Determination of proton affinity from the kinetics of proton transfer reactions. III. The measurement of the equilibrium constant at various temperatures

Abstract

The flowing afterglow measurements of the equilibrium constants for selected proton transfer reactions have been extended to temperatures other than room temperature in order to completely specify the thermodynamics of such reactions. The van't Hoff analysis of the data for the reaction ${\mathrm{CO}}_2{\mathrm{H}}^{+}+{\mathrm{CH}}_4\rightleftarrows {\mathrm{CH}}_5^{+}+{\mathrm{CO}}_2$ results in a value for the standard enthalpy change, ΔH° (196 to 553°K), of −0.064±0.004 eV and for the standard entropy change, ΔS° (196 to 553°K), of +1.4±0.6 e.u. Studies of the reaction ${\mathrm{N}}_2{\mathrm{OH}}^{+}+\mathrm{CO}\rightleftarrows {\mathrm{COH}}^{+}+{\mathrm{N}}_2\mathrm{O}$ led to a value of −0.151±0.009 eV for ΔH° (277 to 505°K) and a value of −1.8±1.1 e.u. for ΔS° (277 to 505°K). ΔH° is a direct measure of the difference between the proton affinities of CO₂ and CH₄ in the former case and of N₂O and CO in the latter. The measured values for ΔS° yield differences in the standard entropies of the protonated species: ${\mathrm{S̄°}}_{298}({\mathrm{CH}}_5^{+}{\mathrm{)-S̄°}}_{298}({\mathrm{CO}}_2{\mathrm{H}}^{+}\text{)= −5.2± 0.6\hspace{0.17em}}\mathrm{e.u}$ and ${\mathrm{S̄°}}_{298}({\mathrm{COH}}^{+}{\mathrm{)-S̄°}}_{298}({\mathrm{N}}_2{\mathrm{OH}}^{+}\text{)= −7.1± 1.1\hspace{0.17em}}\mathrm{e.u.}$ Reference to calculated values for the standard entropy of ${\mathrm{CH}}_5^{+}$ and COH⁺ yields values for the standard entropy of CO₂H⁺ and N₂OH⁺ and some indication of the structure of these species.