Equilibrium Concepts

Calculating Equilibrium

Usually solvents, pure solids, and pure liquids are ignored in calculations of equilibrium constants.

A homogeneous reaction is one in which all reactants and products are in the same phase. Reactions involving only gases and solutions are examples of homogeneous reactions. In such cases all the reactants and products must appear in the formula.

Substances that do not undergo a specific change are not a part of the formula. This often happens in reactions in which all substances are in the same phase. For example, if water is a solvent in a reaction, the concentration of water is ignored in calculating chemical equilibrium. If water is a reactant or a product, it is noted in the formula of chemical equilibrium. Also, if water is in the gaseous state, it must appear in the formula.

Consider the following reaction:
CN(aq)+H2O(l)HCN(aq)+OH(aq){\rm{CN^-}}(aq)+{\rm{H_2O}}(l)\rightleftharpoons{\rm{HCN}}(aq)+{\rm{OH^-}}(aq)
In this example the equilibrium constant is
Kc=[HCN][OH][CN]K_{\rm{c}}=\frac{\left[\rm{HCN} \right]\left[\rm{OH^-}\right]}{\left[\rm{CN^-}\right]}
where the water component is ignored because it is the solvent. However, water is not always ignored. In some reactions, water is a byproduct of a reaction, such as esterification. In such cases, water is a product and not a solvent and so should not be ignored. Likewise, if water is a reactant but not the solvent, then it too should not be ignored when writing the equilibrium constant.

A heterogeneous reaction is one in which reactants exist in two or more phases. For example, two immiscible liquids, a solid and a gas, and mixtures of solids exist in two or more phases.

Substances that are solids or pure liquids are excluded in the equilibrium expression. The reason why pure samples are ignored is because their effective concentrations stay constant throughout the reaction.
CaCO3(s)CaO(s)+CO2(g){\rm{CaC}}{{\rm{O}}_3}(s) \rightleftharpoons {\rm{CaO}}(s) + {\rm{C}}{{\rm{O}}_2}(g)
Since solids and pure liquids are not included in the expression of the equilibrium constant, Kp=KcPCO2{K_{\rm{p}}} = {K_{\rm{c}}}{P_{{\rm{C}}{{\rm{O}}_2}}} .