Thermodynamics of hexachlorocyclotriphosphazene and octachlorocyclotetraphosphazene from T -> 0 to T=450 K

In adiabatic vacuum and dynamic calorimeters, the temperature dependence of the standard molar heat capacity C-p,m(o) of hexachlorocyclotriphospazene and octachlorocyclotetraphosphazene has been determined at temperatures in the range T = 5 K to T = 450 K: from T = 5 K to T = 340 K with an accuracy of about 0.2 per cent, and with an accuracy of 0.5 to 1.5 per cent between T = 340 K and T = 450 K. The temperatures, enthalpies. and entropies of mel ting of the above compounds have been determined. The experimental data wer e used to calculate the thermodynamic functions C-p,m(o)/R, Delta(0)(T) H-m (o)(R.K), Delta(0)(T) S-m(o)/R, and Phi(m)(o) = Delta(0)(T) S-m(o) - Delta( 0)(T) H-m(o)/T (where R is the universal gas constant) in the range T --> 0 to T = 450 K, and the standard thermochemical parameters of formation Delt a(f) S-m(o), Delta(f)G(m)(o), and IgK(f) of the cyclophosphazenes from the elements at T = 298.15 K. The isochoric heat capacities C-V,C-m of both chl orocyclophosphazenes have been estimated over the range T --> 0 to T-fus. T he first and the second cryoscopic constants have been determined. The diff erence in the thermodynamic properties for one phosphazene group (-NPCl2-) has been explained in terms of the different compositions and structure of the rings. (C) 1999 Academic Press.