below a table in my thermo book is given the standard molar chemical exergy (kJ/kmol) of selected substances at 298K and p_0
The table contains Model I & Model II
the difference between the models is
"in model I p_0= 1.019 atm. This model attempts to impose a criterion that the reference environment be in equilibrium. The reference substances are determined assuming restricted chemical equilibrium for nitric acid and nitrates and unrestricted thermodynamic equilibrium for all other chemical components of the atmosphere, the oceans,and a portion of the Earths crust. The chemical composition of the gas phase of this model approximates the composition of the natural atmosphere."
" in model II, p_0 =1 atm. in developing this model a reference substance is selected for each chemical element from among substances that contain the element being considered and that are abundantly present in the natural environment, even though the substances are not in completely mutual stable equilibrium. An underlying rationale for this approach is that substances found abundantly in nature have little economic value. On an overall basis, the chemical composition of the exergy reference environment of model II is closer than model I to the composition of the natural environment, but the equilibrium criterion is not always satisfied."
so, is the a certain model that should be used in a given situation based on the notes above?