7.1.4—Expressions for equilibrium constants

For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / ([A]^a[B]^b), using equilibrium concentrations in mol dm⁻³.

Products appear in the numerator, reactants in the denominator, and equation coefficients become powers. Pure solids and liquids are omitted because their effective concentration is constant.

For N₂ + 3H₂ ⇌ 2NH₃, Kc = [NH₃]²/([N₂][H₂]³). Do not use initial concentrations unless the question states they are equilibrium values.

Kc is not the same as a simple product-minus-reactant ratio. Coefficients become indices, and the expression depends on the balanced equation.