Le Châtelier's Principle and Its Applications
Analogy- Consider that you’re sitting in a crowded room when someone opens a window, letting in a cold draft.
- You might instinctively reach for a jacket or move to a warmer spot to counteract the chill.
- This natural response to restore comfort mirrors how chemical systems respond to disturbances.
When a system at equilibrium experiences a change in concentration, pressure, or temperature, it adjusts to minimize the disturbance and reestablish equilibrium.
The Principle: Systems Resist Change
Definition
Le Châtelier's Principle
Le Châtelier's Principle states:
"If a dynamic equilibrium is disturbed by a change in the reaction conditions, the system adjusts to counteract the disturbance and restore a new equilibrium."
The “disturbances” can include:
- Concentration changes: Adding or removing reactants or products.
- Pressure changes: Altering the volume or overall pressure in systems involving gases.
- Temperature changes: Heating or cooling the system.
Effect of Concentration Changes
How Does Concentration Affect Equilibrium?
When the concentration of a reactant or product changes, the system shifts to reduce the impact of this disturbance.
- Adding Reactants: The system shifts toward the products to consume the excess reactants.
- Removing Reactants: The system shifts toward the reactants to replenish the loss.
- Adding Products: The system shifts toward the reactants to reduce the excess products.
- Removing Products: The system shifts toward the products to compensate for the removal.
- Consider the synthesis of ammonia: $$
\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)
$$ - At equilibrium, if more hydrogen ($\text{H}_2$) is added, the forward reaction is favored, producing more ammonia ($\text{NH}_3$) to counteract the disturbance.
- The equilibrium shifts to the right.
- Many students incorrectly assume that adding more of a substance will always increase its equilibrium concentration.
- Remember, the system adjusts to partially counteract the change, so the final concentration may still differ from the initial equilibrium state.
Effect of Pressure Changes
How Does Pressure Affect Equilibrium?
Pressure changes only affect systems with gaseous reactants or products.
HintThe direction of the shift depends on the number of gas molecules on each side of the reaction.
- Increasing Pressure: The system shifts toward the side with fewer gas molecules to reduce pressure.
- Decreasing Pressure: The system shifts toward the side with more gas molecules to increase pressure.
- In the same ammonia synthesis reaction: $$
\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)
$$ - The left-hand side has 4 moles of gas ($1\text{N}_2 + 3\text{H}_2$), while the right-hand side has 2 moles of gas ($2\text{NH}_3$).
- Increasing the pressure shifts the equilibrium to the right, favoring the formation of ammonia.
- Pressure changes have no effect on systems where the number of gas molecules is the same on both sides of the equation.
- For example, in $\text{H}_2(g) + I_2(g) \rightleftharpoons 2\text{HI}(g)$, pressure changes do not shift equilibrium.
