Why are pure solids and liquids excluded from the equilibrium expression?

Pure solids and liquids are excluded from the equilibrium expression because their concentrations remain constant throughout the reaction.

In a chemical equilibrium, the concentrations of the reactants and products are used to express the position of equilibrium. This is typically done using the law of mass action, which states that the rate of a chemical reaction is proportional to the product of the concentrations of the reactants. However, this law only applies to substances that are in the gas phase or are dissolved in solution.

Pure solids and liquids are not included in the equilibrium expression because their concentrations do not change during the reaction. The concentration of a pure solid or liquid is essentially its density, which remains constant regardless of the amount present. This is because the particles in a solid or liquid are closely packed together and do not have the freedom to move and spread out like particles in a gas or solution. Therefore, adding or removing a pure solid or liquid from a reaction does not affect the equilibrium position.

Furthermore, the activity of pure solids and liquids is considered to be one. In thermodynamics, the activity of a substance is a measure of its 'effective concentration' in a mixture. For pure solids and liquids, this is always one, regardless of the actual amount present. This is another reason why they are not included in the equilibrium expression.

In contrast, the concentrations of gases and solutions do change during a reaction, as the particles are free to move and spread out. Therefore, they are included in the equilibrium expression. The concentrations of gases are usually expressed in terms of partial pressures, while the concentrations of solutions are expressed in terms of molarity.

For a deeper understanding of how the exclusion of solids and liquids aligns with the principles governing reactions at equilibrium, you may explore topics like Dynamic Equilibrium and the calculation of Equilibrium Constant Kc. Additionally, understanding how the position of equilibrium shifts when conditions change can be further explained by Le Châtelier’s Principle.

IB Chemistry Tutor Summary: Pure solids and liquids are left out of the equilibrium expression because their concentrations, essentially their densities, don't change during a reaction. Since they stay constant, they don't influence the reaction's equilibrium position. Their "effective concentration," or activity, is always one, making them irrelevant to the calculation of equilibrium, unlike gases and solutions whose concentrations can vary.