The Thermodynamics of Ion-Solvent Interactions

A very simple experiment that has been carried out for many electrolytes in water is the measurement of the enthalpy associated with the dissolution of the electrolyte, which is often a solid, in water. This process can be either exothermic or endothermic, and has an enthalpy change which depends on the relative amounts of electrolyte and water. By studying the enthalpy of solution for one mole of electrolyte as a function of the number of moles of water, which increase from one experiment to the next, one can determine the enthalpy of solution associated with the formation of an infinitely dilute solution. In the case of NaCl, the relevant process is 

A much more suitable point of reference for assessing ion-solvent interactions is the unsolvated ion in the gas phase. Thus, one would prefer to know the enthalpy change associated with the reaction 

Thermodynamic data for electrolyte solvation have been found experimentally for many different electrolytes. Ultimately, one would like to be able to analyze these results further to obtain separate contributions from the cation and anion. However, that is not possible without making an extrathermodynamic assumption. As a result, a scale of single ion solvation parameters has been defined relative to those for the H ion. For example, the enthalpy associated with the process 

Another way of describing solvation parameters on the conventional scale is by means of an exchange reaction between the ion in question and the proton. Thus, the solvation parameters for the Na ion are defined by the reaction 

From thermodynamic tables, the enthalpy of formation of H in an infinitely dilute aqueous solution is zero by definition the same quantity for Na from tabulated data is -240.1 kJmol On the basis of these results, the enthalpy associated with reaction (3.3.5) is 686.7 kJmol This result agrees within experimental error with that obtained by comparing the heats of formation of infinitely dilute aqueous solutions of NaCl and HCl. 

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