How fast is a fast equilibrium? A new view of reversible reactions.

Reversible reactions are described in terms of kinetic cycles. From this description, two useful parameters arise: 1) average cycle duration and 2) average number of cycles. The latter applies to cycles with at least one unstable species. These parameters allow answering the question "How fast is a fast equilibrium?" in absolute and in relative terms, respectively. The general interest of the approach is demonstrated by application to reactions drawn from several areas: Acid-base equilibria, enzyme kinetics, excited-state proton transfer, thermally activated delayed fluorescence, monomer-excimer kinetics and homo-FRET.