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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Authors: | Kamal H Halawani Cecil F Wells |
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Institution: | (1) Department of Chemistry, Faculty of Applied Science, Umm Al-Qura University, Makkah, P.O. Box 3711, Kingdom of Saudi Arabia;(2) School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, England |
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Abstract: | 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, 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)5Cl3- arising from G
t
o
Co(CN)5Cl3-]> 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)5Cl3-] are found. When G
G
t
o
Co(CN)
5
2-
]- G
G
t
o
CO(CN)5Cl3-] 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. |
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Keywords: | Solvolysis kinetics water + cosolvent CoIII complexes Gibbs energies |
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