SUBLIMATION ENTHALPIES AND ENTROPIES OF THIOUREA, 1-METHYL-2-THIOUREA, 1,3-DIMETHYL-2-THIOUREA, AND 1,1,3,3-TETRAMETHYL-2-THIOUREA

The torsion-effusion technique was used to determine the vapour-pressure dependence on temperature for thiourea (TU); 1-methyl-2-thiourea (MMTU); 1,3-dimethyl-2-thiourea (DMTU); and 1,1,3,3-tetramethyl-2-thiourea (TMTU). The following pressure-against-temperature equations were obtained: for TU(cr), lg(p/kPa) = (11.40 ± 0.25) - (5711 ± 100)(K/T); for MMTU(cr), lg(p/kPa) = (12.60 ± 0.50) - (5779 ± 150)(K/T); for DMTU(1), lg(p/kPa) = (11.33 ± 0.50) - (4971 ± 150)(K/T); and for TMTU(cr), lg(p/kPa) = (11.39 ± 0.50) - (4331 ± 150)(K/T). By a second-law treatment of the vapour-pressure results, the sublimation enthalpies and entropies of TU, MMTU, and TMTU were derived in the experimental temperature ranges, whereas for DMTU, only the vaporization quantities were obtained. For thiourea, ΔsubHom(298.15K) = (111 ± 3) kJ·mol-1 and Som (298.15 K, g) = (302 ± 5) J·K-1·mol-1 were obtained by using Cp, m values that were taken from the literature or estimated. © 1994 Academic Press Ltd.