Standard molar Gibbs function changes

It is AG, the standard molar Gibbs function change, that is directly related to the equilibrium constant. The two terms are related via the equation... 

Here, AjG° is the standard molar Gibbs energy change and is the equiUbrium constant for a reaction R is the gas constant (8.314 472 J K mol ). The subscripts T and 9 denote the temperature to which a quantity pertains, the subscript p denotes constant pressure, and the subscript r denotes that the quantity refers to a reaction. Combination of equations (1) and (2) yields the following equation that gives pK as a function of temperature ... 

The terms enclosed within parentheses in Eqs. (2.10) and (2.11) are respectively called the standard molar reaction Gibbs function change A,G and the standard chemical affinity A°. By setting up... 

In order to determine the standard molar reaction Gibbs function change ArG°, the standard states of the reaction reactants and products must be defined. They must be chosen or, at a minimum, they must correspond to physical states, such as the physical property differences between them being endowed with an unambiguous physical meaning. As an example, a possible standard state of a solute is the state in which its concentration is 1 mol/L and in which the solution it forms with the solvent is an ideal one. Standard states are chosen conventionally for practical reasons. Fortunately, the conventions are universally agreed upon. 

The fact that standard states are conventional may be somewhat troublesome. Indeed, the question immediately arises about equilibrium constant values with different arbitrarily chosen standard states. Quite evidently, when standard states other than the usual ones are chosen, the value of the standard molar reaction Gibbs function change ArG° is different according to Eq. (2.14), it is also the case with K°. However, this is not the case with A G and AGgyst, which remain constant for a given process regardless of the adopted conventions. Actually, in Eq. (2.12),... 

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) ... 

The molar standard-state free-energy change of a reaction (AG°) is a function of the equilibrium constant K) and is related to changes in the molar standard-state enthalpy (AH°) and entropy (A5°), as described by the Gibbs-Helmholtz equation ... 

It is interesting to note that the standard molar reaction Gibbs function A G° does not have the same mathematical status as ArG at each extent value. A G°, indeed, is not a derivative, unlike ArG. ArG is simply the system free enthalpy change AGsyst between two particular states, which happen to be the standard ones. 

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