Thermodynamic time invariances for dual kinetic experiments: Nonlinear single reactions and more

For dual kinetic experiments, i.e., experiments performed in batch or steady-state plug flow reactors from reciprocal initial conditions, thermodynamic (space-)time invariances for all reversible single reactions of the first and second order have been found explicitly.In all analyzed cases, quotient-like functions of concentrations can be defined which equal the equilibrium constant of the reaction during the whole course of the experiment, and not only at the end, i.e., under equilibrium conditions. The obtained invariances can be used as simple fingerprints for distinguishing the types of reactions.For multistep reactions: (a) a similar invariance was obtained for the two-step catalytic reaction (single route complex reaction) under pseudo-steady-state assumption; (b) for the two-step non-steady-state reaction, an approximation for the thermodynamic invariance was found to be valid in two domains, (1) at the very beginning of the reaction (2) at the end of the reaction, near equilibrium conditions. We hypothesize that such two-domain validity of the thermodynamic invariance is a general feature of dual kinetic experiments performed in complex chemical systems.

► Thermodynamic time-invariances express equilibrium constant at finite time.
► They combine concentration dependencies from reciprocal experiments.
► We establish them for second-order reactions such as A+B<−>C+DA+B<−>C+D.
► We apply them to trajectory analysis and generalize to two-step mechanisms.