Showing papers on "Standard molar entropy published in 2005"

Monte Carlo simulations of gas solubility in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate.

Abstract: The Henry's constants of water, carbon dioxide, ethane, ethene, methane, oxygen, and nitrogen are computed in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) using test particle insertion and expanded ensemble Monte Carlo methods. The partial molar enthalpy and partial molar entropy of solvation are also computed for water, carbon dioxide, and oxygen. The results from the simulations are compared against experimental data from the literature. In addition, the accuracy and precision of the two methods in determining the Henry's constant are examined. Local organization of the ionic liquid around a solute molecule is analyzed, and the interactions responsible for the experimentally observed solubility trends are identified.

Solubilities of CO2 in 1-butyl-3-methylimidazolium hexafluorophosphate and 1,1,3,3-tetramethylguanidium lactate at elevated pressures

Abstract: The solubilities of CO2 in 1-butyl-3-methylimidazolium hexafluorophosphate and 1,1,3,3-tetramethylguanidium lactate at the temperatures ranging from (297 to 328) K and the pressures ranging from (0 to 11) MPa were determined. The extended Henry's law was applied to correlate the solubility data, and the thermodynamic properties such as standard enthalpy, standard Gibbs free energy, standard entropy, and standard heat capacity of these two systems were obtained. Experimental results showed that the solubilities of CO2 in TMGL are slightly higher than those in [bmim][PF6].

Solubility of CO2 in Sulfonate Ionic Liquids at High Pressure

Abstract: In this work, the solubility of CO2 in sulfonate ionic liquids (ILs), such as trihexyl (tetradecyl) phosphonium dodecylbenzenesulfonates ([P6,6,6,14][C12H25PhSO3]) and trihexyl (tetradecyl) phosphonium mesylate ([P6,6,6,14][MeSO3]), was determined at temperatures ranging from (305 to 325) K and pressures ranging from (4 to 9) MPa. It was found that the difference of the solubility of CO2 in two kinds of sulfonate ILs is not dramatic on the basis of molality. The solubility of CO2 in [P6,6,6,14][MeSO3] is higher than that in [P6,6,6,14][C12H25PhSO3]. The solubility data were correlated by means of the extended Henry's law, and the thermodynamic functions, such as the standard enthalpy, standard Gibbs free energy, and standard entropy, were obtained. The Henry's law constant for CO2 in all the investigated ionic liquids increases with increasing temperature.

Thermal and mechanical properties of BaWO4 crystal

Abstract: A large single crystal of barium tungstate (BaWO4) with dimensions of 22-mmdiameter×80-mm length was grown by the Czochralski method using an iridium crucible. The melting point, molar enthalpy of fusion, and molar entropy of fusion of the crystal were determined to be 1775.10 K, 96913.80Jmol−1, and 54.60JK−1mol−1, respectively. The average linear thermal-expansion coefficients are αa=10.9526×10−6∕K, αb=10.8069×10−6∕K, and αc=35.1063×10−6∕K in the temperature range from 303.15 to 1423.15 K along the three respective crystallographic axes. The density of the crystal follows an almost linear decrease from 6.393×103 to 6.000×103kgm−3 when the temperature is increased from 303.15 to 1423.15 K. The measured specific heat are 115.56–130.96JK−1mol−1 in the temperature range of 323.15–1173.15 K. The thermal diffusion coefficient of the crystal was measured in the temperature range of 297.15–563.15 K. The calculated thermal conductivity is 2.256Wm−1K−1 along the [001] direction and 2.324Wm−1K−1 along the [100] dir...

Calorimetric Study of Mg2Zn3

Abstract: The thermodynamic properties of Mg 2 Zn 3 were investigated by calorimetry. The standard entropy of formation at 298 K, Δ f S o 2 9 8 , was determined from measuring the heat capacity, C p , from near-absolute zero (2 K) to 300 K by the relaxation method. The standard enthalpy of formation at 298 K, Δ f H o 2 9 8 , was determined by solution calorimetry in hydrochloric acid solution. The standard Gibbs energy of formation at 298 K, Δ f G o 2 9 8 , was determined from these data. The results obtained were as follows: Δ f H o 2 9 8 (Mg 2 Zn 3 ) = -69.80 ′ 20 kJ . mol - 1 ; Δ f S o 2 9 8 (Mg 2 Zn 3 ) = -10.95 ′ 1.80 J . K - 1 . mol - 1 ; Δ f G o 2 9 8 (Mg 2 Zn 3 ) = -66.55 ′ 20 kJ mol - 1 . The electronic contribution to the heat capacity of Mg 2 Zn 3 was found to be similar to pure magnesium, indicating that the density of states in the vicinity of the Fermi level follows the free-electron parabolic law.

A Microcalorimetric Study of Host–Guest Complexes of α-Cyclodextrin with Alkyl Trimethyl Ammonium Bromides in Aqueous Solutions

Abstract: Interactions between α-CD and three alkyl trimethyl ammonium bromides, a homologues series of surfactants, in aqueous solutions have been investigated with titration microcalorimetry at 298.15 K. The results are discussed in the light of the amphiphilic interaction and the iceberg structure of water molecules existing around the hydrophobic tail of the surfactant. The stoichiometry of the host–guest complex changes from 1:1 to 2:1, as the number of carbon atoms (n) in the hydrophobic chain, CnH2n+1, increases from 8 to 14. All the complexes are quite stable, with the apparent experiential stability constants being β1 = 2.65 × 103 dm3-mol−1, β2 = 4.85 × 106 dm6-mol−2, β2 = 6.50 × 106 dm6-mol−2, respectively for n = 8, 12, 14. All the complexation processes are shown to be enthalpy driven, and the standard enthalpy effect (−ΔH0) increases while standard entropy change (ΔS0) decreases with elongation of the hydrophobic chain.

Thermodynamic Properties of Uvarovite Garnet (Ca3Cr2Si3O12).

Abstract: The low-temperature heat capacity of uvarovite (Ca3Cr2Si3O12) was measured between 2 and 400 K, and thermochemical functions were derived from the results The measured heat-capacity curve shows a signiÞ cant lambda-shaped anomaly peaking at around 9 K The nature of this transition is unknown From our data, we suggest a standard entropy for uvarovite at 29815 K of 3209 ± 06 J/(mol·K)

Stabilities of the Aqueous Complexes Cm(CO3)33- and Am(CO3)33- in the Temperature Range 10−70 °C

Abstract: The carbonate complexation of curium(III) in aqueous solutions with high ionic strength was investigated below solubility limits in the 10-70 C temperature range using time-resolved laser-induced fluorescence spectroscopy (TRLFS). The equilibrium constant, K3, for the Cm(CO3)2- + CO32- Cm(CO3)33- reaction was determined (log K3 = 2.01 ± 0.05 at 25C, I = 3 M (NaClO4)) and compared to scattered previously published values. The log K3 value for Cm(III) was found to increase linearly with 1/T, reflecting a negligible temperature influence on the corresponding molar enthalpy change, rH3 = 12.2 ± 4.4 kJ mol-1, and molar entropy change, rS3 = 79 ± 16 J mol-1 K-1. These values were extrapolated to I = 0 with the SIT formula (rH3 = 9.4 ± 4.8 kJ mol-1, rS3 = 48 ± 23 J mol-1 K-1, log K3 = 0.88 ± 0.05 at 25C). Virtually the same values were obtained from the solubility data for the analogous Am(III) complexes, which were reinterpreted considering the transformation of the solubility-controlling solid. The reaction studied was found to be driven by the entropy. This was interpreted as a result of hydration changes. As expected, excess energy changes of the reaction showed that the ionic strength had a greater influence on rS3 than it did on rH3.

Low-Temperature Heat Capacities and Derived Thermodynamic Functions of 1,4-Dichlorobenzene, 1,4-Dibromobenzene, 1,3,5-Trichlorobenzene, and 1,3,5-Tribromobenzene

Abstract: The heat capacities of 1,4-dichlorobenzene, 1,4-dibromobenzene, and 1,3,5-trichlorobenzene from 5 K to 380 K and 1,3,5-tribromobenzene from 5 K to 410 K were measured by adiabatic calorimetry. The experimental data were used to calculate the molar entropy and enthalpy values relative to 0 K. Apart from 1,4-dichlorobenzene, the substances do not show any solid−solid transitions. Molar enthalpies of fusion and melting-point temperatures were determined. The results, given in order, are (17 907 ± 15) J·mol-1 and (326.24 ± 0.03) K, (20 387 ± 15) J·mol-1 and (360.48 ± 0.03) K, (17 557 ± 35) J·mol-1 and (335.92 ± 0.03) K, and (21 721 ± 20) J·mol-1 and (394.96 ± 0.07) K.

Low-temperature heat capacity of magnesioferrite (MgFe2O4)

Abstract: The low-temperature heat capacity of magnesioferrite (MgFe2O4) was measured between 1.5 K and 300 K, and thermochemical functions were derived from the results. No heat capacity anomaly was observed. From our data, we suggest a standard entropy (298.15 K) for magnesioferrite of 120.8±0.6 J mol−1 K−1, which is about 2.4 J mol−1 K−1 higher than previously reported calorimetric studies; but is in rough agreement with predictions from sets of internally consistent thermodynamic data.

Dielectric study of aqueous solutions of alanine and phenylalanine

Abstract: Dielectric relaxation measurements on aqueous solutions of alanine and phenylalanine were carried out using time domain reflectometry (TDR) at 25, 30, 35, and 40 °C in the frequency range from 10 MHz to 20 GHz. Aqueous solutions of alanine and phenylalanine are prepared for five different molar concentrations of the respective amino acid. For all the solutions considered, only one relaxation peak was observed in this frequency range. The relaxation peaks shift to lower frequency with an increase in alanine and phenylalanine molar concentration. The molar enthalpy of activation and molar entropy of activation show endothermic interactions.

Thermodynamics of Reaction of Carbon with Oxygen

Abstract: The standard Gibbs energies of reactions proceeding in the C-O system are calculated per mole of carbon dioxide, per mole of the reaction product, and per mole of atoms of the initial substances. A comparative analysis of the calculation results is made.

Energy and entropy of crystals, glasses and melts

Abstract: The molar entropy, S, and enthalpy (energy), H, of crystals, glasses and melts of the same one-component systems have been suitably visualized including the transformation from the melt into a glass or crystallization. For the temperature T → 0 K the enthalpy and entropy of the glass are larger by ΔH 0 and ΔS 0 as compared to the stable crystal. The S and H functions of glasses correspond to a simple continuation of these functions from the molten state to lower temperatures. Crystallization occurs as a spontaneous process under production of entropy. Extrapolating the entropy of the molten and crystalline states from the melting range to lower temperatures, which is the basis of Kauzmann's paradox, is ambiguous and misleading, as the extrapolated data deviate considerably from the experimental temperature dependencies of S of glasses and crystals. A proper extrapolation does not cause an entropy catastrophe as claimed in Kauzmann's paradox, since the enthalpy difference between the undercooled melt and the corresponding crystals must be taken into account, and the respective entropies in both states are not connected by an isothermal process. The molar entropy and enthalpy are visualized as functions of temperature by numerical results of a Debye model. The molar entropy is a universal function of the ratio T/T D , wherein T D is the Debye temperature of the well known specific heat capacity, C C . Between 0 K and T D the entropy increases by 1.36 x 3R 4R irrespective of T D . Above T D it increases approximately as 3R × ln(T/T D ). The entropy capacity, C D /T, scales with 1/T D and the enthalpy with T D , both considered as functions of T/T D . The entropy capacity shows a maximum of 2.033 x 3R/T D for T/T D = 0.28.