Microscopic reversibility

At equilibrium, each elementary molecular process is balanced by its time-reversed process at an equal rate. This principle connects equilibrium behavior to reversible molecular dynamics.

Detailed balance: A condition in which each transition between states is balanced by its reverse at equilibrium. It states the equilibrium transition-by-transition balance that microscopic reversibility describes.
Chemical equilibrium: A state in which forward and reverse chemical processes balance, leaving macroscopic composition unchanged. Microscopic reversibility specifies how that macroscopic balance holds at the elementary-step level.
Wegscheider conditions: Relations among rate constants in a reaction network required for thermodynamic consistency at equilibrium. They enforce cycle-by-cycle compatibility with microscopic reversibility.
Nonequilibrium thermodynamics: The study of thermodynamic systems that exchange energy or matter and may sustain gradients or flows. Away from equilibrium, forward and reverse events need not balance individually.
Time-reversal symmetry: A symmetry under reversing the direction of time in the equations governing a system. It underlies the equality between equilibrium molecular trajectories and their reversals.
Elementary reaction: A single molecular event represented as one step in a reaction mechanism. The principle applies to elementary events and their specific reverse steps.
Chemical kinetics: The study of reaction rates and the mechanisms that determine them. The principle supplies equilibrium constraints on kinetic mechanisms and rate constants.
Entropy production: A measure of irreversible thermodynamic change, nonnegative for processes in isolated systems. Persistent net transition currents produce entropy rather than equilibrium balance.
Transition-state theory: A framework for estimating reaction rates from passage through a dividing surface near a potential-energy barrier. Its forward and reverse rate descriptions must respect equilibrium balance.
Reaction rate: The change in concentration of a chemical species per unit time due to reaction. Equal rates of paired molecular processes are the principle’s central condition.

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