Le Châtelier’s Principle
Quick Notes:
- Le Châtelier’s Principle: if a system at equilibrium is subjected to a change, the equilibrium shifts to oppose the change.
- Factors affecting equilibrium position:
- Concentration: Increasing reactants shifts equilibrium right and increasing products shifts equilibrium left.
- Pressure (gases): Increasing pressure shifts equilibrium to the side with fewer gas molecules.
- Temperature:
- Endothermic (+ΔH) forward direction : Increasing temperature shifts equilibrium right.
- Exothermic (−ΔH) forward direction: Increasing temperature shifts equilibrium left. .
- Catalysts:Do not shift equilibrium, but increase the rate of both forward and reverse reactions equally.
- K changes only with temperature, not with concentration or pressure.
- Applies to homogeneous and heterogeneous equilibria.
Full Notes:
What Is Le Châtelier’s Principle?
Le Châtelier’s Principle states:
"If a system at equilibrium is subjected to a change, the position of equilibrium will shift to oppose that change."
This helps predict how a system reacts qualitatively to external changes.
Factors Affecting Equilibrium Position
Changing Concentration
- Increasing reactant concentration shifts equilibrium right (product concentration increases).
- Increasing product concentration shifts equilibrium left (reactant concentration increases).
Example Haber Process
Adding more N2 shifts equilibrium right, producing more NH3.
Changing Pressure (for Gaseous Equilibria)
- Increasing pressure shifts equilibrium towards the side with fewer gas molecules.
- Decreasing pressure shifts equilibrium towards the side with more gas molecules.
- No effect if the same number of gas molecules are present on both sides.
Example Haber Process and Pressure
4 moles (N2 + 3H2) ⇌ 2 moles (NH3)
Higher pressure shifts equilibrium right, increasing NH3 yield.
Changing Temperature
- Increasing temperature favours the endothermic direction (+ΔH).
- Decreasing temperature favours the exothermic direction (−ΔH).
Example Haber Process and Temperature
Forward reaction is exothermic (−ΔH).
- Increasing temperature shifts equilibrium left, reducing NH3 yield.
- Decreasing temperature shifts equilibrium right, increasing NH3 yield.
Catalysts and Equilibrium
A catalyst speeds up both the forward and reverse reactions by providing an alternative route with a lower activation energy.
- The position of equilibrium does not change.
- The system reaches equilibrium faster.
- This is useful in industrial processes to improve efficiency without affecting yield.
Heterogeneous Equilibria
Le Châtelier’s Principle also applies to equilibria involving different phases.
Example Gas–solution equilibrium
X(g) ⇌ X(aq)
- Increasing [X(aq)] shifts equilibrium left (less dissolves).
- Increasing [X(g)] shifts equilibrium right (more dissolves).
Summary
- Le Châtelier’s Principle predicts how equilibrium shifts when conditions change.
- Only temperature affects the actual value of K.
- Equilibrium shifts can be caused by changes in concentration, pressure, or temperature.
- Catalysts do not change equilibrium position, only the rate of reaching it.
- Applies to gaseous, aqueous, and heterogeneous systems.
Linked Course Question
Why do catalysts have no effect on the value of K or on the equilibrium composition?
Catalysts speed up both the forward and reverse reactions equally, so they help the system reach equilibrium faster, but they do not shift the position of equilibrium. Since the equilibrium constant (K) depends only on temperature, and not on reaction speed, a catalyst has no effect on the value of K or the final equilibrium composition.