Homogenous and heterogeneous equilibrium

• Expression for the equilibrium constant of a reaction is written in the form of a ratio. The numerator consists of the molar concentration (or partial pressure) terms of the products each raised to a power equal to its stoichiometric coefficient in the balanced chemical equation, and the denominator consists of the molar concentration (or partial pressure) terms of the reactants each raised to a power equal to the stoichiometric coefficient in the balanced chemical equation.

Equilibrium in which all the reactants and products are in the same phase is called homogeneous equilibrium reaction. For example,

• The concentration term of the solvent, which is present in large excess is not included in the equilibrium constant expression.

For example, for the reaction,

The equilibrium constant may be written as:

By convention we write [H₂O] = constant so that,

The [H₂O] term is merged with the equilibrium constant.

• Equilibrium reactions in which the reactants and products are present in different phases are called heterogeneous equilibrium reactions.

For heterogeneous reactions, the concentration terms for pure solids and pure liquids are not included in the expression for equilibrium constant. The concentration of pure solids and liquids remain constant, and these terms are merged into the equilibrium constant or by convention their concentrations are taken as unity, i.e., [solid] = 1, [liquid] = 1.

For example, the equilibrium constant for the reaction,

By convention [Fe_(s)] = 1 and [Fe₃O_4(g)] = 1,

so the equilibrium constant is,

For the reaction,

The equilibrium constant is,

K_sp = [Ag⁺_(aq)] [Cl^-_(aq)]

The constant for this reaction is called solubility product constant (K_sp).

Problem

7. Classify each of the following equilibrium as homogeneous or heterogeneous:

Solution

(i) Homogeneous

(ii) Heterogeneous