Robert G. Behrens

Metal-carbon bond dissociation energies and enthalpies of formation for gaseous metal carbonyls

Abstract Fundamental principles of thermodynamics are used to estimate mean metal-carbon bond dissociation energies for gaseous carbonyls of the transition, lanthanide and actinide metals. The results suggest that all these carbonyls should exhibit similar thermodynamic stabilities, each having a value of (−37.7 ± 2.2) kcal (mol CO)−1 for the enthalpy of formation. Mean metal-carbon bond energies are correlated with ΔFCO, the difference between the Cotton-Kraihanzel C-O stretch force constant for the carbonyl and that for molecular CO(g). ΔFCO values for first transition period carbonyls are found to exhibit a periodic trend analogous to that of the atomic radii for the elemental metals.

Growth of large uraninite and thorianite single crystal from the melt using a cold-crucible technique

Centimeter-size single crystals of uraninite, UO2, and thorianite, ThO2, were grown from their respective melts using the cold-crucible technique of cage melting. This is believed to be the first report of ThO2 single crystals having been grown from the melt. UO2 single crystals grown in air had a composition of UO2.19 while single crystals grown in an inert atmosphere had a composition believed to be near stoichiometric UO2. X-ray diffraction, ion microprobe, and electron microprobe analyses were used to characterize the UO2 and ThO2 single crystals.

VAPORIZATION THERMODYNAMICS OF ALUMINUM CARBIDE.

Abstract The vapor pressure of aluminum carbide has been measured over the temperature range 1321 to 1607 K using Knudsen-effusion mass spectrometry. Vaporization occurs incongruently to give Al(g) and graphite as reaction products. The vapor pressure of aluminum above the (Al4C3+C) mixture over the experimental temperature range is ( R J K − mol − ) 1n ( p Pa )= −(3.4470±0.036)× 10 5 ( K T ) + (218.518±2.470) The third-law enthalpy for the reaction: Al4C3(s) = 4Al(g)+3C(s), obtained from the present vapor pressures is ΔHo(298.15 K) = (1493 ± 1) kJ mol−1. The corresponding second-law result is ΔHo(298.15 K) = (1408 ± 14) kJ mol−1. The enthalpy of formation of Al4C3(s) calculated from the present third-law vaporization enthalpy and the enthalpy of formation of Al(g) is ΔHfo(Al4C3, s, 298.15 K) = −(187 ± 34) kJ mol−1.

Use of 2-D-adaptive mesh in simulation of combustion front phenomena

Abstract Solutions to models with different length scales may contain regions such as shocks, steep fronts and other near discontinuities. Adaptive meshing strategies, in which a spatial mesh network is adjusted dynamically so as to capture the local behavior accurately, will be described. The algorithm will be tested on an example of solid-solid combustion.

Thermodynamics of transition metal carbonyls I. Fe(CO)5, Ru(CO)5, Os(CO)5

Abstract The thermodynamic properties of Fe(CO)5 liquid and vapor have been reviewed and re-evaluated. Standard third law thermodynamic functions for Fe(CO)5 (g) have been computed using updated spectroscopic data from the literature. Third law thermodynamic functions for Ru(CO)5 (g) and Os(CO)5(g) have been computed using estimated fundamental vibrational wave numbers and assuming the molecules to have structures similar to Fe(CO)5(g). Standard third law entropies are computed as: (104.96 ± 0.4) cal K−1 mol−1 for Fe(CO)5(g); (110.45 ± 1.0) cal K−1 mol−1 for Ru(CO)5(g); (110.73 ± 1.0) cal K−1 mol−1 for Os(CO)5(g). Mean Ru-CO and Os-CO bond energies for Ru(CO)5 (g) and Os(CO)5 (g) are estimated and the standard enthalpies and entropies of formation for the gaseous molecules are calculated.

VAPOR PRESSURE AND VAPORIZATION THERMODYNAMICS OF SCANDIUM TRIFLUORIDE.

Abstract The vapor pressure of scandium trifluoride is measured using a calibrated Knudsen effusion mass spectrometer. The vapor pressure of ScF3 (s) between 1159 and 1411 K is log 10 P ( atm ) = {− (17774 ± 85) T } + (8.698 ± 0.068) The vaporization enthalpy and entropy are 81.3 ± 0.4 kcal mol−1 and 39.8 ± 0.3 cal K−1 mol−1 respectively at 1275 K. The present vapor pressures give a second-law value ΔH° (298.15 K) of 85.2 ± 2.8 kcal mol−1 for the reaction ScF3 (s) = ScF3 (g) . The corresponding third-law value is Δ°(298.15 K) = 89.7 ± 2.7 kcal mol−1, computed using an estimated value of S° (298.15 K) for ScF3 (s) of 23.4 ± 2.0 cal K−1 mol−1. A selected thirdlaw vaporization enthalpy, ΔH°(298.15 K) = 89.9 ± 2.7 kcal mol−1, and Δf°(298.15 K) =-295.3 ± 3.1 kcal mol−1 for ScF3 (g) reported in the literature give Δunf°(298.15 K) = −385.2 ± 4.1 kcal mol−1 for ScF3 (s) . This result is compared with enthalpies of formation obtained from calorimetric and e.m.f. measurements and with values derived from previously reported vapor pressure results.

An electron impact mass spectrometry investigation of VOCl3(g), VCl3(g) and their dissociative fragments

Abstract Knudsen effusion quadrupole mass spectrometry investigations on a mixture of VOCl3(g) and VCl3(g) were performed. Mass spectra and electron impact appearance potentials for the singly charged ions formed from these molecules were obtained. Differences exist in the mass spectra and appearance potential values for fragment ions obtained in the present work and those reported in previous investigations.

A mass spectrometric investigation of the vaporization thermodynamics and vapor composition of cadmium oxide

Abstract Mass spectrometric Knudsen effusion vaporization experiments on CdO(s) were performed over the temperature range 886–1090 K. The results confirm directly that CdO(s) vaporizes by decomposition according to the reaction CdO (s) = Cd (g) + 1 2 O 2 (g) The equilibrium constant for this reaction was determined to be log 10 K = − (1.980 ± 0.016) × 10 4 T + (11.706 ±0.164) over the experimental temperature range. The second-law enthalpy change ΔH°(298.15 K) for the decomposition of CdO(s) derived from the present results was 92.82 ± 0.74 kcal mol−1 and the corresponding third-law value was 87.10 ± 2.03 kcal mol−1. Averaging the present third-law value for ΔH°(298.15 K) with third-law values derived from literature vapor pressures gave a recommended value for ΔHdg(298.15 K) of 87.89 ± 0.15 kcal mol−1. This result is in excellent agreement with ΔH°(298.15 K) = 87.92 ± 0.3 kcal mol−1 derived independently from the enthalpies of formation for CdO(s) and Cd(g). CdO(g) was not observed in the equilibrium vapor of CdO(s) for temperatures up to 1150 K. This result gave a revised lower limit for the enthalpy of formation for CdO(g), i.e. ΔH°f(298.15 K) ⩾ 27.2 kcal mol−1.

VAPOR PRESSURE AND SUBLIMATION ENTHALPY OF ELEMENTAL TECHNETIUM.

Abstract The vapor pressure of technetium metal was measured over the temperature range 2051–2348 K using Knudsen effusion mass spectrometry. The vapor pressure of technetium measured over this temperature range is R In P(atm) = {− (128.1 ± 1.5) × 10 3 T} + (23.19 ± 0.68) . The present vapor pressures are a factor of 3–6 higher than previously reported values. The third-law enthalpy for the reaction Tc ( s ) = Tc ( g ) derived from the present vapor pressures is ΔH ° (298.15 K ) = 152.7 ± 2.2 kcal mol −1 . The corresponding second-law result is ΔH ° (298.15 K ) = 129.9 ±1.5 kcal mol −1 . An approximate value of T m = 2435 ± 40 K was obtained for the melting temperature of technetium.

Vapor pressure of liquid cesium by a Knudsen-effusion radiotracer technique

Abstract The vapor pressure of liquid cesium has been measured over the temperature range 327 to 459 K using a Knudsen-effusion radiotracer technique. The vapor pressure over this temperature range is ( R cal th K −1 mol −1 ) ln ( p atm ) = −(16493±522) ( K T )+(16.02±1.38) The third-law enthalpy for the reaction Cs(c) = Cs(g) obtained from the present results is ΔHo(298.15 K) = (18.44 ± 0.36) kcalth mol−1. The second-law result is ΔHo(298.15 K) = (17.31 ± 0.52) kcalth mol−1. Standard thermodynamic functions for Cs2(g) are computed using a spectroscopically derived CsCs bond distance. The entropy of Cs2(g) is computed to be So(298.15 K) = 68.053 calth K−1 mol−1, 0.2 calth K−1 mol−1 higher than the previously reported value. Literature vapor pressures above 500 K are re-evaluated and compared with the present and literature low-temperature vapor pressures. On the basis of this evaluation, the vaporization enthalpy is chosen to be ΔHo(298.15 K) = (18.33±0.36) kcalth mol−1.