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|Type:||Artigo de periódico|
|Title:||Thermochemical Study Of Adducts Of Urea With Zinc, Cadmium And Mercury: Some Correlations For Urea Derivatives|
|Abstract:||The adducts ZnX2·2ur (X = Cl, Br, I), CdX2·2ur (X = Br, I), CdX2· ur (X = Cl, Br, I) and HgX2 ur (X = Cl, Br) have been characterized. The shifts of the stretching vibrations to low frequency (vCO) and to high frequencies (vCN and vNH) indicated that urea (ur) is oxygen-bonded to the metals. The standard molar enthalpies for the reaction MX2(c)+ nur(c) = MX2· nur(c) ΔrHm ⊖ were determined by solution-reaction calorimetry, given the following values: ZnCl2 ·2ur(c), -59.72±0.30; ZnBr2 · 2ur, -59.41±0.89; ZnI2 · 2ur, -81.74 ±0.89; CdCl2 · ur, -21.54±0.18; CdBr2 · ur, -13.88±0.28; CdI2 · ur, -30.75±0.36; CdBr2 · 2ur, -24.40+0.21; CdI2 · 2ur, -19.69±0.17; HgCl2 · ur, -9.39±0.13 and HgBr2 · ur, +1.00 ±0.78 kJ mol-1. From these values, the standard molar enthalpies of formation (ΔfHm ⊖) and of decomposition (ΔDHm ⊖), and lattice enthalpy (ΔMHm ⊖) were calculated and the mean molar enthalpy of the metal-oxygen bond [t̄D(M-O)] was estimated. The thermochemical results indicate a decrease in stability of the adducts from zinc to mercury. The degree of hydrogen bonding affects differently the enthalpies of formation of adducts in solid state for urea and its derivatives. © 1988.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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