A thermodynamic investigation of the transitions in carbon tetrabromide and ammonium chloride

Abstract

Measurements have been made of (i) the heat capacity at constant pressure of carbon tetrabromide from 0" to 60" C, (ii) the temperature and heat of transition of the phase change in this substance, (iii) its isothermal compressibility, coefficient of expansion, and density above and below the transition, (iv) the isothermal compressibility and the coefficient of expansion of ammonium chloride above and below the &point. The Compressibility measurements have been made over the range 1 to 50atm. For each substance, values of Cwres have been calculated above and below the transition, where Cwres is the molar heat capacity at constant volume less the contribution from intramolecular or interionic degrees of freedom. For carbon tetrabromide, Cvres does not alter at the transition, but remains almost equal to 6R, showing that the motion of the molecules at the transition does not change to rotation, but remains one of three-dimensional, simple harmonic torsional oscillations. For ammonium chloride, while the results are in agreement with the established view about the nature of the A-point, they indicate that there is an "anomalous" contribution to Cwres above the A-point. This contribution for a time actually increases with rising temperature, and at 50" above the &point amounts to about 2-6 cal/mole. Incidental to the above measurements, determinations have also been carried out of the compressibility of water at 0" and 34.8" C, and of the compressibility and coefficient of expansion of toluene between -60" and 0" C.

A thermodynamic investigation of dynamical disorder in Phase II of CBr4

“…The heat capacity as a function of temperature is smooth with no singularities due to phase transitions in the temperature range of 30-305 K, extending the conclusion of earlier differential scanning calorimetry results ͑which had been carried out down to 115 K͒. 18 The heat capacity of CBr 4 reported previously 8 above 295 K is also shown in Fig. 1, and agreement with the present results is within experimental error.…”

“…This method was reported to be more effective for purification of CBr 4 than recrystallization from solution. 8 The mass of the sample used in the calorimetric experiments was 9.2904 g.…”

“…Values of the thermodynamic variables required to calculate C V from C P for CBr 4 are not available for the whole temperature range of the experimental heat capacity data. However, the thermal expansion of CBr 4 is known 20 in the temperature range from 210 K to 320 K ͑and we have extrapolated it to lower temperatures͒ and the isothermal compressibility is known 8 at 273 K and 310 K. This allowed us to calculate C V within about Ϯ2%. The smoothed values of the isochoric heat capacity are given in Table I.…”

“…11 Furthermore, an analysis of the hightemperature heat capacity of CBr 4 led to the conclusion that there is essentially no change in the molecular motion at the II→I transition. 8 We report here experimentally determined heat capacities of CBr 4 in phase II, from 30 K to 305 K. The simplicity of solid CBr 4 and the level of knowledge of other properties for this material make it possible to use the heat capacity data to quantify dynamical disorder in phase II of CBr 4 . In addition, with the present results there is sufficient experimental information for this system to calculate related thermodynamic properties, viz., isobaric heat capacity and the Grüneisen parameter.…”

Solid-state phase transition in carbon tetrabromide. II. The crystal structure of phase I

“…In CBr 4, ASt=4.98 cal K -~ mol -~ (Marshall, Staveley & Hart, 1956;Hildebrand & Scott, 1950). Guthrie & McCullough (1961) suppose that the orientational disorder corresponds to the 10 distinguishable orientations of the combined 7",1 and C3,, symmetry (R In 10 = 4.58 cal K -~ mol-l).…”