ISSN:
1572-8927
Keywords:
Solvolysis kinetics
;
water + cosolvent
;
CoIII complexes
;
Gibbs energies
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
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, ΔG t o (i), can be applied. The acceleration of the reaction found with both these cosolvents results from the compensation of the retarding positive ΔG t o (Cl- by the negative term [G t o [Co(CN) 5 2- ]-ΔG t o [Co(CN)5Cl3- arising from ΔG t o [Co(CN)5Cl3-]〉 ΔG 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 ΔG t o [Co(CN) 5 2- ]-Δ t o [Co(CN)5Cl3-] are found. When ΔG G t o [Co(CN) 5 2- ]-ΔG G t o [CO(CN)5Cl3-] becomes low enough not to compensate for the positive ΔG t o (Cl-), as with added hydrophilic glucose, the reaction is retarded. Compensating contributions of the various ΔG 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1022644419971
Permalink
