Gibbs extrathermodynamic assumptions

The following are the practical procedures for obtaining the Gibbs energies of transfer and transfer activity coefficients of ionic species based on the extrathermodynamic assumptions (i), (ii) and (iii) described above ... 

This fundamental parameter quantifies the relative affinity of an ion in the two phases, but it is not directly accessible experimentally because it is associated with a single ionic component. Therefore, to make AGf 1 0 or logP1,0 amenable to direct measurement, an extrathermodynamic assumption must be introduced such as the Grunwald or TATB assumption [139], which states that the cation and the anion of tetraphenylarso-nium tetraphenylborate (TPAs+TPB or TATB) have equal standard Gibbs energies for any pair of solvents [140,141] ... 

The thermodynamic parameters of hydration for many ions have been determined [121,125,126]. Table 3 gives the values of the standard molar Gibbs energy of hydration AGh and standard molar enthalpy of hydration AH , at 25°C for the alkali metal cations. The tabulated values are based on the respective choices A= - 1056 kj/mol and AHh(H ) = -1103 kj/mol, which result from the extrathermodynamic assumption that the thermodynamic parameters of the tetraphenylarsonium cation and tetraphen-ylborate anion are equal [127]. This reasonable and useful assumption, often... 

Upon making an extrathermodynamic assumption such as the TATB assumption, the standard Gibbs energy of the salt partitioning process given by Eq. (33) becomes simply... 

Standard molar Gibbs energy of solvation (kJ/mol) added subscripts el, neut, and unsym refer respectively to the electrostatic [Eq. (5)], neutral, and unsymmetric terms Standard molar Gibbs energy of transfer (kJ/mol) refers here to the transfer of an ion from pure water to pure solvent and employs the TATB extrathermodynamic assumption added subscripts el, neut, and unsym refer respectively to the electrostatic, neutral, and unsymmetric terms clarifying subscripts W—> S and W(S)—> S(W) distinguish between respectively transfer from pure water to pure solvent and partitioning from solvent-saturated water to water-saturated solvent... 

We close this section with a reminder of a fnndamental issue in electrochemistry Not all the quantities in Equations 13.8 throngh 13.13 are accessible to measurement by electrochemical or thermodynamic methods. Only the electrochemical potential ( i ), the work function (W ) or equivalently the real potential (a ) and the Volta potential ( / ) are. Equations 13.9, 13.11, and 13.13 are therefore formal resolutions. It is not possible to assign actual values to the separate terms, the chemical potential ( t ), the Galvani potential (cp ), nor the surface potential (x ), without making extrathermodynamic assumptions. These quantities must therefore be considered unphysical, at least from the point of view of thermodynamics. This statement, which is called the Gibbs-Guggenheim Principle in [42], is often met with disbelief from theoretical and computational chemists, particularly in the case of the chemical potential (Equation 13.10). The standard chemical potential is essentially the (absolute) solvation free energy AjG of species i. One would hope that a molecular simulation contains all information needed to compute AjG . Indeed, there seems to be a way around this thermodynamic verdict for computation and also mass spectroscopic. This continues to be, however, hazardous territory, particularly for DFT calculations in periodic systems. ... 

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