Standard Gibbs function

When adequate enthalpy and entropy data are available, the calculation of AG t- is a matter of substitution into Equations (7.26). 

Generally, data for AH and A5m are available at least at one temperature. The conversion of the data for the Gibbs function from one temperature to another can be carried out by the methods outlined in Chapter 7. 

Statistical thermodynamic methods and the use of spectroscopic information lead to the function (G r H o)/T, and this function is tabulated in Table 12.4. Tables 12.5 and 12.6 give the alternative function (G j-— 77m298.i5)/2 which is equal to (I r — 1 298.15). This method of tabulation avoids the use of empirical 

To calculate the Gibbs function for the formation of a compound C, we use Equations (12.32) through (12.36), which are analogous to the equations used to calculate Af// . In each case we use the relationship [Equation (11.9)] at 0 K 

To express the result in terms of the statistical thermodynamic expression, we use (GmT-—instead of //mo)- Hence, we have the following 

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From an equation like Equation (4.43), G = G + RT n(p/p ), so each G term in Equation (4.50) may be converted to a standard Gibbs function by inserting a term like Equation (4.43) ... 

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]. 

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

CALCULATION OF A STANDARD GIBBS FUNCTION FROM STANDARD ENTROPIES AND STANDARD ENTHALPIES... 

Now that we can calculate standard entropies from the third law (as in Chapter 11), let us consider their primary use, which is the calculations of the standard Gibbs function. 

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). 

Now we can obtain the standard Gibbs function change for Equations (20.36), because the sum of Equations (20.37) through (20.39) yields... 

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],... 

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

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 ... 

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... 

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... 

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... 

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