Proton hydration enthalpy

Finally, let us point out that the absolute standard electrode potential value of the couple H+w/H2(g) is actually about 4.5 V. This value cannot be verified since we cannot measure an absolute potential. It was obtained by using thermodynamic cycles, taking into account some thermodynamic data such as the proton hydration enthalpy and entropy. These last ones have been approached by considering the quadrupole model of water (see Chap. 1). It is quite evident that the value of 4.5 V differs considerably from the conventional one (0.00 V). However, it does not change the redox phenomena provision since only the standard electrode potential differences are taken into account. 

This is an exothermic process, due largely to the large hydration enthalpy of the proton. However, unlike the metallic elements, non-metallic elements do not usually form hydrated cations when their compounds dissolve in water the process of hydrolysis occurs instead. The reason is probably to be found in the difference in ionisation energies. Compare boron and aluminium in Group III ... 

It also includes the enthalpy of ionization of the hydrogen atom (equal to, but opposite in sign to, the electron attachment enthalpy of the gaseous proton), the enthalpy of atomization of dihydrogen and the enthalpy of hydration of the proton. The enthalpy of formation of the cation is estimated by use of the equation ... 

Much the same information can be displayed on an enthalpy diagram, in which we plot the enthalpies of formation (relative to the proton) of aqueous species instead of free energies. This facilitates the inclusion on the diagram of unknown and presumably unstable species, making reasonable assumptions concerning their hydration enthalpies. As an example, Fig. 5.5 presents enthalpy diagrams for M"+(aq) (M = Al, Tl) at a pH of zero. The enthalpy changes are AH° values for the processes ... 

W. A. Donald and E.R. Williams, An improved cluster pair correlation method for obtaining the absolute proton hydration energy and enthalpy evaluated with an expanded data set, J. Phys. Chem. B 114 (2010), pp. 13189-13200. 

FIGURE 3.2.13 Standard hydration enthalpy and activation enthalpy Ea of proton mobility... 

A hypothetical dissociation of the compound EOH can be allowed, giving three fragments E + 0 + H". This arranges a competition for the doubly negative oxide ion between the proton and the ion E, the outcome of which will depend on the electrostatic attractions and the subsequent hydration enthalpies of the two ions produced. The two possibilities are EO (aq) + H (aq) if the attraction between E and is dominant, and E (aq) + OH"(aq) if the proton has the greater claim on the oxide ion. In both cases the added contributions to the exother-micity of the overall outcome from the enthalpies of hydration of the ions are important. If EO (aq) + H (aq) are the energetically preferred products, the oxide is called acidic. If E (aq) + OH (aq) are energetically preferred, the oxide is called basic. 

Sr- and Mg-substituted LaGaOs (LSGM) is another member where no protons are found [31]. Lattice modeling of this material has indeed predicted the hydration enthalpy to be positive (endothermic) [32]. 

First, we examine the sequential hydration of a doubly protonated diamine, H3N(CH2)12NH2+. The sequential enthalpies, Atf n, free energies AG , and entropies obtained from van t Hoff plots of the hydration equilibria,62 are... 

It is quite difficult to measure an accurate enthalpy of solution A//( olutioni with a calorimeter, but we can measure it indirectly. Consider the example of sodium chloride, NaCl. The ions in solid NaCl are held together in a tight array by strong ionic bonds. While dissolving in water, the ionic bonds holding the constituent ions of Na+ and Cl- in place break, and new bonds form between the ions and molecules of water to yield hydrated species. Most simple ions are surrounded with six water molecules, like the [Na(H20)6]+ ion (VI). Exceptions include the proton with four water molecules (see p. 235) and lanthanide ions with eight. 

The general picture is such that the majority of excess protons are located in the central part of the hydrated hydrophilic nanochannels. In this region, the water is bulklike (for not too low degrees of hydration) with local proton transport properties similar to those described for water in Section 3.1.1.1.1. Therefore, the transport properties are indeed a function of the considered length and time scales,22 225 activation enthalpies of both... 

The conventional thermodynamic standard state values of the Gibbs energy of formation and standard enthalpy of formation of elements in their standard states are A(G — 0 and ArH = 0. Conventional values of the standard molar Gibbs energy of formation and standard molar enthalpy of formation of the hydrated proton are ArC (H +, aq) = 0 and Ar// (H +, aq) = 0. In addition, the standard molar entropy of the hydrated proton is taken as zero 5 (H+, aq) = 0. This convention produces negative standard entropies for some ions. 

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