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Kinetics of Aquation of 2+ Ions in Aqueous Solutions as a Function of Temperature and Pressure. The Isochoric Controversy and Analysis of Equilibrium and Kinetic Data

DOI: 10.1039/f19898503733

Source and publish data:

Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases p. 3733 - 3746 (1989)

Update date:2022-08-11

Topics:

Authors:

Blandamer, Michael J.Blandamer, Michael J.

Burgess, JohnBurgess, John

Cowles, Heather J.Cowles, Heather J.

Horn, Ian M.Horn, Ian M.

Engberts, Jan B. F. N.Engberts, Jan B. F. N.

et al.et al.

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Article abstract of DOI:10.1039/f19898503733

Rate constants are reported for the aquation of 2+ ions in aqueous solutions as a function of temperature, T, and pressure, p, yielding volumes and enthalpies of activation for the rate-determining loss of the first 5-Br-phen ligand.Data describing the dependence of molar volume of water on T and p are analysed to obtain T-p pairs at which the molar volume of water, V*(H2O; 1) equals the molar volume at 298.15 K and 101325 N m-2; i.e. under an isochoric condition which is extrinsic to the aqueous solution containing the iron(II) complex.Kinetic data for the aquation reaction are analysed to obtain rate constants, k, and related activation parameters under corresponding extrinsic isochoric conditions.This quantitative approach is applied to data describing the dependences of equilibrium constants characterising the acid dissociation of ethanoic acid(aq), the first ionization constant for orthophosphoric acid, and self-dissociation of water.The treatment is also applied to kinetic data describing the solvolysis in aqueous solution of benzyl chloride.For the three chemical equilibria the dependences are calculated of lnK<*>, enthalpy ΔrHinfinite and volume of reaction ΔrVinfinite on T at fixed p, on p at fixed T, on T with p changing to keep V*(H2O; 1) constant, and on p with T again changing to hold V*(H2O; 1) constant.Similar treatments are applied to lnk, enthalpies and volumes of activation.The procedures show how the suggestion made by Evans and Polanyi in 1935 can be acted on, at the same time rataining thermodynamic consistency with respect to standard and reference states for solutes in solution.Differences between isochoric *(H2O; 1) and both isobaric and isothermally derived parameters are discussed in terms of the role of solvent structure in controlling reaction and activation.The isochoric *(H2O)> dependence of rate constants on temperature for aquation of the above iron(II) complex differs from the corresponding isobaric dependence.The pattern for the solvolysis in aqueous solutions of benzyl chloride differs from that for the iron(II) complex.The differences are consistent with the changes in hydrophilic-hydrophobic character for a given substrate on going from initial to transition state.

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