Law of mass action

The law of mass action relates reaction rates to reactant concentrations and describes equilibrium compositions through concentration-based relationships set by the reaction.

Elementary reaction: A single molecular event in a reaction mechanism, with a rate law determined by its molecularity. For an elementary step, reactant concentration powers in the rate law follow directly from molecularity.
Chemical equation: A symbolic representation of a chemical reaction showing reactants, products, and their stoichiometric proportions. Its balanced coefficients set the exponents in the equilibrium expression.
Peter Waage: A Norwegian chemist who formulated the law of mass action with Cato Guldberg. Waage helped establish the principle's mathematical statement in chemical equilibrium.
Michaelis–Menten kinetics: A rate model for enzyme-catalyzed reactions that produces a saturating dependence on substrate concentration. Enzyme saturation departs from the simple proportional concentration dependence of elementary mass-action rates.
Reaction rate: The change in concentration of a reactant or product per unit time. The kinetic form of mass action specifies how reactant concentrations affect this rate.
Stoichiometry: The quantitative relationships among substances in a chemical reaction, determined by its balanced equation. Stoichiometric coefficients define powers in the mass-action expression.
Cato Guldberg: A Norwegian mathematician and chemist who formulated the law of mass action with Peter Waage. Guldberg connected reacting quantities to the equilibrium relations that became the law.
Hill equation: An empirical equation describing cooperative binding or sigmoidal responses to ligand concentration. Cooperative behavior can produce concentration-response curves unlike ordinary mass-action predictions.
Rate law: An equation relating reaction rate to species concentrations and a rate constant. Mass-action kinetics is a common form of rate law, though it does not describe every reaction mechanism.
Molar concentration: The amount of a substance dissolved per unit volume of solution, commonly measured in moles per liter. Concentration is the quantity used in idealized mass-action rate and equilibrium equations.

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