Gibbs energies formation, hydrated

Estimation of reasonably correct A,G" values [8] results from the sum of the electrostatic terms, Equations 4.13 and 4.14, and the cavity formation term, Equation 4.10. Such values are shown in Table 4.1. It should be noted that the ionic standard molar Gibbs energies of hydration need to be compatible with the corresponding enthalpies and entropies of hydration (see the following text) according to A G, = A /f,°°-TA Sj . The latter quantities are more directly available from experimental data, so that values adopted from Ref 8 are presented in Table 4.1... 

Fig. 6.5. The Gibbs energy for the formation of a hydrated cation from a gaseous cation (AGi) and for the formation of three complexes (A( 2, curves 1,2,3) from gaseous cation and ligands of various types. Curve 1 a complex with the hydrated cation will not be formed ...

The thermodynamically stable form of bulk platinum in oxygen saturated water at ambient conditions is the completely hydrated platinum(IV)oxide, Pt02 4H20, also referred to as platinic acid H2Pt(OH) 14yl5) with a standard Gibbs energy of formation of -84 kJ mol" The formation of this compound will be even more favoured in the case of incompletely coordinated platinum. 

Gibbs Energies and Enthalpies of Formation of Hydrated Ions... 

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. 

Table 2.3 contains the standard Gibbs energies of formation and the standard enthalpies of formation of a selection of main group cations at 25 °C. They refer to the formation of 1 mol dm- solutions of the cations from their elements and are relative to the values for the hydrated proton taken as zero. 

The basis of the estimations of the absolute enthalpies of hydration of the main group ions is dealt with extensively in Chapter 2. In this section, the same principles are applied to the estimation of the enthalpies of hydration of the monatomic cations of the transition elements, i.e. those of the ions M" +. The standard enthalpies of formation of the aqueous ions are known from experimental measurements and their values, combined with the appropriate number of moles of dihydrogen oxidations to hydrated protons, gives the conventional values for the enthalpies of hydration of the ions concerned. Table 7.4 contains the Gibbs energies of formation and the enthalpies of formation of some ions formed by the first-row transition elements, and includes those formed by Ag, Cd, Hg and Ga. 

In this section two prediction techniques are discussed, namely, the gas gravity method and the Kvsi method. While both techniques enable the user to determine the pressure and temperature of hydrate formation from a gas, only the KVSI method allows the hydrate composition calculation. Calculations via the statistical thermodynamics method combined with Gibbs energy minimization (Chapter 5) provide access to the hydrate composition and other hydrate properties, such as the fraction of each cavity filled by various molecule types and the phase amounts. 

Note that gw0 is the molar Gibbs energy of formation of the standard hydrate at the reference conditions (To and Pq), ht is the molar enthalpy, and vt is the molar... 

The Gibbs energy minimization method allows for calculations of the formation conditions for any phase (including the hydrate). It also allows for the calculation of phases present at any T and P (whether hydrates are present or not). Therefore, included are the options to perform all thermodynamic calculations with every phase and not just the hydrate. The types of calculations, combined with plotting capability, included in CSMGem are... 

Evaluation of the constants J and I by application of Eqs. (15.20) and (15.21) at 298.15 K requires values of AH s and AG 98 for the hydration reaction. These are found from the heat-of-formation data of Table 4.4 and the Gibbs-energy-of-formation data of Table 15.1 ... 

Today, all modem hydrate prediction programs are carried out using this new technique, which is based on Gibbs energy minimization. It is extremely useful to determine phase firaction in the aforementioned cases of two-phase hydrate formation (without a free water phase), one can use the Gibbs energy minimization to estimate whether there will be a sufficient hydrate amount to form a blockage in a process. 

Table 6.5 Solubilities and values of the changes in Gibbs energy, enthalpy and entropy of solution at 298 K for the halides of sodium and silver the entropy change is given in the form of a TASsot° term (T = 298 K). Hydrate formation by solid NaBr, Nal and AgF has been neglected in the calculation of AsoiG° for these compounds.

Specific effects described below are known for shifting the equilibrium in the direction of either the T or the R state. These effects are explicable in terms of the AT -mechanism for moving the T,-divide and the approximately equivalent Gibbs free energy of hydrophobic association, AGha, and its component the apolar-polar repulsive free energy of hydration, AGap. In all cases ion-pair formation associated with hydrophobic domains drives hydrophobic association. 

---