Hydrated ions entropy changes

This expression took care of the compression entropy and the numerical coefficients pertain to ion hydration. The entropy change corresponding to the formation of the solvation shell was obtained from the temperature derivative of the Bom expression for the Gibbs energy of solvation (Section 2.3.1.4) ... 

Consider the following example. AG° for dissociation of acetic acid in water is +27.2 kj/mol (Table 6-5). The enthalpy change AH0 for this process is almost zero (-0.1 kj/mol) and AS0 is consequently -91.6 J I< This large entropy decrease reflects the increased amount of water that is immobilized in the hydration spheres of the H+ and acetate- ions formed in the dissociation reaction. In contrast, dissociation of NH4+ to NH3 and H+ converts one positive ion to another. AH0 is large (+52.5 kj/mol) but the entropy change AS0 is small (-2.0 J K-1, Table 6-5). 

In these calculations, the electron affinity of the methyl radical has been taken1 as 27 kcal.mole-1. The other enthalpy terms are all well-known quantities the enthalpies of hydration of individual ions have been assigned as done by Valis ev (see ref. 2) and the enthalpy of hydration of the gaseous methyl anion has been taken as that of the bromide ion. It can be seen from Table 1 that not only is the formation of the methyl anion energetically very unfavoured in the gas phase, but it is also endothermic to the extent of 54 kcal.mole-1 in aqueous solution. A check on this final result can be made by consideration of the standard entropy change for the reaction... 

Some of these categories of ions show distinctive structural and thermodynamic properties. The nonacidic metal cations and nonbasic anions are distinct from aU of the other categories, in that there is a positive entropy change accompanying their hydration as they dissolve from the solid state (Figure 2) they are known to physical chemists as electrostatic structure breakers because these large ions... 

The precipitation of salts according to Rule I is accompanied by a large favorable entropy change, as the strongly hydrated cations and anions release numerous waters of hydration. In contrast, the dissolution of salts according to Rule II is accompanied by very little entropy change, since one ion is a stmctme breaker, while the other is a structure maker the dissolution occurs because the ions are mismatched... 

In addition, every reaction which produces entropy generates disorder within the reacting substances. In the case under consideration, the entropy change is in the H20, in the small number of ions in solution, and in the interactions between H20 and the ions. H20 is the dominant component, and because it has a high degree of disorder, solution of carbonate minerals tends to order the H20, primarily by hydration of carbonate and calcium ions, thereby providing for a decrease in entropy in that part of the system. 

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