Darrell W. Osborne

Thermodynamics of the lanthanide trifluorides. I. The heat capacity of lanthanum trifluoride, LaF3 from 5 to 350°K and enthalpies from 298 to 1477°K

The heat capacity of a sample of LaF3 was determined in the temperature range 5–350°K by aneroid adiabatic calorimetry and the enthalpy from 298.15 to 1477°K by drop calorimetry. The heat capacity at constant pressure C°p(298.15°K), the entropy S° (298.15°K), the enthalpy [H° (298.15°K)−H° (0)] and the Planck function −[G° (298.15°K)−H° (0)]/298.15°K; were found to be (90.29±0.09) J °K−1⋅mole−1, (106.98±0.11) J °K−1⋅mole−1, (16717±17) J mole−1, and (50.91±0.05) J °K−1⋅mole−1. The thermal functions from the present research were extended up to the melting temperature (1766°K) by combination with previously published results. The anomalously high heat capacity from about 1100 to 1766°K is discussed.

The Entropy of Dimethyl Acetylene from Low Temperature Calorimetric Measurements. Free Rotation in the Dimethyl Acetylene Molecule

The separation of the methyl groups in the dimethyl acetylene molecule, H3C-C≡C-C-CH3, is much greater than it is in ethane, and if the potential barrier [1] of about 3000 cal./mole restricting internal rotation in the latter is due to interactions between the methyl groups, then this barrier should be much smaller in dimethyl acetylene. If, on the other hand, the restricting potential in ethane is largely due to resonance with double bonded structures as proposed by Gorin, Walter, and Eyring [2], that fact that the length of the C-C single bonds in dimethyl acetylene [3] is such as to indicate considerable double bond character might lead one to expect a barrier about as large as in ethane. In order to determine the magnitude of this barrier we have calculated the entropy of dimethyl acetylene from low temperature calorimetric measurements and have compared this experimental value with that computed from molecular data.

Heat capacity of 242PuO2 from 12 to 350°K and of 244PuO2 from 4 to 25°K. Entropy, enthalpy, and Gibbs energy of formation of PuO2 at 298.15°K

Heat capacity measurements have been made on a 22.5 g sample of 242PuO2 from 12 to 350°K and on a 3.6 g sample of 244PuO2 from 4 to 25°K. The heat capacity curve has the normal sigmoid shape. At 298.15°K the heat capacity C°p, entropy S°, enthalpy H°−H°0, and tempered Gibbs energy (G°−H°0)/T are, respectively, (66.25±0.26) J °K−1⋅mole−1, (66.13±0.26)J °K−1⋅mole−1, (10 784±43) J mole−1, and −(29.96±0.12) J °K−1⋅mole−1. This value of the entropy is significantly lower than the one previously derived from heat capacity measurements on 239PuO2. At 298.15°K the standard entropy of formation and the standard Gibbs energy of formation are −(195.2±0.4) J °K−1⋅mole−1 and −(998.0±0.7) kJ mole−1, respectively.

The Entropy of Dimethyl Sulfide from Low Temperature Calorimetric Measurements. Restricted Rotation of the Methyl Groups

In order to arrive at a satisfactory theory for the potential restricting the rotation of methyl groups in many molecules, it is desirable to determine how the magnitude of the barrier depends on the kind of atom to which the methyl groups are bonded. Recently we have obtained an estimate of the barriers in dimethyl sulfide by comparing the entropy obtained from calorimetric measurements extending to low temperatures with that computed from molecular data.