Gibbs energy of formation calculations

The selected Gibbs energy of formation calculated from the selected equilibrium constants and auxiliary data are ... 

The corresponding standard molar Gibbs energies of formation, calculated using these constants and AfG° (Th", 298.15 K) = -(704.8 + 5.3) kJ-mol selected by this review are ... 

The excess function of the Gibbs energy of formation, calculated with Eq. (3.48),... 

In the case of hydrogen, for example, at a teiuperamre of 2500 K, the equilibrium constant for dissociation has the value, calculated from the tlrermo-dynamic relation between the Gibbs energy of formation and the equilibrium constant of 6.356 x 10 " and hence at a total pressure of 10 atmos, the degree of dissociation is 0.126 at 2500 K, which drops to 8.32 x 10 at 2000 K. 

A simple example of the analysis of multicomponent systems will suffice for the present consideration, such as the calculation of the components in a gaseous mixture of oxygen, hydrogen and sulphur. As a first step, the Gibbs energy of formation of each potential compound, e.g. S2, H2S, SO, SO2, H2O etc. can be used to calculate the equilibrium constant for the formation of each compound from the atomic species of the elements. The total number of atoms of each element will therefore be distributed in the equilibrium mixture in proportion to these constants. Thus for hydrogen with a starting number of atoms and the final number of each species... 

Molten halides are liquid electrolytes in many instances, and their decomposition may be canned out in principle to produce the metal and halogen, usually in the gaseous state. The theoretical decomposition voltage, E°, is calculated from the Gibbs energy of formation tlrrough the equation... 

The concentration of this species in liquid sulfur was estimated from the calculated Gibbs energy of formation as ca. 1% of all Ss species at the boihng point [35]. In this context it is interesting to note that the structurally related homocyclic sulfur oxide Sy=0 is known as a pure compound and has been characterized by X-ray crystallography and vibrational spectroscopy [48, 49]. Similarly, branched long chains of the type -S-S-S(=S)-S-S- must be components of the polymeric S o present in liquid sulfur at higher temperatures since the model compound H-S-S-S(=S)-S-S-H was calculated to be by only 53 kJ mol less stable at the G3X(MP2) level than the unbranched helical isomer of HySs [35]. 

The standard Gibbs energy of formation of NaCl is — 384 kJ mol 1 and that of NiCl2 is — 62kJmol 1. Calculate the ideal voltage of a ZEBRA cell. 

ArV is not necessarily positive, and to compare the relative stability of the different modifications of a ternary compound like AGSiOs the volume of formation of the ternary oxide from the binary constituent oxides is considered for convenience. The pressure dependence of the Gibbs energies of formation from the binary constituent oxides of kyanite, sillimanite and andalusite polymorphs of A SiOs are shown in Figure 1.10. Whereas sillimanite and andalusite have positive volumes of formation and are destabilized by pressure relative to the binary oxides, kyanite has a negative volume of formation and becomes the stable high-pressure phase. The thermodynamic data used in the calculations are given in Table 1.7 [3].1... 

The first step is to list the compounds of the type 1 component together with their Gibbs energies of formation at the chosen temperature, or the function [AH°(f, 298) + G°(T) - H°(298)] which is much easier to calculate in a data-bank. The two functions must not be used for different substances in the same calculation. 

This equation permits the calculation of AG for any arbitrary condition when AGe is known from a study of the equilibrium condition or calculable from standard Gibbs energies of formation. 

The first term on the right-hand side of this equation is the standard Gibbs energy of formation of methane, which is listed [2] as -50.460 kj/mol and thus EXch,cH4(g) can be calculated to be 831.6 kj/mol. Chapter 9 illustrates the use of this exergy value in the analysis of a natural gas-driven power station. 

Table 7.2 gives the standard chemical exergy of H20 and 02, respectively. Calculate with the help of Equation 7.13 the standard chemical exergy of H2. The standard Gibbs energy of formation for H20 can be found in [1],... 

In treating the fundamental equations of thermodynamics, chemical potentials of species are always used, but in making calculations when T and P are independent variables, chemical potentials are replaced by Gibbs energies of formation AfG . Therefore, we will use equation 3.1-10 in the form... 

In making actual calculations, standard formation properties are used rather than chemical potentials, and so the standard Gibbs energy of formation of an isomer group is given by... 

Thus the standard enthalpy of formation Af H I) and standard Gibbs energy of formation Af G (I) of an ionic species at 298.15 K in kJ mol 1 can be calculated using (Clark and Glew, 1980 Goldberg and Tewari, 1991)... 

This equation reproduces the third column of Table 3.1 to 1% accuracy. The calculations of these three functions are shown in Problem 3.5, and they are used in the calculation of standard Gibbs energies of formation and standard enthalpies of formation of species at other temperatures in Problems 3.6 and 3.7. 

The standard transformed heat capacity at constant pressure of a reactant is discussed later in Chapter 10 on calorimetry. The calculation of A H ° using equation 4.5-3 looks simple, but note that the standard transformed Gibbs energies of formation of all of the species are involved in the calculation. These equations were applied to the ATP series by Alberty and Goldberg (1992). 

When the reactant of interest consists of two species with different numbers of hydrogen atoms, the pK of the weak acid is needed to calculate ArC °(/ = 0) of the two species, and the calculation is more complicated. The standard transformed Gibbs energy of formation of a pseudoisomer group containing two species is given by... 

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