The effect of the failure of isotropy of a gas in an effusion cell on the vapor pressure and enthalpy of sublimation for alkyl derivatives of carbamide

The divergence of the values of enthalpy of sublimation for urea,methylurea,ethylurea, 1,1-dimethylurea, 1,3-dimethylurea, (1-methylethyl)urea, n-butylurea, (1-methylpropyl)urea, (1,1-methylethyl)urea, 1,1-diethylurea, 1,3-diethylurea, 1,3-bis(1,1-dimethylethyl)urea obtained calorimetrically and from the effusion measurements [Izv. Akad. Nauk SSSR. Ser. Khim. 4 (1990) 750] was analyzed. The results of the calorimetric measurements were shown to be independent of the evaporation rate. The influence of the isotropy failure of a gas on the effusion results was analyzed. The vapor pressures and enthalpies of sublimation were corrected according to the proposed technique. The decrease of divergence of the values of enthalpies of sublimation for alkyl derivatives of carbamide from effusion and calorimetric measurements while taking into account the isotropy failure of a gas was shown. The average weighted value of enthalpy and entropy of sublimation were evaluated. The additive scheme for enthalpy of sublimation was proposed. (C) 2003 Elsevier B.V. All rights reserved.