Nonequilibrium Effects in the Kinetics of Gas-Phase Reactions

A theoretical rate equation derived for exchange reactions in the gas phase from an assumed Boltzmann equation yields statistical rate expressions for theoretical rate coefficients which in general are functions of concentration with a ratio not equal to the equilibrium constant K. However, the theoretical rate equation may be rearranged to yield rate coefficients defined to obey the relation κf,c/κr,c = K. The rate coefficients κf,c and κr,c are also functions of concentration except in three linear cases: (1) a reaction with one of the components present in large excess; (2) a chemical reaction proceeding unaffected by inelastic processes; (3) an isomerization reaction proceeding in a large excess of inert gas. Thus for these three cases the usual phenomenological rate equation [Eq. (2)] applies. In Cases 1 and 3 the rate constants are determined not only by chemical-reaction cross sections but also by inelastic cross sections for at least some and possibly all reactants and products.