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.
Characteristics of Chemical Equilibrium
| Feature | Description | Example |
|---|---|---|
| Dynamic | Forward and reverse reactions continue | N₂ + 3H₂ ⇌ 2NH₃ |
| Constant concentrations | No net change in [reactants] or [products] | [NH₃] remains constant at equilibrium |
| Closed system | No reactants added or products removed | Sealed container |
| Temperature dependent | K changes only with temperature | Kc increases with T for endothermic rx |
| Reversible | Can be approached from either direction | Same K from N₂+H₂ or from NH₃ |
| Condition independent | Same K regardless of initial amounts | K is constant at fixed T |
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Related Terms
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).
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.
Kc (equilibrium)
Kc is the equilibrium constant expressed in terms of molar concentrations (mol/L) of reactants and products at equilibrium. Each concentration is raised to the power of its stoichiometric coefficient, and pure solids and pure liquids are excluded from the expression because their concentrations are constant. Kc is temperature-dependent and is the most commonly used form of the equilibrium constant in solution-phase and heterogeneous equilibria.
From Latin "aequus" (equal) and "libra" (balance, scale). The concept was formalised by Norwegian chemists Cato Guldberg and Peter Waage in 1864 through the Law of Mass Action.