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Determination of the reaction heat and studies of thermodynamic functions

Hydrolytic polymerization of chromium(III) at relatively high concentrations by microcalorimetry

Journal of Thermal Analysis and Calorimetry
Authors: Lu Chengxue, Yu Xiufang, and Zhang Honglin

The hydrolytic polymerization of Cr3+ at relatively high concentrations was studied by microcalorimetry. The thermal curves were determined with a 2277 thermal activity monitor. From the curves, the identified reaction heats (δrH m σ), hydrolysis constants (K) and thermodynamic functions (δrS m σrH m σ) were calculated at different temperatures.

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The sample of LiCoO2 was synthesized, and the heat capacity was measured by adiabatic calorimetry between 13 and 300 K. The smoothed values of the heat capacity were calculated from the data. The thermodynamic functions, standard enthalpy, entropy and Gibbs energy, of LiCoO2 were calculated from the heat capacity and the numerical values are tabulated at selected temperatures from 15 to 300 K. The heat capacity, enthalpy, entropy, and Gibbs energy at T=298.15 K are 71.57 J K–1mol–1, 9.853 kJ mol–1, 52.45 J K–1 mol–1, –5.786 kJ mol–1, respectively.

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The thermodynamic functions of southern taiga forest litter samples were determined by means of DSC. It was found that more thermostable compounds (at least in the interval 50–150C) with higher absolute values of the Gibbs function for intermolecular structures are formed in the course of forest litter decomposition. The data obtained indicate the spontaneous nature of the processes relating to the transformation of organic matter in soil.

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The EMF concentration cell method was applied to determine excess partial molar thermodynamic functions for some sections of the systems Tl−Ag−Te and Tl−Sb−Te. The presence of binary and ternary associates is assumed in Tl−Ag−Te solutions.

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The low-temperature heat capacities of cyclohexane were measured in the temperature range from 78 to 350 K by means of an automatic adiabatic calorimeter equipped with a new sample container adapted to measure heat capacities of liquids. The sample container was described in detail. The performance of this calorimetric apparatus was evaluated by heat capacity measurements on water. The deviations of experimental heat capacities from the corresponding smoothed values lie within 0.3%, while the inaccuracy is within 0.4%, compared with the reference data in the whole experimental temperature range. Two kinds of phase transitions were found at 186.065 and 279.684 K corresponding solid-solid and solid-liquid phase transitions, respectively. The entropy and enthalpy of the phase transition, as well as the thermodynamic functions {H(T)-H 298.15 K} and {S (T)-S298.15 K}, were derived from the heat capacity data. The mass fraction purity of cyclohexane sample used in the present calorimetric study was determined to be 99.9965% by fraction melting approach.

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The solubility and solubility product of erbium trifluoride in aqueous solution were measured using three different techniques. The more acceptable radiometric values for the solubility and solubility product are 1.98×10–5M and 3.5×10–18, respectively. The values for the thermodynamic functions, H 298 o , G 298 o and S 298 o for the dissolution of ErF3 and the stability constant for ErF2+ have also been measured.

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Calorimetric measurement of adsorption enthalpies of native lysozyme(Lyz) on a moderately hydrophobic surface at 25°C, pH 7.0 and various salt concentrations was performed. Based on the thermodynamics of stoichiometric displacement theory (SDT), we calculated the fractions of thermodynamic functions involving four subprocesses during a displacement adsorption process from the directly determined enthalpies in combination with adsorption isotherm measurements. The thermodynamic fractions reveal the relative degree of the four subprocesses for contributions to enthalpy, entropy and free energy. The results show that native Lyz adsorption on a moderately hydrophobic surface is an entropy driven process contributed mainly by conformational loss of adsorbed Lyz.

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The solubility, solubility product and the thermodynamic functions for the CeF3–H2O system have been measured using the radiometric, conductometric and potentiometric techniques. The radiometric values for the solubility and solubility product, the lowest and more acceptable for reasons cited in previous papers, are 3.14·10–5 M and 2.17·10–17 respectively. The enthalpy change measured by the conductometric method is almost twice as that obtained by potentiometric method due to abnormal conductances registered at higher temperatures. The average values for Ho and Go and So at 298 K are 53.0±17.4, 91.7±4.0 and –129.7±58.2 KJ·mol–1 respectively. The positive values for Ho and Go and the negative value for So are indicative of the low solubility of this salt in water. The stability constants for the mono- and difluoride complexes of Ce(III) have been determined potentiometrically using unsaturated solution mixtures of Ce(III) and F. These values for CeF+ and CeF 2 + are 997±98 and (1.03±0.44)·105, respectively. Studies on pH dependence of the solubility shows that the solubility reaches a minimum value at a pH of about 3.2.

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The solubility products and thermodynamic functions for the praseodymium and ytterbium fluoride-water systems have been measured using different analytical techniques for comparison among the measured values as well as with values reported in literature. Although there is not much difference among our values obtained by potentiometric, conductometric and radiometric methods, the values for PrF3 disagree with at least two of the reported values. The effects of precipitant, aging of the precipitate and pH of the saturated solutions on the solubility product were studied and found to be significant. The standard enthalpy and free energy changes for the dissolution of both fluorides were found to be positive although the enthalpy for YbF3 is about twice as high as that for PrF3. The standard entropy change for PrF3 is negative, but that for YbF3 is positive. The overall stability constants for the mono- and difluoride complexes of both Pr and Yb have also been measured potentiometrically using a simplified approach.

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The values of thermodynamic functions of the following eight organic compounds under non-reacting conditions within the temperature range 298.15 – 550 K are given: N,N dimethylformamide, CHO · N(CH3)2; 4-nitro-1-chlorobenzene, Cl · C6H4 · NO2; sodiump-nitrophenoxide, NO2 · C6H4 · ONa; 1-methyl-2-pyrrolidone, 4,4′-dinitrodiphenyl ether, (NO2 · C6H4)2O; 4,4′-diaminodiphenyl ether, (NH2 · C6H4)2O; bis-(4-aminophenyl)methane, (NH2 · C6H4)2CH2; bis(4-maleic acidimidphenyl)methane,

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