HEAT-CAPACITIES OF UREA, N-METHYLUREA, N-ETHYLUREA, N-(N)PROPYLUREA, AND N-(N)BUTYLUREA IN THE RANGE 200 TO 360K

The heat capacities at constant pressure of urea, N-methylurea, N-ethy lurea, N-(n)propylurea and N-(n)butylurea were measured by differentia l scanning calorimetry over the range 200-360 K, in gas-flow condition s. A He + Ne mixture and N-2 only were selected as the purge gases bel ow and above room temperature, respectively. Benzoic acid was used as the reference material and the accuracy of the measurements was estima ted to be within 2% across the temperature range. The heat capacities at 298.15 K were obtained by interpolating the smoothed fitting equati ons. No anomalies were detected for urea or N-methylurea. One fully re versible solid-solid transition was observed for N-ethylurea at T = (2 94.5 +/- 2.0) K, with Delta(trs)H(m) = (1.0 +/- 0.3) kJ mol(-1) and fo r N-(n)propylurea at T = (289.6 +/- 1.0) K, with Delta(trs)H(m) = (3.0 +/- 0.2) kJ mol(-1). Two solid-solid transitions were found for N-(n) butylurea at T = (315.0 +/- 2.0) K, with Delta(trs)H(m) (7.0 +/- 2.0) kJ mol(-1) and at T = (346.0 +/- 1.2) K, with Delta(trs)H(m) = (1.0 +/ - 0.1) kJ mol(-1), the first one being recoverable at temperatures low er than 240 K only.