Le Chatelier's Principle
When a system at equilibrium is subjected to a change, the position of equilibrium shifts to counteract that change and partially restore the original conditions.
Summary Table
For the reaction: \( aA(g) + bB(g) \rightleftharpoons cC(g) + dD(g) \quad \Delta H = ? \)
| Change | Shift | Effect on K |
|---|---|---|
| Increase [A] or [B] | → (towards products) | No change |
| Decrease [A] or [B] | ← (towards reactants) | No change |
| Increase pressure (fewer moles side) | Towards fewer moles of gas | No change |
| Increase T (exo forward) | ← (endothermic direction) | K decreases |
| Increase T (endo forward) | → (endothermic direction) | K increases |
| Add catalyst | No shift | No change |
Key Distinction
Only temperature changes the value of K. Changes in concentration, pressure, or adding a catalyst shift the position of equilibrium but do not change K.
Applying to the Haber Process
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹
- Low temperature favours products (exothermic forward) — but rate is too slow
- High pressure favours products (4 mol gas → 2 mol gas) — but expensive
- Compromise: ~450°C, 200 atm, iron catalyst
Think About It
Adding an inert gas to a fixed-volume container at equilibrium — does the position shift?
No. The total pressure increases, but the partial pressures and concentrations of each reactant and product are unchanged. Le Chatelier's only responds to changes in the concentrations of reacting species.