Extra-thermodynamic assumptions effect

The extra thermodynamic assumptions which have been proposed to split the medium effect for electrolytes, that is for electrically neutral combinations of ions, into values for individual ions are discussed in sect. 2.11.4b. 

The standard molar entropy of hydration of an ion is A 5" = 5 - 5, , the difference between its standard molar entropy in the aqueous solution (Table 2.8) and the standard molar entropy of the isolated ion in the ideal gas phase (Table 2.3). The latter, S°, are calculated from the third law of thermodynamics and spectroscopic data without invoking any extra-thermodynamic assumptions. The former, do involve the assumptions leading to A5 (H+, aq)=-22.2 2J K" mol" for the hydrogen ion (Section 2.3.1.2). With 5°(H% g)=108.9J K" mol", the standard molar entropy of hydration of the hydrogen ion is then A,5"(H ) = -22.2 2-108.9 =-131.1 2 J-K" mor. The standard molar entropies of hydration of ions are shown in Table 4.1, derived from A 5,°° = S -S,° but also obtainable from the conventional values by use of the absolute value of the hydrogen ion. They are related to the effect that ions have on the structure of water according to various approaches. This aspect is fully dealt with in Section 5.1.1.7. 

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