Why do we omit the concentrations of pure liquids and pure solids from the mass action expression of heterogeneous reactions?

The concept of chemical equilibrium is a fundamental one in the study of chemical reactions. It refers to the state where the rates of the forward and reverse reactions are equal, leading to a constant ratio of product and reactant concentrations over time. This does not mean the reactions have stopped; they are still occurring, just at a rate that maintains a balance.

At equilibrium, the mass action expression, or equilibrium constant expression (\( K \text{c} \)), is derived from the reaction quotient (\( Q \) by using the concentrations of the reacting species when the reaction has reached this balanced state. The equilibrium constant is a measure of the extent of the reaction and is only affected by temperature, not by the concentrations of reactants or products.

The reaction quotient, denoted as (\( Q \) is a snapshot of a reaction at any point in time and helps predict the direction in which a reaction will proceed to reach equilibrium. The formula is similar to that of the equilibrium constant, but while (\( K \text{c} \) is specific to the equilibrium state, (\( Q \) can be calculated for any concentration values of the reactants and products during the reaction.

When comparing (\( Q \) to the equilibrium constant (\( K \text{c} \) we can deduce if a reaction will shift to form more products (\( QK \text{c} \) or if the system is already at equilibrium (\( Q=K \text{c} \) This concept is crucial in understanding chemical processes and in predicting the results of mixing various compounds.

Stoichiometry comes from the Greek words for 'element' and 'measure' and is the part of chemistry that involves calculating the amounts of reactants and products in chemical reactions. For a balanced reaction, stoichiometry provides the ratios in which reactants combine and products form.

In the mass action expression, stoichiometric coefficients become the exponents for the concentration terms. These coefficients indicate the relative number of moles of each substance involved in the reaction. By applying laws of conservation of mass and of charge, stoichiometry helps in making precise predictions about the quantitative outcomes of chemical reactions and is essential for proper chemical formulation and experimentation.

Pure substances, whether they are solids or liquids, feature prominently in chemical reactions. However, their inclusion in the mass action expression differs from that of gases and solutions.

This is because, under typical conditions, the concentration of a pure solid or liquid is effectively constant, a physical property dictated by its density. Hence, in heterogeneous reactions that involve pure substances alongside gases and aqueous solutions, their concentrations do not appear in the mass action expression. By omitting them, we simplify the expression without affecting the calculation of equilibrium since these concentrations do not change as the reaction progresses.