Le Chatelier's Principle
Le Chatelier's Principle states that if an external stress is applied to a system at equilibrium, the system will shift in the direction that partially counteracts the applied stress and re-establishes equilibrium. Stresses include changes in concentration, pressure, volume, or temperature. This principle is fundamental to industrial process optimisation — for example, the Haber process for ammonia synthesis uses elevated pressure to favour product formation.
Effect of Stresses on Equilibrium: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol
| Stress Applied | System Response | Shift Direction | Effect on K |
|---|---|---|---|
| Increase [N₂] | Consume excess N₂ | Forward (→) | No change |
| Decrease [NH₃] | Produce more NH₃ | Forward (→) | No change |
| Increase pressure | Reduce moles of gas (4→2) | Forward (→) | No change |
| Decrease temperature | Release heat (exothermic) | Forward (→) | K increases |
| Increase temperature | Absorb heat (reverse is endothermic) | Reverse (←) | K decreases |
| Add catalyst | Reach equilibrium faster | No shift | No change |
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Related Terms
Chemical Equilibrium
Chemical equilibrium is the state in a reversible reaction where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products over time. The system appears static but is actually dynamic — molecules continuously react in both directions at matching rates. The equilibrium state is quantified by an equilibrium constant (K), whose value depends only on temperature for a given reaction.
Equilibrium Constant
The equilibrium constant (K) is a dimensionless number that expresses the ratio of the concentrations (or partial pressures) of products to reactants, each raised to the power of their stoichiometric coefficients, for a reversible reaction at equilibrium at a given temperature. A large K (K >> 1) indicates the equilibrium favours products, while a small K (K << 1) indicates reactants predominate. K changes with temperature but is independent of initial concentrations, catalysts, or pressure (for Kc).
Kp (equilibrium)
Kp is the equilibrium constant expressed in terms of the partial pressures of gaseous reactants and products, each raised to the power of their stoichiometric coefficients. It is used exclusively for reactions involving gases and is related to Kc through the ideal gas equation, with the conversion factor depending on the change in moles of gas in the reaction. Kp is particularly useful in industrial gas-phase reactions such as the Haber process and the Contact process.
Named after French chemist Henry Louis Le Chatelier (1850–1936), who stated the principle in 1884. "Le Chatelier" is a French surname; the principle is sometimes called the "equilibrium law" or "Le Chatelier–Braun principle" (Karl Ferdinand Braun independently formulated it).