Showing papers on "Standard molar entropy published in 1997"

Differential molar heat capacities to test ideal solubility estimations.

Abstract: Purpose. Calculation of the ideal solubility of a crystalline solute in a liquid solvent requires knowledge of the difference in the molar heat capacity at constant pressure of the solid and the supercooled liquid forms of the solute, ΔCp. Since this parameter is not usually known, two assumptions have been used to simplify the expression. The first is that ΔCp can be considered equal to zero; the alternate assumption is that the molar entropy of fusion, ΔSf, is an estimate of ΔCp. Reports claiming the superiority of one assumption over the other, on the basis of calculations done using experimentally determined parameters, have appeared in the literature. The validity of the assumptions in predicting the ideal solubility of five structurally unrelated compounds of pharmaceutical interest, with melting points in the range 420 to 470K, was evaluated in this study.

Thermodynamics of the Conversion of Chorismate to Prephenate: Experimental Results and Theoretical Predictions

Abstract: A thermodynamic investigation of the Claisen rearrangement of chorismate2-(aq) to prephenate2-(aq) has been performed using microcalorimetry and high-performance liquid chromatography. The study used a well-characterized monofunctional chorismate mutase from Bacillus subtilis that was devoid of prephenate dehydrogenase and prephenate dehydratase activities. The calorimetric measurements led to a standard molar enthalpy change = −(55.4 ± 2.3) kJ mol-1 at 298.15 K for this reaction. An estimated value of the standard molar entropy change = 3 J K-1 mol-1 for the above reaction was used together with the experimental value of to obtain a standard molar Gibbs free energy change ≈ −56 kJ mol-1 and an equilibrium constant K ≈ 7 × 109 for the conversion of chorismate2-(aq) to prephenate2-(aq) at 298.15 K. Thus, for all practical purposes, this reaction can be considered to be “irreversible”. Quantum mechanics (Gaussian 94 with a B3LYP functional and a 6-31G* basis set) was used to calculate values of absolute and...

Tetramethyl Carboxylic Acids Derived from o-Phenylenediamines as Sequestering Agents for Iron(III): Thermodynamic Studies. X-ray Crystal Structure of Sodium Aqua(4-chloro-1,2-phenylenediamine-N,N,N',N'-tetraacetato)ferrate(III)-Water (1/1.5)

Abstract: The coordinating ability of the ligands 3,4-toluenediamine-N,N,N‘,N‘-tetraacetic (3,4-TDTA), ortho-phenylenediamine-N,N,N‘,N‘-tetraacetic (o-PhDTA), and 4-chloro-1,2-phenylenediamine-N,N,N‘,N‘-tetraacetic (4-Cl-o-PhDTA) acids, H4L, with Cu(II) and Fe(III) is described. The stability constants and the partial molar enthalpies of the complexes formed (25 °C, I = 0.5 mol dm-3 in KNO3) were respectively determined by means of emf and calorimetry measurements. For the system 3,4-TDTA−Fe(III) (25 °C, I = 0.5 mol dm-3 in NaClO4), the stability constants of the complexes FeL- and [FeL(OH)]2- were also determined spectrophotometrically. The calorimetric measurements show that the complex formation is entropy driven, exothermic for Cu(II) and endothermic for Fe(III). The partial molar entropy of complexation (ΔS°) is much the same as that of the EDTA complexes. X-ray diffraction structural analysis of Na[Fe(4-Cl-o-PhDTA)(H2O)]·1.5H2O revealed that, in [FeL(OH2)]- (monoclinic C2/c, a = 10.693(3) A, b = 13.931(3) A, ...

Thermodynamics of various polyvalent main group elements in a borosilicate glass melt

Abstract: The thermodynamics of redox equilibria were studied by square-wave-voltammetry in a borosilicate glass melt at temperatures in the range of 700 to 1100°C. The glass melts were doped with oxides of As, Sb, Bi, Te, Sn and Pb. While lead and bismuth are reduced in one redox step to the metallic state, glass melts doped with the other elements have two well separated redox steps. Standard potentials linearly depend on temperature and hence both the standard enthalpy and the standard entropy are constant. The electrochemical series of elements is nearly the same as that reported in soda–lime–silica glass melts. With the exception of the Pb2+/Pb0-redox step, all standard potentials in the borosilicate glass melt are notably shifted to more negative potentials by comparison with those reported from soda–lima–silica glass melts.

Chapter 5 – calorimetric properties

Abstract: Publisher Summary The specific and molar heat capacities and the latent heats of crystallization or fusion are referred to as calorimetric properties. Both groups of properties can be calculated as additive molar quantities. Furthermore, starting from these properties, the molar entropy and enthalpy of polymers can be estimated. This chapter discusses the heat capacity. The specific heat capacity is the heat that must be added per kilogram of a substance to raise the temperature by one degree. The molar heat capacity is the specific heat multiplied by the molar mass (the molar mass of a structural unit in the case of polymers). Specific and molar heat capacity may be defined at constant volume or at constant pressure. The heat added causes a change in the internal energy and in the enthalpy of the substance. The complete course of the specific heat capacity as a function of temperature has been published for a limited number of polymers only. For all the polymers investigated, the curves for the molar heat capacity of solid and liquid may be approximated by straight lines, except for the solid below 150 K.

Thermochemical parameters of complexes of di-isobutyldithiocarbamate with phosphorus-group elements

Abstract: The standard molar enthalpy of formation of crystalline di-isobutyldithiocarbamate complexes of P, As, Sb and Bi(III) has been derived by solution calorimetry at 298.15 K. The corresponding standard molar enthalpies of sublimation were estimated by means of differential scanning calorimetry. From the standard molar enthalpies of formation of the gaseous chelates the homolytic and heterolytic mean metal-sulphur bond-dissociation enthalpies were calculated.

Equilibrium constants for the liquid-phase synthesis of isopropyl tert-butyl ether from 2-propanol and isobutene

Abstract: Equilibrium constants for the liquid-phase addition of 2-propanol to isobutene to give isopropyl tert-butyl ether (IPTBE) were determined experimentally in the temperature range 303−353 K and at 1.6 MPa. To reach etherification equilibrium, the macroporous sulfonic resin Bayer K-2631 was used as the catalyst. The UNIFAC estimates of activity coefficients were used to describe the liquid-phase nonideality. The thermodynamic equilibrium constants and the enthalpy, free energy, and entropy changes of the reaction were given as a function of temperature. At 298.15 K, the standard molar reaction enthalpy ΔrH°m(298.15 K) is −(22.9 ± 1.3) kJ·mol-1, in agreement with literature data, and the standard molar reaction entropy ΔrS°m(298.15 K) and free energy ΔrG°m(298.15 K) are −(60.3 ± 0.5) J·(mol·K)-1 and −(4.9 ± 1.4) kJ·mol-1, respectively. Finally, at 298.15 K the standard molar enthalpy of formation ΔfH°m(l, 298.15 K) and the standard molar entropy of IPTBE S°m(l, 298.15 K) were estimated to be −(378.5 ± 2.7) kJ...

Experimental determination of the upper stability of scorzalite, FeAl2[OH/PO4]2, and the occurrence of minerals with a composition intermediate between scorzalite and lazulite(ss) up to the conditions of the amphibolite facies

Abstract: The stability field of scorzalite (FeAl2[OH/PO4]2) was investigated in the P-T range from 487 to 684 °C and 0.1 to 0.3 GPa. in hydrothermal experiments. The oxygen fugacity was fixed by the Ni/NiO buffer. Scorzalite shows a decomposition according to the reaction: FeAl2[OH/PO4]2) → FeAlPO5 + AlPO4 (berlinite) + H2O. The mean standard enthalpy and standard entropy of reaction were determined as ΔH R 0 = 94(13) kJ, ASR = 180(16) JK−1. A57Fe-Mosbauer spectroscopic examination showed that about 4 atomic % of the total Fe in scorzalite is trivalent.

Measurement of the partial molar volume and solubility of hydrogen in its dilute solutions in palladium-silver alloys

Abstract: Linear expansion of thin-wall tubes of palladium-silver alloys containing 10, 20, 27, and 40 wt% silver was measured on absorption of small amounts of hydrogen (hydrogen-to-metal atom ratio, 0.005 to 0.25) at temperatures between 150°C (423 K) and 350°C (623 K) for equilibration with gaseous hydrogen at pressures up to around 1 bar. Hydrogen uptake by the test specimens was determined concurrently with the expansion measurements. Thermal expansion coefficients for the hydrogen-free materials were also obtained. Analysis of the data indicates that fractional length change is linearly dependent on hydrogen-to-metal atom ratio within the experimental range of hydrogen concentrations, independent of temperature, and varies only slightly with alloy composition. Partial molar volumes of hydrogen (with 95% confidence limits) vary from 1.77±0.08cm3·(mol H)−1 for the 10% Ag alloy to 1.92±0.03cm3·(mol H)−1 for the 40% Ag alloy. Hydrogen solubility results are correlated by a relation that facilitates linear extrapolation for determination of limiting values for equilibrium constants. Heats of solution of hydrogen at infinite dilution and standard entropy changes are essentially independent of temperature within the range of experimental conditions.

Ion Exchange of Bovine Serum Albumin at a Natural Organic Anion Exchanger: Thermodynamics and Energetics

Abstract: The thermodynamics and energetics of the ion exchange of bovine serum albumin (BSA) at a cellulosic ion exchanger (Whatman QA52) have been studied. Equilibrium isotherm analysis allowed the determination of the thermodynamic equilibrium constants (K) for a range of pH. The heats of exchange of the BSA at Whatman QA52 were obtained by high-sensitivity titration microcalorimetry. A thermodynamic analysis procedure was developed allowing calculation of the standard free energy, the standard enthalpy, and the standard entropy of exchange. The results show that the relative contributions of the enthalpy and entropy change radically depend on the solution conditions. The knowledge of these thermodynamic properties provided useful insights into the relative importance of the factors contributing to the overall ion-exchange process. This assessment hence provides a basis for guidelines for more effective separation and for establishing a more practical model for theoretical prediction of ion-exchange performance.

Effect of Various Factors on the Sorption of Endosulphan on Two Indian Soils

Abstract: The effects of exchangeable cations (H + and Na + ), organic matter, non-ionic and anionic surfactants, and temperature on the sorption of endosulphan on two different types of soils were studied. Adsorption studies were performed using the batch technique and adsorption isotherms for all effects/treatments were in close agreement with the Freundlich equation. The adsorptive capacity of endosulphan towards organic matter and clay content in the natural forms of both soils was also evaluated by calculating the corresponding values of the specific adsorption capacity for organic matter and the clay content. various thermodynamic parameters such as the thermodynamic equilibrium constant, standard free energy changes, standard enthalpy changes and standard entropy changes have been calculated in order to predict the nature of the isotherms.

Thermodynamic properties of the molten Tl-GeTe4 system

Abstract: From the electromotive force method, the chemical potential or activity of Tl in the molten Tl-GeTe 4 system has been measured as a function of composition and temperature. The chemical potential of Tl has then been separated in terms of partial entropy and enthalpy, the both quantities showing an abrupt change from negative to positive values around the stoichiometric composition corresponding to Tl 8 GeTe 4 . The integral molar entropy and enthalpy, ΔS m and ΔH m , show sharp minima at the composition Tl 8 GeTe 4 where the electrical conductivity has a sharp minimum and the thermoelectric power changes its sign. From these results it is suggested that the strong interaction between Tl and Te leads to form a chemical entity Tl 2 Te and the observed thermodynamic properties can be interpreted as a mixture of Tl 2 Te and unbounded atomic Ge and Te in the GeTe 4 -rich side and or atomic Ge and Tl in the Tl-rich side.