Chemical equilibrium

A state in a reversible reaction where forward and reverse reaction rates are equal, keeping reactant and product concentrations constant over time.

Law of mass action: A relation connecting reaction rates or equilibrium composition to reactant and product concentrations. It relates equilibrium composition to the concentrations of the reacting species.
Reversible reaction: A chemical reaction that can proceed in both forward and reverse directions under the same conditions. Both directions must be possible for their rates to balance.
Le Chatelier's principle: A rule predicting that an equilibrium shifts to oppose a change in conditions imposed on a system. It gives a qualitative way to predict responses to concentration, pressure, or temperature changes.
Chemical steady state: A condition in which concentrations remain constant because formation and consumption rates balance, not necessarily through reversible equilibrium. A steady state can persist with unequal opposing reaction rates and ongoing energy input.
Equilibrium constant: A value describing the ratio of product to reactant activities at equilibrium for a specified reaction and temperature. It quantifies the composition associated with equilibrium for a given reaction.
Chemical activity: An effective concentration that accounts for interactions among particles in a nonideal mixture. Equilibrium expressions are fundamentally written using activities, not raw concentrations.
Acid–base equilibrium: The equilibrium between proton-transfer forms of acids and bases in solution. Acid strengths and solution pH depend on equilibrium between protonated and deprotonated species.
Chemical equilibrium versus steady state: A comparison between thermodynamic equilibrium and nonequilibrium states with constant observable concentrations. This distinction prevents constant concentrations from being mistaken for equilibrium.
Reaction quotient: A ratio of product to reactant activities calculated using the current composition of a reaction mixture. Comparing it with the equilibrium constant predicts the direction of net reaction.
Stoichiometry: The quantitative relationships among reactants and products in a chemical reaction. Stoichiometric coefficients determine the powers in equilibrium expressions.

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