Thermodynamic Quantities Pertaining to Ion Association

Results for the standard thermodynamic functions, temperature derivatives thereof leading to and TA S° are shown in the review by Marcus and Hefter [29] for many electrolytes in several solvents. Some of these data are shown in Table 7.4. For the ion pairing of the thallium halides in the solvents shown in this table, the association is enthalpy driven, pointing to CIP formation with some coordinative bonding. For the other ion pairs shown, the association is entropy driven, for even if there are only 2SIPs being formed, fewer particles result in the solution than when only free solvated ions exist. 

The volume change on ion association has led to information regarding the stepwise pairing in Equation 7.21. According to Hemmes [25], when SIPs or CIPs are formed and solvent molecules are liberated from the solvation shell of the ions, the volume changes must reflect this process. The relevant quantity, is obtained from the pressure derivative of the association constant  

The -RTKj. term is needed for expressed in M . It may also be calculated according to the Bjerrum theory (Section 7.2.1) as  

TABLE 7.4 Thermodynamic Functions of Ion Pairing of Some Electrolytes in Several Solvents at 25°C  

Marcus [40,41 ] has used the V° data, some of which are shown in Table 7.5, to calculate the number of solvent molecules released when the ion pairing proceeds beyond the 2SIP stage to form a SIP and/or a CIP  

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