Le Chatelier's Principle
When a system at dynamic equilibrium is subjected to an external change (stress in concentration, temperature, or pressure), the system responds by shifting its equilibrium position in the direction that partially opposes the applied change.
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 |
Factors Changing the Equilibrium Constant K
Critical Examiner Rule: Only temperature changes the numerical value of the equilibrium constant (\(K\)). Changes in concentration, partial pressure, total pressure, volume, or adding a catalyst alter the position of equilibrium or reaction quotient \(Q\), but leave \(K\) strictly unchanged.
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
Adding an Inert Gas at Constant Volume
Adding an unreactive noble gas (e.g. \(\text{Ar}\)) to an equilibrium mixture in a container of fixed volume increases the total pressure, but does not change the partial pressure or concentration of any reacting species. Hence, the equilibrium position does not shift.
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