Heterogeneous Reaction Systems
A system that has two for more phases is heterogenous.
eg. CaCO3(s)  + dil. HCl   <===>  CO2(g) + CaO(s)
Increasing the amount of solid present in a heterogenous equilibrium increases the total, surface area at which both the forward and reverse reactions are occurring. The total number of moles of solid reactant being converted into product per unit time increases. However, the total number of moles of product being converted back into reactant increases proportionally, This results in no net change in the total amount of reactant or product.
Therefore Keq for the above reaction is dependant upon the [CO2] only.
ie. the Keq equation for the above reaction is Keq = [CO2] only.
The equilibrium constant expression for a heterogeneous reaction involving gases does not include the concentrations of pure solids.
eg. NH4Cl(s)  <====>  NH3(g) + HCl(g)
Keq = [NH3][HCl]