Determination of proton affinity from the kinetics of proton transfer reactions. II. Kinetic analysis of the approach to the attainment of equilibrium

A detailed kinetic analysis of the approach to and attainment of equilibrium in proton transfer reactions proceeding in a flowing afterglow at 300°K has been developed by extending existing analyses for ion molecule reactions to include axial diffusion and back reaction. The analysis has been applied to the experimental data obtained for the reactions ${\mathrm{H}}_3^{+}+{\mathrm{N}}_{2^{\rightleftarrows }}{\mathrm{N}}_2{\mathrm{H}}^{+}+{\mathrm{H}}_2$ and ${\mathrm{CO}}_2{\mathrm{H}}^{+}+{\mathrm{CH}}_{4^{\rightleftarrows }}{\mathrm{CH}}_5^{+}+{\mathrm{CO}}_2$ proceeding in a large excess of H₂ gas. The latter reaction was investigated independently in both directions. The forward and reverse rate constants, k_f and k_r, were uniquely determined under nonequilibrium conditions for both reaction systems, and in the case of the CO₂H⁺+CH₄ system, this was accomplished both in the forward and in the reverse direction. The ratios of rate constants determined under nonequilibrium conditions were found to be equal, within experimental error, to the equilibrium constant, K, determined from the equilibrium concentrations. Furthermore, in the case of the CO₂H⁺+CH₄ system, for which it was possible to attain equilibrium conditions from either side of the reaction, the rate constants and the equilibrium constant were observed to be independent of the direction of approach to equilibrium.