Equilibrium Constants: Kc

Le Chatelier's principle gives qualitative predictions about equilibrium shifts, but for quantitative work we need equilibrium constants. The equilibrium constant Kc expresses the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficient.

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Writing the Kc Expression

For the general equilibrium: aA + bB ⇌ cC + dD

Kc = [C]^c [D]^d / ([A]^a [B]^b)

Where [X] represents the equilibrium concentration of species X in mol dm⁻³.

Rules for Writing Kc Expressions

1. Products on top, reactants on bottom.
2. Each concentration is raised to the power of its coefficient in the balanced equation.
3. Pure solids and pure liquids are omitted from the expression (their concentrations are effectively constant).
4. Only species in solution (aq) or in the gas phase (g) are included.

Examples

For: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Kc = [NH₃]² / ([N₂][H₂]³)

For: CaCO₃(s) ⇌ CaO(s) + CO₂(g)

Kc = [CO₂] (the solids are omitted)

For: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l)

Kc = [CH₃COOC₂H₅][H₂O] / ([CH₃COOH][C₂H₅OH]) (all species are liquids in the same phase — they are included because they form a homogeneous mixture, not pure separate liquids)

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