Formation Enthalpies Of Compounds Research Articles

Influence of carbon on the formation of γ/γ′ microstructure and κ-phase in the WC/Co–Ni–Al–W system: ab initio calculations and experimental studies

Modifications of the binder phase (γ) of cemented carbides have the potential to increase the hardness and wear resistance of the whole material. Partially, coherent precipitations with L12 structure (γ’) promise these improved properties without sacrificing tensile strength or toughness. γ’ is a metastable phase in the Al–Co–W ternary system in the form of Co3(Al,W) which is stabilized by the substitution of cobalt with nickel. Superalloys of the composition Co–(30Ni)–9Al–7 W with different carbon contents were prepared by inductive melting, and the resulting microstructures were analysed using SEM–EDS, XRD and Vickers hardness. Cemented carbides with γ/γ’ binder microstructure were prepared via DTA, and the phase equilibria in the composite material were investigated experimentally and in silico. It was shown that nickel stabilizes the γ’ phase in superalloys as well as in cemented carbides. Carbon leads to the formation of an additional phase with E21 structure (κ). DTA measurements of cemented carbides with different aluminium–cobalt–nickel mixtures as binder gave an overview of the compositional influence. Enthalpies of formation for compounds with L12 and E21 structure were calculated using ab initio methods and compared to experimental results.Graphical

Thermodynamic properties of some isomeric 5-(nitrophenyl)-furyl-2 derivatives

BackgroundThe aim of the current work was to determine thermodynamical properties of 5-(nitrophenyl)-2-furaldehyde oximes and 3-[5-(nitrolphenyl)-2-furyl]acrylic acids.ResultsThe temperature dependences of saturated vapor pressures of 5-(nitrophenyl)-2-furaldehyde oximes and 3-[5-(nitrolphenyl)-2-furyl]acrylic acids were determined by the Knudsen effusion method. The results are presented by the Clapeyron–Clausius equation in linear form, and via this form, the standard enthalpies of sublimation of compounds were calculated at 298.15 K. The standard molar formation enthalpies of compounds in crystalline state at 298.15 K were determined indirectly from the corresponding standard molar combustion enthalpy, obtained using combustion bomb calorimetry. The non-nearest neighbour interactions (strain) in molecule were defined. The ideal-gas enthalpies of investigated compounds formation and the data available from the literature were used for calculation of group-additivity parameters and the correction terms useful in the application of the Benson correlation.ConclusionDetermining the thermodynamic properties for these compounds will contribute to solving practical problems pertaining to optimization processes of their synthesis, purification and application. It will also provide a more thorough insight regarding the theoretical knowledge of their nature and are necessary for the application of the Benson group-contribution correlation for calculation of Delta {}_{f}H_{{m(298.15{text{K}})}}^{o} (g)calc.

Gas-phase standard enthalpy of formation of 3,5-dimethyl-4-nitroisoxazole determined by semi-micro-combustion calorimetry and thermal analysis

The present paper reports the determination of the standard (p° = 0.1 MPa) molar enthalpy of formation in gas phase at T = 298.15 K of 3,5-dimethyl-4-nitroisoxazole, which was derived from the enthalpy of combustion in solid state and the sublimation enthalpy. The combustion enthalpy was derived from the respective combustion energy value, which was obtained from combustion calorimetry using a static bomb combustion calorimeter in oxygen atmosphere at T = 298.15 K. The sublimation enthalpy was obtained at the same temperature by using thermogravimetry experiments in isothermal regime. The purity and the constant pressure heat capacity of the compound were determined by using differential scanning calorimetry, priori to carry out the combustion experiments. The value obtained of formation enthalpy of compound in gas phase presents high precision and low uncertainty.

A Reliable Method for Prediction of the Condensed Phase Enthalpy of Formation of High Nitrogen Content Materials through their Gas Phase Information

The condensed phase standard enthalpy of formation of energetic materials plays important role in assessment of their detonation and combustion performance. High nitrogen content materials can be considered as high-performance energetic materials because they have relatively high positive values of the condensed phase heat of formation. Two new models were presented for prediction of the condensed phase standard enthalpy of formation for compounds containing more than 50 % of nitrogen. The new models need the calculated values of gas phase standard enthalpies of formation, which were calculated on the basis of B3LYP/6-31G* method or the semiempirical PM6 method. Moreover, the effects of some structural parameters, such as hydrogen bonding, were considered in order to increase the accuracy of models. Reliability of the new models has been confirmed as compared with several of the best and new available methods.

Thermal Decomposition of Dinitro-chloro-azido Benzenes: A Comparison of Theoretical and Experimental Results

1,2-Dichloro benzene (I) and 1,2,3-trichloro benzene (II) were nitrated under appropriate strong conditions. Nitrated mixtures were reacted with NaN3 in polar solvents. As a result of the nucleophilic substitution, mixtures of nitro-chloro-azido benzene were prepared. After fractional crystallization, two crystalline substances were obtained from the mixtures. These crystalline substances were characterized by infrared (IR), 1H-NMR spectroscopy, mass spectroscopy, and elemental analysis. It was understood that the substance obtained from compound I was 2-chloro-4,6-dinitro azido benzene (III). The crystals of compound III were suitable for single-crystal X-ray diffraction (XRD). The molecular structure of this substance was determined by the mentioned method, XRD. It was revealed that the crystalline substance obtained from compound II was most probably 1,5-dinitro-2,4-diazido-3-chloro benzene (IV). In addition, compounds III and IV were investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC). Compound III turned into 4-nitro-6-chloro phenoxazine (V) at about 120°C with an exothermic reaction. Compound IV completely decomposed at about 150°C with an exothermic reaction similar to that of explosive materials. In addition, theoretical formation enthalpies of compounds III and IV were calculated with Gaussian 09, CBS-4 M algorithm. The enthalpies of thermal reactions observed in the TG curves were calculated using the theoretical formation enthalpies. Finally, the theoretical results were compared with the experimental findings obtained from DSC.

Experimental and Computational Studies on the Thermal Degradation of Nitroazidobenzenes

AbstractIn this combined experimental and theoretical study the thermal degradation of 2‐nitroazidobenzene (I), 2,4‐dinitroazidobenzene (II) and 2,4,6‐trinitroazidobenzene (III) to yield benzofuroxan (IV), 4‐nitrobenzofuroxan (V), and 4,6‐dinitrobenzofuroxan (VI) were investigated by thermoanalytical (TG, DSC) and computational methods. In contrast, furoxan formation was not observed for 4‐nitroazidobenzene (VII) under heating due to the para position of the nitro group in the benzene ring. All compounds were characterized by elemental analysis, vibrational (IR) spectroscopy and mass spectrometry. The crystal structure of compound III was determined by single‐crystal X‐ray diffraction. DFT‐based structure optimizations and frequency analyses were performed at the B3LYP/cc‐pVDZ level of theory. The structural parameters of the fully optimized compound III showed very good agreement with the single‐crystal X‐ray data. The enthalpies of formation for compounds I, II, III, IV, V, and VI were calculated using the complete basis set (CBS‐4M) method of Petersson and co‐workers in order to obtain accurate energies. The enthalpies of degradation for compounds I, II, and III were obtained from calculated enthalpies of formation according to the Hess Law and were compared with the experimental values, which were available from DSC analysis and were found to be in very good agreement.

Some new energetic benzaldoximes

5-Nitro-2-hydroxy benzaldoxime (I), 3-nitro-4-hydroxy benzaldoxime (II), 3,5-dinitro-2-hydroxy benzaldoxime (III), and 3,5-dinitro-4-hydroxy benzaldoxime (IV) were prepared from their respective nitrated aldehydes. Prepared oximes were characterized by IR spectroscopy, elemental analysis, and mass spectrometry. Suitable crystals of compounds II and III were obtained and molecular structures were determined by means of the single crystal XRD method. All benzaldoximes were investigated by TG. At temperatures above 140 °C, it was observed that compounds II and IV lost one H2O and was converted to the respective benzonitriles. Only thermal analysis peaks of 3,5-dinitro-4-hydroxy benzonitrile (V) were found proper for both experimental and theoretic calculations; whereas, compounds I and III were converted to phenoxazines by Beckmann rearrangement along with dehydration. Beckmann product of compound III is referred as compound VI and its tautomer as compound VII. Similarly only 3,5-dinitro phenoxazine (VIII) was investigated experimentally and theoretically since its thermal analysis peaks were proper for the purpose. DFT-based structure optimizations and frequency analyses were performed at the B3LYP/cc-pVDZ level of theory. The enthalpies of formation for compounds III–VIII were calculated by means of the complete basis set (CBS-4M) method of Petersson and coworkers to obtain accurate energies. The enthalpies of decomposition for compounds III and IV were obtained from calculated enthalpies of formation according to Hess’ law and were compared with the experimental values which were available from DSC analyses and were found to be in good agreement with the theoretic values.

Enthalpies of Formation of DNA and RNA Nucleobases from G3X Theory

The Gaussian-3X (G3X) method has been used to calculate accurate values of the gas-phase enthalpies of formation of all five DNA and RNA nucleobases: thymine, adenine, cytosine, guanine, and uracil, whose experimental gas-phase study is a difficult problem because of the low vapor pressures. Only for uracil, the published experimental value agrees well with the theoretical one. However, the revision of available experimental data on enthalpies of formation for compounds in the solid phase and enthalpies of sublimation allowed us to recommend a set of self-consistent thermochemical data for thymine, adenine, and cytosine, which lead to good agreement between experimental and theoretical gas-phase enthalpies of formation. To obtain more reliable values of enthalpy of sublimation at 298.15 K, the heat capacity values of gaseous compounds were calculated by standard statistical thermodynamics formulas using molecular parameters determined from B3LYP/6-31G(2df,p) calculations. The following values are recommen...

ThermoData Engine (TDE): Software Implementation of the Dynamic Data Evaluation Concept. 3. Binary Mixtures

ThermoData Engine (TDE) is the first full-scale software implementation of the dynamic data evaluation concept, as reported recently in this journal. The present paper describes the first application of this concept to the evaluation of thermophysical properties for binary chemical systems. Five activity-coefficient models have been implemented for representation of phase-equilibrium data (vapor-liquid, liquid-liquid, and solid-liquid equilibrium): NRTL, UNIQUAC, Van Laar, Margules/Redlich-Kister, and Wilson. Implementation of these models in TDE is fully described. Properties modeled individually are densities, surface tensions, critical temperatures, critical pressures, excess enthalpies, and the transport properties-viscosity and thermal conductivity. Extensions to the class structure of the program are described with emphasis on special features allowing close linkage between mixture and pure-component properties required for implementation of the models. Details of gas-phase models used in conjunction with the activity-coefficient models are shown. Initial implementation of the dynamic data evaluation concept for reactions is demonstrated with evaluation of enthalpies of formation for compounds containing carbon, hydrogen, oxygen, and nitrogen. Directions for future enhancements are outlined.

Calorimetric Determination of the Enthalpies of Formation of Hydrotalcite-Like Solids and Their Use in the Geochemical Modeling of Metals in Natural Waters

AbstractInterest in hydrotalcite-like compounds has grown due to their role in controlling the mobility of aqueous metals in the environment as well as their use as catalysts, catalyst precursors and specialty chemicals. Although these materials have been studied in a number of contexts, little is known of their thermodynamic properties. High-temperature oxide melt solution calorimetry was used to measure the standard enthalpy of formation for compounds M(II)1−xAlx(OH)2(CO3)x/2·mH2O (0.2 < x < 0.4, M(II) = Mg, Co, Ni and Zn). The enthalpy of formation of these compounds from the relevant single cation phases was also determined. The formation of HTLCs results in a 5–20 kJ/mol enthalpy stabilization from the single cation hydroxides and carbonates and water. The data are correlated to two variables: the ratio of divalent to trivalent cation in the solid (M(II)/Al) and the identity of the divalent cation. It was observed that the M(II)/Al ratio exerts a minor influence on the enthalpy of formation from single-cation phases, while greater differences in stabilization resulted from changes in the chemical nature of the divalent cation. However, the data do not support any statistically significant correlation between the composition of HTLCs and their heats of formation. Equilibrium geochemical calculations based upon the thermodynamic data illustrate the effect of HTLCs on the speciation of metals in natural waters. These calculations show that, in many cases, HTLCs form even in waters that are undersaturated with respect to the individual divalent metal hydroxides and carbonates. Phase diagrams and stability diagrams involving Ni-bearing HTLCs and the single-cation components are presented. The Ni(II) concentration as a function of pH as well as the stability diagram for the equilibrium among minerals in the CaO-NiO-Al2O3-SiO2-CO2-H2O system at 298 K are plotted.

Review: Experimental enthalpies of formation of compounds in Al-Ni-X systems

The published experimental enthalpies of formation for compounds in the Al-Ni-X ternary system are reviewed. Most of these experimental data were obtained by either solution calorimetry or high temperature direct synthesis calorimetry. Data for the Al-Ni-Cu compounds determined by the current authors using direct synthesis calorimeter are presented for the first time.

Thermochemistry of rare-earth orthophosphates

The enthalpies of formation for the compounds (RE3+)PO4, (where RE = Sc, Y, La–Nd, Sm–Lu) were determined by oxide-melt solution calorimetry. Calorimetric measurements were performed in a Calvet-type twin microcalorimeter in sodium molybdate (3Na2O · 4MoO3) and lead borate (2PbO · 2B2O3) solvents at 975 K. The experiments were carried out using both powdered single crystals grown by a flux technique and powders synthesized by precipitation. Formation enthalpies were derived from the drop-solution enthalpies for (RE)PO4, RE oxides, and P2O5. Enthalpies of formation for the (RE)PO4 compounds with respect to the oxides at 298 K become more negative with increasing RE3+ ionic radius; i.e., in going from ScPO4 (−209.8 ± 1.0 kJ/mol), to LuPO4 (−263.9 ± 1.9 kJ/mol), to LaPO4 (−321.4 ± 1.6 kJ/mol). From structural considerations, a similar trend is expected for the isostructural RE vanadates and arsenates, as well as for the tetravalent actinide orthosilicates.

Elementary steps in the formation of horseradish peroxidase compound I: direct observation of compound 0, a new intermediate with a hyperporphyrin spectrum.

The reaction of horseradish peroxidase (HRP) with H2O2 has been studied in 50% v/v methanol/water over the 25.0 to -36.0 degrees C temperature range by using the low-temperature stopped-flow technique. All reactions were carried out under pseudo-first-order conditions with [H2O2] much greater than [HRP]. Arrhenius plots for the pseudo-first-order rate constant kobs were linear over the 17.6 to -36.0 degrees C temperature range studied with an activation energy of 4.8 +/- 0.5 kcal/mol. Above 0 degrees C, kobs varies linearly with peroxide concentration. However, saturation kinetics are observed below -16.0 degrees C, indicating that there is at least one reversible elementary step in this reaction. Double-reciprocal plots at -26.0 degrees C at pH* 7.3 for the reaction give kappa max(obs) = 163 s-1 and KM = 0.190 mM. Rapid-scan optical studies carried out at -35.0 degrees C with [H2O2] much greater than KM reveal the presence of a transient intermediate referred to as compound 0 whose conversion to compound I is rate limiting. The Soret region of the optical spectrum of compound 0 resembles that of a "hyperporphyrin" with prominent bands near 330 and 410 nm. The temperature dependencies of kappa max(obs) and KM have been measured over the -16.0 to -26.0 degrees C range and give an activation energy for kappa max(obs) of 1.6 +/- 0.7 kcal/mol and an enthalpy of formation for compound 0 of 4.0 +/- 0.7 kcal/mol.

Thermochemical studies of organometallic compounds I. Heats of formation of dimethylchlorosilane methyldichlorosilane and dimethyldichlorosilane

1. A method of combustion for organosilicon compounds including those containing up to 70% of chlorine has been developed, which allows complete combustion to be achieved with an explosion in a calorimetric bomb.2. Combustion enthalpies for three methylchlorosilanes have been evaluated in a rotating tantalum bomb, and the values: ΔH°c. 298 ((CH3)2HSiCl)liq. = −570.5 ± 0.6; ΔH°c. 28 (CH3HSiCl2)liq. = −353.6 ± 0.8, and ΔH°c. 298 ((CH3)2SiCl2)liq. = −503.4 ± 0.5 kcal/mole have been obtained.3. The formation enthalpies of compounds have been calculated, and the values: ΔH°f. 298 ((CH3)2HSiClliq.) = −79.8 ± 0.6; ΔH°f. 298 (CH3HSiCl2)liq. = −105.9 ± 0.8, and ΔH°f. 298 ((CH3)2SiCl2)liq. = −118.4 ± 0.5 kcal/mole, have been obtained.There is only one indirectly determined value in the literature5, ΔH°f ((CH3)2SiCl2)liq. = −116.0 kcal/mole, that is near our data.