Equilibrium constant

A dimensionless value equal to the product-to-reactant ratio of chemical activities at equilibrium, with each activity raised to its stoichiometric coefficient. Its value depends on temperature and the reaction as written.

Law of mass action: A relation connecting reaction rates or equilibrium composition to reactant and product concentrations raised to stoichiometric powers. It provides the concentration-based form from which equilibrium-constant expressions are derived.
Chemical equilibrium: A dynamic state in which forward and reverse reaction rates are equal, leaving macroscopic composition constant. The equilibrium constant characterizes the composition at this state.
Chemical equilibrium calculation: A calculation that determines equilibrium composition from initial amounts and thermodynamic constraints. The equilibrium constant supplies the condition used to solve for final concentrations or activities.
Chemical kinetics: The study of reaction rates and the mechanisms that determine them. An equilibrium constant specifies the final balance, not how quickly it is reached.
Chemical activity: An effective concentration that accounts for nonideal interactions in a chemical system. The thermodynamic equilibrium constant is defined using activities rather than raw concentrations.
Standard state: A reference state assigned to a substance for defining thermodynamic properties and activities. Activities are normalized to standard states, making the thermodynamic constant dimensionless.
Solubility product: The equilibrium constant for the dissolution of a sparingly soluble ionic solid. It predicts saturation and precipitation from dissolved-ion activities.
Equilibrium position: The relative amounts of reactants and products present at equilibrium. It is what the constant indicates, but it is not itself a numerical constant.
Reaction quotient: The product-to-reactant activity ratio for a reaction at a particular composition, with activities raised to stoichiometric coefficients. At equilibrium, this changing ratio reaches the equilibrium constant.
Chemical potential: The change in a system’s Gibbs free energy when the amount of a substance changes at fixed conditions. Equality of chemical potentials at equilibrium leads to the activity-ratio expression.

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