Equilibrium Expressions for Heterogeneous Reactions

Since the activities of pure solids are defined as unity, only the concentrations or partial pressures of the gaseous components are included in the equilibrium expression, thereby simplifying the calculation of $K_{eq}$.

The exclusion of pure solids from the equilibrium expression indicates that solids have a negligible effect on the equilibrium and can be ignored in all calculations.

Because pure solids are omitted, the value of $K_{eq}$ becomes highly sensitive to changes in the concentrations of gases, leading to large fluctuations in the equilibrium state.

The absence of pure solids in the equilibrium expression demonstrates that the reaction cannot reach equilibrium unless only gases are present in the system.