Standard Gibbs function of formation

CALCULATION OF A GIBBS FUNCTION OF A REACTION FROM STANDARD GIBBS FUNCTION OF FORMATION... 

We have discussed in some detail the various methods that can be used to obtain the standard Gibbs function of formation of a pure gaseous compound such as HCl(g). As many of its reactions are carried out in aqueous solution, it also is desirable to know AfGm for HCl(aq). 

Standard Gibbs Function of Formation of Individual Ions HCI... 

With this definition it is possible to calculate the standard Gibbs function of formation of other ions. For example, for Cl ion, we proceed by adding appropriate equations to Equation (20.44). For the reaction... 

From the standard Gibbs function of formation of the aqueous electrolyte, we also can obtain that for the Na ion alone by subtracting Equation (20.46) from Equation (20.51). Thus, we obtain... 

The standard reaction Gibbs function ArG° can easily be evaluated from tabulated values of the standard Gibbs functions of formation (ArG ), as ArG° can also be expressed by the sum of the ApG° values of all products and reactants from the elements (and thus ArG° of elements is by definition zero) ... 

Figure 4.2.1 Stability diagram standard Gibbs functions of formation AFG°for selected species (for hydrocarbons, ApG° is related here to one carbon atom).

The standard reaction Gibbs function ArC° can be evaluated from the standard Gibbs functions of formation (ApC ) ... 

Figure 6.8.2 Stability diagram of hydrodesulfurization of dibenzothiophene (for principle aspects of a stability diagram based on the standard Gibbs functions of formation ApC° see Section 4.2.1).

Figure 1.10 The standard Gibbs energy of formation from the binary constitutent oxides of the kyanite, sillimanite and andalusite modifications of A SiOs as a function of pressure at 800 K. Data are taken from [3]. All three oxides are treated as incompressible.

TABLE 7.2. Standard Gibbs Function for Formation of a Compound at 298.15 K ... 

By convention, the standard Gibbs function for formation AfG of graphite is assigned the value of zero. On this basis, AfG gg of diamond is 2900 J mol Entropies and densities also are listed in Table 8.2. Assuming that the entropies and densities are approximately constant, determine the conditions of temperature and pressure under which the manufacture of diamonds from graphite would be thermodynamically and kinetically practical [2]. 

Having obtained the standard Gibbs function change accompanying the transfer of HCl from the gaseous to the aqueous state, we can add it to the standard Gibbs function for formation of gaseous HCl [11],... 

As it has been shown that the Gibbs function for formation of an individual ion has no operational meanings [12], no way exists to determine such a quantity experimentally. However, for the purposes of tabulation and calculation, it is possible to separate AfGm of an electrolyte arbitrarily into two or more parts, which correspond to the number of ions formed, in a way analogous to that used in tables of standard electrode potentials. In both cases, the standard Gibbs function for formation of aqueous H" " is defined to be zero at every temperature ... 

Solute Very Soluble Sodium Chloride. As the standard Gibbs function for formation of NaCl(s) is available [11], the AfGm 29s for NaCl(aq) can be obtained... 

Figure 3.4. Ellingham diagram showing standard Gibbs energy of formation of various oxides as a function of temperature.

We consider only aqueous solutions here, but the methods used are applicable to any solvent system. The standard Gibbs energy of formation of a strong electrolyte dissolved in water is obtained according to Equation (11.28). In such solutions the ions are considered as the species and we are concerned with the thermodynamic functions of the ions rather than the component itself. We express the chemical potential of the electrolyte, considered to be MVtAv, in its standard state as... 

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 following program calculate the function of ionic strength that gives the standard Gibbs energy of formation of a species. 

The input speciesmat is a matrix that gives the standard Gibbs energy of formation at 298.15 K, the standard enthalpy of formation at 298.15 K, the electric charge, and the numbers of hydrogen atoms in each species. There is a row in the matrix for each species of the reactant, gfnsp is alist of the functions for the species. )... 

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