Gibbs energy of hydration

Privalov, P. L., and Makhatadze, G. I., 1993. Contributions of hydration to protein folding thermodynamics. II. The entropy and Gibbs energy of hydration. y(9wra z/ of Molecular Biology 232 660-679. 

Large differences in cation and anion radii, mirrored by large differences in their enthalpies and Gibbs energies of hydration. 

Furthermore, what is the real origin of hydrophobieity (or solvophobieity), that is, whieh moleeular property of water is primarily responsible for the positive Gibbs energy of hydration of nonpolar solutes and their tendeney to assoeiate The two main physieal explanations are ... 

Table 2-8. Standard molar Gibbs energies of hydration, AG+, , of some representative single ions at 25 °C [241, 242] ).

Considering the first equality, Is measurable. It Is the Isothermal reversible work required to extract ions from phase a for electrons In a metal It represents the electronic work function. For metals, a, can also be obtained from thermo-emission or the photo-electric effect. Sometimes a is called the real (Gibbs) energy of hydration of ion i. The logic behind this last definition stems from the second equality In [3.9.61. The standard molar Gibbs energy of solvation of an Ion [1.5.3.11 equals when Is referred to the gas... 

Table 3.7. Real potentials of monovalent ions in aqueous solution, after Randles, ionic Gibbs energies of hydration taken from table 1.5.4, and the difference between them. All data in kJ mole" (25 mV in x " and a corresponds with 2.4 kJ mole" in Fx and Faj). The sign convention is such that a positive value of x " implies that water is positive with respect to air.

Subsequently, there has been no lack of Improved methods to narrow down the uncertainty margin. These approaches invoke information on standard potentials, on improved models to establish individual ionic activities (to determine x " only for one ion is such a datum required) or individual (Gibbs) energies of hydration. For instance, Alfenaar and de Ligny" experimented with... 

The result obtained with Fe(III) is not the only example of discrepancy between the results obtained using data from Table 2.2 and literature data. According to Ref. [18] Ni(OH)2 is more stable than NiO, while the present data suggest an opposite, but the positive Gibbs energy of hydration calculated using the data from Table 2.2 is very small. 

This equation is often called the Nernst equation for the ITIES, and the term A is in fact the standard Gibbs energy of transfer AG°t ° expressed on a potential scale, since, by definition, AG is equal to the difference between the standard Gibbs energy of solvation fif0 and the standard Gibbs energy of hydration q. 1 ... 

In this section, we are mainly concerned with redox processes in aqueous solution see Section 16.1 for a list of relevant topics already covered in the book. Values of E° for halfreactions 16.78 can be measured directly for X = Cl, Br and I (Table 16.1) and their magnitudes are determined by the X—X bond energies (Figure 16.3), the electron affinities of the halogen atoms (Table 16.1) and the standard Gibbs energies of hydration of the halide ions (Table 16.1). This can be seen from scheme 16.79 for X = Br and I, an additional vaporization stage is needed for the element. 

Average number of water molecules in the electrostricted hydration shell, estimated from standard molar Gibbs energies of hydration [121]. 

The thermodynamic parameters of hydration for many ions have been determined [121,125,126]. Table 3 gives the values of the standard molar Gibbs energy of hydration AGh and standard molar enthalpy of hydration AH , at 25°C for the alkali metal cations. The tabulated values are based on the respective choices A= - 1056 kj/mol and AHh(H ) = -1103 kj/mol, which result from the extrathermodynamic assumption that the thermodynamic parameters of the tetraphenylarsonium cation and tetraphen-ylborate anion are equal [127]. This reasonable and useful assumption, often... 

Cs (g) q Na (S)[Eq. (10)]. Values of4G s(Na) and 4 (7s(Cs) were obtained from the relation AG° = AGl AGh [Eq. (13)], where the standard molar Gibbs energies of hydration AGh were taken from Table 3 and the standard molar Gibbs energies of transfer AG°r were taken from columns 4 and 7. 

Note that the ratio ot the Born entropy of hydration to the Born enthalpy of hy.dration is extremely smalt This is not justified by the treatment ol entropies of hydration of ions given in Section 2,7 2. but IS indicative of the relatively secondary importance of the hydraticn entropy in determining the Gibbs energy of hydration. The Born equations assume that the water solvent is a continuous medium and give no credence to its unique properlies. 

Plyasunov AV, Shock EL. (2000). Standard state Gibbs energies of hydration of hydrocarbons at elevated temperatures as evaluated from experimental phase equilibria studies. Geochimica et Cosmochimica Acta 64(16) 2811-2833. 

The standard molar Gibbs energy of hydration of an ion, AhydrGi° , can now be obtained from a combination of the standard molar enthalpy and entropy of hydration ... 

The standard molar Gibbs energy of hydration of the hydrogen ion noted above, AhydrG° (H+, aq)= -l,064 7kJmol is compatible with the estimates -1,056 6 kJ mor (Marcus 1991) and -1,066 17 kJ mol (Conway 1978) but not with -1,113 8 kJ mol obtained from the cluster pair approximation used by Kelly et al. (2006). The assumptions involved in obtaining the latter value lead to a surface potential of water of Ax = 0.34 0.08 V (Marcus 2008), which, in turn, is not consistent with the recent estimate of Ax = 0.1 V (Parfenyuk 2002) deemed to be the most nearly correct one. 

So called real standard molar Gibbs energies of hydration are obtained from the electromotive force of specially constructed cells. These consist of a jet of aqueous solution flowing downward in the middle of a tube, along the inner surface of which another solution, concentric with the jet, flows with a narrow vapor gap between them. The measurable real standard molar Gibbs energy of hydration is ... 

The interaction of the solute ion with the water in its surrounding (step 4 in the thought process) is described by the electrostatic terms ATEiifc f) + AFei2(z, r). The first pertains to the first hydration shell of the ion and the second to water outside this shell. For the Gibbs energy of hydration these two terms read ... 

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