It is shown that equilibrium constants determine the time-dependent behavior of particular
ratios of concentrations for any system of reversible first-order reactions. Indeed, some
special ratios actually coincide with the equilibrium constant at any moment in time. This is
established for batch reactors, and similar relations hold for steady-state plug-flow reactors,
replacing astronomic time by residence time. Such relationships can be termed time
invariants of chemical kinetics.