Some thermodynamic properties of the hydrated electron

Recent determinations of rate constants in both directions permit an estimate of the free energy change at 25° for the process H₂O(1) + e_aq^- = H_aq + OH_aq^-. This value can be combined with estimates of absolute thermodynamic properties of individual ions to indicate that ΔG° = -39.4 kcal/mole when electrons in the gas phase are transferred to water at the same concentration (provided any electrostatic potential change at the interface has been compensated). This quantity is smaller in magnitude than for other ionic species and indicates that the charge of the hydrated electron is dispersed in a region of radius about 3.0 A. Data on other anions have been extrapolated to this radius and used to estimate the comparatively small entropy change accompanying hydration. The calculations permit the prediction that the acid dissociation of aqueous hydrogen atom proceeds with positive ΔH° and negative ΔS° and with rate constant about 4 sec^-1. They also predict that the unimolecular decomposition of the hydrated electron has an activation energy between 4 and 13 kcal/mole and probably not near either limit. In view of the rapid bimolecular decomposition of solvated electrons in water, it is not clear why this species is so inert to the same sort of process in ammonia.