Entropy Changes Accompanying Hydration

The is the entropy change that accompanies the transition of an ion in the gas phase to an ion in solution (with the arbitrary neglect of any change in crossing the gas-liquid interface). Thus, 

The second term in the right-hand side of Eq. (2.86) is the entropy of the ion in the gas phase and the first term represents the total entropy change caused upon the entry of the ion into the solution. will be given as 

The substitution into Eq. (2.86) of the values of from Eqs. (2.87) and (2.88) gives the entropy ofhydration. Relevant terms will be evaluated next. 

One uses the Sackur-Tetrode equation for the translational entropy of a monatomic ion in the gas phase and it gives 

Relevant Parameters in Calculations oi the Entropy of ionic Hydration 

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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... 

P-Cyclodextrin undecahydrate undergoes a first-order transition at 226 K. accompanied by an entropy change of 45 JK mol The large transition entropy is related to disorder of the water molecules in the channel of cyclodextrin. Disorder of the water molecules is also involved in a clathrate-like hydrate crystal [H31014] [CdCu2(CN)7] studied by calorimetry and dielectric measurement. 

Perhaps the most significant finding reported in a series of articles," describing these research results, was that a large positive entropy change was found to accompany the association of calcium with polyphosphates. This positive entropy change was attributed to the release of waters of hydration upon association." The enthalpies of association for calcium complexes of the polyphosphates were found to be small. 

The higher the charge of die ion and the shmter its radius, die higher the change in the enthalpy during hydration. Hydration is accompanied by a decrease in the entropy of the system AS,. < 0. The dissolution of ions is acconqianied by a change in the solution volume. 

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