Showing papers on "Standard molar entropy published in 1996"

Adsorption of aromatic compounds in large MFI zeolite crystals

Abstract: Very large (180 × 40 × 40 µ3) and well defined crystals have been used in a gravimetric system to investigate the adsorption of aromatic compounds in MFI (silicalite) zeolite. The isotherms and isosteric heats of adsorption (Qst) were reported for future comparison with various adsorption models. Three types of phase transition were found. Isotherms of p-xylene showed a sharp step rise from ca. 4 to 8 molecules uc–1. A clear phase boundary could be outlined. Adsorbed benzene behaved as a dual-phase system in the range between 4.6 and 6 molecules uc–1. There were more phase transitions above 6 molecules uc–1 but it was not possible to outline the phase boundary. Molar entropy changes of 210 and 180 J mol–1 K–1 were found for the observed p-xylene–MFI and benzene–MFI phase transitions. For ethylbenzene and toluene, a dual-phase region was also observed but the transition was less pronounced. Isosteric heats, calculated from isotherms, showed a complex variation with loading and a strong dependence on temperature. It was also found that the adsorption kinetics are strongly influenced by the previous adsorption history. For freshly calcined samples the uptake rate was relatively fast. However, re-calcination after the adsorption of p-xylene created a diffusion barrier inside the crystalline material and subsequent adsorption of other aromatic compounds became much slower and displayed two-step kinetics.

Maximum entropy analysis of chemical reaction energy dependence

Abstract: In this article the energy dependence of chemical and ionic reactions is examined and a new model is developed for the calculation of reaction probability and postreaction energy disposal for exchange reactions. The new model is based on the principles of the maximum entropy method. For the exchange reactions the reaction probability is based on either the translational or the vibrational energy of the reagents as appropriate, whereas for dissociation the vibrational energy is used as the controlling factor. The new method is compared with other methods used for direct simulation Monte Carlo calculations. Examples of flowfield solutions are presented and the differences between the results of the new method, those using Bird's method, and continuum solvers are discussed and evaluated.

Voltammetry in silicate and borosilicate melts

Abstract: With the aid of square-wave-voltammetry, redox equilibria of various multivalent elements were studied in a soda-lime-silicate and a borosilicate glass melt in the temperature range of 600 to 1300°C. The peaks in the current-potential curves could be attributed to certain redox couples. The peak potentials measured dependend linearly on temperature proving that both standard enthalpy and standard entropy possess constant values within the temperature range studied. The peak potentials measured in the silicate and the borosilicate melt were in fairly good agreement. A systematic shift caused by the higher alkali content of the borosilicate glass could not be observed.

Separation of Amino Acids at a Synthetic Ion Exchange Resin: Thermodynamics and Energetics

Abstract: The thermodynamics and energetics of the ion exchange of four amino acids at a synthetic ion exchange resin have been studied. Experimental work included determination of ion exchange isotherms and the use of high-sensitivity titration microcalorimetry. A rigorous thermodynamic analysis of the data is developed allowing calculation of the standard free energy, the standard enthalpy, and standard entropy of exchange, and also the differential free energy, incremental enthalpy, and incremental entropy of exchange. The results show that the relative contributions of the enthalpy and entropy to the overall free energy differ markedly for the chosen amino acids. From a knowledge of these fundamental thermodynamic properties, the solution conditions giving enhanced separation are predicted. These predictions are experimentally confirmed. The paper establishes experimental protocols and data analysis methods that allow progression from a single isotherm and a single set of calorimetric data to a selection of pro...

Correlating thermochemical data of the oxygen non-stoichiometric compound YBa2Cu3O7 –x with the oxygen content

Abstract: Equations correlating the standard enthalpy of formation, the standard entropy and the heat capacity of the oxygen non-stoichiometric compound YBa2Cu3O7-x with the oxygen contents have been derived based on equilibrium thermogravimetric data from the literature. The results show that the oxygen content exhibits a marked influence upon the thermochemical data. The derived equations correlating the enthalpy are in good agreement with the calorimetric results from the literature.

Monolayer Properties of Long-Chain Alkylammonium Perfluorohexanoates on the Concentrated Sodium Chloride Solution

Abstract: Surface pressure (pi)- and surface potential (DeltaV)-area (A) isotherms were obtained for monolayers of alkylammonium perfluorohexanoates of four different alkyl chains (m) on water and 4.4 M NaCl solutions as a function of molecular area of various temperatures by the Langmuir and the ionizing electrode methods. As for octadecylammonium perfluorohexanoate (C18F11), the pi-A curves of these long-hydro-short-fluoro chain salt showed three phase transition points. Judging from the surface potential and the apparent molar quantity together with our previous data on the transitions, the first was assigned to the transition from the expanded (E) phase to the condensed (C-I) phase, the second from the C-I phase to another condensed (C-I') phase, and the third from the (C-I') phase to another condensed (C-II) phase. From the surface potential, the apparent molar entropy change, and the phase diagram, the orientation of long-hydro-short-fluoro chain salts in the monolayer state was discussed.

Thermodynamics of hexadecyltrimethylammonium bromide in water and in aqueous NaBr solution

Abstract: Density and ultrasound velocity of hexadecyltrimethylammonium, bromide in water and in aqueous NaBr solutions have been measured as functions of concentration and temperature. Using the values of critical micelle concentration (cmc) at several temperatures evaluated from the sound velocity versus concentration plots, the standard free energy change ΔG m 0 , the standard enthalpy change ΔH m 0 and the standard entropy change ΔS m 0 of micellization in water as well as in aqueous NaBr solution have been determined. The ΔG m 0 , and ΔH m 0 values were found to be negative and to decrease with increasing temperature, while ΔS m 0 values are positive and also show a decrease. The exothermic micellization process has been attributed to diminishing hydrophobicity at higher temperature, demonstrating a structure breaking process. A transition in the values of ΔH m 0 ( and ΔS m 0 was observed in the case of aqueous NaBr as has been predicted. The apparent adiabatic compressibilities in monomeric form (β 1 ), micellar form (β m ), and the change upon micellization (Δβ m ) have been computed. These values show a consistent pattern of behaviour; the overall decrease in Δβ m values with increasing temperature is ascribed to the dehydration of ionic head groups and the consequent partial destruction of hydrophobic hydration.

Base-Catalyzed Acyl Exchange Reactions of N,O-Diacyl-o-aminophenols. Substituent and Temperature Effects on the Equilibrium Constants

Abstract: Substituent and temperature effects on the equilibrium constants for the triethylamine-catalyzed acyl exchange reactions of N-(substituted benzoyl)-O-acetyl-o-aminophenols have been investigated by 1H NMR spectroscopy. It was found that the standard enthalpy change ΔH° in acyl exchange processes, estimated from temperature dependence of the equilibrium constant, correlates well with the standard free energy change ΔG° which was used as a measure of the relative stability of isomer pairs, while the standard entropy change ΔS° is approximately constant (−6 ± 2 J K−1 mol−1) irrespective of the substituents in benzoyl introduced. The finding of enthalpy-controlled acyl exchange reactions confirms that the relative stability of a given isomer pair is determined by the difference in electrostatic force of attraction between the amide nitrogen with a partial positive charge and the ester carbonyl oxygen having a partial negative charge in both isomers.

Base-Catalyzed Acyl Exchange Reactions of N,O-Diacyl-o-aminophenols. Substituent and Temperature Effects on the Equilibrium Constants.

Abstract: Substituent and temperature effects on the equilibrium constants for the triethylamine-catalyzed acyl exchange reactions of N-(substituted benzoyl)-O-acetyl-o-aminophenols have been investigated by 1H NMR spectroscopy. It was found that the standard enthalpy change ΔH° in acyl exchange processes, estimated from temperature dependence of the equilibrium constant, correlates well with the standard free energy change ΔG° which was used as a measure of the relative stability of isomer pairs, while the standard entropy change ΔS° is approximately constant (−6 ± 2 J K−1 mol−1) irrespective of the substituents in benzoyl introduced. The finding of enthalpy-controlled acyl exchange reactions confirms that the relative stability of a given isomer pair is determined by the difference in electrostatic force of attraction between the amide nitrogen with a partial positive charge and the ester carbonyl oxygen having a partial negative charge in both isomers.