Kinetics of aquation of [Fe(5-Br-phen)3]2+ ions in aqueous solutions as a function of temperature and pressure

Abstract

Rate constants are reported for the aquation of [Fe(5-Br-phen)₃]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 101 325 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 In K^#, enthalpy Δ_rH^∞ and volume of reaction Δ_rV^∞ on T at fixed p, on p at fixed T, on T with p changing to keep V*(H₂O; l) constant, and on p with t again changing to hold V^*(H₂O;l) constant. Similar treatements are applied to In k, 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 retaining thermodynamic consistency with respect to standard and reference states for solutes in solution. Differences between isochoric [V^*(H₂O;l)] and both isobaric and isothermally derived parameters are discussed in terms of the role of solvent structure in controlling reaction and activation. The isochoric [V^*(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.