Formation reaction standard Gibbs-energy change

The standard Gibbs-energy change of reaction AG° is used in the calculation of equilibrium compositions. The standard heat of reaclion AH° is used in the calculation of the heat effects of chemical reaction, and the standard heat-capacity change of reaction is used for extrapolating AH° and AG° with T. Numerical values for AH° and AG° are computed from tabulated formation data, and AC° is determined from empirical expressions for the T dependence of the C° (see, e.g., Eq. [4-142]). 

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... 

According to reaction (1) the standard Gibbs energy change -AGi°, allowing calculating the standard anode potential E° = -jp- - can be evaluated from the standard energy of formation AGf... 

Standard changes in the Gibbs energy for a proposed reaction at 298 K may be calculated from tables of standard Gibbs energies of formation, if a value for the reactant is available. 

If we know the standard Gibbs energies of formation of each substance for the original system AfG and the reaction system AfG, the change in standard Gibbs free energy AG is calculated by... 

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... 

---