The equilibrium constants for reactions that contain substances that are all in the same phase, and reactions that contain substances in different phases, need to be calculated differently. The former are called homogenous reactions, and the later are called heterogeneous reactions.

Equilibrium constants

The equilibrium constant K for a given reaction is defined as the ratio of the products of a reaction to the reactants, measured at equilibrium. In a general reaction

$aA + bB \leftrightarrow cC + dD$

The reaction quotient measured at equilibrium is the equilibrium constant K.

$Q \text{ at equilibrium}= \dfrac{[C]^c[D]^d}{[A]^a[B]^b} = K$

Homogenous Equilibria

A homogeneous equilibrium is one in which all of the reactants and products are present in a single solution (by definition, a homogeneous mixture). Reactions between solutes in liquid solutions belong to one type of homogeneous equilibria. The chemical species involved can be molecules, ions, or a mixture of both.

For the homogenous reaction

C₂H₂(aq) + 2Br₂(aq) $\leftrightarrow$ C₂H₂Br₄(aq)

The equilibrium constant $K=\dfrac{[C_2H_2Br_4]}{[C_2H_2][Br_2]^2}$.

Heterogeneous Equilibria

A heterogeneous equilibrium is a system in which reactants and products are found in two or more phases. The phases may be any combination of solid, liquid, or gas phases, and solutions. When dealing with these equilibria, remember that solids and pure liquids do not appear in equilibrium constant expressions.

For example, for the reaction

Br₂(l) $\leftrightarrow$ Br₂(g)

The equilibrium constant K is simply [Br₂], with the concentration of the pure liquid Br₂ excluded.