Showing papers on "Standard molar entropy published in 1972"

Thermodynamics of the fcc Fe-Mn-C and Fe-Si-C alloys

Abstract: The activity of carbon in austenitic Fe-Mn-C and Fe-Si-C alloys has been studied by equilibration with controlled CH4-H2 atmospheres at temperatures in the range 848° to 1147°C and for composition up to about 60 pct Mn and 7 pct Si. The activity coefficient of carbon is diminished by manganese and is increased by silicon. Activity coefficients and derived values of the partial molar free energy, enthalpy, and entropy of solution of graphite in the alloy are expressed in mathematical form. The heat of solution of graphite, which is positive in the Fe-C binary alloys, decreases with increasing manganese and increases with increasing silicon concentrations. The partial molar entropy is independent of manganese, but is decreased by silicon.

Phase equilibria and kinetics of evolution of dilute solutions of hydrogen in niobium

Abstract: Ultrahigh vacuum techniques have been used to measure the equilibrium pressure of hydrogen dissolved in niobium in dilute solutions over the pressure, concentration and temperature ranges, pe=10-7-10-2 Torr, H/Nb=0-0.035, and T=351-487 K. Sievert's law which indicates ideal solution behaviour was observed for hydrogen concentrations below about H/Nb=0.023; the solubility data are described by the expression square root pe=c(395.4+or-33.8)exp(-(4.345+or-0.161)*103/T) where c=H/Nb*100. The standard partial molar entropy and enthalpy of solution are Delta s0=49.6+or-2.3 J K-1 g-atom-1 and Delta h0=-36.12+or-1.34 kJ g-atom-1. The value of Delta s0 is compared with theoretical entropies calculated from configurational and vibrational contributions. The rate of evolution of hydrogen from a niobium filament has been studied over the pressure and temperature ranges 10-7-10-3 Torr and 465-553 K. The rate limiting process is identified as the recombination of hydrogen atoms on the metal surface rather than diffusion within the bulk provided the overall concentration is less than H/Nb approximately=0.02. The activation energy for the process is 67.56+or-1.26 kJ mol-1.

Xenon Tetrafluoride: Heat Capacity and Thermodynamic Functions from 5 to 350°K. Reconciliation of the Entropies from Molecular and Thermal Data

Abstract: The heat capacity of a very pure sample of xenon tetrafluoride, XeF4, was determined by adiabatic calorimetry between 5 and 350°K. No irregular thermal behavior was observed, except in the region near 210°K, where slightly enhanced heat capacity values, ascribed to a small amount of impurity, were observed. The values of the thermodynamic quantities Cp°, S°, (H° − H °0)/T, and (G° − H °0)/T at 298.15°K are 118.39 ± 0.12, 167.00 ± 0.17, 77.24 ± 0.08, and −89.76± 0.09 J °K−1 · mole−1, respectively. The standard entropy of the gas at 298.15°K was calculated from the thermal data to be 323.2± 2.0 J °K−1· mole−1, in excellent agreement with the value 323.98 ± 0.4 J °K−1· mole−1 calculated from electron‐diffraction data and the frequency assignment of Tsao, Cobb, and Claassen. The standard entropy of formation of solid XeF4 at 298.15°K was found to be −407.95 ± 0.17 J °K−1 · mole−1, and the standard Gibbs energy of formation at 298.15°K is −145.48 ± 0.88 kJ mole−1. The corresponding quantities for gaseous XeF4 ...

Activity coefficients of single ions III: The difference between the standard chemical potentials of single ions in methanol and methanol-water mixtures and in water at 25°C, and some related quantities

Abstract: The method described by De Ligny et al. for the estimation of activity coefficients of single ions, which originated from ideas of Alfenaar and De Ligny, Buckingham, Halliwell and Nyburg and Muirhead-Gould and Laidler, has been improved further. The modified method was applied successfully to the transfer of ions from water to methanol and methanol-water mixtures. A similar method was used to estimate the standard partial molar enthalpy of transfer ΔH° of single ions, and from these data values for the standard partial molar entropy of transfer ΔS° were obtained. The differences between the standard real potentials of the chloride ion in methanol and methanol-water mixtures, and in water have been determined by Case and Parsons. From these data, the corresponding differences for the standard chemical potentials and the surface potential of water, the surface potentials of methanol and methanol-water mixtures, were determined.

Far from equilibrium synthesis of small polymer chains and chemical evolution

Abstract: A number of simple models for the synthesis of small polymers are considered. The role of autocatalysis and primitive template processes is examined. It is shown that far from equilibrium such systems may exhibit multiple steady state or sigmoidal transitions, favouring the enhancement of the (oligomer/monomer) ratio in the medium. A thermodynamic analysis in the non-linear range shows that the molar entropy production passes through a maximum when the system undergoes the non-equilibrium transition. The results are discussed in connection with the problems of prebiotic synthesis and developmental biology.

Beiträge zur Chemie der Oxidhalogenide von Molybdän und Wolfram. V. Molybdänoxidtetrachlorid und Molybdänoxidtrichlorid

Abstract: Die Bildungsenthalpien ΔH° (MoOCl4, f, 298) = −158,3 (± 1) kcal/Mol und ΔH° (MoOCl3 f, 298) = −148,92 (± 0,5) kcal/Mol wurden aus den Losungsenthalpien in 2 n NaOH bzw. 2 n NaOH mit 1% H2O2 ermittelt. Ergebnisse der Gleichgewichtsmessungen in den Systemen liefern die Bildungsenthalpien und Standardentropien: . Die Standardentropie des gasformigen MoOCl4 wurde zu 91 (±3) cl abgeschatzt, Mit den Sublimationsenthapien und -entropien: wurden gewonnen: . The formation enthalpies were ascertained from the solution enthalpies in 2 n NaOH resp. 2 n NaOH + 1% H2O2. The results of equilibrium measurements in the systems give the formation enthalpies and standard entropies: . The value of the standard entropy of the gaseous MoOCl4 was estimated to be 91 (±3) cl. From the enthalpies and entropies of sublimation the values were obtained.

Heat of Formation and Entropy of C3 Molecule

Abstract: : Partial pressures of C3(g) have been measured with a high-resolution mass spectrometer and a Knudsen effusion cell in the temperature range from 2300 to 2800K. The third-law heat of formation was derived with the thermodynamic functions of Strauss and Thiele as was the second-law value. The entropy values were also determined.

Determination of the Thermodynamic Functions of Solutions by Gas-liquid Chromatography

Abstract: This review of the latest progress in gas-liquid chromatography is devoted to a description of the methodological grounds of experimental technique for the gas-chromatographic determination of the thermodynamic properties of solutions. Methods are given for the calculation of the chief thermodynamic functions — the distribution constant, activity coefficients, molar entropy, and free energy of the solution. The bibliography contains 152 references.

Xenon Difluoride: Heat Capacity from 5 to 350°K and Some Derived Thermodynamic Properties

Abstract: The heat capacity of a 99.9% pure sample of xenon difluoride, XeF2, was measured between 5 and 350°K by an adiabatic method, and various thermodynamic properties were calculated from the heat capacity. No irregularities in the heat capacity nor solid‐solid transitions were observed. At 298.15°K Cp°, S°,(H°−H°0)/T, and (G°−H°0)/T were found to be 75.60±0.08, 115.09±0.12, 51.17±0.05, and −63.92± 0.06 J °K−1· mole−1, respectively. The standard entropy of the gas at 298.15°K calculated from the thermal data is 258.6±1.9 J °K−1 mole−1, in excellent agreement with the value 259.4±0.1 J °K−1 mole−1 from molecular data. For solid XeF2 at 298.15°K the standard entropy of formation is −257.17± 0.12 J °K−1· mole−1, and the standard Gibbs energy of formation is −86.08±0.88 kJ mole−1; the corresponding quantities for gaseous XeF2 are −112.86± 0.1 J °K−1· mole−1 and −73.40±0.89 kJ mole−1, respectively. The equilibbrium constant for the formation of gaseous XeF2 from the elements at various temperatures is calculated an...

Relationship between Sonic Velocity and Entropy in Molten Salts

Abstract: Sonic velocity in molten salts can be estimated from absolute molar entropy by means of an equation developed from the Debye theory of specific heats. When tested on 29 molten halide compounds, for which the required data are available, the equation reliably predicts sonic velocity and its negative temperature dependence. When applied to molten alumina, the equation predicts the magnitude of the sonic velocity, but does not yield the correct temperature dependence. For more complex molten salt compounds (such as nitrates and sulfates), a substantial reduction in the molar entropy is necessary for the predicted sonic velocity to agree with the experimental. When entropy contributions, associated with interatomic vibrations and free rotation of the complex ion, are subtracted from the total molar entropy, the equation yields excellent results for these complex molten salt compounds. The nature of the subtracted entropy terms implies that ``gaslike'' degrees of freedom, if present in the liquid, have very little effect on sonic volocities in salt melts containing complex ion species.

Heats of Atomization and Some Thermochemical Constants of AIIIBV Compounds

Abstract: An analysis was made of the published experimental and calculated values of the thermochemical constants of gallium phosphide (the standard entropy and the specific heat, the enthalpy, and the free energy of formation of this compound) and of the equilibrium constants and vapor pressure of phosphorus. Approximate methods were used to calculate the remaining constants: the specific heat was found by the Landiya method and the standard entropy was deduced from the Eastman equation and by summing the entropies of elemental group IV semiconductors.

Entropy and Free Energy

Abstract: Entropy is a thermodynamic function, which was first defined in engineering thermodynamics regarding the performance of heat engines. The entropy of a system is a measure of the disorder or randomness of the system. Entropy depends only on the state of the system and not on the way in which that state was reached. The entropy of a system is a measure of the disorder or randomness of the system. The value of the entropy change is dependent on the temperature and the pressure. It is possible to determine molar entropy values for individual substances, using calculations based on the third law of thermodynamics, which states that the entropy of a pure crystalline substance at 0 K is zero. Molar entropy values are usually quoted for the substance in its standard state. Standard entropy changes for reactions can be added and subtracted in the same way as heats of reaction, to enable unknown values to be obtained from known values.

Thermodynamic study of disorder in zinc fluoride tetrahydrate

Abstract: The heat capacity of zinc fluoride tetrahydrate has been measured from 10 K to 300 K, giving a value for Scal, the calorimetric entropy, at 25°C. The entropy Seq of this salt has been determined by studying the solution thermodynamics of zinc fluoride at 25°C. Seq exceeds Scal by 8.9 ± 1.5 J K–1 mol–1, so that the crystalline tetrahydrate is disordered at 0 K, with a residual entropy of approximately R ln 3. The reason for this disorder is believed to be the same as that for iron(II) fluoride tetrahydrate at ordinary temperatures, namely that the metal atom is octahedrally surrounded by four water molecules and two fluorine atoms, and that the crystal does not distinguish between the oxygen and fluorine atoms. If the fluorine atoms in an octahedron are in a trans position to each other, each octahedron would have three possible orientations.To obtain an improved estimate of the standard entropy of the fluoride ion (needed in the evaluation of Seq), the heat of solution of sodium fluoride in water at 25°C has been redetermined. This value, together with existing information on the salt and its saturated solution, gives –13.1 ± 0.4 J K–1 mol–1 for the standard entropy of the fluoride ion at 25°C.

THERMODYNAMIC PROPERTIES OF AISb AND AISb-GaSb SOLID SOLUTIONS*

Abstract: Thermodynamic data are frequently used in an analysis of the laws governing the change of properties ofA1II.Bv semiconducting compounds with change of pOSition of the constituent elements in the periodic table [1-3]. To resolve this problem, it is necessary to obtain sufficiently reliable data on the thermodynamics of the formation of such compounds. The thermodynamic properties of aluminum antimonide have been previously studied by the electromotive force (emf) method using aluminum chloride as the electrolyte in fused lithium and potassium chlorides [4]. The calculated value [4] of the standard entropy of solid aluminum antimonide Sg98 = 6.0 ± 0.8 eu/g-atom and that obtained by Piesbergen [5] from measurements of low-temperature specific heat, Bg98.16 = 7.68 ± 0.05 eu/ g-atom do not agree even within the limits of e1.!Perimental error (here, eu = entropy unit = cal! deg). For the isovalent solid solutions of AlII BV semiconductors, there are no published data on the free energy and the energy of their formation. We studied the thermodynamic properties of aluminum antimonide and the AlSb GaSb system by the emf method, using a solid electrolyte. The emf! s of the following galvanic cells were measured