📘 IB Understanding
Le Chatelier's principle enables the prediction of the qualitative effects of changes in concentration, temperature, and pressure to a system at dynamic equilibrium.
The Principle
If a change (stress) is applied to a system at dynamic equilibrium, the position of the equilibrium will shift in the direction that opposes the change, partially restoring the original conditions.
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 |
💡 Key Point
Changing concentration shifts the equilibrium position but does not change the value of K. The system adjusts until the concentrations again satisfy the equilibrium expression.
Effect of Pressure (Gases Only)
📋 Rule
When pressure is increased, the equilibrium shifts towards the side with fewer moles of gas (to reduce the pressure). When pressure is decreased, it shifts towards the side with more moles of gas.
Worked Example: The Haber Process
What happens if pressure is increased?
Left side: 1 + 3 = 4 moles of gas
Right side: 2 moles of gas
Equilibrium shifts right (towards fewer moles) to reduce the pressure. More NH₃ is produced.
Effect on K: No change (K only changes with temperature).
⚠️ Special Case
If both sides have the same number of moles of gas, changing pressure has no effect on the equilibrium position. Example: H₂(g) + I₂(g) ⇌ 2HI(g) has 2 moles on each side.
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?
Changes K
- Temperature (the ONLY factor)
Does NOT Change K
- Concentration
- Pressure
- Adding a catalyst
What About Catalysts?
Catalysts and Equilibrium
A catalyst increases the rate of both forward and reverse reactions equally. This means equilibrium is reached faster, but the position of equilibrium and the value of K are unchanged.
Think About It
In the Haber process (exothermic forward), a compromise temperature of ~450°C is used. Why not use a lower temperature if lower T favours more NH₃?
At low temperatures, K is larger and the equilibrium favours products. However, the rate becomes too slow to be commercially viable. The compromise temperature gives a reasonable rate AND a reasonable yield. An iron catalyst is used to increase the rate further without affecting the yield.