Standard Enthalpy Of Formation ΔH Research Articles

Experimental determination of the standard enthalpy of formation of 4H,8H-bis(furazano)[3,4-b:3′,4′-e]pyrazine and evaluation of its performance as a dispersant of solid fuels

The energy of combustion ΔH°c of 4H,8H-bis(furazano)[3,4-b:3′,4′-e]pyrazine (BFP) was measured by calorimetry in the oxygen atmosphere and the standard enthalpy of formation ΔH°f was calculated. The real ΔH°f value was found to be 33 kJ mol−1 higher than reported before. The obtained results were used for correction of the evaluated efficiency of using BFP as a solid fuel dispersant for rocket ramjet engines, which showed high BFP performance.

Role of Ni addition on hydrogen storage characteristics of ZrV2 Laves phase compounds

In this work, the Zr(V1−xNix)2 (x = 0.02, 0.05, 0.1, 0.15, 0.25) intermetallic compounds were prepared by the arc-melt method and annealed 1273 K for 168 h. The alloys have been characterized for the structure, morphology, pressure composition isotherms over a H2 pressure range of 150 Pa to 50 MPa at different temperatures, hydrogen storage capacity, hydrogen absorption kinetics in detail. The solidification segregation of as-cast Zr(V1−xNix)2 alloys almost disappear under the homogenizing treatment at 1273 K for 168 h. With increasing Ni content in the alloys, the hydrogen absorption capacities decrease and equilibrium pressure increases. Meanwhile, the standard enthalpy of formation ΔH and standard entropy of formation ΔS for the alloys' hydrides have been obtained from Van't Hoff's equation in the (α + β) two-phase region of Zr(V1−xNix)2 alloys. It is found that the Zr(V1−xNix)2 alloys with higher Ni content preserve higher absolute values of ΔH and more stable metal hydride roughly. The alloys also exhibit fast absorption kinetics at room temperature and 773 K. After 100 hydrogen absorption/desorption cycles, Zr(V0.05Ni0.95)2 still preserves 94.1% of the first hydrogen storage capacity at 823 K and shows excellent cyclic stability.

Investigation of the thermodynamic and kinetic properties of La–Fe–B system hydrogen-storage alloys

La–Fe–B hydrogen-storage alloys were prepared using a vacuum induction-quenching furnace with a rotating copper wheel. The thermodynamic and kinetic properties of the La–Fe–B hydrogen-storage alloys were investigated in this work. The P–C–I curves of the La–Fe–B alloys were measured over a H2 pressure range of 10−3 MPa to 2.0 MPa at temperatures of 313, 328, 343 and 353 K. The P–C–I curves revealed that the maximum hydrogen-storage capacity of the alloys exceeded 1.23 wt% at a pressure of approximately 1.0 MPa and temperature of 313 K. The standard enthalpy of formation ΔH and standard entropy of formation ΔS for the alloys' hydrides, obtained according to the van't Hoff equation, were consistent with their application as anode materials in alkaline media. The alloys also exhibited good absorption/desorption kinetics at room temperature.

Hydroxyoxiranone: an ab initio MO investigation of the structure and stability of a model for a possible α-lactone intermediate in hydrolysis of sialyl glycosides

The standard enthalpy of formation ΔH°f,298 for hydroxyoxiranone is estimated as –377 ± 10 kJ mol–1 by means of ab initio molecular orbital calculations at the QCISD(T)(full)/6-311G(2df,p)//MP2(full)/6-311G(d,p) level of theory, corresponding to a conventional ring strain energy of 104 kJ mol–1. The hydroxy substituent on Cα stabilizes the α-lactone by 65 kJ mol–1 with concomitant elongation of the bond from Cα to the endocyclic oxygen On. There is a much larger stabilization (205 kJ mol–1) by the hydroxy substituent upon the zwitterion obtained by heterolysis of the Cα–On bond, and upon the carbene obtained by decarboxylation of the zwitterion. The relative energies of the α-lactone, zwitterion and carbene have been determined by MP2(fc)/6-31+G(d) calculations in vacuo and with the IPCM method for aqueous solvation. Solvation by this continuum method preferentially stabilizes the zwitterion, although at the IPCM-MP2(fc)/6-31+G(d) level the α-lactone is still 21 kJ mol–1 lower in energy than the zwitterion.

Substituent Effects on the C‐C Bond Strength, 15 [1] Geminal Substituent Effects, 7 [2] . Thermochemistry and Thermal Decomposition of Alkyl‐substituted Tricyanomethyl Compounds

The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile (10a) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile (10b) decomposed heterolytically into carbenium ions and (CN)3C− anions, while 9-methyl-9-(tricyanomethyl)fluorene (10c) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH (10c) = 155· 2 kJ mol−1, ΔS (10c) = 58· 5 J mol−1 K−1]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile (2) (422.45 kJ mol−1), 2,2-dicyanohexanenitrile (3) (349.74 kJ mol−1), 2,2-dicyano-3-phenylpropionitrile (4) (540.75 kJ mol−1), 2-butyl-2-methylhexanentrile (5) (-133.20 kJ mol−1), 2,2-dimethylpentanenitrile (6) (-45.78 kJ mol−1), and 2-methylbutyronitrile (7) (2.44 kJ mol−1) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1RSE (1) = -18 kJ mol−1 was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol−1) and α,α-dicyanoalkyl (27 kJ mol−1) radicals, which were recalculated from bond homolysis measurements[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2).

Vaporization behavior of V 3P(s)

The vaporization behavior of V 3P has been studied by mass-loss effusion in the temperature range 1830–1900 K. The vaporization is kinetically inhibited, and vaporization under grossly non-equilibrium conditions is congruent. As equilibrium is approached, the vaporization becomes incongruent with phosphorus being lost preferentially. Data from runs which are at equilibrium yield a standard enthalpy of formation ΔH f,298.15 o = − 49.7 ± 1.0 kcal mol − 1 for V 3P(s).

Thermodynamic stability of the molecules YC2, YC3, YC4, YC5, YC6, YC7, and YC8 by high temperature Knudsen effusion mass spectrometry

The molecules YC2, YC3, YC4, YC5, YC6, YC7, and YC8 have been observed in a Knudsen effusion mass spectrometric investigation of the gas phase above the Y–Ir–Au–graphite system. The enthalpies ΔH○0 of the reactions Y(g)+n C(graph)=YCn(g)(n=2–8) were evaluated by the third law method; the second law method was employed for the gaseous molecules with up to six carbon atoms. The selected values of the reaction enthalpies ΔH○0 were combined with ancillary literature data to yield the atomization enthalpies ΔH○a,0 and the standard enthalpies of formation ΔH○f,298.15 in kJ mol−1 of the gaseous yttrium carbides. The values of ΔH○0 and ΔH○a,0 (in kJ mol−1) are 197±5 and 1225±8 for YC2; 377±10 and 1757±12 for YC3; 361±8 and 2484±10 for YC4; 499±10 and 3057±15 for YC5; 536±15 and 3731±20 for YC6; 684±35 and 4294±35 for YC7, and 693±35 and 4727±35 for YC8. The values of ΔH○f,298.15 (in kJ mol−1) are 623±8, 804±12, 790±10, 924±15, 966±20, 1105±35, and 1124±35 for YCn (n=2–8), respectively.

The photoionization spectra of effusive and supersonic molecular beams of monosilane

Effusive and supersonic molecular beams of monosilane have been ionized by photons from the Berlin synchrotron light source (BESSY). Photoionization efficiency curves have been taken as a function of wavelength (50 to 120 nm) for a number of fragment ions. The main products observed are SiH+3 and SiH+2 . No appreciable SiH+4 could be detected and less than 5% of the signal was due to lower fragments (SiH+, Si+). The threshold energies Ethr, the standard enthalpy of formation ΔH○f,g of the fragment ions SiH+2 and SiH+3 and the ionization potentials Iz of the corresponding radicals have been determined to be SiH+2: Ethr=11.67±0.04 eV ΔH○f,g=276.3±0.9 kcal/mol, Iz(SiH2)=9.47±0.03 eV. SiH+3: Ethr=12.23±0.02 eV, ΔHf,g=237.1±0.6 kcal/mol, Iz(SiH3)=8.32±0.07 eV. Discrete structure in the photoionization curves is observed in the range of Ethr=15.8–17.6 eV and interpreted as vibrational progressions of a highly excited autoionizing state of SiH4 which lies 15.8 eV above the ground state. Vibrational constants have been found to be 1790±80 and 680±50 cm−1 which correspond to the ground state constants 2187 and 711 cm−1 of the ν1 and the ν2 modes of monosilane. The broadening of some lines in the progression is interpreted as being caused by a transition into a dissociative excited neutral state with lifetimes as short as 3×10−14 s for v=3 of the ν2 mode. Additional ions (predominantly Si2H+4, Si2H+6, Si2H+7) have been observed when photoionizing a supersonic molecular beam of monosilane. These are shown to stem from the ionization and subsequent fragmentation of dimers and possibly higher multimeres. Threshold values have been determined to be Si2H+4: 11.58±0.02 eV; Si2H+6: 11.4 eV; and Si2H+7: 11.4 eV.

ChemInform Abstract: ENTHALPIES OF FORMATION OF NAPHTHALENE TCNQ, ANTHRACENE TCNQ, TMPD, AND TMPD TCNQ, AND EXPERIMENTAL CRYSTAL BINDING ENERGIES OF MIXED SIMPLE REGULAR LATTICES

The standard enthalpies of formation ΔH°f[naphthalene TCNQ(c), 298.15 K]=716.95±7.30 kJ/mol, ΔH°f[anthracene TCNQ(c), 298.15 K] =763.84±4.87 kJ/mol, and ΔH0f[TMPD(c), 298.15 K]=39.60±3.85 kJ/mol were measured by oxygen combustion calorimetry. Then, the states naphthalene TCNQ(c), anthracene TCNQ(c), and TMPD TCNQ(c) are more stable by 30.01±7.63, 30.31±5.26, and 82.08±4.34 kJ/mol than the states [naphthalene(c), TCNQ(c)], [anthracene(c), TCNQ(c)], and [TMPD(c), TCNQ(c)], respectively; they are more stable than the gaseous neutral ions by 203±12, 232±12, and 230±10 kJ/mol, respectively. Therefore, the choices of different lattices and ionicities are determined not by the thermochemical results given above, but by a subtle interplay between the cost of ionization ID–AA and the various charge‐dependent contributions to the theoretical cohesive energy.

Enthalpies of formation of naphthalene TCNQ, anthracene TCNQ, TMPD, and TMPD TCNQ, and experimental crystal binding energies of mixed simple regular lattices

The standard enthalpies of formation ΔH°f[naphthalene TCNQ(c), 298.15 K]=716.95±7.30 kJ/mol, ΔH°f[anthracene TCNQ(c), 298.15 K] =763.84±4.87 kJ/mol, and ΔH0f[TMPD(c), 298.15 K]=39.60±3.85 kJ/mol were measured by oxygen combustion calorimetry. Then, the states naphthalene TCNQ(c), anthracene TCNQ(c), and TMPD TCNQ(c) are more stable by 30.01±7.63, 30.31±5.26, and 82.08±4.34 kJ/mol than the states [naphthalene(c), TCNQ(c)], [anthracene(c), TCNQ(c)], and [TMPD(c), TCNQ(c)], respectively; they are more stable than the gaseous neutral ions by 203±12, 232±12, and 230±10 kJ/mol, respectively. Therefore, the choices of different lattices and ionicities are determined not by the thermochemical results given above, but by a subtle interplay between the cost of ionization ID–AA and the various charge-dependent contributions to the theoretical cohesive energy.

The low-temperature (5 to 310 K) heat capacity and thermodynamic functions of cesium fluoroxysulfate (CsSO 4F) and the standard potential at 298.15 K for the half-reaction: SO 4F −(aq) + 2H +(aq) + 2e − = HSO 4−(aq) + HF(aq)

The low-temperature (5 to 310 K) heat capacity of cesium fluoroxysulfate, CsSO 4F, has been measured by adiabatic calorimetry. At T = 298.15 K, the heat capacity C p o( T) and standard entropy S o( T) are (163.46±0.82) and (201.89±1.01) J · K −1 · mol −1, respectively. Based on an earlier measurement of the standard enthalpy of formation ΔH f o the Gibbs energy of formation ΔG f o(CsSO 4F, c, 298.15 K) is calculated to be −(877.6±1.6) kJ · mol −1. For the half-reaction: SO 4F −(aq)+2H +(aq)+2e − = HSO 4 −(aq)+HF(aq), the standard electrode potential E at 298.15 K, is (2.47±0.01) V.

Thermochemistry of uranium compounds XII. Standard enthalpies of formation of the α and β modifications of uranium pentafluoride The enthalpy of the β-to-α transition at 298.15 K

A solution-calorimetric determination of the enthalpies of reaction of the α and β modifications of UF 5 with Ce(SO 4) 2 + H 2SO 4 is described. Auxiliary measurements were made of the enthalpies of reaction of UO 2, γ-UO 3, and HF(aq). From these results are calculated the standard enthalpies of formation ΔH f o(298.15 K) of β-UF 5, −(2083.0 ± 6.3) kJ·mol −1 and of α-UF 5, −(2075.5 ± 6.7) kJ·mol −1. The enthalpy of the (β-to-α) transition is (7.5 ± 2.1) kJ·mol −1 at 298.15 K. Suggested ΔH f o values are also given for U 4F 17 and U 2F 9.

The standard enthalpy of formation of potassium sulfide (K 2S) by fluorine bomb calorimetry

The standard energy of combustion in fluorine of potassium sulfide K 2S has been measured by bomb calorimetry. The standard enthalpy of formation ΔH f o(K 2S, c) = −(406.2±2.9) kJ mol −1 was derived. This value is compared with literature values obtained by reaction calorimetry.

The standard enthalpy of formation of chalcopyrite (CuFeS 2) by fluorine bomb calorimetry

The standard energy of combustion of chalcopyrite, CuFeS 2, has been measured by fluorine bomb calorimetry. The standard enthalpy of formation ΔH f o(CuFeS 2, c) = −(173.1±8.7) kJ mol −1 was derived. This value was compared with ΔH f o values obtained in equilibrium studies and by chromic-acid solution calorimetry. Based on the measured value and data from the literature, the thermodynamic properties of chalcopyrite at 298.15 K are ΔH f °=−(173.1±8.7) kJ kmol −1, ΔS f °=−(0.32±0.21) J K −1 mol −1 and ΔG f °=−(173.1±8.7) kJ kmol −1

The standard enthalpy of formation of uranium dioxide by fluorine bomb calorimetry

The standard energies of combustion in fluorine of two samples of uranium dioxide of compositions UO 2.000 and UO 2.023 have been determined in a bomb calorimeter. The standard enthalpies of formation ΔH f o(UO 2.000, c) = − (1085.1 ± 2.5) kJ mol −1 and ΔH f o(UO 2.023, c) = − (1090.2 ± 2.8) kJ mol −1 were derived. These values were compared with literature data and the recommended CODATA standard enthalpy of formation ΔH f o(UO 2, c).

The enthalpies of formation of SbF 5(l) and Sb 2S 3(c) and the high-temperature thermodynamic functions of Sb 2S 3(c) and Sb 2S 3(l)

The energies of combustion of Sb(c) and Sb 2S 3(c) in fluorine were measured in a bomb calorimeter. The standard enthalpies of formation ΔH f o(298.15 K), of SbF 5(l) and Sb 2S 3(c) were derived to be −(1327.95 ± 0.93) kJ mol −1 and −(141.8 ± 4.1) kJ mol −1, respectively. High-temperature enthalpy increments were determined for crystalline and liquid Sb 2S 3 by drop calorimetry. The enthalpy and entropy of melting of Sb 2S 3(c) at 823 K were derived to be (40.64 ± 0.20) kJ mol −1 and (49.38 ± 0.24) J K −1 mol −1, respectively.

Enthalpy of combustion of diphenyl diselenide

The standard enthalpy of the combustion reaction ( C 6 H 5) 2 Se2( c)+16.5 O 2( g)+2263 H 2 O(l)=12 CO 2( g)+2{ SeO 2+1134 H 2 O}( l) has been measured in a rotating-bomb combustion calorimeter to be ΔH c o = − (615.3 ± 2.8) kJ mol −1. This leads to a standard enthalpy of formation ΔH f o{(C 6H 5) 2Se 2, c} = + (120.6 ± 8.2) kJ mol −1. A Knudsen-effusion technique has been used to determine the enthalpy of sublimation of diphenyl diselenide as ΔH sub = + (116.7 ± 2.5) kJ mol −1. The bond dissociation energy D(Se-Se) = + (280.0±19.0) kJ mol −1 is calculated.

The enthalpy of formation of thulium trifluoride

The energy of formation of thulium trifluoride was measured by direct fluorination of thulium metal in a bomb calorimeter. From the experimental results the standard enthalpy of formation ΔH f o(TmF 3, c, 298.15 K) was determined to be −(1656 ± 7) kJ mol −1 and the standard Gibbs energy of formation ΔG f o(TmF 3, c, 298.15 K) was estimated to be −(1579 ± 7) kJ mol −1.

The enthalpies of formation and high-temperature thermodynamic functions of As 4S 4 and As 2S 3

The energies of combusion of β-As 4S 4 and vitreous As 2S 3 in fluorine were measured in a bomb calorimeter. High-temperature enthalpy increments were also determined by drop calorimetry. The standard enthalpies of formation ΔH f o(298.15 K), were found to be −(134.6±6.7) kJ mol −1 and −(69.6 ± 4.2) kJ mol −1 for β-As 4S 4 and vitreous As 2S 3, respectively. Based on the measured results and data from the literature, the thermodynamic properties of the minerals realgar, α-As 4S 4, and orpiment, As 2S 3(c) were derived. For realgar at 298.15 K, ΔH f o = −(138.1 ± 6.7) kJ mol −1, ΔS f o = −(22.3 ± 3.1) J K −1 mol −1, and ΔG f o = −(131.5 ± 6.8) kJ mol −1 are recommended and, for orpiment at 298.15 K, ΔH f o = −(91.6 ± 4.8) kJ mol −1, ΔS f o = −(3.1 ± 4.3) J K −1 mol −1, and ΔG f o = −(90.7 ± 5.0) kJ mol −1.

The enthalpy of formation of neodymium trifluoride

The energy of formation of neodymium trifluoride was measured by direct fluorination of neodymium metal in a bomb calorimeter. From the experimental results the following values were determined: the standard enthalpy of formation ΔH f o(NdF 3, c, 298.15 K) = −(1660.6 ± 4.9) kJ mol −1, and the standard Gibbs energy of formation ΔG f o(NdF 3, c, 298.15 K) = −(1582.5 ± 5.1) kJ mol −1.