Heat Of Dissociation Research Articles

Heat of dissociation of As4(g) and the heat of formation of As2(g)

The equilibrium As4(g)=2As2(g) has been studied in detail over solid phase MoAs2 and Mo2As3 using the Knudsen effusion technique combined with a mass spectrometer. A ``see-through'' ion source and a liquid-nitrogen cooled condensation plate were used to eliminate the spurious As4(g) which has been encountered by other investigators. Excellent agreement was obtained between the second- and third-law enthalpies. From equilibrium constants measured between 807 and 1050°K, the following third-law heats were determined: As4(g)=2As2(g), Δ H0°=53.95± 1.1, Δ H298°=54.26± 0.7, Δ H0°(f)[As2(g)]=45.65± 0.7, Δ H298(f)°[As2(g)]=45.45± 0.7, all in kilocalorie mole−1, the errors being estimates of total error. The results obtained when GaAs, Mo5As4, and Zn3As2 were used as source phases are briefly described. The results of previous investigations of the As4(g)=2As2(g) equilibrium are critically discussed.

Combustibility of hydroxylamine and hydrazine salts

A thermochemica l evaluation of the combustibil i ty of the hydrazine salts of oxygen-containing acids, whose anions have oxidative proper t ies , was previously made in [7]. In connection with the experimental ly observed combustion of hydroxylamine sulfate it has been suggested that the hydrogen is completely oxidized as a resul t of the reduction of sulfuric acid to SO 2 [8]. In view of the exothermici ty of NH2OH and N2H t decomposit ion and the relat ively small heat of dissociat ion of their salts , the salts of acids that do not have oxidative proper t ies may also be expected to prove combustible. When these salts burn, the acid, like the water in the combustion of aqueous solutions of NH2OH or N2H4, will absorb heat.

The monomer−dimer equilibria of liquid aliminum alkyls: III. Trimethylaluminum: the monomer−dimer equilibria of liquid and gaseous trimethylaluminum and triethylaluminum

There has been a dichotomy of opinion concerning the values for the heats of dissociation of trimethylaluminum (TMA) and triethylaluminum (TEA) in the liquid phase. It is believd that the result reported herein will enable these differences to be resolved. In the present study, the heat of dissociation (ΔHod of liquid TMA is evaluated as 19.40±0.30-kcal · (mole of dimer)−1 from the previously determined ΔH0d of liquid TEA and new heat of mixing data for TMA−TEA. Essentially the same result is derived from the established ΔH0d of gaseous TMA by application of a thermodynamic relationship between heats of dissociation and heats of vaporization (the “Dissociation−Vaporization Rule”). It is concluded that the experimental ΔH0d values for liquid TEA (16.93 kcal · (mole of dimer)−1) and gaseous TMA (20.40 are thermodynamically consistent. The ΔS0dof liquid TMA is evaluated as 29.3±0.3 cal · K−1 · (mole of dimer)−1. The ΔH0d valuesfor liquid adn gaseous TMA are shown to be thermodynamically consistent with vapor pressure data. Boiling points derived for pure TMA monomer and pure dimer are 8.1±2.1 and 131.95±0.02°C, respectively. Degrees of dissociation of TMA in the pure liquid state and at various mole fractions in aliphatic hydrocarbon solution are tabulated over a wide temperature range. The results indicate that the extent of dissociation of liquid TMA is 0.0047% at 20, 0.053% at 70, and 0.32% at 120°C.The ΔH0d and ΔS0d of gaseous TEA are derived as 18.17±0.30 kcal·(mole of dimer)−1. and 45.51±0.70 cal·K−1·(mole of dimer)−1 from the experimental values for liquid TEA. The ΔH0d adn ΔS0d values for liquid and gaseous TEA are shown to be thermodynamically consistent with vapor pressure data. It is also shown that the values ΔH0d(I)11±3 kcal·(mole of dimer)−1 and ΔH0á(g)10.2±1.0 kcal·(mole of dimer)−1 proposed in the literature are far too low. They are thermodynamically inconsistent with the established ΔH0d(g) for TMA and also with TEA vapor pressure data.

The monomer-dimer equilibria of liquid aluminum alkyls : IV. Triethylaluminum in mesitylene

The monomer-dimer equilibrium of triethylaluminum (TEA) in mesitylene solution has been studied by measuring heats of dilution. The value obtained for the heat of dissociation of one mole of TEA dimer in mesitylene, 13.3 1 kcal, is significantly smaller than the corresponding value in n-hexadecane (16.9 3 kcal). The difference is ascribed primarily to the (exothermic) complexation of mesitylene with TEA monomer. Degrees of dissociation of TEA in mesitylene at various mole fractions are tabulated over a wide temperature range and compared with corresponding values in aliphatic (saturated) hydrocarbons. The results indicate that TEA is about eight times as dissociated in mesitylene as in an aliphatic hydrocarbon at −50°C. The ratio decreases with rising temperature and becomes unity at about 170° C. The results have been extended to trimethylaluminum (TMA) and to other aromatic solvents. The estimated value for the heat of dissociation of TMA in toluene (15.6±1.4 kcal· (mole of dimer) −1) is consistent with the literature value (15.4±2.0 kcal·mole −1) for the activation energy for the bridge—terminal exchange of methyl groups. This supports the literature contention that the exchange in toluene solution occurs via dissociation of dimeric molecules to monomers as the rate determining step. Since the literature value for the activation energy for exchange in cyclopentane solution (15.6±0.2 kcal·mole −1) is well below the heat of dissociation of TMA in cyclopentane (19.4 kcal·mole −1), there is a distinct possibility that the exchange mechanism is different in the two solvents.

Heats of dissociation of 2:1 and 1:1 molecular complexes formed between toluene and copper(I) tetrachloroaluminate

Heats of dissociation of 2:1 and 1:1 molecular complexes formed between toluene and copper(I) tetrachloroaluminate

Studies of Catalysis by β-Glucuronidase: ACTIVE SITE

Abstract In an attempt to understand the structure of the active center of β-glucuronidase (EC 3.2.1.31), the behavior of the kinetic parameters of the liver lysosomal enzyme as a function of pH was examined. The pH optimum of the enzyme in the presence of 0.65 m NaCl is 5.1. This dependence of maximal velocity on pH suggests that two dissociable groups are involved, for which the pK values 4.14 and 6.21 were calculated. These values, and those of the heats of dissociation calculated for these two groups, are suggestive of the participation of a carboxylate group and perhaps a carboxyl group in the catalysis. The inactivation of the enzyme by a carbodiimide and by isoxazolium salts is compatible with this hypothesis. The interaction between β-glucuronidase and phenolphthalein-β-d-glucopyranosiduronic acid was found to involve ionic forces, as suggested by the absence of substrate activity or inhibitory effects of p-nitrophenyl-β-d-glucopyranosiduronamide, by the greater inhibitory activity of an isoxazolium salt bearing a sulfonate group than of one that did not contain this group, and by the effect of ionic strength on the kinetic constant Vmax/Km. It was inferred from these findings that a cationic group at the active center of the enzyme is probably involved in attacking the substrate. That this cationic group is probably an imidazolium group was suggested by the inactivation effected by photooxidation, which was pH-dependent, and by diazonium salts.

Kinetics of the Mercury-Photosensitized Decomposition of Neopentane. Part II. Reactions of the Methyl and Neopentyl Radicals

The results of Part I are further analyzed with reference to certain of the elementary free-radical processes occurring. A fall-off in the methyl radical combination is observed at low pressures. Comparison of this process with the CH3 + neopentane abstraction yields for the latter an activation energy of 11.5 kcal/mol and a frequency factor of 4.9 × 1011 cc mol−1 s−1. The relative importance of CH3 + neopentyl and neopentyl + neopentyl is compared. The decomposition of the neopentyl radical into i-C4H8 + CH3 shows a fall-off at low pressures; the limiting activation energy at high pressures is 29.0 kcal/mol, while that at low pressure is 17.1 kcal/mol. The former value leads to 6.7 kcal/mol for the heat of formation of the neopentyl radical at 25 °C, to 21.3 kcal/mol for the heat of its dissociation into i-C4H8 + CH3, and to 98.5 kcal/mol for the heat of dissociation of neopentane into neopentyl + H. Entropy values are also calculated in an approximate manner.

Calorimetric studies of polycarboxylic acids in aqueous solution. II. Maleic acid–butyl vinyl ether copolymer

AbstractA calorimetric investigation of the heat of dissociation of a maleic acid‐butyl vinyl ether copolymer sample in aqueous solution has been carried out. Measurements were performed using dilute copolymer solutions (ca. 10−2 equiv/liter) in water, 0.04M NaCl, and 0.20M NaCl, respectively, at 25°C. Heat of dissociation data for the copolymer in water at 45°C are also reported. The calorimetric data allow evaluation in each case, (except for the solutions in 0.2M NaCl) of the enthalpy for the conformational transition which the maleic acid‐copolymer undergoes upon ionization. With the aid of potentiometric data the associated entropy changes are finally estimated. Our data point out the notable influences of temperature and of added simple salts on the energetics of the conformational transition.

The equilibrium between sodium hydrogen acetylide and disodium acetylide

Attempts to determine the equilibrium pressure of acetylene over the solid phases in the system 2NaHC2⇌ Na2C2+ C2H2 were unsuccessful when appreciable dissociation (>5%) of the hydrogen acetylide occurred; this is due to the polymerisation of acetylene, which is catalysed by the hydrogen acetylide. True dissociation pressures (P) are obtainable when dissociation is small (<0·5%). These are given by log10P/mm = 9·942 – 3363/T, and the heat of dissociation is +15·4 kcal/mol. Dissociation rates for the forward reaction have been measured for the temperature range 120–170 °C, and activation energies for the forward and back reactions found to be 30·8 and 15·4 kcal/mol respectively.

Kinetics of dehydration of single crystals of copper formate tetrahydrate

Reproducible kinetic data for the dehydration of individual single crystals of copper formate tetrahydrate have been obtained from –45° to +25°C in vacuo. Although structural studies suggest the presence of two differently coordinated types of water molecule, all of the water molecules behave identically with respect to their kinetics of dehydration. The reactant/product interface penetrates into the crystal at a constant rate in only the two dimensions parallel to the planes (001) which contain the copper and formate ions. A two-dimensional contracting envelope equation describes the α(fraction dehydration) against time curves. The energy of activation, 11.2 ± 0.2 kcal/mol H2O, for penetration of the interface is less than the overall heat of dissociation, 12.5 kcal/mol H2O. The kinetics of dehydration are unaffected by an antiferroelectric transition at –37.7°C. The inhibition of the dehydration process by water vapour has been examined and discussed quantitatively.

Mass spectrometric study of the evaporation of lithium and sodium molybdates and tungstates

The vapor phases in equilibrium with lithium molybdate, lithium tungstate, sodium molybdate and sodium tungstate salts were investigated by a mass spectrometric Knudsen effusion technique in the temperature range 1200–1800°K. Molybdenum and tungsten Knudsen cells were used as containers. Lithium molybdate and tungstate evaporate mainly as molecular Li 2MoO 4( g) and Li 2WO 4 ( g). Sodium molybdate and tungstate are reduced by the crucible material to yield Na( g) and MoO 2( s) and WO 2( s), respectively. Heats of dissociation of Li 2MoO 4( g) and Li 2WO 4( g) to Li 2O( g) and MoO 3( g) and to Li 2O( g) and WO 3( g) are 119±5 kcal/mole and 132±5 kcal/mole, respectively.

Thermodynamic Studies of Solid Polyethers. I. Poly(oxacyclobutane), [–(CH2)3O–]n and Its Hydrate

Specific heat of semicrystalline and glassy poly(oxacyclobutane) was measured from 90 to 320 °K by using an adiabatic calorimeter and the following thermodynamic values were determined: Tg=195 °K, ΔCp (at Tg) =0.895 J/g°K, T2=155 °K, S0=0.189 J/g°K, Tm=305 °K, ΔHm=162.5 J/g°K and ΔSm=0.533 J/g°K.As for the hydrated poly(oxacyclobutane), isotherms and dissociation pressures were measured from −25 to 20°C. The result confirmed that water molecules in the hydrated poly(oxacyclobutane) exist as stoichiometric water in the crystalline part of the solid polymer. The temperature dependence of the dehydration pressures was also represented by the following equation; logP=10.82−2.93×103T−1, and the dissociation heat was evaluated to be 3.25×103 J/mol.

The monomer-dimer equilibria of liquid aluminum alkyls: II. Triisobutylaluminum

The monomer-dimer equilibrium of liquid triisobutylaluminum (TiBA) has been studied by a thermochemical method applied earlier to triethylaluminum (TEA). The values obtained for the heat and entropy of dissociation of 1 mole of TiBA dimer are 8.1 6±0.1 2 kcal and 30.4 9±0.3 4 cal/deg. These values, compared with corresponding values for triethylaluminum (16.9 kcal and 32.2 cal/deg), indicate that a large steric effect is involved in the formation of the dimeric molecule. This suggests an appreciable increase in the (average) lengths of the Al CH 2 and Al Al bonds in the bridge accompanied by a significant increase in rotational restriction. Degrees of dissociation of TiBA in the pure liquid state and at various mole fractions in hydrocarbon solution are tabulated over a wide temperature range. The results, which are consistent with literature data, indicate that TiBA is 39.4% associated at 10° and 16.4% associated at 40°. This contradicts the widely held belief that (pure) TiBA is either “completely monomeric” or practically unassociated.

Free Energy Changes vis-a-vis other Thermodynamic Parameters as a Measure of Stability -A solid Phase Study on Heat of Dissociation and Pressure-Composition Isothermals of β-picoline Complexes of Fluoborates of Some Transitional Metals. Part I.

Free Energy Changes vis-a-vis other Thermodynamic Parameters as a Measure of Stability -A solid Phase Study on Heat of Dissociation and Pressure-Composition Isothermals of β-picoline Complexes of Fluoborates of Some Transitional Metals. Part I.

EPR Spectrum and Heat of Dissociation of the Perylene Dimer Cation

The EPR spectrum of the perylene dimer cation, obtained by dissolving (perylene2)+ ClO4− in CH2Cl2 and in CH3CN, is reported. The proton coupling constants in CH2Cl2 are 1.913, 1.492, and 0.212 G, while the g value at − 80°C is 2.002601. The heat of dissociation in CH2Cl2 is estimated to be 8.6 kcal/mole.

Electronic spectrum of NSe

Isotope shift studies and fine structure analysis of the emission spectrum observed in the visible region have established the existence of the NSe molecule. Molecular constants are reported identifying the transition as 2π r - X 2π r. The nature of the molecular formation and heat of dissociation are discussed.

Formation and Electrolytic Dissociation of the Potassium Compounds of Monohydronaphthalene and Monohydroanthracene

The molar heats of dissociation of the potassium compounds of monohydronaphthalene and monohydroanthracene (MH⊝K⊕) in 1,4-dioxane have been determined. Furthermore it has been shown that even high-purity solvents will eventually act as proton donors for dinegative ions of aromatic hydrocarbons.

Thermochemical and spectroscopic properties of transition-metal complexes. Part II. Cobalt complexes of the type CoL2X2(L =o-, m-, or p-fluoroaniline, and X = chlorine, bromine, or iodine)

The title compounds have been prepared and the heats of decomposition of the solid compounds have been determined by differential scanning calorimetry. The electronic spectra of the solid compounds have been recorded and the ligand-field parameters have been derived. These parameters have been compared with the differences in the heats of dissociation of the compounds.

Thermochemistry of some complexes of titanium tetrachloride with organic ligands

The heats of formation in the condensed phase of the 1 : 1 and 1 : 2 complexes of titanium tetrachloride with tetrahydrofuran, tetrahydropyran, tetrahydrothiophen, acetonitrile, acetophenone, benzophenone, and nitrobenzene have been determined by use of a reaction calorimeter. The heats of sublimation were also measured and the data thereby obtained enabled the gas-phase heats of dissociation of the complexes to be calculated. Tetrahydrofuran is a strong ligand to TiCl4, as also is benzophenone. Tetrahydrothiophen and acetophenone are also moderately strong electron donors. Acetonitrile, however, is a weak ligand.

The Second Dissociation Constant of Sulphuric Acid at a Number of Temperatures and Heat of Dissociation of HSO-4 Ion

The Second Dissociation Constant of Sulphuric Acid at a Number of Temperatures and Heat of Dissociation of HSO-4 Ion