Thermochemical data from electrode potentials

Solubility products can be derived indirectly from standard electrode potentials and other thermochemical data, and directly from tabulated standard Gibbs energies of formation, AfG°, of the ions in aqueous solution [12]. Thus, the use of... 

Standard electrode potential data are available for an enormous number of halfreactions. Many have been determined directly from electrochemical measurements. Others have been computed from equilibrium studies of oxidation/reduction systems and from thermochemical data associated with such reactions. Table 18-1 contains standard electrode potential data for several half-reactions that we will be considering in the pages that follow. A more extensive listing is found in Appendix 5. ... 

These have been obtained as photoemission work function w (cf. Fig. 1) at the electron electrode equilibrium potential (which for hexamethylphosphotriamide and liquid ammonia was measured by experiment, and, for water, calculated from the thermochemical data ) by making a correction for the ideal gas entropy according to Eq. (3) ri . It should be noted that the aforementioned value, computed in this manner, is independent of the solvated electron concentration (for the same standard concentration of localized and delocalized electrons). 

DuBois and co-workers introduced the thermochemical cycle in Scheme 4 as a means to determine the hydride donor power (AG°h-) or hydricity, of a cationic metal hydride. The method requires the knowledge of metal-hydride acidity (p- a) data and the electrode potentials for the oxidation of the metal-hydride conjugate base to two-electron oxidized counterpart, either by two successive one-electron processes (Equation (10), Scheme 4), or by one two-electron process (Equation (11), Scheme 4). The thermochemical cycle is derived from one that was introduced by Parker and co-workers for use in organic systems/ The accuracy of values on an absolute scale rests upon the... 

Scheme 4 Thermochemical cycle for determination of metal-hydride hydricities from pKa and electrode potential data. The quantity C in Equations (10) and (11), when electrode potentials and pKa data are obtained in the same solvent and E° data are referred to the Cp2Fe/Cp2Fe scale, is 333 kJ mor (solv = MeCN) and 387 kJ mor (solv = DMSO).

The thermochemical cycle in Scheme 6 has been used to estimate the effect of a one-electron oxidation on the thermodynamic acidities of metal hydrides. The method has been similarly used on organic systems. Measurement of the oxidation potentials for the metal hydride and its conjugate base gives access to relative values for the metal hydride and its one-electron oxidized counterpart through Equation (14), Scheme 6. In many reported cases, one (or even both) electrode potentials are obtained from chemically irreversible voltammograms, with consequential uncertainties in the derived thermodynamic data. Table 7 gives a comprehensive list of M-H data comparing MH and MH species as determined with this thermochemical cycle. 

Thermodynamic cycles involving standard electrode potentials obtained by cyclic voltammetry have also been used to provide thermochemical information on organometallic compounds. This so-called electrochemical method leads to Gibbs energies of reaction in solution, from which bond dissociation enthalpies may be derived using a number of auxiliary data that are often estimated. For example, the derivation of a metal-hydrogen bond dissociation enthalpy in an L MH species requires (i) an estimate of the reduction potential of in the same solvent where the experiments were carried out (ii) an estimate of the solvation entropies of L MH, L M, and H and (iii) the knowledge of the pK of... 

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