Showing papers on "Standard molar entropy published in 2002"

Entropy Explained: The Origin of Some Simple Trends

Abstract: Density functional theory computational methods were used to calculate the entropies of various molecules; computed entropies correlated closely with measured values. For organic systems, an average of 8.4 kcal/mol for the reaction entropy (one particle to two at 298.15 K) was observed; this value is largely determined by translational entropy gain. The average reaction entropy is slightly lower for reactions that produce two linear molecules and up to 4 kcal/mol higher when no linear molecules are produced, due to differences in rotational entropy of the reactants and products. Translational and rotational entropy are generally independent of molecular identity except for increases in mass and generation of additional moments of inertia; vibrational entropy, which is more dependent on the molecule itself, is a small contributor to the nearly constant entropy of reaction. A variety of inorganic and non-hydrocarbon main group reaction entropies were also calculated; there is an increased contribution of vi...

Thermodynamic properties of nickel chromite (NiCr2O4) based on adiabatic calorimetry at low temperatures.

Abstract: The hitherto unknown low-temperature heat capacity of nickel chromite (NiCr2O4) was measured between 8 and 381 K using adiabatic calorimetry, and some thermochemical functions [CP(T), S(T), S°298, H(T)−H(0)] were derived from the results. The standard entropy (S°298=140.0 ± 0.3 J mol−1 K−1) for nickel chromite was calculated from the results. Our calorimetric measurements indicate three major anomalies in the heat-capacity curve at temperatures between 8 and 381 K. A short literature review indicates that two of these anomalies can be accounted for, whereas an anomaly peaking at 29 K has not been reported previously.

Thermodynamics and crystal chemistry of the hematite–corundum solid solution and the FeAlO 3 phase

Abstract: High-temperature oxide-melt calorimetry and Rietveld refinement of powder X-ray diffraction patterns were used to investigate the energetics and structure of the hematite–corundum solid solution and ternary phase FeAlO3 (with FeGaO3 structure). The mixing enthalpies in the solid solution can be described by a polynomial ΔHmix=WX hem(1−X hem) with W=116 ± 10 kJ mol−1. The excess mixing enthalpies are too positive to reproduce the experimental phase diagram, and excess entropies in the solid solution should be considered. The hematite–corundum solvus can be approximately reproduced by a symmetric, regular-like solution model with ΔG excess=(W H −TW S )X hem X cor, where W H= 116 ± 10 kJ mol−1 and W S =32 ± 4 J mol−1 K−1. In this model, short-range order (SRO) of Fe/Al is neglected because SRO probably becomes important only at intermediate compositions close to Fe:Al=1:1 but these compositions cannot be synthesized. The volume of mixing is positive for Al-hematite but almost ideal for Fe-corundum. Moreover, the degree of deviation from Vegard's law for Al-hematite depends on the history of the samples. Introduction of Al into the hematite structure causes varying distortion of the hexagonal network of oxygen ions while the position of the metal ions remains intact. Distortion of the hexagonal network of oxygen ions attains a minimum at the composition (Fe0.95Al0.05)2O3. The enthalpy of formation of FeAlO3 from oxides at 298 K is 27.9 ± 1.8 kJ mol−1. Its estimated standard entropy (including configurational entropy due to disorder of Fe/Al) is 98.9 J mol−1 K−1, giving the standard free energy of formation at 298 K from oxides and elements as +19.1 ± 1.8 and −1144.2 ± 2.0 kJ mol−1, respectively. The heat capacity of FeAlO3 is approximated as C p (T in K)= 175.8 − 0.002472T − (1.958 × 106)/T 2− 917.3/T 0.5+(7.546 × 10−6) T 2 between 298 and 1550 K, based on differential scanning calorimetric measurements. No ferrous iron was detected in FeAlO3 by Mossbauer spectroscopy. The ternary phase is entropy stabilized and is predicted to be stable above about 1730 ± 70 K, in good agreement with the experiment. Static lattice calculations show that the LiNbO3-, FeGaO3-, FeTiO3-, and disordered corundum-like FeAlO3 structures are less stable (in the order in which they are listed) than a mechanical mixture of corundum and hematite. At high temperatures, the FeGaO3-like structure is favored by its entropy, and its stability field appears on the phase diagram.

Structure and Thermodynamics of Uranium(VI) Complexes in the Gas Phase: A Comparison of Experimental and ab Initio Data

Abstract: Ab initio methods were applied for the calculation of the total energy and the molar entropy and heat capacity of the compounds UO2F2, UO2(OH)(2), UF6, and UO3 in the gas phase with the purpose to ...

Structure-function relationships in high-density octadecylsilane stationary phases by Raman spectroscopy. 1. Effects of temperature, surface coverage, and preparation procedure.

Abstract: Raman spectroscopy is used to examine the effects of temperature, surface coverage, nature of the alkylsilane precursor (octadecyltrichlorosilane, methyloctadecyldichlorosilane, or dimethyloctadecylchlorosilane), and surface grafting method (surface or solution polymerized) on alkyl chain conformational order in a series of high-density octadecylsilane stationary phases ranging in surface coverage from 3.09 to 6.45 micromol/m2. Conformational order is assessed using the intensity ratio of the antisymmetric and symmetric v(CH2) modes as well as the frequency at which these Raman bands are observed. Conformational order increases with surface coverage. Temperature-induced surface phase changes are observed between 258 and 343 K for this homologous series of stationary phases that are demonstrated to adhere to the Clapeyron equation for a simple first-order transition. Phase changes are discussed in terms of variation of the molar enthalpy, molar entropy, and molar volume of the stationary phase, all of which depend on surface coverage. For the limited range of systems investigated, a correlation between stationary-phase preparation (surface versus solution polymerized and nature of the silane precursor) and extent of alkyl chain order is not clearly observed.Instead, akyl chain order is largely dependent on bonding density. A molecular picture of temperature-induced disorder in octadecylsilane stationary phases is proposed, with disorder originating at the distal carbon and propagating toward the proximal carbon.

Complexation of the La(III) cation by p-sulfonatocalix[4]arene A 139La NMR study

Abstract: The complexation of La(III) by the water-soluble p-sulfonatocalix[4]arene was thermodynamically characterized by 139La NMR. The 139La NMR data are consistent with the formation of a 1:1 complex resulting from electro static interactions between the sulfonato groups and La(III). The complexation is entropy-driven and is characterized by a positive standard enthalpy (ΔrH° = 11.0 ± 0.5 kJ mol–1) and a positive standard entropy (ΔrS° = 108 ± 2 J K–1 mol–1), which are in very good agreement with the ones determined previously by microcalorimetry. The linear relationship between the 139La NMR linewidth of the free and of the complexed cation, obtained at temperatures ranging from 290 to 340 K, excludes the formation of complexes or aggregates other than the 1:1 complex. It shows also that upon complexation, the effective radius of La(III) undergoes an increase of 50%, related to the replacement of water molecules of the hydrated cation by sulfonato groups of the ligand.Key words: complexation, water-soluble cal...

Heat capacity of hafnium mononitride from temperatures of 5 to 350 K: An estimation procedure

Abstract: Measurements of the heat capacity by quasi-adiabatic, intermittent energy increments from 5 to 350 K show small high heat-capacity anomalies near 7 and 10 K which are attributed to superconducting transitions seen by magnetic measurements on the same carefully synthesized and well-characterized sample of (Hf0.934Zr0.057)(N0.97). Although no previous heat capacity measurements over the cryogenic region are known, the estimated 298.15 K standard entropy values (S/R) vary in the literature from about 200 per cent higher to 5 per cent lower than our measured value of (5.28±0.01)R−1 when the formula is represented as above. A simple scheme to represent and predict values based on both molar volumes and atomic masses for related materials is presented which seems more reliable on a limited sample than do others despite the intrusion of lanthanide contraction.

Kinetics and mechanisms of oxidation of 1-octene and heptanal by crown ether-solubilized potassium permanganate in non-aqueous solvents

Abstract: The kinetics of oxidation of 1-octene and heptanal by 18-crown-6-ether-solubilized KMnO4 in benzene and CH2Cl2 have been investigated. In benzene, the oxidation of 1-octene is first order with respect to the oxidant and zero order with respect to the substrate, whereas in CH2Cl2 the reaction is first order with respect to both substrate and oxidant. The reaction of heptanal followed different kinetics being first order with respect to both substrate and oxidant, regardless of whether benzene or CH2Cl2 was employed as the solvent. The values of activation energy Ea, standard enthalpy DH � , standard entropy change DS � , and standard free energy DG � , for the reaction, are reported. Mechanistic pathways for the studied reactions are also proposed.

Comparative study of the adsorption of n-aliphatic alcohols on metal-doped alumina samples

Abstract: A study has been made of the adsorption isotherms of n-aliphatic alcohols (methanol, ethanol, 1-propanol, and n-butanol designated C1, C2, C3, and C4, respectively) on parent and metal-doped γ-alumina samples in the temperature range 273–313K as a function of temperature and coverage, using CAHN 1000 electrobalance. It is noted from the data that at a given relative pressure, the adsorbed amount for the sample treated at the same temperature decreases in the order From the adsorption data, thermodynamic parameters such as isosteric heat of adsorption (q st), free energy (ΔG 0), differential enthalpy (ΔH), and molar entropy (ΔS 0) of adsorption have been calculated as a function of temperature and coverage. The values of q st are found to be higher for metal-doped alumina which may be due to the coordination of n-aliphatic alcohols to metal vacancies present on the surface of alumina. It has been observed that at a given relative pressure, the enthalpy of adsorption for n-aliphatic alcohols (C1–C4) treated...

Thermodynamics of the micellization of cationic surfactants 1-hexadecyltrimethylammonium bromide and 1-hexadecylpyridinium bromide. Part 1: Effect of urea

Abstract: The critical micelle concentrations and degrees of counterion binding of the ionic surfactants 1-hexadecyltrimethylammonium bromide (HTMABr) and 1-hexadecylpyridinium bromide (HPBr) in water and aqueous solutions of urea in a wide range of concentrations were determined at various temperatures (20-50 °C) by the electrical conductivity method. While the c.m.c. values increased with increasing temperature and concentration of urea in the solution, the values for the degree of counterion binding decreased. The thermodynamic parameters of micellization -standard molar Gibbs energy, enthalpy, entropy, heat capacity of micellization and transfer values - were determined. Negative values for molar Gibbs energy and positive values for molar entropy of micellization were observed in all the systems studied. The micellization process was exothermic in all cases.

The thermodynamic properties of sodium uranoborate

Abstract: The standard enthalpy of formation of crystalline NaBUO 5 at 298.15 K (-2257.5 ′ 4.0 kJ/mol) was determined by reaction calorimetry. The heat capacity of the substance in the temperature range 10-300 K was studied by adiabatic vacuum calorimetry. The thermodynamic functions of the compound were calculated. The standard entropy and the Gibbs function of formation of sodium uranoborate at 298.15 K were found to be -452.7 ′ 2.1 J/(mol K) and -2122.5 ′ 4.5 kJ/mol, respectively. The standard thermodynamic functions of synthesis and thermal decomposition of sodium uranoborate were calculated and analyzed.

A thermodynamic analysis of the dispersive interactions between atoms of hydrogen, carbon, chlorine, and bromine and the stationary phase n-octadecane

Abstract: The standard enthalpies and entropies of hydrogen, carbon, chlorine, and bromine atoms, when interacting dispersively with n-octadecane, are calculated from retention measurements made on some substituted methanes. A clean linear correlation between the standard enthalpy and standard entropy of interaction is demonstrated and it is also shown that there is a linear relationship between the standard free enthalpy of interaction of the elements investigated and their atomic polarizability. The results also indicate that there is another factor, independent of the element polarizability that contributes to the standard enthalpy of interaction. It is postulated that this factor may be a function of some property of the stationary phase.

The heat capacity and thermodynamic functions of tripropylarsine

Abstract: The heat capacity of tripropylarsine was measured in an adiabatic vacuum calorimeter in the temperature range 6-300 K. The vitrification and melting temperatures were determined and the thermodynamic functions of tripropylarsine were calculated for the vitreous, crystalline, and liquid states in the temperature range of measurements. The standard entropy of formation of As(n-C 3 H 7 ) 3 at 298.15 K was calculated. The zero entropy of the substance in the vitreous state and the difference of its zero enthalpies in the crystalline and vitreous states were estimated.

Thermoelectric power studies on MgO-stabilized β″-alumina

Abstract: A study of the temperature dependence of thermopower is known to yield auxiliary information about the electronic conductivity of a mixed conductor. In light of the above, thermoelectric power (TEP) measurements were made on MgO-stabilized β″-alumina over the temperature range from 773 to 1223 K under conditions of different sodium activities in the ambient in order to substantiate the existing information on the electronic conductivity of sodium beta alumina (SBA). A mixture of NaxSimO2m+x/2 and SiO2 in an environment of fixed \(P_{{\rm O}_{\rm 2} } \) served as electrodes reversible to Na+. The heat of transport obtained using the thermopower data at higher temperatures (973–1223 K) was in fair agreement with the activation energy of electrical conduction determined by other studies like impedance measurements and molecular dynamics simulation. It could be inferred from these results that there is negligible electronic conductivity in SBA under the conditions of measurement. The average TEP for SBA was determined to be 700–800 µV/K and the partial molar entropy of Na+ in SBA was found to be ~98 J mol–1 K–1.

A computational study of entropy rules for charged fullerenes

Abstract: Thermochemical data on fullerenes are relatively scarce. However, some thermochemical information can be derived from gas-phase experiments using the Knudsen cell mass spectrometry method. The third-law treatment can be carried out on the observed data, though one has to make the crucial presumption that the change in the thermodynamic potential ΔΦ T o in the course of the reactions considered is negligible: ΔΦ T o =0. It would be difficult to check the presumption directly in the experiment, but it can be checked computationally. Model reactions like C60+ 70 − = 60 − +70 are selected. The change in the thermodynamic potential ΔΦ T o and the change in the standard entropy ΔS T o are computed. For example, at a temperature of T=1000 K, the standard changes for the reaction evaluated using the SAM1 method are ΔΦ T o =1.513 cal/(mol K) and ΔS T o =−0.054 cal/(mol K). Overall, the computations support the critical thermodynamic presumption.

Hydrogen Absorption Thermodynamic Properties of Rare-Earth Based Hydrogen Storage Alloy in Benzene

Abstract: The hydriding/dehydriding thermodynamic properties of the slurry system formed by suspending La-rich mischmetal nickel hydrogen storage alloy (M1Ni5) in Benzene (C6H6) were investigated. Theisotherms for both the alloy powder and the slurry suspended with MINi5 were measured at several temperatures( 10,20, 30, 40℃ ). The standard enthalpy of formation ΔH° and standard entropy of formation ΔS° for the alloy pow-der with and without benzene were determined respectively. The experimental results show that the values of ΔH°and ΔS° for the hydriding reaction of hydrogen storage alloy (MINi5) of the slurry system and the gas-solid systemare all very close.