Entropy effusion

Powell and Searcy [1288], in a study of CaMg(C03)2 decomposition at 750—900 K by the torsion—effusion and torsion—Langmuir techniques, conclude that dolomite and C02 are in equilibrium with a glassy phase having a free energy of formation of (73 600 — 36.8T)J from 0.5 CaO + 0.5 MgO. The apparent Arrhenius parameters for the decomposition are calculated as E = 194 kJ mole-1 and activation entropy = 93 JK-1 (mole C02)-1. 

Effusive beam technique, 157-158 Electron bombardment flow radiolysis, 238 Electrospray ionization and ionic clusters, 168 Enantiomers, separation techniques, 154-155 Enantioselectivity of enzymes, 148 Enthalpy-entropy compensation plots, 261 Enthalpy of activation, and quantum tunneling, 67, 70-71... 

Two papers reported powder pattern crystallographic results. The paper by Santos et al. (7) stood out from the rest because it presented a collection of more classical physical chemistry experiments. In this paper the authors described the use of micro-combustion calorimetry, Knudsen effusion to determine enthalpy of sublimation, differential scanning calorimetry, X-ray diffraction, and computed entropies. While this paper may provide some justification for including bomb calorimetry and Knudsen cell experiments in student laboratories, the use of differential scanning calorimetry and x-ray diffraction also are alternatives that would make for a crowded curriculum. Thus, how can we choose content for the first physical chemistiy course that shows the currency of the discipline while maintaining the goal to teach the fundamentals and standard techniques as well ... 

Eleven different species were distinguished during an effusion equilibrium study of the non-stoicheiometric zirconium iodides.240 Enthalpy and entropy changes for the disproportionation reactions leading to these species were evaluated as were the heats of formation of Zrl (n = 4, 3.2, 3.17, 3, 2.82, or 2.5). A distorted tetrahedral co-ordination of the fluorines around hafnium in HfF4 appears likely according to... 

In the first experiments to measure the vapor pressure of metallic Bk, using Knudsen effusion target-collection techniques, the preliminary data were fitted with a least-squares line to give a provisional vaporization equation for the temperature range 1326-1582 K, and Ai 298 was calculated to be 382 18 kJ/mol (128). The crystal entropy of berkelium metal at 298 K (S s) had been estimated earlier to be 76.2 1.3 J K 1 mol-1 (129), and later, to be 78.2 1.3 J K 1 mol-1 (124). 

Oxides. Decomposition pressure measurements on the TbO system by Eyring and his collaborators (64) have been supplemented by similar and related studies on the PrO system (46) and on other lanthanide-oxygen systems (43, 44). Extensive and systematic studies of vaporization processes in lanthanide-oxide systems have been undertaken by White, et al. (6, 188,196) using conventional Knudsen effusion measurements of the rates of vaporization of the oxides into high vacuum. Combination of these data with information on the entropies and Gibbs energy functions of reactants and products of the reaction yields enthalpies of reaction. In favorable instances i.e., if spectroscopic data on the gaseous species are available), the enthalpies of formation and the stabilities of previously undetermined individual species are also derived. The rates of vaporization of 17 lanthanide-oxide systems (196) and the vaporization of lanthanum, neodymium, and yttrium oxides at temperatures between 22° and 2700°K. have been reported (188). 

Metals. Kruglikh, et al. (104) measured saturated vapor pressures of erbium, samarium, and ytterbium by the Knudsen effusion method, and standard (average) sublimation entropies of 18.4, 20.7, and 25.6 cal./(gram atom °K.) were derived. Nesmeyanov, et al. (146) studied the vapor pressure of yttrium by an integral variant of the effusion technique. Similar studies at higher temperatures by Herrick (70) on samarium metal have been interpreted in good accord by both first and second law methods. Ideal gas thermodynamic functions have been derived from 100 °K. to 6000°K. at 100° intervals for both actinide and lanthanide elements by Feber and Herrick (45). 

In addition to the estimated properties, we measured the thermochemistry of several important vapor species. These measurements were conducted in a Knudsen effusion cell using special line-of-sight vaporization under subambient pressures with flowing O2 and H2O vapor mixtures [4]. The gaseous species over silica [5], manganese oxide [6], lanthana, alumina, and palladium metal were detected and relative partial pressures measured as a function of temperature. These vapor pressure measurements were calibrated by using the known metal atom or binary metal oxide volatility as a calibration source. Oxide species concentrations were measured relative to that of a reference compound, e.g., metal atom. The identification of oxide and hydroxide compounds was facilitated by Ae technique of threshold electron ionization [7]. These data were then evaluated using estimated entropy functions and the third law temperatures. 

The total vapour pressure of selenium in equilibrium with a mixture of Au(cr) and a-AuSe was measured in the temperature range 505 to 602 K using the Knudsen effusion method in [71RAB/RAU]. The result is presented as the partial pressure of Sc2(g) at equilibrium and the enthalpy of formation and entropy at 298.15 were evaluated to be Af//°(AuSe, a, 298.15 K) = -7.9 kJ-moP and S°(AuSe, a, 298.15 K) = 80.8 J K -moP, respectively. However, Sc2(g) is not the major species in the gas phase at the temperatures and total pressures of the study, and it is not clearly stated how the partial pressure was derived from the experiments. The result is therefore questionable and impossible to re-evaluate using the selected thermodynamic properties of the gaseous selenium species. No values were selected by the review. 

The total pressure of the vapour in equilibrium with (3-SnSe was measured in the temperature range 862 to 920 K using the Knudsen effusion technique. The enthalpy and entropy of sublimation according to the reaction a-SnSe SnSe(g) were calculated by the review from the reported vapour pressure expression and the selected heat capacities of SnSe(g) and a-SnSe, the enthalpy of transformation a-SnSe —> (3-SnSe being 1.28 kJ-mof (cf V.7.4.1.2), and a heat capacity expression of (3-SnSe being identical to that of a-SnSe, yielding (SnSe, a, 298.15 K) = (188.2 + 20.0) kJ-mol and... 

The partial pressures of the species in gaseous aluminium-selenium mixtures were determined in the temperature range 1232 to 1352 K using mass spectrometry and Knud-sen effusion cells. The measured ion intensities were converted to partial pressures by normalising the ion intensity of Al(g) in equilibrium with Al(l) to the known total vapour pressure. The derived thermodynamic quantities were recalculated by the review using the selected thermodynamic properties of Se2(g), the CODATA [89COX/WAG] properties of Al(g), and the entropies and heat capacity expressions of the aluminium selenides given in Sections V.8.2.1.1 to V.8.2.1.3. The results are summarised in Table A-59. 

In the present review, the enthalpies of formation at 625 K in Table III of the paper were recalculated to 298.15 K using the selected heat capacities of Se(cr, I), Se2(g), and Se5(g)-Ses(g) to provide for a comparison with other investigations. Similarly, third law enthalpies of formation and second law entropies were evaluated by combining the data of Table 111 in the paper and the total pressure measurements by the Knudsen torsion-effusion method in the same paper with the selected values for the heat capacities and entropies. The results are summarised in the Table A-70. 

The evaporation of PbSe(cr) was studied in the temperature range 835 to 1047 K using the torsion effusion technique. The original evaluation employed auxiliary data different from those used by the review and the vapour pressure expression was therefore reevaluated using the selected values of the entropies and heat capacities of PbSe(cr) and PbSe(g). The second law entropy of PbSe(g) and the second law enthalpy of sublimation of PbSe(cr) were calculated to be S°(PbSe, g, 298.15 K) = (260.6 + 7.0)... 

Tables 23 and 24 show some selected enthalpies and entropies of dissociation determined by Knudsen effusion mass spectrometry. In addition, partial pressures and equilibrium constants were evaluated. An account of partial pressures and equilibrium constants is given in a recent review by Hilpert [438].

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