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e, 616 ET * 1987 No. 4 The Compensation Effect on the Ionic Hydration in Thermodynamics Naokata TAKEYAMA* and Kimie NAKASHIMA Department of Industrial Chemistry, Faculty of Science and Engineering, Saga University Honjyo-machi, Saga-shi 840 Japan Four linear equations have been found from the 'data of 64 ions at 298 K between the standard enthalpy and entropy of. hydration, 4Hh and 4S These These 'equations can be expressed in terms of r (A2) using the following effective ionic radius re as introduced in the previous paper (J. Phys. Soc. Jpn., 55, 1021(1986)) : 4H h=4ll0-70. 7 z I fr: (kj mol-') ( a) where 4,,l/h0 is the constant,characteristic of a group of ions, and d z f is an effective ionic charge. According to the equations, 64 ions are classified into Group I (14 ions ; Mrh=-163,o kj mol-'and)z f=1),group I ' (9 ions;2111,1,0= 168 kj.rnol-' and I zi f=1/3 or 2/3), 'Group II (15 ions ; 4H0= 265 kj mol-' and I z I f=2), and Group III (26 ions ; 4H0=0 and I z 11=3), in which H+ is fit for Group Ill, ncj. SO427 ta]es I z I f-=-- 3/2 in Group II. Using Eq. (a) and 45;,= 26. 1 r: (J mol-i K-') from the,,,,previous result, the standard Gibbs energy of hydration at 298 K can 1)e expressed as 4Ch= i/hh,0 (70. 7 I, z, I f 7. 78) r: (kj mo1-1) (b) to examine the individual ionic values estimated in comparison with the thermodynamic values. The second term on the right side of Eq. ( b ) describes the compensation effect on the ionic hydration through the mediation of r2 Moreover, the second term on the right side of Eq. ( a ) can be ascribed to an ion-water dipole interaction energy, by which a restriction is imposed between re and the distance R from the central ion to the water dipole ; e R re =2. 74A.2 with E representing the relative permittivity' of the hydration sphere. For 51 monoatomic ions, the following linear relationship was 'obtained between the value of.v e R or 2. 74/7-0,(ii) and the Pauling ionic radius r;i,/r=2 7417-0=A+Br, (c) where A and B are the constants depending mainly on the,absolute magnitude of, ionic charge ;1z1.