Dynamic equilibrium

Dynamic equilibrium is a state in which opposing processes continue at equal rates, so observable quantities remain constant despite ongoing change.

Chemical equilibrium: A chemical state in which forward and reverse reactions occur at equal rates, keeping concentrations constant. It is the best-known case: reacting molecules continue changing partners while bulk concentrations stay steady.
Reversible reaction: A chemical reaction that can proceed in both forward and reverse directions under the same conditions. Both directions must remain possible for chemical dynamic equilibrium to persist.
Haber process: An industrial process that synthesizes ammonia from nitrogen and hydrogen using an iron-based catalyst. Its reversible reaction reaches equilibrium, and operating conditions trade yield against production rate.
Chemical steady state: A condition in a reaction network where concentrations remain constant because production and consumption rates balance. Unlike equilibrium, a steady state may involve irreversible reactions and sustained energy consumption.
Law of mass action: A relation connecting reaction rate to reactant concentrations, each raised to a reaction-dependent power. It helps calculate how concentrations set the opposing reaction rates.
Steady state: A condition in which system variables remain constant over time, often because flows balance. A steady state can require continuous external energy or matter flow, unlike equilibrium.
Oxygen–hemoglobin dissociation curve: A curve showing how hemoglobin oxygen saturation varies with oxygen partial pressure. Oxygen binding and release approach reversible equilibrium as blood encounters different oxygen pressures.
Equilibrium thermodynamics: The branch of thermodynamics describing systems in equilibrium and the constraints on their macroscopic states. It distinguishes equilibrium conditions from merely constant readings in driven systems.
Equilibrium constant: A temperature-dependent ratio of product and reactant activities at chemical equilibrium. It specifies the equilibrium composition without implying that reactions have stopped.
Static equilibrium: A condition in which a system is balanced and has no ongoing macroscopic motion or change. Dynamic equilibrium preserves ongoing opposing activity rather than eliminating it.

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