There are two types of chemical equilibria:
1. Homogeneous equilibrium
2. Heterogeneous equilibrium
1. Homogeneous equilibrium:
The equilibrium reactions in which all the reactants and the products are in the same phase are known as homogeneous equilibrium reactions. These are divided into two categories:
(a) The number of product molecules is equal to the number of reactant molecules. For example:
N2 (g) + O2 (g) ⇌ 2NO (g)
H2 (g) I2 ⇌ 2HI (g)
(b) The number of product molecules is not equal to the number of reactant molecules
COCl2 (g) ⇌ CO (g) + Cl2 (g)
2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
2. Heterogeneous equilibrium
The equilibrium reactions in which the reactants and the products are present in different phases are known as Heterogeneous equilibrium reactions. For example:
The dissociation of solid calcium carbonate to give solid calcium oxide and gaseous carbon dioxide:
CaCO3 (S) ⇌ CaO (s) + CO2 (g)
The equilibrium expression for Heterogeneous equilibrium reactions does not include the concentrations of pure solids because their concentrations remain constant. Similarly, the concentrations of pure liquids are also assumed to be constant and these do not appear in equilibrium concentration expression. By convention [solid] = 1 and [liquid] = 1
