Termoquimica de quelatos de 2,2,6,6-tetrametil-3,5-heptanodiona com elementos lantanidicos

AUTOR(ES)

Luiz de Souza Santos Junior

DATA DE PUBLICAÇÃO

1992

RESUMO

The chelates [Ln(thd)3, cr] (Ln = La, Pr, Nd, Sm, Gd, Tb, Ho, Er, Tm and Yb; thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) were prepared, characterized and thermochemically studied through reaction-solution calorimetry, by using HCl.26,64 H2O (aq) as solvent, based on the following reaction LnCl3.6H2O(cr) + 3 Hthd(l) + 73,92H2O(l) = [Ln(thd)3,cr] + 3HCl.26,64H2O(aq); DrHmº By combining DrHmº values [4,70 ± 1,52 (La) to 57,71 ± 1,46 (Yb) kJmol] with the standard molar enthalpies of formation of the components of the above reaction, the standard enthalpies of formation of chelates DfHmº (quel, cr) in solid phase were calculated. The enthalpies of sublimation of these chelates were obtained through the determination of the molar heat capacities of the chelates in solid and liquid states, the standard molar enthalpy of fusion and vaporization obtained by the use of differential scanning calorimetry, and the molar heat capacities in gaseous phase were estimated. The mean enthalpies of the lanthanide-oxygen, (Ln-O), for chelates [278,40 ± 3,55 (La) to 220,16 ± 3,50 (Yb) kJmol] correlate linearly with the respective values of the crystalline metallic oxides. These data suggest that the lanthanide-oxygen interactions are similar in chelates and metallic oxide. The values of the enthalpies of formation of chelates, DfHmº (quel, cr) [-2434,6 ± 4,2 (La) to -2343,7 ± 4,2 (Yb) kJmol] and (Ln-O) [278,40 ± 3,55 (La) to 220,16 ± 3,50 (Yb) kJmol] decrease with the increase of the atomic number, showing clearly the lanthanide contraction. Other thermochemical values also change behaviour neighbourhood the gadolinium.

ASSUNTO(S)

entalpia termoquimica

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