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 PLUS ETHANOL AND WATER PLUS UREA
Kh. Halawani et Cf. Wells, 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 PLUS ETHANOL AND WATER PLUS UREA, Journal of solution chemistry, 27(3), 1998, pp. 273-284
The kinetics of the solvolysis of Co(CN)(5)Cl3- 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 solvo
lysis involves a rate-determining dissociative step corresponding clos
ely to a 100% separation, Co3+... Cl-, in the transition state, a Gibb
s energy cycle relating Gibbs energies of activation in water and in t
he mixtures to Gibbs energies of transfer of individual ionic species
between water and the mixtures, Delta 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 Delta G(t)(o)(Cl-) by
the negative term [Delta G(t)(o)[Co(CN)(5)(2-)]-Delta G(t)(o)[Co(CN)(5
)Cl3- arising from {Delta G(t)(o)[Co(CN)(5)Cl3-] > Delta G(t)(o)[Co(CN
)(5)(2-)]}. Moreover, only a small tendency to extrema in the enthalpi
es 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 {Delta G(t)(o)[Co(CN)(5)(2-)]
- Delta(t)(o)[Co(CN)(5)Cl3-] are found. When {Delta G(t)(o)[Co(CN)(5)
(2-)] - Delta GGP[Co(CN)(5)Cl3-]} becomes low enough not to compensate
for the positive Delta G(t)(o)(Cl-), as with added hydrophilic glucos
e, the reaction is retarded. Compensating contributions of the various
Delta G(t)(o)(i) involved in the Gibbs energy cycle with added methan
ol or ethane-1, 2-diol allow log (rate constant) to vary linearly with
the reciprocal of the relative permittivity of the medium.