THERMODYNAMIC PROPERTIES OF METHYLUREA

The heat capacity of crystalline methylurea was measured by vacuum adiabatic calorimetry (T = 8.7 K to T = 300 K) and by the triple heat-bridge method (T = 300 K to T = 350 K). The value of ΔfGom(cr, 298.15 K) = -(166.06±0.76) kJ·mol-1 was found on the basis of our and literature results. The molar enthalpy of sublimation of CH3NHCONH2 was directly determined by a heat-conduction differential microcalorimeter of the Calvet type: ΔsubHom(343 K) = (94.38±0.84) kJ·mol-1. The vapour pressure p of the liquid was measured by the integral Knudsen-effusion method at temperatures from 377.2 K to 410.3 K: ln(p/Pa) = (30.528±0.579)-(10314±223)(K/T), and the molar enthalpy of vaporization was calculated: Δvapom(394 K) = (85.75±1.89) kJ·mol-1. Using these and literature results, the weighted average value of the molar enthalpy of sublimation: ΔsubHom(343 K) = (95.73±0.39) kJ·mol-1, was calculated and the molar entropy of CH3NHCONH2(g): Som (g, 343 K) = (337.32±1.33) J·K-1·-1, was found. Statistical calculations of the standard thermodynamic properties CH3NHCONH2(g) were carried out with account taken of the contributions of geometric and rotational isomers. The calculated standard molar entropy: Som(g, 343 K) = 335.93 J·K-1·mol-1, is in a satisfactory accordance with the experimental value. Standard molar thermodynamic functions of CH3NHCONH2 (cr or l) (T → 0 to T = 450 K) and of CH3NHCONH2(g) (T = 298.15 K to T = 1000 K) were calculated and tabulated. © 1993 Academic Press Ltd.