Molar Enthalpies Of Formation Research Articles

Experimental Determination and Assessment of Enthalpies of Formation of Lithium Silicides

Recently, extensive studies using silicon anodes in lithium ion batteries (LiB) have shown that the Li-Si-system is an attractive alternative to the currently dominating graphite anode in view of costs and specific capacity. Currently the main obstacle of silicon is the poor charge/discharge cycling stability and capacity fading due to the large volume variation during charging/discharging. However, remarkable progress has recently been achieved by using thin films of silicon or silicon nanowires to relieve the mechanical stress [1]. It is indisputable that future progress in lithium silicide applications will require fundamental knowledge of the physicochemical properties of the various lithium silicide phases. Especially accurate thermodynamic quantities are crucial for the understanding as well as modelling of phase, reaction and electrochemical equilibria. In contrast to the numerous phase diagram studies reported in the literature, only few reliable thermodynamic quantities were available. Motivated by this situation, we recently reported experimental heat capacities and entropies of the lithium silicides Li17Si4, Li16.42Si4, Li13Si4, Li7Si3 and Li12Si7 [2,3]. Presently, the standard molar enthalpies of formation of the lithium silicides seem to be one of the most unreliable fundamental data because available values scatter by about 10 % which is unacceptable in view of reliable thermodynamic equilibrium calculations. This contribution is focused on the accurate experimental determination of the standard enthalpies of formation ΔFH°(298) of the lithium silicides Li17Si4, Li13Si4, Li7Si3 and Li12Si7. For this purpose the hydrogenation/dehydrogenation equilibria in the system Li-Si-H were investigated at three different temperatures (450 K, 475 K, 500 K) using a Sievert’s type apparatus. The measurements based on two different sample types: (i) pure silicide and (ii) a stoichiometric mixture of lithium hydride and silicon. The calculation of the standard enthalpies of formation by using the slopes of the classical van’t Hoff plots results in uncertainties of at least 10 %. Due to this unsatisfying large error, an alternative evaluation method is demonstrated linking the hydrogen equilibrium pressures peq(H2) from the sorption measurements with our precise heat capacity and entropy data of the lithium silicides. The obtained values for the standard enthalpies of formation show strongly reduced uncertainties of less than 2%. Finally, it will be shown that the phase diagram of the Li-Si-system can now be calculated (CALPHAD) with excellent quality using a complete set of experimental thermodynamic data. [1] M. T. McDowell, S. W. Lee, W. D. Nix, Y. Cui, Adv. Mater. 2013, 25, 4966–4985. [2] D. Thomas, M. Abdel-Hafiez, T. Gruber, R. Hüttl, J. Seidel, A. U. B. Wolter, B. Büchner, J. Kortus, F. Mertens, J. Chem. Thermodyn. 2013, 64, 205–225. [3] D. Thomas, M. Zeilinger, D. Gruner, R. Hüttl, J. Seidel, A. U. Wolter, T. F. Fässler, F. Mertens, The Journal of Chemical Thermodynamics 2015, 178–190.

Purification and thermochemical properties of dihydromyricetin

This work reports the purification of dihydromyricetin (abbreviation: DMY) by the recrystallization method, with a yield of secondary recrystallization of up to 77 %. The products were confirmed by element and spectral analysis. The results showed a sharp R-bond absorption peak located around the near-ultraviolet band, which is close to K-bond absorption, illustrating the occurrence of a large conjugation of carbonyl with adjacent phenol and hydroxyl. This was further confirmed by the infrared (IR) spectrum, i.e., the carbonyl stretching vibration peaks were red-shifted from 1700 to 1641 cm−1. The standard thermodynamic energy was determined to be ΔcUmθ[DMY(s), 298.15 K] = −(5437.44 ± 5.14) kJ mol−1 by an oxygen bomb calorimeter. Furthermore, the standard molar enthalpy of combustion was calculated to be ΔcHmθ[DMY(s), 298.15 K] = −(5434.96 ± 5.14) kJ mol−1, and the standard molar formation enthalpy of DMY was estimated to be ΔfHmθ[DMY(s), 298.15 K] = −(2182.67 ± 5.33) kJ mol−1.

5,5'-Bis(2,4,6-trinitrophenyl)-2,2'-bi(1,3,4-oxadiazole) (TKX-55): Thermally Stable Explosive with Outstanding Properties.

The novel, thermally stable explosive 5,5'-bis(2,4,6-trinitro-phenyl)-2,2'-bi(1,3,4-oxadiazole) (TKX-55) is reported. This compound can be prepared by means of a facile synthetic procedure and shows outstanding properties (detonation velocity, detonation pressure, sensitivity toward mechanical stimuli, and temperature of decomposition). TKX-55 was isolated and characterized by means of mass spectrometry, multinuclear (1 H, 13 C) NMR spectroscopy, and vibrational spectroscopy (IR and Raman). The structure in the crystalline state was determined by low-temperature single-crystal X-ray diffraction. From the calculated standard molar enthalpy of formation (CBS-4M) and the densities, the Chapman-Jouguet detonation properties were predicted by using the EXPLO5 V6.01 thermochemical computer code. The sensitivity of TKX-55 towards impact, friction, and electrostatic discharge was determined. The shock reactivity (explosiveness) of TKX-55 was measured by applying the small-scale shock reactivity test.

Phase equilibria of the systems of CsCl + HoCl3 + H2O and CsCl + HoCl3 + HCl (∼10%) + H2O at T = 298.15 K and the thermodynamic and fluorescent properties of the new solid-phase compounds

(Solid+liquid) equilibria in the ternary system CsCl+HoCl3+H2O and in the quaternary system CsCl+HoCl3+HCl (∼10%)+H2O at T=298.15K and atmospheric pressure were investigated by using isothermal solution saturation and moist residue method. Based on the measured solubility data, the corresponding phase diagrams were plotted. In the ternary system, three crystallization regions corresponding to CsCl, Cs2HoCl5·6H2O and HoCl3·6H2O were found. However, there were four crystallization regions corresponding to CsCl, Cs5HoCl8·4H2O, Cs2HoCl5·6H2O and HoCl3·6H2O in the quaternary system. The phase diagrams of the ternary and quaternary systems were compared, and it showed that (1) new solid-phase compound Cs2HoCl5·6H2O was obtained which was congruently soluble in the two systems; (2) a new double salt Cs5HoCl8·4H2O was formed in the quaternary system which was congruently soluble in an average medium of ∼10mass% HCl, and (3) the area of the crystallization region of CsCl decreased with the increasing concentration of HCl in the equilibrium liquid phase. The new solid-phase compounds Cs5HoCl8·4H2O and Cs2HoCl5·6H2O were characterized by chemical analysis, XRD and TG-DTG techniques. The standard molar enthalpies of solution of Cs5HoCl8·4H2O and Cs2HoCl5·6H2O in water were confirmed to be (53.09±0.53)kJ·mol−1 and (4.26±0.24)kJ·mol−1 by microcalorimetry in the condition of infinite dilution and their standard molar enthalpies of formation were determined to be −(4530.1±1.4)kJ·mol−1 and −(3776.6±0.9)kJ·mol−1. The fluorescence excitation and emission spectra of the new solid phase compounds were measured. The results indicate that upconversion spectra of the compounds Cs5HoCl8·4H2O and Cs2HoCl5·6H2O at the same wavelength 474nm, when the former was excited at 701nm and the latter was excited at 713nm.

Synthesis, characterization and determination of standard molar enthalpies of formation of two copper borophosphates by solution calorimetry

Two pure microporous materials for copper borophosphates, Cu(H2O)2[B2P2O8(OH)2] and Cu3[B2P3O12(OH)3], have been synthesized and characterized by XRD, FTIR, simultaneous TG–DTA and chemical analysis. The molar enthalpies of solution of Cu(H2O)2[B2P2O8(OH)2] (s) and Cu3[B2P3O12(OH)3] (s) in 3 mol dm−3 HCl (aq) were measured, respectively. The molar enthalpies of solution of CuCl2·2H2O (s) in the mixture solvent of (HCl + H3BO3) (aq), and of H3PO4 (s) in the mixture solvent of (HCl + H3BO3 + CuCl2·2H2O) (aq) were also measured. With the incorporation of the previously determined enthalpy of solution of H3BO3 (s) in 3 mol dm−3 HCl (aq), together with the standard molar enthalpies of formation for H3PO4 (s), CuCl2·2H2O (s), H3BO3 (s), HCl (aq) and H2O (l), the standard molar enthalpies of formation of −(4078.8 ± 1.7) kJ mol−1 for Cu(H2O)2[B2P2O8(OH)2] (s) and −(4805.7 ± 2.0) kJ mol−1 for Cu3[B2P3O12(OH)3] (s) at 298.15 K were obtained on the basis of the appropriate thermochemical cycles.

Crystal structure and thermochemistry of anhydrous potassium 2-pyrazine carboxylate (C5H3KN2O2)

Potassium 2-pyrazine carboxylate C5H3KN2O2 [abbreviated as K(PCA)(s)] was synthesized, and the synthesis was carried out in the mixed solvent of ethanol and water. Its crystal structure was characterized by X-ray crystallography. And its low-temperature heat capacities were measured by use of a precise automatic adiabatic calorimeter with a small sample in the temperature region of 78–400 K. A polynomial equation of experimental molar heat capacities as a function of the temperature has been obtained. The smoothed molar heat capacities and thermodynamic functions of the compound were calculated based on the fitted polynomial equation. A reasonable thermochemical cycle was designed on the basis of the synthesis reaction of the substance, and the standard molar enthalpies of dissolution for the reactants and products of the reaction in the selected solvent were measured by an isoperibol solution-reaction calorimeter, respectively. The enthalpy change of the reaction was calculated as—(65.396 ± 0.388) kJ mol−1. Finally, the standard molar enthalpy of formation of the title complex was calculated as—(532.106 ± 1.196) kJ mol−1 by using the Hess thermochemical cycle. Furthermore, the reliability of the designed thermochemical cycle was verified by UV–Vis spectra of the solutions obtained from the dissolution of reactants and products.

Thermochemical properties for a series of transition metal borates of M[B12O14(OH)10] (MII = Mn, Zn, Fe, Co, Ni)

A pure zinc borate, Zn[B12O14(OH)10], has been synthesized and characterized by XRD, FT-IR, simultaneous TG-DTA techniques, and chemical analysis. The molar enthalpy of solution of Zn[B12O14(OH)10] (s) in 1mol·dm−3 HCl(aq) was measured by microcalorimeter at T=298.15K. The molar enthalpy of solution of ZnO(s) in the mixture solvent of 2.00cm3 of 1mol·dm−3 HCl(aq) and calculated amount of H3BO3 was also measured. With the incorporation of the previously determined enthalpy of solution of H3BO3(s) in 1mol·dm−3 HCl(aq), together with the use of the standard molar enthalpies of formation for ZnO(s), H3BO3(s), and H2O(l), the standard molar enthalpy of formation of −(9646.6±9.6)kJ·mol−1 at T=298.15K was obtained on the basis of an appropriate thermochemical cycle. In addition, the molar enthalpy of formation of −9492.7kJ·mol−1 for [B12O14(OH)10]2− has also been estimated by a group contribution method, which has been used to predict the ΔfHmθ of a series of transition metal borates of M[B12O14(OH)10] (MII=Mn,Zn,Fe,Co,Ni). These estimated data have been used to further compare the stability of this series of transition metal borates.

Hydrogen Bonding in Liquid Water and in the Hydration Shell of Salts.

A near-IR spectral study on pure water and aqueous salt solutions is used to investigate stoichiometric concentrations of different types of hydrogen-bonded water species in liquid water and in water comprising the hydration shell of salts. Analysis of the thermodynamics of hydrogen-bond formation signifies that hydrogen-bond making and breaking processes are dominated by enthalpy with non-negligible heat capacity effects, as revealed by the temperature dependence of standard molar enthalpies of hydrogen-bond formation and from analysis of the linear enthalpy-entropy compensation effects. A generalized method is proposed for the simultaneous calculation of the spectrum of water in the hydration shell and hydration number of solutes. Resolved spectra of water in the hydration shell of different salts clearly differentiate hydrogen bonding of water in the hydration shell around cations and anions. A comparison of resolved liquid water spectra and resolved hydration-shell spectra of ions highlights that the ordering of absorption frequencies of different kinds of hydrogen-bonded water species is also preserved in the bound state with significant changes in band position, band width, and band intensity because of the polarization of water molecules in the vicinity of ions.

Thermodynamic properties of alkyl 1H-indole carboxylate derivatives: A combined experimental and computational study

The standard (po=0.1MPa) molar enthalpies of formation, in the crystalline phase, of methyl 1H-indole-3-carboxylate and ethyl 1H-indole-2-carboxylate, at T=298.15K, were derived from measurements of the standard massic energies of combustion using a static bomb combustion calorimeter. The Knudsen effusion technique was used to measure the vapour pressures as a function of the temperature, which allowed determining the standard molar enthalpies of sublimation of these compounds. The standard (po=0.1MPa) molar enthalpies of formation, in the gaseous phase, at T=298.15K, were calculated by combining, for each compound, the standard molar enthalpy of formation, in the crystalline phase, and the standard molar enthalpy of sublimation, yielding −(207.6±3.6)kJ·mol−1 and −(234.4±2.4)kJ·mol−1, for methyl 1H-indole-3-carboxylate and ethyl 1H-indole-2-carboxylate, respectively.Quantum chemical studies were also conducted, in order to complement the experimental study. The gas-phase enthalpies of formation were estimated from high level ab initio molecular orbital calculations, at the G3(MP2) level, for the compounds studied experimentally, extending the study to the methyl 1H-indole-2-carboxylate and ethyl 1H-indole-3-carboxylate. The results obtained were compared with the experimental data and were also analysed in terms of structural enthalpic group contributions.

Experimental and computational thermochemical studies of 6-azauracil derivatives

The standard (p°=0.1MPa) molar enthalpies of formation, in the crystalline phase, of 6-azauracil, 6-azathymine and 6-aza-2-thiothymine at T=298.15K, were derived from the standard molar energies of combustion, in oxygen, measured by combustion calorimetry. The standard molar enthalpies of sublimation, at T=298.15K, were measured by high temperature Calvet microcalorimetry. For 6-azauracil, the standard molar enthalpy of sublimation, at T=298.15K, was determined from the temperature-vapour pressure dependence, obtained by the Knudsen mass-loss effusion method.From the experimental studies, the standard molar enthalpies of formation, in the gaseous phase, at T=298.15K, of the 6-azauracil, 6-azathymine and 6-aza-2-thiothymine were derived. The gas-phase enthalpies of formation were also estimated by G3 and G4 calculations which were further extended to the computation of the standard molar enthalpy of formation of 6-aza-2-thiouracil. We compare the values obtained computationally with the experimental data available and find a good agreement between them.

The effect of ketone groups on the energetic properties of phthalan derivatives

In the present work the energetic properties of phthalide and phthalic anhydride are assessed experimental and computationally allowing to calculate the standard molar enthalpy of formation, in the gaseous phase, of each compound. These results enabled to analyze and interpret the enthalpic and structural molecular effects of one and two ketone groups in the main structure of phthalan.The high-temperature Calvet microcalorimetry and the static bomb combustion calorimetry were used to measure, respectively, the enthalpy of sublimation and the massic energy of combustion of the two compounds. These data were combined to derive the standard molar enthalpy of formation, in the gaseous phase, of phthalide and phthalic anhydride.The gas-phase enthalpies of formation of phthalide and phthalic anhydride compounds were estimated using the composite G3(MP2)//B3LYP approach together with adequate gas-phase working reactions. The computational study was also extended to phthalan and the reliability of the value obtained for the correspondent gas-phase enthalpy of formation, when compared with the experimental value reported in the literature, contributes to validate the computational methodology used. The good agreement verified between computational and experimental results for the other two phthalan derivatives studied gave us confidence to estimate the gas-phase enthalpy of formation of the 2,5-dihydrofuran that was not studied experimentally.Complementary, natural bond orbital (NBO) calculations were also performed, allowing an advance on the analysis of the structural and reactivity characteristics of this type of compounds.

5-Isopropylbarbituric and 2-thiobarbituric acids: An experimental and computational study

The standard (po=0.1MPa) molar energies of combustion, ΔcUmo, for the crystalline 5-isopropylbarbituric and 2-thiobarbituric acids were determined, at the temperature of 298.15K, by static bomb or rotating bomb combustion calorimetry, respectively. For 5-isopropylbarbituric acid, the standard molar enthalpy of sublimation, ΔcrgHmo, at T=298.15K, was derived by Calvet microcalorimetry. The standard molar enthalpy of sublimation, at T=298.15K, was derived by the Clausius–Clapeyron equation, from the temperature-vapor pressure dependence, measured by the Knudsen mass-loss effusion method. These experiments allowed the determination of the standard molar enthalpy of formation, in the gaseous phase, at T=298.15K for 5-isopropylbarbituric acid. For 2-thiobarbituric acid, the gas-phase enthalpy of formation was calculated combining the result derived for the crystalline phase with literature data for the enthalpy of sublimation, at T=298.15K.These values were compared with estimates obtained from very accurate computational calculations using the G3 and G4 level theory composite methods.

A Low Sensitivity Energetic Salt Based on Furazan Derivative and Melamine: Synthesis, Structure, Density Functional Theory Calculation, and Physicochemical Property

Reaction of 4,4′-oxybis[3,3′-(1H-5-tetrazol)]furazan (H2BTFOF, 1) with melamine (MA) leads to a energetic salt (MA+)·(HBTFOF¯)·H2O (2). The structure of 2 was characterized by single-crystal X-ray analysis. Thermogravimetry analysis indicates that the main framework of 2 possesses good thermal stability up to 501 K. Optimized structure, total energy and the frontier orbital energy of 2 were investigated in detail. As to the thermodecomposition reaction of the main framework for 2, the nonisothermal thermokinetics parameters were obtained by Kissinger’s and Ozawa’s methods. The standard molar enthalpy of formation of 2 was calculated as being (518.55 ± 6.30) kJ·mol–1 from the constant-volume combustion energy determined by a rotating-bomb microcalorimeter. Importantly, 2 has a very low sensitivity to friction (>360 N) and impact (38 J), which makes it of interest in new environmentally friendly, insensitive energetic materials.

Enthalpy of formation of natural hydrous copper sulfate: Chalcanthite

This paper presents the results of the thermochemical study of hydrous copper sulfate CuSO4·5H2O performed on a high-temperature heat-flux Tian–Calvet microcalorimeter. The samples of two natural chalcanthite (Kosmurun ore deposit, Kazakhstan, and Lavrion deposit, Greece) and synthetic hydrous copper sulfate (blue vitriol) were characterized by X-ray microprobe analysis, X-ray powder diffraction, thermal analysis, FTIR and Raman spectroscopy. The enthalpy of dehydration at T=298.15K was measured, and the standard molar enthalpies of formation from the elements were determined by the melt solution calorimetry in accordance with Hess’s law. The values of ΔfHm0 (T=298.15K) were found to be −(2267.2±4.1)kJ⋅mol−1 for natural chalcanthite and −(2272.6±6.0)kJ·mol−1 for synthetic hydrous copper sulfate (blue vitriol).

Computational Thermo-chemical Study of Enthalpies of formation of β-Alkylthiophenes Using Ab Initio and DFT Calculations.

The values for the standard molar enthalpies of formation of a series of the B-ring position alkyl-substituted thiophenes are calculated at 298.15 K using the Hartree-Fock (HF) and density functional theory (DFT) techniques. The results obtained are discussed in term of the substituent effect on the structural, electronic, and energetics of the titled molecules. The alkyl substitution with electron donating plays a fine-tune effect on the geometries, electronics, and energetics of the species. In the atomization energy route, the standard enthalpies of formation in the gas-phase, DHº(f,298)(g), obtained using the B3LYP/6-31G(d,p) method, can be successfully correlated to the substituent length via a linear dependence. However, DHº(f,298)(g), obtained using the HF/6-31G(d,p) method, is not able to predict their experimental behavior. In the formation reaction route, both the DFT and HF calculations reveal the same trend for predicting the values for the standard enthalpies of formation in the condensed-phase. It could be anticipated that the calculations can be extended to estimate the relative thermodynamic stabilities of oligo- and polymers consisting of this building blocks.

A Highly Energetic N-Rich Metal-Organic Framework as a New High-Energy-Density Material.

Metal-organic framework (MOF)-based energetic material [Cu3 (MA)2 (N3 )3 ] (1; MA=melamine) was synthesized and structurally characterized (47.55 % N). The structural analysis revealed the existence of unusual multiwalled tubular channels and interweaving of single and double helical units in 1. The standard molar enthalpy of formation was found to be 1788.73 kJ mol(-1) , which is the highest value among previously reported MOF-based energetic materials. The calculated detonation properties showed that 1 can be used as a potential explosive. Sensitivity tests revealed that 1 is insensitive and thus can function as a high-energy-density material with a favorable level of safety.

Thermodynamic properties of 5(nitrophenyl) furan-2-carbaldehyde isomers.

BackgroundThe aim of the current work was to determine thermo dynamical properties of 5(2-nitro phenyl)-furan-2-carbaldehyde, 5(3-nitro phenyl)-furan-2-carbaldehyde and 5(4-nitro phenyl)-furan-2-carbaldehyde.ResultsThe temperature dependence of saturated vapor pressure of 5(2-nitro phenyl)-furan-2-carbaldehyde, 5(3-nitro phenyl)-furan-2-carbaldehyde and 5(4-nitro phenyl)-furan-2-carbaldehyde was determined by Knudsen’s effusion method. The results are presented by the Clapeyron–Clausius equation in linear form, and via this form, the standard enthalpies, entropies and Gibbs energies of sublimation and evaporation 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 by the corresponding standard molar combustion enthalpy, obtained using bomb calorimetry combustion.ConclusionsDetermination of the thermodynamic properties for these compounds may contribute to solving practical problems pertaining optimization processes of their synthesis, purification and application and it will also provide a more thorough insight regarding the theoretical knowledge of their nature.Graphical abstract:Generalized structural formula of investigated compounds and their formation enthalpy determination scheme in the gaseous state

Hydrogen Storage: Thermochemical Studies of N-Alkylcarbazoles and Their Derivatives as a Potential Liquid Organic Hydrogen Carriers

Although N-ethylcarbazole is considered as a most promising potential liquid organic hydrogen carriers (LOHC) substance, a major drawback for stationary and particularly mobile applications is its high melting point of 343 K. Study of other possible N-alkylcarbazole-derivatives having lower melting points but keeping a high storage density is of practical importance. This contribution presents thermochemical properties (enthalpy of formation, enthalpy of vaporization, enthalpy of sublimation, and fusion enthalpy) for N-alkylcarbazoles (with alkyl = isopropyl, n-propyl, and n-butyl) derived from experiments in order to investigate the applicability of the carbazole derivatives as potential LOHCs. Additionally, high-level quantum chemical calculations were applied to determine molar enthalpies of formation of the gaseous carbazole derivatives and thus validated the experimental findings. Using a combination of the quantum-chemical calculations with vaporization enthalpy data measured in this work, the stand...

Low-temperature heat capacities and standard molar enthalpy of formation of 2-pyrazinecarboxylate cupric(II) complex Cu(pyza)2(H2O)2

2-Pyrazinecarboxylate cupric(II) complex Cu(pyza)2(H2O)2(s) was synthesized by the method of liquid phase synthesis. The composition and structure of the compound were characterized by chemical and elemental analyses and X-ray crystallography. Low-temperature heat capacities of the compound were measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 400 K. A polynomial equation of the heat capacities as a function of the temperature was fitted by the least-square method. The smoothed heat capacities and the thermodynamic functions of the compound relative to 298.15 K were calculated and tabulated at an interval of 5 K. In accordance with Hess’ law, a thermochemical cycle was designed, the enthalpy change in the reaction of 2-pyrazinecarboxylic acid with cupric acetate monohydrate was determined by the use of an isoperibol solution-reaction calorimeter, and the standard molar enthalpy of formation of the complex was calculated to be −(1191.282 ± 2.457) kJ mol−1 by the use of the dissolution enthalpies and other auxiliary thermodynamic quantities.

Investigation on specific heat capacity and combustion enthalpy of ivermectin

The specific heat capacity (Cp) of ivermectin (IVM) was measured by differential scanning calorimetry (DSC) and expressed as a function of temperature. Moreover, the standard molar combustion enthalpy of IVM was determined to be −26963.6 kJ mol−1 by XRY-1C calorimeter. The standard molar enthalpy of formation was calculated to be −862866.3 kJ mol−1 based on the standard molar combustion enthalpy. The thermodynamic basic data obtained are available for exploiting new synthesis methodology, engineering design and industry production of IVM.