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

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The kinetics of the solvolysis of Co(CN)5Cl3− 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, ΔGot(i), can be applied. The acceleration of the reaction found with both these cosolvents results from the compensation of the retarding positive ΔGot(Cl−) by the negative term [ΔGot[Co(CN)2−5]-ΔGot[Co(CN)5 Cl3− arising from {ΔGot[Co(CN)5Cl3−] > Δ Got[Co(CN)2−5]}. 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 {ΔGot[Co(CN)2−5]-Δot[Co(CN)5Cl3−] are found. When {Δ Got[Co(CN)2−5]-Δ GGot[Co(CN)5Cl3−]} becomes low enough not to compensate for the positive ΔGot(Cl−), as with added hydrophilic glucose, the reaction is retarded. Compensating contributions of the various ΔGot(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.

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