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Equilibrium Constant (Kc)

Definition

A number describing where equilibrium lies: products over reactants, each raised to its coefficient power. Big K means mostly products at equilibrium.

Kc=[C]c[D]d[A]a[B]bK_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}

Symbol: KcKc ·

Technical definition

Kc is the equilibrium constant expressed in molar concentrations: the ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium. K much larger than 1 favors products; K much smaller than 1 favors reactants.

Example

For N2 + 3H2 forming 2NH3, Kc = [NH3]^2 / ([N2][H2]^3). A large Kc means the mixture sits mostly as ammonia at equilibrium.

Common confusion

Pure solids and liquids are left OUT of the Kc expression. Only gases and aqueous species appear, which trips up beginners writing heterogeneous equilibria.