Chemical equilibrium is a dynamic state in a reversible reaction where the forward and reverse reactions occur at the same rate. Reactants continuously turn into products, and products simultaneously turn back into reactants at the molecular level. A system at equilibrium shows no observable change in the concentrations of the species involved over time. Quantifying this balanced state is crucial for predicting the extent to which a reaction will proceed.
Defining the Equilibrium Constant Expression
The chemical balance in a reversible reaction is quantified by the equilibrium constant, \(K\). This constant is a numerical value representing the ratio of product concentrations to reactant concentrations once the system has reached equilibrium. This expression is based on the Law of Mass Action.
For a general reaction, the expression is written by multiplying the concentrations of the products in the numerator and the reactants in the denominator. Each concentration term is raised to the power of its corresponding stoichiometric coefficient from the balanced chemical equation. The equilibrium constant can be expressed in terms of molar concentration (\(K_c\)) or in terms of partial pressures for gaseous reactions (\(K_p\)).
Why Pure Liquids and Solids Are Excluded
Pure liquids and pure solids are excluded from the equilibrium constant expression because their effective concentration remains constant throughout the entire reaction. The concentration of any pure substance is defined as its density divided by its molar mass. Since density and molar mass are intrinsic properties, this ratio is a fixed value that does not change with the amount present. As long as some pure solid or liquid is present, its concentration does not vary as the reaction progresses toward equilibrium. This constant concentration is mathematically absorbed into the overall numerical value of the equilibrium constant, \(K\).
By convention, the concentration of a pure solid or liquid is represented by the number one, simplifying the final expression. This rule applies only to “pure” substances; aqueous solutions and gases have concentrations that change significantly and must be included.
Calculating \(K\) in Heterogeneous Reactions
When a reversible reaction involves substances in more than one physical state, it is termed a heterogeneous reaction. In such cases, the rule of excluding pure solids and liquids is applied when calculating \(K\). Only the species whose concentrations or partial pressures are variable—gases and aqueous solutes—are written into the equilibrium constant expression.
Consider the thermal decomposition of solid calcium carbonate: \(CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)\). Since calcium carbonate and calcium oxide are pure solids, their concentration terms are omitted. The equilibrium constant expression simplifies to \(K_c = [CO_2]\), or \(K_p = P_{CO_2}\).
This simplified expression shows that the equilibrium position is determined solely by the concentration or pressure of the gaseous carbon dioxide. The amount of the solids present does not affect the final ratio of products to reactants at a given temperature. Therefore, only species capable of having a variable concentration are included in the final mathematical expression.