Values For Enthalpies Of Formation Research Articles

Mechanical properties and electronic structures of M23C6 (M = Fe, Cr, Mn)-type multicomponent carbides

The mechanical and electronic properties of M23C6- (M = Fe, Cr, Mn) and M23C6-type multicomponent carbides are investigated systematically by first-principles calculations. The values of cohesive energy and formation enthalpy exhibited thermodynamically stable structures of these carbides. The stress–strain method and Voigt-Reuss-Hill approximation were used to calculate the elastic constants and moduli, respectively. The mechanical properties of the doped Fe or Mo in M23C6 compounds are superior to the pure phases of Cr23C6, Mn23C6 and Fe23C6. Mechanical anisotropy of these compounds was illustrated from the anisotropic index and different shapes of the surface contour by Young's modulus. Moreover, the total density of states of M23C6 compounds indicated that the bonding behaviors of M23C6 compounds were combinations of metallic and covalent bonds.

Half-Metallic Properties in the New Ti2NiB Heusler Alloy

The electronic structure and magnetic properties of the new Ti2NiB Heusler alloy, which has a high-ordered CuHg2Ti structure, were examined using the self-consistent full-potential linearised augmented plane wave method within density functional theory. Analysis of the electronic band structures and state densities of the Ti2NiB full-Heusler compound revealed that whereas the spin-up electrons are metallic, the spin-down bands are characterised by a gap of 0.62 eV, resulting in stable half-metallic ferromagnetism with a magnetic moment of 3 μ B/f.u. The total magnetic moment (M t) and number of valence electrons (Z t) in the Ti2NiB compound were found to obey the Slater-Pauling (SP) rule of M t = Z t − 18. Due to their different surroundings, the magnetic moments of Ti (1) and Ti (2) exhibit distinct differences, with the magnetic contribution of Ti (1) slightly larger than that of Ti (2). A negative formation enthalpy value was also obtained, thus encouraging the experimental manufacture of the alloy. The band gap was elucidated to be mainly determined by the bonding and antibonding states created from the hybridisations of the d states between the Ti (1)-Ti (2) coupling and the Ni atom. Finally, the Ti2NiB compound was found to maintain 100 % polarisation at the Fermi level, with the half-metallic character present for lattice constants ranging from 5.60 to 6.50 A, thus making the alloy favourable for practical applications.

Effect of Cu substitution for Ni on microstructural evolution and hydrogen storage properties of the Mg77Ni20−xCuxLa3 (x = 0, 5, 10 at%) alloys

The effect of Cu-substitution on the microstructures and hydrogen storage thermodynamic properties of Mg77Ni20−xCuxLa3 (x=0, 5, 10at%) alloys prepared by vacuum induction melting has been studied. XRD result shows that the samples consist of Mg, Mg2Ni and LaMg12 phases. With Cu substituting for Ni, the structures of all phases do not change except for a slight increase in lattice parameters of Mg2Ni phase. The P–C-isotherms of the Mg-based alloys have two distinct hydrogen absorption/desorption plateaus, which correspond to the Mg–H2 system and the Mg2(Ni+Cu)–H2 system, respectively. The hydrogen storage capacities of Mg77Ni20−xCuxLa3 (x=0, 5, 10at%) alloys are 4.18wt%, 4.20wt% and 4.51wt%, respectively. In the hydrogenated samples, these alloys consist of MgH2, monoclinic Mg2NiH4, orthorhombic Mg2NiH4, LaH3 as well as MgCu2. The presence of MgCu2 indicates the reaction of Mg2Cu with hydrogen. The hydride formation enthalpy of alloys were calculated by the van’t Hoff equation. With the addition of Cu, the thermodynamic property is greatly improved, and the formation enthalpy values decrease to −51.02, −33.71 from −56.05kJmol−1 H2 for the Mg2(Ni+Cu)–H2 plateaus of Mg77Ni20−xCuxLa3 (x=0, 5, 10at%) alloys, respectively.

Gas-Phase Enthalpies of Formation and Enthalpies of Sublimation of Amino Acids Based on Isodesmic Reaction Calculations

Accurate gas-phase enthalpies of formation (ΔfH298°) of 20 common α-amino acids, seven uncommon amino acids, and three small peptides were calculated by combining G4 theory calculations with an isodesmic reaction approach. The internal consistency over a set of ΔfH298°(g) values was achieved by sequential adjustment of their values through the isodesmic reactions. Four amino acids, alanine, β-alanine, sarcosine, and glycine, with reliable internally self-consistent experimental data, were chosen as the key reference compounds. These amino acids together with about 100 compounds with reliable experimental data (their accuracy was supported by G4 calculations) were used to estimate the enthalpies of formation of remaining amino acids. All of the amino acids with the previously established enthalpies of formation were later used as the reference species in the isodesmic reactions for the other amino acids. A systematic comparison was made of 14 experimentally determined enthalpies of formation with the results of calculations. The experimental enthalpies of formation for 10 amino acids were reproduced with good accuracy, but the experimental and calculated values for 4 compounds differed by 11–21 kJ/mol. For these species, the theoretical ΔfH298°(g) values were suggested as more reliable than the experimental values. On the basis of theoretical results, the recommended values for the gas-phase enthalpies of formation were also provided for amino acids for which the experimental ΔfH298°(g) were not available. The enthalpies of sublimation were evaluated for all compounds by taking into account the literature data on the solid-phase enthalpies of formation and the ΔfH298°(g) values recommended in our work. A special attention was paid to the accurate prediction of enthalpies of formation of amino acids from the atomization reactions. The problems associated with conformational flexibility of these compounds and harmonic treatment of low frequency torsional modes were discussed. The surprisingly good agreement between the ΔfH298°(g) values calculated from the atomization and isodesmic reactions is largely the result of a fortuitous mutual compensation of various corrections used in the atomization reaction procedure.

Synthesis of 5‐(1H‐Tetrazolyl)‐1‐hydroxy‐tetrazole and Energetically Relevant Nitrogen‐Rich Ionic Derivatives

AbstractSodium 5‐cyanotetrazolate sesquihydrate (1) was prepared from sodium azide and two equivalents of sodium cyanide under acidic conditions. Sodium 5‐cyanotetrazolate sesquihydrate (1) reacts with hydroxylammonium chloride to form 5‐aminohydroximoyl tetrazole (2). 5‐Aminohydroximoyl tetrazole (2) is treated with sodium nitrite and hydrochloric acid to form 5‐chlorohydroximoyl‐tetrazole (3). The chloride azide exchange yields 5‐azidohydroximoyl‐tetrazole monohydrate (4). When compound 4 is treated with hydrochloric acid, 5‐(1H‐tetrazolyl)‐1‐hydroxytetrazole (5) is obtained in good yield. Compound 5 can be deprotonated twice by various bases. Different ionic derivatives such as bis(hydroxylammonium) (6), bis(hydrazinium) (7), bis(guanidinium) (8), bis(aminoguanidinium) (9), bis(ammonium) (10), and diaminouronium (11) 5‐(1‐oxidotetrazolyl)‐tetrazolate were synthesized and characterized. With respect to energetic use salts 6 and 7 are most relevant. Compounds 3–9 and 11 were characterized using low temperature single‐crystal X‐ray diffraction. All compounds were investigated by NMR and vibrational (IR, Raman) spectroscopy, mass spectrometry and elemental analysis. The thermal properties were determined by differential scanning calorimetry (DSC). The sensitivities towards impact (4: 4 J, 5: 40 J, 6: 12 J, 7: 40 J), friction: (4: 60 N, 5: 240 N, 6: 216 N, 7: 240 N), and electrical discharge (5: 0.40 J, 6: 0.75 J, 7: 0.75 J), were investigated using BAM standards and a small scale electrostatic discharge tester. The detonation parameters of 5–7 were calculated using the EXPLO5.06 code and calculated (CBS‐4 M) enthalpy of formation values.

Formation Enthalpy of Bli Intermediate Phase

Abstract The formation enthalpy of the BLi (B48Li52) intermetallic phase was measured with the use of the water reaction and the direct reaction calorimetric method. The phase was prepared of weighed amounts of B and Li in a glove-box filled with high purity argon. The BLi phase was confirmed with using the X-ray diffraction investigations. The experiments were carried out with the use of a water reaction calorimeter (water reaction calorimetric technique) at 25°C (298 K) in air, as well as a direct reaction calorimeter at 752°C in high purity argon. The formation enthalpy value obtained for the BLi phase in the case of the water reaction calorimetric method equaled -30.3±2.2 kJ/mole of atoms and in the case of the direct synthesis technique - 29.4±2.0 kJ/mole of atoms.

Determination of the energetics of formation of semiconductor/dendrimer nanohybrid materials: implications on the size and size distribution of nanocrystals

The synthesis of inorganic–organic hybrid nanomaterials has attracted considerable interest in recent years because of their multifaceted applications, such as in optoelectronics, cellular imaging, drug delivery, etc. The maneuvering of controlling parameters is key to the successful fabrication of good quality materials. However, the fundamental aspects pertaining to the thermodynamics of growth of such nano hybrid materials has so far not been unraveled. Here, we have investigated the energetics behind the formation of semiconductor–dendrimer nanohybrid materials using isothermal calorimetry. It is apparent from the observed energy release profile that the heat change for the formation of nanoclusters in phase II saturates faster with an increase in starting materials or monomer concentrations. We have also shown variation of the thermodynamic parameters with changes in the synthesis conditions, such as temperature, dendrimer generation and dendrimer core or surface groups. Based on a bi-phasic thermogram and the dependence of thermodynamic parameters on the dendrimer core and surface functionalities, it is suggested that nanoparticles are formed inside dendrimer molecules in the initial time period and on the outer surface at a longer time scale. Furthermore, it is observed that the formation of quantum dot–dendrimer hybrid materials is an exothermic, spontaneous and enthalpy driven process. Also, a lower temperature thermodynamically favors formation in the core of dendrimer molecules leading to smaller particles with a narrower distribution. The observed results suggest that higher values of formation constant and enthalpy are likely to make dendrimers of higher generation better templates for the synthesis of nanoparticles. The dependence of the ratio of concentrations of reacting metal ions (Cd or Zn) to sulfide ions shows a differential size pattern for CdS and ZnS nanoparticles, which has been interpreted in terms of binding constants determined calorimetrically. It is shown that enthalpy–entropy compensation takes place in the synthesis process affording favorable free energy. Such investigation can provide useful guidelines for the synthesis of better quality semiconductor–dendrimer hybrid nanomaterials.

The enthalpy of formation of selenocysteine: A G3 and G4 quantum chemical study

The gas phase enthalpy of formation of the amino acid containing selenium, selenocysteine, has been calculated using G3 and G4 calculations, the values being (−347.7 and −345.2)kJ·mol−1, respectively. The enthalpies of formation of the counterpart amino acids containing oxygen and sulfur, serine and cysteine, have also been calculated. The enthalpies of formation of compounds H2Z, C2H5ZH and (C2H5)2Z (ZO, S, Se) have been calculated and compared with their available experimental values. The agreement between calculated and experimental values gives us confidence in our calculated values for the gas phase enthalpy of formation of selenocysteine.

Intermetallic phase formation and evolution during homogenization and solution in Al-Zn-Mg-Cu alloys

The effects of major alloy element contents of Zn, Mg, Cu in Al-Zn-Mg-Cu alloys on the formation and evolution of intermetallic phases during casting, homogenization and solution treatment have been investigated through using X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Experimental results showed that a relatively higher Zn content with lower Mg and Cu contents was beneficial to the formation of MgZn 2 phase instead of the Al 2 CuMg phase, which resulted in the unicity of the intermetallics in the Al matrix, and that the MgZn 2 phase was easier for diffusion and dissolution during homogenization and solution than the Al 2 CuMg phase. Additionally, the results of the first-principles calculations gave support for explaining the experimental phenomena. A larger absolute value of formation enthalpy and a smaller value of binding energy of the MgZn 2 phase, as compared with the Al 2 CuMg phase, give it priority to precipitate during casting and make it easier to re-dissolve during homogenization and solution treatment. What's more, higher elastic constants with severe anisotropy of Young's modulus make undissolved blocks of Al 2 CuMg phase act as crack initiation, which degrade the performance of the materials.

Experimental Study of Chemical Equilibria in the Liquid-Phase Reaction between 1-Octanol and Ethanol to 1-Ethoxyoctane

The equilibrium constants for the liquid-phase dehydration reaction between 1-octanol and ethanol to 1-ethoxyoctane (EOE) and water and the dehydration reaction between two ethanol molecules to ethoxyethane (DEE) and water were determined over Amberlyst 70 in the temperature range (410 to 463) K in a batch reactor. Both reactions were greatly shifted to products at the chemical equilibrium. In particular, product concentrations found for EOE synthesis were higher than those measured for DEE formation. Besides, both liquid-phase etherification reactions proved to be exothermic, with a reaction enthalpy change of (−18.9 ± 1.3) kJ·mol–1 for EOE synthesis and (−13.1 ± 0.9) kJ·mol–1 for DEE one, at 298 K. The EOE standard formation enthalpy value of (−436 ± 7) kJ·mol–1, in good agreement with the estimated value by a modified Benson method, and the EOE standard molar entropy value in the liquid phase of (434 ± 11) J·mol–1·K–1, slightly underestimated by the Benson method, are found.

Thermodynamically sick molecules: searching for defective experimental enthalpies of formation values using empirical and quantum chemistry methods

A revised parameterization of the extended Laidler bond additivity method and quantum chemistry calculations were independently used to assess the standard molar enthalpies of formation of 20 non-polycyclic hydrocarbons in the gas phase. The detected discrepancies between predicted and experimental values are discussed, illustrating how this methodology can be useful in curing thermochemical data.

Conformational Analysis of Some β-Halohydrins via G3 Calculations

The conformational analysis of fifteen β-halohydrins, XCR1R2CR3R4OH, (X=F, Cl and Br) had been studied by ab initio G3 method. The enthalpies of formation values were calculated for nine conformers of each β-halohydrin. The (g , g + ) and (g + , g ) conformers were found to be the most stable due to intramolecular H-bond formation. These H-bonds are weaker than intermolecular H-bonds formed between alkyl halides and alcohols. This trend is more obvious in flouro-species. The rotation of C- X bond of (g , g + ) conformers by 360 ° gives three transition states, this rotation was found to require 6-7 kcal/mol.

Electronic, magnetic and elastic properties of Mo2FeB2: First-principles calculations

The first-principles calculations are performed to investigate structure stability, magnetism, electronic structure and elastic properties of Mo2FeB2 in nonmagnetic (NM), ferromagnetic (FM) and antiferromagnetic (AF) cases. Density functional theory and ultrasoft pseudopotentials are used in this study. It is found that the lattice parameters of the AF case are more close to the experimental data than the ones of the NM and FM cases, and the AF one has the lowest energy, indicating it is the ground state. All of the cases are stable thermodynamic structures. Furthermore, the AF case expresses more stability than the FM and NM cases because it has the largest formation enthalpy value. The density of states and overlap populations are analyzed for NM, FM and AF cases of Mo2FeB2. B–B and B–Mo are covalent bonding and give the positive to shear modulus. According to the analysis of magnetism, Fe atoms play the key point. The FM and AF cases of Mo2FeB2 have a lower bulk modulus, but higher shear modulus and Young’s modulus than of the NM case. It is obvious that the spin restriction can significantly affect the moduli of Mo2FeB2, and the FM and AF cases show brittle character while the NM one ductile character.

The stability and elastic properties of NaCl-type MN (M = Ti, V, Zr, Nb, and Ta) compounds investigated by first principles

The first principles calculations based on density functional theory (DFT) are adopted to investigate the stability, elastic constants, Debye temperature, and hardness of MN (M = Ti, V, Zr, Nb, Ta). The NaCl-type cubic structures of MN (M = Ti, V, Zr, Nb, and Ta) compounds are optimized, and the lattice parameters are in good agreement with the experimental values. The calculated values of cohesive energy and formation enthalpy indicated that they are stable structures. The mechanical properties are presented by our calculation, which are elastic constants, bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio. The Debye temperature of MN (M = Ti, V, Zr, Nb, Ta) are obtained and the results show that the value of TaN is the lowest of them. The hardness of MN (M = Ti, V, Zr, Nb, and Ta) compounds is 23.2, 25.0, 14.0, 16.0, 16.5 GPa, respectively.

Energetics of Oxygen Adatoms, Hydroxyl Species and Water Dissociation on Pt(111)

Calorimetric measurements of the adsorption enthalpy of gaseous oxygen (O2,g) to make two oxygen adatoms (2 Oad) on Pt(111) were performed by Fiorin et al.(1) However, we show that they used a calibration value for the optical reflectivity of Pt(111) that was incorrectly reported in the literature. This error in reflectivity led to a 40% error in the adsorption energies originally reported. We use our more accurate reflectivity of 76% to recalibrate their oxygen adsorption enthalpy data and show that it gives nearly identical results below 0.15 ML to the heats of adsorption determined from the temperature programmed desorption (TPD) experiments of two separate groups.(2, 3) Differences arise above 0.15 ML, but we attribute these to the very low sticking probability of O2,g on Pt(111) (<0.05) above 0.15 ML, which can lead to large errors in the adsorption energies measured by calorimetry. Given this, we propose that the most reliable values for the adsorption enthalpy of oxygen on Pt(111) up to 1/4 ML are those derived from TPD experiments rather than the more recent calorimetry data. The best values are well described by (−217 + 151θ) kJ/mol O2 below 1/4 ML, where θ is the Oad coverage in ML (where 1 ML is defined as 1 adsorbate per Pt surface atom). We also report calculations of coverage-dependent adsorption energies for oxygen on Pt(111) from density functional theory (DFT-RPBE) and find the results to be within ca. 20 kJ/mol of the integral heats measured by TPD. We further use these corrected adsorption enthalpies to amend the energetics of deuterated hydroxyl species on Pt(111) that we previously measured.(4) This gives revised values for the standard enthalpies of formation of the coadsorbed water–hydroxyl complex ((D2O–OD)ad) of −511 ± 7 kJ/mol and a Pt–OD bond energy of 248 ± 7 kJ/mol for the OD species within this complex. DFT compares reasonably well, calculating an enthalpy of formation for the (H2O–OH)ad complex of −456 kJ/mol and an O–Pt bond energy of 217 kJ/mol for the OH species within this complex. These revised values are used to estimate reaction enthalpies for the dissociation of adsorbed water and hydroxyl on Pt(111) and are compared to DFT.

Enthalpy of formation of the Li22Si5 intermetallic compound

The solution-reaction calorimetric method was used for the determination of the formation enthalpy of the Li22Si5 intermetallic compound and the heat of the reaction of the Li-component with an acetic acid bath. The phase was prepared in a glove box with high purity argon protective atmosphere. The phase was homogenized and then analyzed by means of the X-ray diffraction technique. Also, the topography analysis was performed with the use of a scanning electron microscopy (SEM), whereas the microstructure analysis was performed by means of a transmission electron microscopy (TEM). The phase analysis was also run by the selected area electron diffraction pattern (SAEDP).The experiments in the calorimeter were carried out at room temperature with acetic acid as the calorimetric solvent. The enthalpy of the lithium's reaction with the acetic acid was measured and it equaled −261.1±1.6kJ/g·atom. The formation enthalpy value ΔfH for the Li22Si5 compound was equal to −24.4±3.0kJ/g·atom.

Enthalpy of formation of Ln2O2CO3 II (Ln = La, Nd, Eu) and thermodynamics of decomposition equilibria

High temperature oxide melt solution calorimetry using molten 2PbO·B2O3 at 1078K and molten 3Na2O·4MoO3 at 976K has been applied to determine standard enthalpies of formation, ΔfHm° (298K), of Ln2O2CO3 II (Ln=La, Nd, Eu) from their oxide components Ln2O3 and CO2 at 298K. Two solvents were applied to cross-check the thermodynamic cycles. Using molten 2PbO·B2O3, ΔfHm°(298K) for La2O2CO3 II, Nd2O2CO3 II and Eu2O2CO3 II were found as −198.0±1.7, −176.2±3.1 and −151.4±6.5kJmol−1, respectively. Using molten 3Na2O·4MoO3 the corresponding values for Nd2O2CO3 II and Eu2O2CO3 II were determined as −176.6±3.5 and −154.9±3.2kJmol−1, respectively. These enthalpies of formation are discussed in view of prior thermodynamic data, discrepancies are analyzed, and recommended values for enthalpy, entropy and free energy of formation for La2O2CO3 and Nd2O2CO3 are given.

Taming the Beast: Measurement of the Enthalpies of Combustion and Formation of Triacetone Triperoxide (TATP) and Diacetone Diperoxide (DADP) by Oxygen Bomb Calorimetry

Low-melting paraffin wax was successfully used as a phlegmatizing agent to perform semi-micro oxygen bomb calorimetry of spectroscopically pure samples of the sensitive explosive peroxides TATP and DADP. The energies of combustion (ΔcU) were measured and the standard enthalpies of formation (ΔfH°) were derived using the CODATA values for the standard enthalpies of formation of the combustion products. Whilst the measured ΔfH° of DADP (ΔfH°=−598.5 ± 39.7 kJ mol−1) could not be compared to any existing literature value, the measured ΔfH° value of TATP (ΔfH°=+151.4 ± 32.7 kJ mol−1) did not correlate well with the only existing experimental value and confirmed that TATP is an endothermic cyclic peroxide.

Correcting density functional theory for accurate predictions of compound enthalpies of formation: Fitted elemental-phase reference energies

Despite the great success that theoretical approaches based on density functional theory have in describing properties of solid compounds, accurate predictions of the enthalpies of formation ($\ensuremath{\Delta}{H}_{f}$) of insulating and semiconducting solids still remain a challenge. This is mainly due to incomplete error cancellation when computing the total energy differences between the compound total energy and the total energies of its elemental constituents. In this paper we present an approach based on GGA $+$ $U$ calculations, including the spin-orbit coupling, which involves fitted elemental-phase reference energies (FERE) and which significantly improves the error cancellation resulting in accurate values for the compound enthalpies of formation. We use an extensive set of 252 binary compounds with measured $\ensuremath{\Delta}{H}_{f}$ values (pnictides, chalcogenides, and halides) to obtain FERE energies and show that after the fitting, the 252 enthalpies of formation are reproduced with the mean absolute error $\text{MAE}=0.054$ eV/atom instead of $\text{MAE}\ensuremath{\approx}0.250$ eV/atom resulting from pure GGA calculations. When applied to a set of 55 ternary compounds that were not part of the fitting set the FERE method reproduces their enthalpies of formation with $\text{MAE}=0.048$ eV/atom. Furthermore, we find that contributions to the total energy differences coming from the spin-orbit coupling can be, to a good approximation, separated into purely atomic contributions which do not affect $\ensuremath{\Delta}{H}_{f}$. The FERE method, hence, represents a simple and general approach, as it is computationally equivalent to the cost of pure GGA calculations and applies to virtually all insulating and semiconducting compounds, for predicting compound $\ensuremath{\Delta}{H}_{f}$ values with chemical accuracy. We also show that by providing accurate $\ensuremath{\Delta}{H}_{f}$ the FERE approach can be applied for accurate predictions of the compound thermodynamic stability or for predictions of Li-ion battery voltages.

Enthalpy of formation of B13Li, B14Li3 and B3Li (B6Li2) compounds measured with reaction calorimetric method

The formation enthalpies of B13Li, B14Li3 and B3Li (B6Li2) compounds were measured using a reaction calorimetric method. The phases were prepared in a glove-box filled with high purity argon. The phases were annealed for two days and next studied with X-ray diffraction and electron microscopy methods. It was experimentally found that the dissolution of boron in liquid aluminum, used as solvent in many earlier experiments, was so slow in comparison with other metals that another method had to be selected. The experiments were conducted using a water calorimeter working at room temperature in air. Samples of Li and phases were delivered from the glove-box to the calorimeter in a closed glass little bottle and transferred quickly into water directly before the experiment. The enthalpy of lithium reaction with water was measured to be equal −205.3 ± 1.2 kJ/gram-atom. As the boron did not react with water, its reaction heat effect with water was assumed to be zero. The formation enthalpy value obtained for the B13Li, B14Li3 and B3Li (B6Li2) compounds were equal to −14.1 ± 0.1, −33.2 ± 1.6 and −43.6 ± 1.8 kJ/mol of atoms, respectively.