Rates of Dissociative Ionic Reactions in Aqueous Mixtures Correlated with Opposing Gibbs Energies of Transfer of Single Ions: Solvolysis of Chloropentacyanocobaltate(III) Anions in Water + Ethanol and Water + Urea

The kinetics of the solvolysis of Co(CN)₅Cl^3– have been investigated in water with an added structure former, ethanol, and with added urea, which has only a weak effect on the solvent structure. As this solvolysis involves a rate-determining dissociative step corresponding closely to a 100%; separation, Co³⁺ Cl^-, in the transition state, a Gibbs energy cycle relating Gibbs energies of activation in water and in the mixtures to Gibbs energies of transfer of individual ionic species between water and the mixtures, G_t^o(i), can be applied. The acceleration of the reaction found with both these cosolvents results from the compensation of the retarding positive G_t^o(Cl^- by the negative term [G_t^o[Co(CN)₅^2-]-G_t^o[Co(CN)₅Cl^3- arising from G_t^o[Co(CN)₅Cl^3-]> G_t^o[Co(CN)₅^2-]. Moreover, only a small tendency to extrema in the enthalpies and entropies of activation is found with both these cosolvents, as was also found with added methanol or ethane-1,2-diol, but unlike the extrema found when hydrophobic alcohols are added to water. With the latter, much greater negative values for G_t^o[Co(CN)₅^2-]-_t^o[Co(CN)₅Cl^3-] are found. When GG_t^o[Co(CN)₅^2-]-GG_t^o[CO(CN)₅Cl^3-] becomes low enough not to compensate for the positive G_t^o(Cl^-), as with added hydrophilic glucose, the reaction is retarded. Compensating contributions of the various G_t^o(i) involved in the Gibbs energy cycle with added methanol or ethane-1, 2-diol allow log (rate constant) to vary linearly with the reciprocal of the relative permittivity of the medium.