Homogenous and Heterogenous Equilibria and Kc

In homogenous equilibria, all species have the same physical state.N₂O₄(g) ⇌ 2 NO₂(g)In heterogenous equilibria there is a mixture of different physical states. When water is a reactant or product, it appears in the expression for the equilibrium constant. When water is a solvent, it does not appear in the expression for the equilibrium constant. This is because the concentration of water stays constant. Solids are not included in the equilibrium expression. For the reaction CaCO₃ (s) ⇌ CaO(s) + CO₂(g), K_c = [CO₂(g)]. Units of K_c depend on the equilibrium expression e.g. 2CHClF₂(g) ⇌ C₂F₄(g) + 2HCl(g) units are moldm^-3