia Synthesis
For the Haber reaction: \(\text{N}_2(\text{g}) + 3\text{H}_2(\text{g}) \rightleftharpoons 2\text{NH}_3(\text{g}) \quad \Delta H^\circ = -92\text{ kJ mol}^{-1}\)
- Increasing pressure: Left side has \(4\text{ mol gas}\); right side has \(2\text{ mol gas}\). Equilibrium shifts right (towards products).
- Increasing temperature: Reaction is exothermic (\(\Delta H < 0\)). Equilibrium shifts left (endothermic direction), and \(K\) decreases.
- Removing \(\text{NH}_3\): Equilibrium shifts right to replenish product.
Equal Moles of Gas on Both Sides
If a gaseous reaction has the same number of gas moles on both sides (e.g. \(\text{H}_2(\text{g}) + \text{I}_2(\text{g}) \rightleftharpoons 2\text{HI}(\text{g})\), with \(2 \rightleftharpoons 2\)), changing pressure or volume has zero effect on the equilibrium position.
Effect of Temperature
Temperature is the only factor that changes the value of K.
| Change | Exothermic Forward (ΔH < 0) | Endothermic Forward (ΔH > 0) |
|---|---|---|
| Increase T | Shifts left (endothermic direction) to absorb heat K decreases |
Shifts right (endothermic direction) to absorb heat K increases |
| Decrease T | Shifts right (exothermic direction) to release heat K increases |
Shifts left (exothermic direction) to release heat K decreases |
Summary: What Changes K?
Temperature Changes K
Temperature is the ONLY factor that changes the value of \(K\): For exothermic reactions (\(\Delta H < 0\)), increasing \(T\) decreases \(K\). For endothermic reactions (\(\Delta H > 0\)), increasing \(T\) increases \(K\).
Factors That Leave K Constant
Never changes K: (1) Concentration changes, (2) Total pressure changes, (3) Adding an inert gas, (4) Adding a catalyst. These may shift position or change rate, but \(K\) remains constant.
What About Catalysts?
Catalysts Accelerate Equilibrium without Shifting Position
A catalyst increases the rate of forward and reverse reactions by the exact same proportion. It allows the system to reach dynamic equilibrium faster, but leaves the equilibrium composition and \(K\) completely unchanged.
Industrial Conditions: Kinetics vs Equilibrium
In the Haber process: low temperature gives higher yield (exothermic), but the rate is too slow. High temperature gives fast rate, but low equilibrium yield. A compromise temperature of ~450 °C with an iron catalyst provides an economically viable rate with acceptable yield.
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