Gibbs energy change of formation

If Gf is arbitrarily set equal to zero for all elements in their standard states, then for compounds Gf = AG°, the standard Gibbs-energy change of formation for species i. In addition, the fugacity is eliminated in favor of the fugacity coefficient by Eq. (4-79),/ = yi jP. With these substitutions, the equation for becomes... 

Stability criteria, 449-454 Standard Gibbs-energy change of formation, 510, 512-513 of reaction, 504-510 effect of temperature on, 507-508 Standard heat (enthalpy change) of formation, 118-123 of reaction, 116-133, 505, 507-508 effect of temperature on, 123-127, 508 Standard state, 117-118, 399-400, 504-505, 515-518... 

The stable crystalline form of sulfur at room temperature is called rhombic sulfur, and another crystalline form is called monoclinic sulfur. The standard-state enthalpy change of formation of monoclinic sulfur at 298.15 K is 0.33 kJ mol , and the standard-state Gibbs energy change of formation of monoclinic sulfur is 0.096 kJ mol. Assume that both and AG° for the conversion of rhombic sulfur to monoclinic sulfur are tempCTature-independent. 

Values of standard-state Gibbs energy changes of formation for a number of substances are included in Table A.8 of Appendix A, and larger tables are available. This table also includes values of the function —— 7fm,298 >)/7, which can also be used to calculate AG° for a reaction, as is done in Example 7.1. This function generally varies more slowly with temperature than does AfG°. If a value of AG° is needed for a temperature that is not included in the table, interpolation of this function usually gives better accuracy than does interpolation in a table of AfG values. 

The standard-state Gibbs energy change for the reaction of Eq. (7.3-7) could be calculated from the Gibbs energy changes of formation of H2O, NaCl, NaOH, and HCl if the data were available. However, actual tables include data for separate ions instead of neutral electrolytes. We use the net ionic equation... 

Note. Most sources give these values at 25°C = 298.15Kfa298K [1, 3]. Same give them at exactly 298.00K. Some use 1 atm pressure, some use 1 bar (faO.987 atm). Some show the Gibbs energy change of formation from the elements as Ag°, but the A here leads to confusion with the... 

---