ΔHf Values Research Articles

An investigation of arsenic and antimony additives in unseeded and potassium-seeded [formula omitted] flames

Studies have been completed of the reactions involved with arsenic and antimony additives in H 2 O 2 flames and H 2 O 2 flames seeded with potassium. The flame was atmospheric and a molecular beam of the frozen species was introduced into the mass spectrometer through a four-stage differential pumping system operating from 1 to 10 −11 atm. Arsenic compounds found in the flame included AsO(g), HAsO 3(g) and H 3AsO 4(g). Heat of formation values, ΔH f298 , were calculated as −145.4±5 kcal/mole for HAsO 3(g) and −259.7±5 kcal/mole for H 3AsO 4(g). Antimony compounds included Sb(g), SbO(g), HSbO 3(g) and H 3SbO 4(g), yielding ΔH f298 values of 18.7±5 kcal/mole for SbO(g), −137.4±5 kcal/mole for HSbO 3(g), and −262.9±5 kcal/mole for H 3SbO 4(g). The formation of the potassium compounds KH 2AsO 4(g) and KH 2SbO 4(g) was also established. Heats of formation values, ΔH f298, obtained for the K-substituted compounds were −299.5±5 and −303.7±5 kcal/mole for KH 2AsO 4(g) and KH 2SbO 4(g), respectively.

Enthalpies of formation of alkali-metal hexachloro-zirconates and -hafnates

Enthalpies of reaction have been determined by solution calorimetry for the process (i) where M′= Na, K, or Cs 2 M′Cl(c)+ MCl4(c)→ M′2[MCl6](c)(i) and M = Zr or Hf. Heats of formation for the complexes, ΔHf⊖(298 K), have been calculated using mean values of ΔHf⊖ for MCl4 taken from the literature. The reaction is marginally more favourable when M = Hf but both ZrCl4 and HfCl4 are somewhat better acceptors of chloride ion than is NbCl4 and both are markedly better acceptors than SnCl4.

Dissociation energies of BeF and BeCl and the heat of formation of BeClF

An effusion-mass spectrometric investigation of the Be–Cl–F system to obtain bond energies of BeF and BeCl and the heat of formation of the mixed halide BeClF has been completed. A study of the ion intensities of the species produced in the simultaneous reaction of chlorine gas and BeF2 vapour with elemental beryllium was made in the temperature range 1415 to 1592 K. The reaction enthalpies calculated from these species yield second and third law ΔHf298 values of –139.0 ± 4 and –140.5 ± 2 kcal mol–1, respectively, for BeClF(g). The dissociation energies of 104 kcal mol–1 obtained for BeCl and 144 kcal mol–1 for BeF agree with those previously reported from spectroscopic, molecular flow effusion, and transport investigations.

Measurement of Negative Ions Formed by Electron Impact. IX. Negative Ion Mass Spectra and Ionization Efficiency Curves of Negative Ions of m⁄e 25, 26, 27, 38, 39, 40 and 50 from Acrylonitrile

Abstract Negative ion mass spectra of acrylonitrile were measured for the electron energies of 80, 40 and 9.5 eV. Emphasis was laid on the abundance of the negative ions relative to the positive ions measured for the energies of 80, 40 and 15 eV, respectively. The ionization efficiency (IE) curves were also determined up to 25 eV for the ions of m⁄e 25(C2H−), 26(CN−), 27(HCN−), 38(C2N−), 39(CHCN−), 40(CH2CN−) and 50(C3N−). The electron impact of 80 eV and 40 eV gave almost the same distribution of m⁄e for negative ion mass spectra. Besides the most intense peak of CN− ions, relatively strong peaks of C3N−, C2H−, C2− and C2N− ions were observed. In 9.5 eV, CN− ions predominated over other ions. Yields of the ions showed a good linearity against the pressure in the range used for usual chemical analysis, irrespective of electron energy. Comparison of the yield of CN− ions with that of C2H3CN+ gave the values 1.2×104 and 1.9×104 for C2H3CN+/CN− at 80 eV and 40 eV, respectively, and 47.6 at 9.5 eV (15 eV for the positive ions). The plausible reaction schemes expected to occur at each onset observed in the IE curves were also sought thermochemically by using ΔHf values of the reactant and products. A value \gtrsim2 eV was obtained for the electron affinity of C2H.

Rates of formation and dissociation of complexes of manganese(II) with the ligands 1,10-phenanthroline, 2,2′-bipyridine, and 2,2′2″-terpyridine in anhydrous methanol

Rate constants for the formation (kf) and dissociation (kd) of complexes of MnII with the ligands 1,10-phenanthroline (phen), 2,2′-bipyridine (bipy), and 2,2′,2″-terpyridine (terpy) have been measured, at low temperatures in anhydrous methanol, by the stopped-flow method. At 298·1 K and I= 0·20 mol l–1(NaClO4) : log (kf/l mol–1 s–1)(extrapolated)= 4·95, 4·24, and 3·72; ΔHf‡= 10·6 ± 0·4, 11·7 ± 0·1, and 12·8 ± 1·0 kcal mol–1; ΔSf‡=–0·4 ± 1·6, 0·1 ± 0·1 and 1·3 ± 3·9 cal K–1 mol–1; log (kd/s–1)= 1·15, 1·57, and –1·33; ΔHd‡= 13·5 ± 0·3. 13·4 ± 0·5. and 13·7 ± 0·6 kcal mol–1; and ΔSd‡=–8·2 ± 0·9, –6·4 ± 1·9, and –18 ± 2 cal K–1 mol–1 for phen, bipy, and terpy respectively. The kinetically determined stability constants in methanol for the complexes (logK1= 3·8, 2·7, and 5·0 for phen, bipy, and terpy respectively) are very similar to values determined in aqueous solution. Values of ΔHf‡ are significantly larger than the activation enthalpy associated with MnII–methanol solvent exchange and possible reasons for this unexpected behaviour are discussed.

Effusion-mass spectrometric study of thermodynamic properties of vanadium and vanadium nitride

Effusion-mass spectrometric studies were performed on the vapour species in equilibrium above VN(c) in the temperature range 1900 to 2412 K. From an analysis of the reactions involved, ΔHf298 values of 122.4 ± 5 kcal/mol (second law) and 120.4 ± 2 kcal/mol (third law) were obtained for VN(g), yielding a dissociation energy, D°298, for gaseous VN of 114.1 kcal/mol. An appearance potential of 8 ± 1 eV was recorded for VN(g). The thermodynamic analysis of the vapour species over the solid state yielded a second law ΔHf of –52 ± 1.5 kcal/mol for VN(c). Studies made in this temperature range resulted in a third law heat of fusion of 4.30 ± 0.45 kcal/mol for vanadium. The second and third law mass spectrometric ΔHf298 values of 122.8 ± 1.5 and 123.0 ± 0.2 kcal/mol, respectively, for V(g) were in good agreement with Knudsen third law weight loss experiments at 2092 and 2122 K, which yielded ΔHf298 values of 123.3 and 122.9 kcal/mol.

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF4 were measured for the reactions 2SiO2(c)+2BeF2(d)⇋SiF4(g)+Be2SiO4(c) (log PsiF4(mm) = [8.790 ‐ 7620/T] ±0.06(500°–640°C)) and Be2SiO4(c) +2BeF2(d)⇋SiF4(g) +4BeO(c)(log PsiF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF2 was present in solution with LiF as molten Li2BeF4. The solubility of SiF4 was low (∼0.04 mol kg‐1 atm‐1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved ΔHf and ΔGf values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO2 and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF4 generated showed an increasing positive deviation from the expression given for the equilibrium involving Be2SiO4 and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO2.

Effusion-Mass Spectrometric Determination of the Heats of Formation of the Gaseous Molecules V4O10, V4O8, VO2, and VO

An effusion-mass spectrometric investigation of the vapor in equilibrium with the vanadium compounds, V2O5, V2O4, and V2O3, was conducted. The only congruently evaporating solid was V2O3, while the others decomposed to various vapors and V2O3 in the temperature range 1000–1200°K. The vapor over V2O5 was found to contain the compounds V4O10(g) and V4O8(g) with resulting ΔHf298 values of −681.2±7 kcal/mole and −590.5±7 kcal/mole, respectively. A study showed that the vapor over V2O3 at 2270°K contained the species V(g), VO(g), and VO2(g), yielding values for ΔHf298 of 35.1±4 kcal/mole and −56.3±4 kcal/mole for VO(g) and VO2(g), respectively.

Differential Scanning Calorimeter Studies of the Thermal Transitions of Collagen: IMPLICATIONS ON STRUCTURE AND STABILITY

Thermal transition enthalpy changes (ΔHf) were investigated in intact and soluble collagen from a variety of sources and species utilizing differential scanning calorimetry. Results indicated that the ΔHf determinations were not dependent on sample size, method of preparation, or sample purity. The ΔHf values for intact collagen were virtually identical with those from tropocollagen preparations, confirming previous conclusions that the state of aggregation or extent of covalent cross-linking plays only a minor role in the thermal stability of collagen. The experimentally determined values for the enthalpy change per residue (ΔHr) were 1055 cal per mole and appeared to be invariant in all the collagens studied. The denaturation energy per triplet averaged 3000 cal per mole, suggesting that forces other than hydrogen bonding must be contributing to denaturation enthalpy. A correlation between thermal denaturation or shrinkage temperature and total pyrrolidine content was evident. The entropy change per residue also increased with decreasing imino acid content. These results tend to support the conclusion that the tertiary superhelix structure of the collagen molecule is stabilized primarily by the steric restrictions imposed by the pyrrolidine residues.

Excited products in the photodissociation of methyl thiocyanate and isothiocyanate in the vacuum ultra-violet

Photolysis of CH3NCS and CH3SCN in the vacuum u.-v. produces fluorescence attributed to SCN excited radicals. In the case of CH3SCN production of CNB2∑+ system is also found. The threshold wavelength of incident photons for SCN formation is 197 ± 1 nm for CH3SCN and 188 ± 1 nm for CH3NCS. These values give D(CH3—SCN)= 3.04 eV and D(CH3—NCS)= 3.37 eV for the production of NCSA2Πi. A value of ΔHf(SCN)= 3.19 eV (76.4 kcal mol–1) has been calculated.

Measurement of Negative Ions Formed by Electron Impact. VIII. Ionization Efficiency Curves of Negative Ions from Methyl and Ethyl Cyanides

Abstract The ionization efficiency (I.E) curves of m/e 26(CN−), 27(HCN−), 38(C2N−) and 39(CHCN−) ions from methyl and ethyl cyanides, besides 40(CH2CN−) ions from methyl cyanide, and 50(C3N−) ions from ethyl cyanide were measured to the extent of about 25 eV electron energies under the pressure of ∼10−6 mmHg by a Hitachi RMU-6D mass spectrometer. Schemes of plausible reactions expected to appear at each onset value observed in the respective IE curves were sought thermochemically by using ΔHf values of the reactants and products. The estimation method of unknown ΔHf values is described also. It should be noted that for CN− ions one process (probably, R+CN−) was observed in lower energies than 2 eV; C2N− ions are expected to be formed with excess energy; also, for CN− and HCN− ions rather strange phenomena of the decrement of their signal intensities with increasing electron energies were observed in the range of higher energies. Moreover, the IE curves of C3N− ions in the higher energy region were well interpreted in terms of the overlapping phenomena of dissociative capture process with ion pair formation process. It was found from this work that EA(HCN) \gtrsim1 eV, ΔHf(CHCN−) ∼3.2 eV, EA(CHCN) \gtrsim1.1 eV and EA(CH2CN) \gtrsim1.6 eV. For the electron affinity of C2N a value \gtrsim2.3 eV is tentatively introduced on the basis of EA(C3N)=2.4 eV, EA(C5N)=2.3 eV, and EA(CN)=3.4 eV.

Heat of Formation of O2−

A series of Born–Mayer-type calculations are used to calculate the lattice energies of simple oxides (MgO, BeO, CaO, and ZnO). Repulsion and other non-Coulombic contributions to the lattice energy are obtained using thermodynamic and recent ultrasonic data for the bulk moduli and the isothermal pressure and temperature derivatives of the elastic constants. Using thermochemical data for the heat of formation of MgO, CaO, and BeO and their cations, the heat of formation of O2−, ΔHf°(O2−), is calculated to be 197 ± 5 kcal/mole. Using the largest value of ΔHf°(O2−), obtained for MgO, presumably the most ionic of the crystals treated, a value of 202.3 kcal/mole is obtained. These values are believed to be more accurate than earlier values given by Morris and by Huggins and Sakamoto who obtained 210 ± 6 and 221 ± 15 kcal/mole. The anomalously low value calculated for ΔHf°(O2−) for ZnO is believed to result from a substantial covalent contribution in the Zn–O bond in this oxide.

Measurement of Negative Ions Formed by Electron Impact. III. The Ionization Efficiency Curves of Negative Ions of m⁄e 26 and 42 from Nitroalkanes

Abstract The studies of this series were extended to the negative ions of m⁄e 26 and 42 obtained from nitroalkanes; nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane. For m⁄e 26 (probably, CN−) ions, the contribution of three or two processes in the range from zero to ∼20 eV of electron energy was observed for nitromethane or other samples. On the basis of ΔHf values of the reactants and products, and of the value of EA(CN), the plausible reactions to be assigned to each process were presented. For the first process, CH3NO2+e−→CN−+H2O+OH, RNO2+e−→CN−+R1OH+OH. For the second process, CH3NO2+e−→CN−+H2O+O+H, CH3NO2+e−→CN−+H2+O2+H, RNO2+e−→CN−+R1+HO2+H, and C3H7NO2+e→CN−+C2H4+H2+O+OH irrespective of kind of nitropropane. For the third process, CH3NO2+e−→CN−+3H+O2. For m⁄e 42 (probably, CNO−) ions, the contribution of two processes was observed. By taking ΔHf (CNO)=x (eV), EA(CNO)=y (eV), and also combining the observed onset values with the calculated values of appearance potential of reactions to be assigned for each process — for the first process, CH3NO2+e→CNO−+H2+OH, RNO2+e−→CNO−+R1OH+H; for the second process, CH3NO2+e−→CNO−+HO+2H, CH3NO2+e−→CNO−+H2+O+H, RNO2+e−→CNO−+R1O+2H– a relation of x−y\simeq2(eV) was derived.

Mass spectrometric studies of structural isomers—II: Mono‐and bicyclic C6H10 molecules

AbstractThe electron‐impact‐induced ionization and fragmentation of six C6H10 structural isomers have been studied in order to determine the effect of isomerism upon their mass spectrometric behavior. The 70 eV mass spectra, metastable transitions and appearance potentials of the principal ions are reported. Significant differences between the mass spectra of the six isomers were observed; however, metastable transition and appearance potential data indicate that the fragmentation path‐ways are the same for all the C6H10 molecules. Experimentally determined ionization potentials for the structural isomers are presented and compared to ionization potentials calculated by the bond orbital method. Utilizing fragmentation pathways deduced from general features in the mass spectra and from observed metastable transitions, we calculated heats of formation (ΔHf) for the observed principal ions and compared these values to ΔHf values for isomeric ions from other molecules.

Ionization Potentials of Molecules and Free Radicals and Appearance Potentials by Electron Impact in the Mass Spectrometer

An improved method of measuring ionization efficiency (I.E.) curves by the retarding potential difference technique in the mass spectrometer (to ∼one ion/sec at onset) gives ionization potentials which agree within ± 0.02 eV of spectroscopic values and ± 0.03 eV of photoionization. Ionization potentials I have been measured for the following free radicals (eV): CH3, 9.87; C2H5, 8.34; n-C3H7, 8.13; i-C3H7, 7.57; n-C4H9, 8.01; C2H5O, 9.11; n-C3H7O, 9.20; i-C3H7O, 9.20; n-C4H9O, 9.22. Appearance potentials V have been measured for R+/RX, where R=CH3, C2H5, n-C3H7, n-C4H9 and X=H, Cl, Br, I, and OR; also for RO+/ROX where R=C2H5, i-C3H7, n-C4H9 and X=H, NO, and R; also for I+/n-C4H9I, NO+/C2H5ONO, NO+/n-C4H9ONO, and C3H6D+/CH3CD2CH3. Values of ΔHf(R+), ΔHf(RO+), D(R−X), and D(RO−X) are reported as well as electron affinities of NO, C2H5O, n-C3H7O, i-C3H7O, and n-C4H9O. Whenever two or more independent measurements of V provide the same ΔHf or D, agreement is satisfactory. The linearity of onsets does not correlate with photoionization efficiency curves according to present threshold theory. The “breaks” in I.E. curves do yield thermochemically consistent results.

Chemical Behaviour of Difluorocarbene, and the Dissociation of the Carbon—Carbon Bond in Tetra-fluoroethylene

Bond strengths. The heat of formation of CF2 has been estimated through electron impact, thermochemical and kinetic studies. The most recent measurements were obtained through a study of the pyrolysis of CF2HC1 at 530°–750° C, which is believed1–3 to follow the mechanism: Either by interpolation from known data, or by assuming the heat of formation of C2F4 (−151.5 kcal/mole (ref. 4) ) and combining this with the thermodynamic data for the equilibrium: it is possible to estimate the heat of formation of CF2HC1 (∼ − 112 kcal/mole); an estimate of the Arrhenius factors of the elementary steps can be derived from the kinetic analysis. Combination of the thermochemical and kinetic data yields ΔHf(CF2) ≲ − 40 kcal/mole, and hence D(F2C−CF2) ≲ 70 kcal/mole. Similar conclusions have been reached by Stull5, on the basis of a study of the formation of C2F4 from carbon and CF4 in a furnace6. Majer and Patrick7 have suggested that higher (less negative) values for ΔHf(CF2), which have been reported on the basis of electron impact studies, result from the production of excited fragments in the primary dissociation.

Reaction of Cyclopropane with Iodine and Some Observations on the Isomerization of Cyclopropane

The kinetic and thermodynamic data of Ogg and Priest on the system I2+cyclopropane⇌ICH2CH2CH2I are reexamined. Using an entropy for ICH2CH2CH2I, estimated from additivity rules of 93.0±1.5 eu, and the experimental value of Keq, ΔH° is found to be −19.4±0.9 kcal. This gives Hf° (ICH2CH2CH2I) = 8.2 kcal/mole and the use of group additivity rules allow the values of ΔHf° for EtI and n—C3H7I to be fixed at −3.3±0.5 and −8.3±0.5 kcal/mole, respectively. For CH3I, ΔHf°=1.6±1 kcal/mole. The kinetic mechanism is also reexamined and from entropy and energy estimates for the free radical species, values of all of the rate constants are evaluated. These are in good agreement with values for similar reactions. It is proposed that the I2-catalyzed isomerization of cyclopropane proceeds through the step I+ICH2CH2CH2I→HI+ICH2–CH=CH2+I, followed by a rapid, atom-catalyzed reaction of allyl iodide with HI to give propylene. The rate constants for such a step are also in good agreement with those for similar reactions. Extension of the data to cyclopropane-propylene isomerization is made and it is shown that the most reasonable path involves the trimethylene radical as an intermediate. Its lifetime is, however, very short∼10−11 sec. Slater's model for this reaction is shown to be implausible. These data indicate an activation energy for the cyclization of the trimethylene free radical of about 8 kcal. This is reasonable only if the end CH2 groups in trimethylene are interacting via a ``bent'' bond. In such case the activation energy arises from the necessity for eclipsed configurations. A similar analysis is made for cyclobutane and the tetramethylene radical with compatible results. For cyclopropane the strain energy is 30 kcal and for cyclobutane 28 kcal.