C. C. Stephenson

Heat Capacities of Cubic and Hexagonal Ammonium Hexafluosilicate from 25° to 300°K

Heat capacities of cubic and hexagonal ammonium hexafluosilicate were measured between 25° and 300°K. The hexagonal heat capacity showed an anomaly at 38.6°K with an entropy of transition of R ln2. This was attributed to an order—disorder reorientation of the anion. These heat‐capacity measurements, when combined with heat of solution measurements, gave a third‐law verification with ΔS0°=0.03±0.2 cal deg—1 mole—1 for the change in state (NH4)2SiF6(hex)=(NH4)2SiF6(cub). The heat capacities were analyzed to determine the motions of the ammonium ions in the two polymorphs. These analyses showed that the ammonium ions in the cubic modification are undergoing torsional oscillation with a frequency of 235 cm—1, and that the ammonium ions in the hexagonal modification are undergoing pseudofree three‐dimensional rotation at room temperature. The thermodynamic functions CP, S°, (H° — E0°)/T, and — (F° — E0°)/T were calculated. The values, in cal deg—1 mole—1 at 298.15°K are: 59.25, 67.99, 35.21, 32.78 for the cubi...

Heat Capacity of Deutero‐Ammonium Bromide from 17° to 300°K

The heat capacity of deutero‐ammonium bromide has been measured from 17° to 300°K. The heat‐capacity curve is characterized by the appearance of two maxima. These are associated with polymorphic transitions in the crystal lattice and with the ordering of the ammonium ion tetrahedra with respect to the two equilibrium positions in the unit cell. The excess entropy associated with the maxima is found to be in agreement with the order‐disorder nature of the upper transition. A frequency of torsional oscillation for the ammonium ion in good agreement with infrared data is obtained from the Debye‐Einstein function representing the heat capacity below 100°K.

The Dissociation of Ammonium Chloride

Two values for the standard free energy of dissociation of ammonium chloride into ammonia and hydrogen chloride, calculated by two independent paths, are 21,860 cal. and 21,780 cal. at 298.1°K. The good agreement serves as additional evidence for the complete dissociation of ammonium chloride in the vapor phase. The corresponding values calculated for ΔH at 298.1 are 42,160 and 42,260 cal. Two independent values for the standard entropy of the aqueous ammonium ion at 298.1°K derived from these data are 26.5 and 27.1 cal. mole−1 deg.−1.

Heat Capacity of Manganese Titanate from 30° to 300°K

The heat capacity of antiferromagnetic manganese titanate was measured between 31° and 300°K. The magnetic heat capacity of MnTiO3 was determined by subtracting the lattice heat capacity from the observed heat capacity. The lattice heat capacity of MnTiO3 may be represented by a sum of Debye and Einstein functions. The characteristic temperatures of MnTiO3 were estimated from the characteristic temperatures of isomorphous diamagnetic compounds. The entropy associated with the antiferromagnetic transition was calculated to be 3.51 cal deg−1, which is in good agreement with Rln6. Only 55% of the magnetic entropy is acquired by the Neel temperature at 62.3°K. The thermodynamic functions Cp, S° (H° − H0°) / T, and −(F° − H0°) / T of MnTiO3 in cal deg−1·mole−1 at 298.15°K are: 23.93, 25.08, 13.38, and 11.70.