Actinide hydrated

E.N. Rizkalla and G.R. Choppin, Lanthanides and actinides hydration and hydrolysis 529... 

The hydration entropy for the ion A y8S°(M" ) represents the standard entropy change (usually at 298K) for the process M" (g)-> M" (aq). This property should reflect lanthanide-actinide differences because the final state represents the ion with all the water molecules in the primary and outer hydration spheres. Bratsch and Lagowski (1985b, table I) proposed a set of hydration parameters Ay and by which hydration entropies could be calculated for the lanthanides. Rizkalla and Choppin (1991, table 11) used these parameters to tabulate entropies of hydration for the lanthanides. However, it is not reasonable to extend these entropies for a lanthanide-actinide comparison because there are no experimental data from which independent actinide hydration entropy parameters Ay and can be calculated (see section 2.2.2 for experimental entropies of aqueous ions). 

Hydration properties are also discussed by Rizkalla and Choppin (1992) and in the chapter by Rizkalla and Choppin (ch. 127) of this volume. For internal consistency, especially when used to calculate ionization enthalpies of the heavy actinides, hydration enthalpies of David (table 3, this chapter) on the absolute scale of yd(H= — 1114kJmol were used to calculate ionization energies in table 2. 

Differences in lanthanide and actinide hydration thermodynamics have been discussed by Bratsch and Lagowski (1986) who attributed the difierences to relativistic effects in the actinides which cause changes in the energies of the s, p, d, and f orbitals. For example, the first and second ionization potentials of the electrons of the 7s state of the actinides are higher than those of the 6s state of the lanthanides whereas the third ionization potentials are similar for both families and the fourth ionization potential is lower for the actinides than the lanthanides. The small decrease in IP3 and IP4 for the f configuration in the actinides results in smooth variations in the relative stabilities of the adjacent oxidation states across the actinide series while the greater spatial extension of the 5f orbitals increases the actinide susceptibility to environmental efiects (Johnson 1982). 

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