Enthalpies and Gibbs Free Energies of Formation

To find a numerical value for AHi, we need to know ArH° at one temperature, while evaluation of I requires ArG° at one temperature. The usual choice is to obtain ArH° and ArG° at T = 298.15 K from standard molar enthalpies of formation and standard molar Gibbs free energies of formation. Earlier in this chapter we referred to examples of these quantities. It is now time to define AfH° and AfG° explicitly and describe methods for their measurement. 

The formation reaction is the one in which the elements in their stable form, as they occur in nature at T — 298.15 K, react to form the chemical substance. The following are examples of formation reactions  

Standard molar enthalpies of formation, AfH°m, and standard molar Gibbs free energies of formation, Af(7, are useful, since they can be used to calculate ArH° and ArG°. The relationships are 

Extensive tables can be found in the literature giving values at T = 298.15 K for AfH°m and AfG (usually along with and C° m) from which ArH° and ArG0 can be obtained. We will now describe some of the methods used to determine AfH°m and AfG°.  

JTwo of the best sources are D. D. Wagman. W. H. Evans. V. B. Parker, R. H. Schumm, 1. Halow, S. M. Bailey, K. L. Churney. and R. L. Nuttall. The NBS Tables of Chemical Thermodynamic Properties. Selected Values for Inorganic and C i and C Organic Substance in SI units , J. Phys. Chem. Ref. Delia, 11. 1982. Supplement No. 2. 

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Table 9.2 Standard heat capacities, entropies, enthalpies, and Gibbs free energies of formation of some common ions in aqueous solution at T= 298.15 K...

TABLE 7.2 Standard Enthalpies and Gibbs Free Energies of Formation (Values are joules per mole of the substance formed)... 

This example illustrates the principle that values of AG° and AH0 may be calculated from values of the enthalpies and Gibbs free energies of formation of the products and reactants. In more general form,... 

Standard enthalpies and Gibbs free energies of formation at 25 °C and 101.3 kPa are as follows. 

Estimates of Enthalpy and Gibbs Free Energy of Formation from Elements by Additivity ... 

We represent A///° and A/G° as the standard enthalpy and Gibbs free energy changes for the reaction in which the chemical substance is formed from the elements in their stable form, as they occur in nature at T = 298.15 K.rr For ions in solution, the values tabulated are relative to the standard enthalpy and Gibbs free energy of formation of the H+ ion being set equal to zero.ss... 

See Section 9.2, Chapter 9, Principles and Applications. Values for A///0 and AfG° at T= 298.15 K for selected substances and ions are tabulated in Tables A5.2 and A5.3 of Appendix 5 of this volume, with some AjH° values also given in Table A5.1 of the same appendix. ss See Section 9.2b, Chapter 9, Principles and Applications. Values for the standard enthalpies and Gibbs free energies of formation of selected ions at T = 298.15 K. are summarized in Table A5.3 of Appendix 5 of this volume. 

Table 5.1 Enthalpies and Gibbs (Free) Energies of Formation,...

ENTHALPIES AND GIBBS (FREE) ENERGIES OF FORMATION, ENTROPIES, AND HEAT CAPACITIES... 

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