Hydrated protons solvation energy

From the definition of acidity in solution as presented in Eq. (7.4), it is clear that in order to compute AG° one must know the proton solvation energy. However, from the previous discussion on the structural models for the hydrated proton one may anticipate some difficulties. For example, how many water molecules should be considered in the calculation of the proton solvation energy In other words How large should one take the H (H20) cluster One reasonable approach should be to examine the convergence of... 

It has been mentioned that = E when the reference system is the oxidation of molecular hydrogen to solvated (hydrated) protons. The standard electrode potential of the hydrogen electrode is chosen as 0 V. Thermodynamically it means that not only the standard free energy of formation of hydrogen (/r ) is zero - which is a rule in thermodynamics (see Table 2) - but also that of the solvated hydrogen ion /U.S+ = O . (The old standard values of were calculated using = atm = 101325 Pa. The new ones are related to 10 Pa (1 bar). It causes a difference in the potential of the SHE of -i- 0.169 mV, that... 

Data indicative of the relative basicities of C=C and C=C bonds and relative solvation energies for protonation processes have been obtained from measurements of hydration rates of RCH=CH2 and RC=CH (R = H, Me, Bu ) in aqueous H2SO441. Enthalpies of hydration of a series of acyclic olefins producing tertiary alcohols have been determined42,43. 

Figure 14. Potential energy diagram for solvation-shell activation of the hydrated proton coupled with quantum-mechanical neutralization and transfer n h at an electrode (a) after Levich et al and (b) comparison with Bell s 2(a) approach and others (Ref. 26b).

We will attempt to (a) tabulate the available measured and derived thermodynamic data for the protonation of important representative bases in aqueous acids, superacids, and the gas phase (b) show how these data may be used to estimate the ionization ratios of the different bases in aqueous acid media, including their pA"-values in water at 25° and at different temperatures (c) estimate the solvation energies of the onium ions of the protonated bases (d) relate the acidity function and hydration behavior of the different classes of bases in order to provide the necessary data for a practical theory of acidity functions. 

A primary hydration number of 6 for Fe + in aqueous (or D2O) solution has been indicated by neutron diffraction with isotopic substitution (NDIS), XRD, 16,1017 EXAFS, and for Fe " " by NDIS and EXAFS. Fe—O bond distances in aqueous solution have been determined, since 1984, for Fe(H20)/+ by EXAFS and neutron diffraction, for ternary Fe " "-aqua-anion species by XRD (in sulfate and in chloride media, and in bromide media ), for Fe(H20)g by neutron diffraction, and for ternary Fe -aqua-anion species. The NDIS studies hint at the second solvation shell in D2O solution high energy-resolution incoherent quasi-elastic neutron scattering (IQENS) can give some idea of the half-lives of water-protons in the secondary hydration shell of ions such as Fe aq. This is believed to be less than 5 X I0 s, whereas t>5x10 s for the binding time of protons in the primary hydration shell. X-Ray absorption spectroscopy (XAS—EXAFS and XANES) has been used... 

According to the energy balance the oxidation of hydrogen requires almost 32 eV about 22 eV are provided by the hydration of the proton, 9-10 eV, twice the work function, by the metal, and the rest by the potential drop between the electrode and the bulk of the solution, which is the only part that we can control experimentally. Thus, solvation plays a dominant part in the energetics, and any model for the hydrogen reaction that neglects the solvent leaves out a most important part. 

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