Ideal Gas Reference for Association and Dissociation Reactions: I. Basic Concepts

Starting with a distance-based definition of molecules consisting of non-interacting atoms, which is in line with IUPAC terminology, we construct an ideal gas reference for chemical association and dissociation reactions. The corresponding ideal equations for equilibria and kinetics reveal the mathematical structure, known for real systems, in comprehensible clarity. The ideal gas reference corresponds to the limit of an entirely flat potential energy surface of the system where chemical equilibria and kinetics are determined by "unspecific" particle number combinatorics according to the reaction stoichiometry. The ideal equilibrium and rate constants provide a reference for the definition of excess equilibrium and rate constants of real reactions that quantify all "system-specific" contributions resulting from the particular shape of the potential energy surface. The ideal gas reference therefore enables a distinction between unspecific and system-specific aspects in the equilibria and kinetics of chemical association/dissociation reactions. Whereas conventional equilibrium and rate constants suffer from incompatibility between reactions of different stoichiometry, excess equilibrium and rate constants can be consistently compared across different reaction orders. Furthermore, whereas the conventional treatment requires an arbitrary specification of reference concentrations, e.g. at standard conditions, the ideal gas framework introduces an intrinsic concentration scale that is equal to the inverse of a molecular volume.