Standard molar exergy

Furthermore, pure oxygen gas, whose molar entropy is lower by an amount of / ln 0.2034 than that of gaseous oxygen molecules in the atmospheric wet air (xQl = 0.2034 ) at the standard temperature and pressure, possesses its standard molar exergy given by Eq. 10.26 relative to the atmospheric wet ain... 

In chemical thermodynamics the standard chemical potential ut of a compound i is defined as the molar free enthalpy Ag° for the formation of the compound from its constituent elements j in their stable molecular form in the standard state, and their chemical potential values are set zero in the standard state fit-Ag°f. In exergy engineering the standard molar exergy e° of a compound i is defined as consisting of the molar free enthalpy Ag°f for the formation of the compound in the standard state from its constituent elements and the stoichiometrical sum of the standard chemical exergy values e° of the constituent elements j in their stable state at the standard temperature T° and pressure p° ef- Ag°f + 2 vy e°. 

In calculating the numerical values of the standard molar exergy e°of chemical elements and compounds, we usually make clear the exergy reference species at zero level of exergy in our natural environment of the atmosphere, the hydrosphere and the lithosphere. 

From the foregoing discussion, it follows that the standard exergy of one of the reactants can be estimated by use of the standard affinity of the reaction, provided that we know the values of the standard exergy of the other reactants and products. The numerical values of the molar exergy thus obtained of various chemical substances in the standard state (temperature T° = 298 K, pressure p° = 101.3 kPa, activity a° = 1) are tabulated as the standard chemical exergy of chemical substances in the literature on engineering thermodynamics [Ref. 9.]. 

Table 10.1. Standard molar chemical exergy of a few substances relative to the reference species in the atmosphere [Refs. 9 and 11.].

In the case of solid substances the reference species is often set at the most stable solid compounds in lithospheric rocks. For example, metallic iron is most stable in the form of its oxides. The standard chemical exergy of metallic iron can then be obtained from the standard affinity Aaf of the formation of iron oxide, Fe +0.75O2 = 0.5Fe2O3 A° = e e + 0.75s 2 - 0.5 pe2Oj and = 0 hence e°c = A° -0.75e° . Table 10.3 shows the standard molar chemical exergy of a few substances relative to the solid reference species in the lithosphere at the standard temperature and pressure. 

TABLE I Standard Molar Chemical Exergy of Various Substances at 298.15 K and Pref... 

The standard molar chemical exergy of any substance not present in the environment can be determined using the change in the specific Gibbs function Ag for the reaction of this substance with substances present in the environment (Bejan, Tsatsaronis, and Moran, 1996 Moran and Shapiro 1998) ... 

Table A.l.Standard molar chemical exergy, (kJ/kmol), of various substances at 298.15 K and...

Table 9.11 Standard molar heat and stemdetrd molar exergy of some substances...

Table 15.3 Temperature, molar flow rate and exergy flow rate for the FP-FC system (at standard conditions see also Figure 15.2).

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