Showing papers on "Standard molar entropy published in 2001"

The assessment of thermodynamic parameters in the Al2O3‐Y2O3 system and phase relations in the Y‐Al‐O system

Abstract: Thermodynamic parameters for solid and liquid phases in the Al 2 O 3 -Y 2 O 3 system are assessed using new calorimetric measurement for the YAG (Y 3 Al 5 O 12 ), YAP (YAlO 3 ) and YAM (Y 4 Al 2 O 9 ) phases. The calculated phase diagram of the Al 2 O 3 -Y 2 O 3 system is in reasonable agreement with experimental data. According to the calculations, the YAP phase melts congruently and is stable down to the low temperatures, while the YAM phase disproportionates to a mixture of YAP and Y 2 O 3 phases at temperatures below 1385 K. The calculated entropy of the YAG phase 300.1 J/(mol.K) is between 2 experimentally determined values 284.8 and 349.1 J/(mol K). However, the difference between calculated and experimental values exceeds uncertainty limits of adiabatic calorimetry data. The enthalpies of melting for the YAG and YAP phases calculated in this study are in reasonable agreement with DTA measurements. The calculated enthalpy of melting for the YAG phase is not consistent with estimates based on solution calorimetric data. New independent measurement of the standard entropy and enthalpy of melting are desirable for the YAG, YAM and YAP phases. The liquidus surface and isothermal section at 2000 K for ternary Al-Y-O system are calculated in this study.

Specific heat capacity and thermal conductivity of NdGaO3 and LaAlO3 single crystals at low temperatures

Abstract: For single-crystalline samples of NdGaO3 and LaAlO3 the specific heat capacity cp(T) (temperature ranges 2.24-100 K and 4.35-350 K, respectively) and the thermal conductivity κ(T) (15-300 K) are determined. From an analysis of the specific heat capacity of NdGaO3 the crystalline electric field contribution and the high-temperature tail from the magnetic ordering are extracted. The thermodynamic standard values for enthalpy and entropy at 298.15 K are calculated: H° = 18.40(18) kJ mol-1, S° = 121.0(1.2) J mol-1 K-1 (NdGaO3); H° = 14.57(22) kJ mol-1, S° = 86.6(1.3) J mol-1 K-1 (LaAlO3). Significant contributions to the entropy from the magnetic ordering of the Nd3+ ions are included in the calculation. A recently published value of the standard entropy for the related NdAlO3 is re-evaluated (S° = 105.1(3) J mol-1 K-1) in this context. The thermal conductivity of NdGaO3 at low temperatures is limited by spin-wave scattering, while that of LaAlO3 in the same temperature range is lessened by the short phonon mean free path, which is due to the presence of twin domains.

Comparison of the thermodynamic properties of structurally related amphiphilic antidepressants in aqueous solution

Abstract: The critical concentrations of amitriptyline, desipramine and nortriptyline hydrochlorides in aqueous solution were determined over the temperature range 288–313 K by a method based on deconvolution into Gaussians of the second derivate of the measured specific conductivity data. The mass-action model was used to calculate the thermodynamic quantities: standard free Gibbs energy, standard enthalpy and standard entropy of aggregate formation. The results are discussed in terms of the structure of the drug aggregates.

Thermodynamic and physico-chemical descriptors of chloronaphthalenes: an attempt to select features explaining environmental behaviour and specific toxic effects of these compounds

Abstract: physico-chemical descriptors were used to characterize all 75 congeners of chloronaphthalene in terms of their environmental stability and specific dioxin-like toxicity. A prepared basic thermodynamic and physico-chemical property data matrix of PCNs was interpreted using Principal Component Analysis (PCA). The PCA of the thermodynamic and physico-chemical data matrix created a four-dimensional model that explained 76% (58% + 9% + 5% + 4%) of the total variance. The loading plot shows that the first PC is influenced by variables describing degree of chlorination, molecular weight, polarizability and lipophilicity. The best positively correlated descriptors are: retention time, standard molar entropy, heat capacity, a first-order molecular connectivity index, logarithm of octanol-water partition coefficient, the Wiener Index, specific polarizability, a third order shape index for molecules, the sum of absolute of the charges on each atom of the molecule, molecular weight, polarizability, refractivity, solvent-accessible surface, van der Waals surface, solvent-accessible volume, van der Waals volume. Negatively correlated descriptors are: standard enthalpy of formation and energy of HOMO. The second PC is strongly influenced by energy of LUMO, while substitution pattern parameters, number of chlorine atoms at α-positions and vicinal (adjac- ent) carbon atoms substitution pattern are less important parameters. The third PC depends on dipole moment and the largest negative charge, and on substitution at position 2 of naphthalene nuclei, while the symmetry group parameter is determined by PC4. There are small groups consisting of compounds which have similar values of LUMO energy and substitution pattern. The congeners of CN substituted with chlorine at positions 1, 2, 3, 6 and 7 (Fv/Fv PCN congeners), and next those substituted at positions 1, 2, 3

Thermodynamics of the Hydrophobic Effect

Abstract: A thermodynamic theory is developed describing the transfer process of nonpolar substances from water to their pure state and the micellization process. The basis of the theory is the consideration, experimentally supported, that to a first-order approximation the change in the standard molar entropy (ΔS°) can be consider proportional to the standard molar heat capacity (ΔCpo) with proportionality constant, r, independent of the temperature and the pressure. In general it is found that the value of the magnitude of r decreases with the size of the solute molecule until a limiting value. This level of approximation is sufficient to predict observations such as linear plots between TΔS° and ΔH°, reduced behavior for surfactants with the temperature, and an important part of accepted phenomenology in the literature about the hydrophobic effect.

Phase transitions in lawsonite a calorimetric study

Abstract: The specific heat of lawsonite, CaAl 2 Si 2 O 7 (OH) 2 .H 2 O, has been measured in the temperature range 125 K - 325 K. An anomaly is seen at 273 K, which is related to the Cmcm — Pmcn phase transition. The magnitude of the total excess entropy associated with this transition is not reproducible, varying in the range 5.93 — 6.24 J K −1 mol −1 . On heating, the specific heat anomaly is consistent with a tricritical phase transition. However, on cooling, significant hysteresis is observed, and the form of the C P anomaly is quite different. In all measurements extensive pre-transitional effects are observed above T C . Analysis of existing specific heat data in the temperature range 75 K — 175 K shows an anomaly associated with the Pmcn — P2 1 cn phase transition. The excess entropy associated with this transition is 6 (1) J K −1 mol −1 . These data are interpreted as showing that both transitions are caused by the interaction of proton ordering and displacive changes in the aluminosilicate framework. The standard entropy of lawsonite at 298 K is recalculated, incorporating the effects of the two transitions. Two methods are used for this recalculation, giving values of S 0 298 = 233.27 and 234.96 JK −1 mol −1 respectively.

Gaseous-phase proton affinity of anilines: A quantum chemical evaluation and discussion in view of aqueous basicity

Abstract: Using the PM3 method, enthalpies and free energies of the gaseous-phase proton affinity (PA ) have been computed for aniline and 62 of its derivatives with different kinds of electron-donor and electron-acceptor substitution in the aromatic ring and at the nitrogen atom. Linear correlations of the type pKa vs. PA have been found. Deviations of the data for ortho substituted anilines from the above relationships was discussed in view of possible hydrophobic hydration of the molecular fragments ajacent to the protonation centre. Linear dependeces Pexper = bPtheor (where P is standard entropy, heat or Gibbs energy of formation, first ionization potential, molecular dipole moment) were found.

An electrochemical investigation of the thermodynamic properties of the NaCl-AlCl3 system at subliquidus temperatures

Abstract: The phase relations in the NaCl-AlCl3 system ( $$0.3 \leqslant N_{AlCl_3 } \leqslant 0.65$$ ) have been determined in the temperature range from 373 to 623 K by isothermal equilibration, electrical conductivity, and electromotive force measurements. Only one ternary compound, NaAlCl4, was found to be stable, with a melting point of 426 K. The standard Gibbs energy of formation of NaCl and NaAlCl4 has been measured in the temperature range from 423 to 623 K by a novel galvanic cell technique involving in-situ electrogenerated chlorine electrode in the Na/β″-alumina/NaCl, NaAlCl4/Cl2,C and Al/NaCl, NaAlCl4/Cl2,C cells along with the Na/β″-alumina/NaCl,NaAlCl4/Al cell. The Δ f G NaCl( )/o and $$\Delta _f G_{NaAlCl_4 (l)}^ \circ $$ values have been calculated as −412.4+0.095 T (±1) kJ mol−1 and −1117.5+0.2460 T (±2) kJ mol−1, respectively. The standard entropy of NaAlCl4 (s) at 298 K, computed from the results of the study and the auxiliary information from the literature (184 J K−1 mol−1), show good agreement with the estimated JANAF value (188.28 J K−1 mol−1). The enthalpy of formation of NaAlCl4 (l) from NaCl (s) and AlCl3 (s) at 550 K obtained in the present study (−1850 J mol−1) is in agreement with that computed from the heat-capacity measurements (−1910 J mol−1). The present measurements are unique, as a new electrochemical technique is employed in a cell with low-melting sodium chloroaluminate electrolyte to obtain the thermodynamic properties of NaCl and NaAlCl4 at significantly low temperatures. The Gibbs energy of formation of NaCl (s) is, thus, measured at temperatures as low as 423 K by an electrochemical technique for the first time, in this work.

NaY and CrY zeolites ion exchange. Thermodynamics

Abstract: Isotherm ion exchange of Y zeolite has been studied at different temperatures. The direct exchanges of chromium in NaY show a very high selectivity to the in-going Cr 3+ ion. It was also shown that exchange increases with increasing temperature. On the other hand, the reverse isotherms in Cr-NaY are non-preferential process. In both cases Kielland plots and thermodynamic properties such as standard enthalpy of ion exchange (∆H o ), standard free energy of ion exchange (∆G o ), and standard entropy of ion exchange (∆S o ) were calculated. Thermodynamic properties show a favorable process for direct exchange. The reverse exchange process is less sensitive to the changes in temperature due to the high energy needed to replace 3Na + by Cr 3+ in the zeolitic phase.

Synthesis, characterization and thermochemical properties of N-acyl-N′,N′-diethylthioureas

Abstract: Three N-acyl-N′,N′-diethylthioureas, RCONHCSNEt2, R = iPr, iBu, tBu, have been prepared and characterised. The standard (po = 0.1 MPa) molar enthalpies of combustion in oxygen of the three crystalline compounds, at T = 298.15 K, have been measured by rotating bomb-combustion calorimetry, and the standard molar enthalpies of sublimation of the compounds by microcalorimetry. These values were used to derive the standard molar enthalpies of formation of the compounds in their crystalline and gaseous phases, respectively. The derived standard molar enthalpies of formation in the gaseous state are discussed comparatively. Acid constants and some complex stabilities have been measured pH-potentiometrically in dioxane–water mixture. The crystal structure of N,N-diethyl-N′-isovaleroylthiourea is presented and shows a delocalization of the π electrons of the CS group over the carbon–amine nitrogen bond, CS–NEt2, stabilising the molecule, in accordance with the thermochemical results.

Appendix B Essential Statistical Thermodynamics

Abstract: Some computational methods, particularly ab initio techniques, produce detailed molecular information but no thermodynamic information directly. Further calculations are needed to generate familiar, ideal-gas quantities such as the standard molar entropy (SE), heat capacity (CpE), and enthalpy change [HE(T)-HE(0)]. This Appendix details the necessary procedures, including worked examples. Thermochemical calculations can be extended to transition states of chemical reactions. Procedures are provided for converting such information into rate constants. Tables are also provided for unit conversions and physical constants.

Thermal properties of a disc-like compound benzene-hexa-n-pentanoate: a precursor of a discotic mesogen

Abstract: The heat capacity of a disk-like compound, benzene-hexa-n-pentanoate (BH5), a precursor of a discotic mesogen, has been measured by adiabatic calorimetry between 13 and 390 K. BH5 showed four different crystalline phases, but no liquid crystalline phase. Molar entropy and transition entropies were determined and compared with those of other homologues (BH6, BH7, and BH8). An odd-even effect with respect to the number of carbon atoms in the alkyl chain was observed for the cumulative entropies of the phase transitions occurring in the solid state. This effect is discussed by comparing with data for the n-alkanes. The molar entropies of the crystalline state, especially below 250 K, exhibited a peculiar 'pairing effect' between BH(2m- 1) and BH(2m), where m is an integer.

Study on Overall Protonation Constants of Tetraphenylporphyrin Derivatives

Abstract: The overall protonation constants of six tetraphenylporphyrin derivatives were determined by spectrophotometric methods. The changes of standard molar enthalpy Hm and the changes of standard molar entropy Sm of the reactions were obtained from the plots of InKs vs 1/ T. The Porphyrin molecules were calculated by using semiempirical PM3 method. The influence of electron effect and steric hindrance of substituting group on overall protonation constants was discussed. Experimental data and theoretical discussion showed that the degree of porphyrin protonation depend on electron and steric effect. The values of the protonation constants are increased as increasing donor conjugate effect, while steric hindrance of substituting group reduces the constants.

Thermodynamic Properties in Gold Phenylacetylide in the Range 0-330 K

Abstract: The temperature dependence of the heat capacity of gold phenylacetylide in the range 13-330 K was measured in an adiabatic vacuum calorimeter with an accuracy of ∼0.3%. These data were used for calculating the thermodynamic functions Cp0(T), H0(T) - H0(0), S0(T) - S0(0), and G0(T) - H0(0) for the range 0-330 K. The standard entropy of formation ΔfS0 of gold phenylacetylide from the elements at T 298.15 K and p 101.325 kPa was calculated. The thermodynamic properties of gold phenylacetylide and related silver and copper compounds were compared.