Calculation of K from Thermochemical Data

To find K, we typically use available thermochemical data (Agj, A/ij, or permutations of these pure species properties), which allow us to calculate the Gibbs energy of reaction. We then solve for K via Equation (9.15). We will first look at how to calculate K from Ag at298 K then we will examine how to determine Ag at any T. Appendix F pro- 

The most common thermochemical data available to calculate the equilibrium constant are in the form of the Gibbs energy of formation, Agf Appendix A.3 shows some representative values for 25°C and 1 bar. The Gibbs energy of formation is defined analogously to the enthalpy of formation, introduced in Section 2.6. It is equal to the Gibbs energy of reaction when the species of interest is formed from its pure elemental constituents, as found in nature, that is, 

The Gibbs energy of formation of a pure element, as it is found in nature, is identically zero. 

if Gibbs energies of formation are available for all the species in the chemical reaction of interest, the Gibbs energy of reaction can be determined by scaling each species Agy by its stoichiometric coefficient. In summary. 

SOLUTION The equilibrium constant can be found from the Gibbs energy of formation. In this case, Equation (9.17) can be written as follows  

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