The calculation and interpretation of the equilibrium constant depends on whether the chemical reaction involves homogeneous equilibrium or heterogeneous equilibrium. All of the products and reactants are in the same phase for a reaction at homogeneous equilibrium. For example, everything could be a liquid or all the species could be gases. More than one phase is present for reactions that reach heterogeneous equilibrium. Usually only two phases are present, such as liquids and gases or solids and liquids. Solids are omitted from the equilibrium expression.
Significance of the Equilibrium Constant For any given temperature, there is only one value for the equilibrium constant. Kc only changes if the temperature at which the reaction occurs changes. You can make some predictions about the chemical reaction based on whether the equilibrium constant is large or small. If the value for Kc is very large, then the equilibrium favors the reaction to the right and there are more products than reactants. The reaction may be said to be “complete” or “quantitative.” If the value for the equilibrium constant is small, then the equilibrium favors the reaction to the left and there are more reactants than products. If the value of Kc approaches zero the reaction may be considered not to occur. If the values for the equilibrium constant for the forward and reverse reaction are nearly the same then the reaction is about as likely to proceed in one direction and the other and the amounts of reactants and products will be nearly equal. This type of reaction is considered to be reversible.