Ions, absolute properties entropy

The entropy of dissolved species is approached quite differently. We cannot know (and do not need to know) their absolute entropies. Instead, we consider the dijfferences in S (or AS) between an aqueous species and ion, assuming St for the H" (aq) = 0. Treatment of the thermodynamic properties of aqueous species is addressed in more detail later. 

The lack of information on the thermodynamic properties of the alkylammonium makes it impossible to calculate absolute values for the AH1, AHh, AS1, AS71 terms. Taking into account the hydrophobic nature of the Pr ion, it seems reasonable to assume that HFrh is smaller in absolute values than HXa7t. The hydration of ions being an exothermic effect, the AHh is expected to yield a negative value. The entropy of hydration is also negative and increases in absolute value when the ion is more hydrated. Thus, AS7 also will be a negative quantity. 

An important quantity required for analysis of ion-solvent interactions and structural properties is the absolute free energy (or enthalpy and entropy) of solvation. Most methods of obtaining these quantities involve some extra-thermodynamic assumption such as the extrapolation of solvation energies versus some function of crystal radii (see sect. 2.11.4). The method based on measurements of volta potential differences avoids the controversy involving the significance of these radii. This method has been used by Frumkin, Klein and Lange, Randles and Parsons et... 

We have now acquired all the tools with which to perform one of the most practical calculations of chemical thermodynamics determining the equilibrium constant for a reaction from tabulated data. Example 13-10, which demonstrates this application, uses thermodynamic properties of ions in aqueous solution as well as of compounds. An important idea to note about the thermodynamic properties of ions is that they are relative to H" (aq), which, by convention, is assigned values of zero for AfH°, AfG°, and S°. This means that entropies listed for ions are not absolute entropies, as they are for compounds. Negative values of S° simply denote an entropy less than that of H (aq). 

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