The thermal dehydration and decomposition of Ba[Cu(C2O4)2(H2O)]·5H2O

The thermal behaviour of Ba[Cu(C₂O₄)₂(H₂O)]·5H₂O in N₂ and in O₂ has been examined using thermogravimetry (TG) and differential scanning calorimetry (DSC). The dehydration starts at relatively low temperatures (about 80°C), but continues until the onset of the decomposition (about 280°C). The decomposition takes place in two major stages (onsets 280 and 390°C). The mass of the intermediate after the first stage corresponded to the formation of barium oxalate and copper metal and, after the second stage, to the formation of barium carbonate and copper metal. The enthalpy for the dehydration was found to be 311±30 kJ mol^–1 (or 52±5 kJ (mol of H₂O)^–1). The overall enthalpy change for the decomposition of Ba[Cu(C₂O₄)₂] in N₂ was estimated from the combined area of the peaks of the DSC curve as –347 kJ mol^–1. The kinetics of the thermal dehydration and decomposition were studied using isothermal TG. The dehydration was strongly deceleratory and the -time curves could be described by the three dimensional diffusion (D3) model. The values of the activation energy and the pre-exponential factor for the dehydration were 125±4 kJ mol^–1 and (1.38±0.08)×10¹⁵ min^–1, respectively. The decomposition was complex, consisting of at least two concurrent processes. The decomposition was analysed in terms of two overlapping deceleratory processes. One process was fast and could be described by the contracting-geometry model withn=5. The other process was slow and could also be described by the contracting-geometry model, but withn=2.The values ofE _a andA were 206±23 kJ mol^–1 and (2.2±0.5)×10¹⁹ min^–1, respectively, for the fast process, and 259±37 kJ mol^–1 and (6.3±1.8)×10²³ min^–1, respectively, for the slow process.Dedicated to Prof. Menachem Steinberg on the occasion of his 65th birthday     

Kinetics of the thermal decomposition of [Co(NH3)6]2(C2O4)3·4H2O

The thermal decomposition of [Co(NH₃)₆]₂(C₂O₄)₃·4H₂O was studied under isothermal conditions in flowing air and argon. Dissociation of the above complex occurs in three stages. The kinetics of the particular stages thermal decomposition have been evaluated. The R_N and/or A_M models were selected as those best fitting the experimental TG curves. The activation energies,E, and lnA were calculated with a conventional procedure and by a new method suggested by Kogaet al. [10, 11]. Comparison of the results have showed that the Arrhenius parameters values estimated by the use of both methods are very close. The calculated activation energies were in air: 96 kJ mol^–1 (R_1.575, stage I); 101 kJ mol^–1 (Ain1.725 stage II); 185 kJ mol^–1 (A _2.9, stage III) and in argon: 66 kJ mol^–1 (A _1.25, stage I); 87 kJ mol^–1 (A _1.825, stage II); 133 kJ mol^–1 (A _2.525, stage III).     

étude thermodynamique de certains médicaments

Two compounds of sulphamide type:p-amino-benzene sulphonamide (I) and 3,4-dimethylisoxazol 5-sulphanylamide (II) were studied by combustion calorimetry and by differential scanning calorimetry (DSC).The enthalpies in solid state at 298,15 K of combustion, _c H _m ^o (I)=-2788,5±1,6 kJ mol^–1, _c H _m ^o (II)=-5036±3,8 kJ mol^–1 and of formation, _f H _m ^o (I)=-458,3±1,6 kJ mol^–1, _fH _m ^o (II)=-180,1±3,8 kJ mol^–1 were determined.The thermal effects concerning the melting and phase transition of this compounds were also measured.

     

Comparative thermal analysis study of two biopolymers, starch and cellulose

The degradation of cellulose and starch samples in air and nitrogen has been investigated by thermal analysis techniques. The techniques employed were differential thermal analysis, rising temperature and temperature jump thermogravimetry. Rate data were obtained from these experiments and Arrhenius parameters calculated from these values. This data was used to determine the mechanism by which the cellulose and starch samples degraded. The Arrhenius parameters were also calculated. The behavior of starch and cellulose upon thermal analysis were compared and are reported.E _act for corn starch was found to be 474 kJ mol^–1 and for a cellulose 242 kJ mol^–1.Dedicated to Prof. Menachem Steinberg on the occasion of his 65th birthday     

Tensimetric investigation of the CrCl<Subscript>3</Subscript>—Cl<Subscript>2</Subscript> system in the temperature range of 600–1200 K

The sublimation pressure of chromium trichloride was measured by the static method with a quartz membrane-gauge manometer in the temperature range of 875–1230 K. An approximating equation for the sublimation pressure vs. temperature was found. The enthalpy (259.4±4 kJ mol^–1) and the entropy (224.2±3.5 J mol^–1 K^–1) of sublimation at 298 K were calculated. For the process 2 CrCl₃(g) + Cl₂(g) = 2 CrCl₄(g), the following values were obtained: _r H°₂₉₈ = –207.1±11.6 kJ mol^–1 and _r S°₂₉₈ = –173.6±10 5 J mol^–1 K^–1.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1561–1564, August, 2004.     

Thermodynamic characteristics of thermal dissociation of platinum tetrachloride

The pressure of thermal dissociation of platinum tetrachloride by the first step PtCl₄(s) = PtCl₃(s) + 0.5 Cl₂(g) was measured by the static method with a quartz membrane-gauge zero-pressure manometer. An approximating equation for the dissociation pressure vs. temperature was found. The enthalpy (52160±880 J mol⁻¹) and entropy (72.1±1.6 J mol⁻¹ K⁻¹) of dissociation were calculated. The heat of formation found for platinum tetrachloride (−246.3±1.3 kJ mol⁻¹) at 298.15 K agrees well with the value obtained by the calorimetric method (−245.6±1.9 kJ mol⁻¹).__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2028–2031, October, 2004.     

Mechanism of oxidation of selenium(IV) by cobalt(III) in perchloric acid — a kinetic study

Summary The kinetics of Co^III oxidation of Se^IV have been studied in aqueous HClO₄. The order with respect to Co^m is two the order with respect to Se^IV is one at low concentrations; two at high concentrations. The latter variation is attributed to the greater reactivity of the Se^IV dimier A mechanism involving complexation between oxidant and substrate is proposed. [CoOH]²⁺ is presumed to be the reactive Co^III species and H₂SeO₃ and HSeO ₃ ^– to be those of Se^IV. At 25° C, E_a, H and S for the monomeric path are 125.6±4.0 kJ mol^–1, 122.1±3.8 kJ mol^–1 and 206±12 JK^–1 mol^–1 respectively and those for the dimeric path are 88.6±3.6 kJ mol^–1, 85.9±3.4 kJ mol^–1 and 62.6±11.3 JK^–1 mol^–1 respectively.     

The kinetics of the dissolution of monoclinic pyrrhotite in aqueous acidic solutions

Using the rotating disc method, the rates of dissolution of natural monoclinic pyrrhotite, FeS_1.14, in oxygen-free aqueous solutionsS([H⁺]=0.1, [Na⁺]=0.9, [ClO ₄ ^– ]=1.0 mol kg^–1) were determined. In the temperature range 40–90 °C the dissolution reaction occurs under kinetic control; the activation energy being 14±1 kcal mol^–1 (50±5 kJ mol^–1).

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Die Kinetik der Auflösung von monoklinem Pyrrhotin in sauren wäßrigen Lösungen

Zusammenfassung Die Auflösungsgeschwindigkeit von natürlichem monoklinen Pyrrhotin, FeS_1.14, wurde in sauerstofffreien LösungenS([H⁺]=0.1, [Na⁺]=0.9, [ClO ₄ ^– ]=1.0 mol kg^–1) mit Hilfe der Methode der rotierenden Scheibe bestimmt. Im Temperaturbereich von 40–90° erfolgt die Auflösungsreaktion kinetisch kontrolliert, wobei eine Aktivierungsenergie von 14±1 kcal mol^–1 (59±5 kJ mol^–1) gefunden wurde.

     

Molecular structure,conformational mobility,vibrational spectra,and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol^–1. The equilibrium molecular structure of AcOOH (C _s symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (46 kJ mol^–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol^–1) and products of radical decomposition of the peroxide — AcO^· (–190.2 kJ mol^–1) and AcOO^· (–153.4 kJ mol^–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol^–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (E _a = 71.7 and 50.9 kJ mol^–1 respectively).     

High levelab initio stabilization energies of benzene

Summary G2 theory is shown to be reliable for calculating isodesmic and homodesmotic stabilization energies (ISE and HSE, respectively) of benzene. G2 calculations give HSE and ISE values of 92.5 and 269.1 kJ mol^–1 (298 K), respectively. These agree well with the experimental HSE and ISE values of 90.5±7.2 and 268.7±6.3 kJ mol^–1, respectively. We conclude that basis set superposition error corrections to the enthalpies of the homodesmotic or isodesmic reactions are not necessary in calculations of the stabilization energies of benzene using G2 theory. The calculated values of the enthalpies of formation of such molecules containing multiple bonds such as benzene ands-trans 1,3-butadiene, which are found from the enthalpies of isodesmic and homodesmotic reactions rather than of atomization reactions, demonstrate good performance of G2 theory. Estimates of theH _f ^o value for benzene from the G2 calculated enthalpies of homodesmotic reaction (2) and isodesmic reaction (3) are 80.9 and 82.5 kJ mol^–1 (298 K), respectively. These are very close to the experimentalH _f ^o value of 82.9±0.3 kJ mol^–1. TheH _f ^o value ofs-trans 1,3-butadiene calculated using the G2 enthalpy of isodesmic reaction (4) is 110.5 kJ mol^–1 and is in excellent agreement with the experimentalH _f ^o value of 110.0±1.1 kJ mol^–1.     

Thermal decomposition and kinetic data of Cd(BF4)2·6H2O

The thermal dehydration and decomposition of Cd(BF₄)₂·6H₂O were studied by means of DTA, TG, DSC and X-ray diffraction methods and the end products of the thermal decomposition were identified. The results of thermal analysis show that the compound is fused first, then it is dehydrated until Cd(BF₄)₂·3H₂O is obtained, which has not been described in the literature so far. The enthalpy of phase transition is H _ph.tr.=115.6 kJ mol^–1 Separation of the compound is difficult since it is highly hygroscopic. Then, dehydration and decomposition take place simultaneously until CdF₂ is obtained which is proved by X-ray diffraction. On further increasing the temperature, CdF₂ is oxidized to CdO and the characteristic curve assumes a linear character.Based on TG data, kinetic analyses were carried out separately for both parts of the curve: first until formation of the trihydrate and then — until formation of CdF₂. The formal kinetic parameters are as follows:for the first phase:E ^*=45.3 kJ mol^–1; rate equationF=^2/3; correlation coefficient 0.9858 for the second phase:E ^*=230.1 kJ mol^–1; rate equationF=(1–)^2/3[1-(1–)^1/3]^–1; correlation coefficient 0.9982.     

Phenacyl dimethyl sulphonium ylide-mercuric chloride complex initiated radical polymerization of methylmethacrylate

Summary The metal-ylide-initiated radical polymerization of methylmethacrylate (MMA) at 85±0.1°C using dioxan as inert solvent was investigated by dilatometry. Kinetic parameters, average rate of polymerization (R _p ) and reaction orders with respect to initiator and monomer have been determined and are 0.33±0.1 and 1.33, respectively. Polymerization was inhibited by hydroquinone and non-polar solvents, but is favoured by polar solvent. The activation energy (E) and k _p ² /k_t values were 64.0 kJ mol^–1 and 3.3×10^–2 l mol^–1 s^–1 respectively. A suitable mechanism consistent with the observed kinetic data is proposed.     

Studies on the kinetics and mechanism of dissociation of copper(II) and nickel(II) complexes of the macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene in perchloric acid media

Summary Acid catalysed dissociation of the copper(II) and nickel(II) complexes (ML²⁺ of the quadridentate macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene (L) has been studied spectrophotometrically. Both complexes dissociate quite slowly with the observed pseudo-first order rate constants (k_obs) showing acid dependence; for the nickel(II) complex (k_obs)=k_O+k_H[H⁺], the k_o path is however absent with the copper(II) complex. At 60°C (I=0.1M) the k_H values areca 10^–4 M^–1 s^–1 for both complexes; k _H ^Cu /k _H ^Ni =ca. 3.9, comparable to some other square-planar complexes of these metal ions. The rate difference is primarily due to H values [copper(II) complex, 29.4±0.5 kJ mol^–1; nickel(II) complex, 35.6±1.5 kJ mol^–1] with highly negative S values [for copper(II), –215.5 ±6.1 JK^–1 mol^–1 and for nickel(II), –208.1 ±5.6 JK^–1 mol^–1] which are much higher than the entropy of solvation of Ni²⁺ (ca. –160 JK^–1 mol^–1) and Cu²⁺ (ca. –99 JK^–1 mol^–1) ions; significant solvation of the released metal ions and the ligand is indicated.     

Thermochemical properties of polycyclic hydrocarbons including fragments of norbornane and small cycles

Combustion enthalpies of three polycyclic hydrocarbons were measured by the precision bomb calorimetry method and their enthalpies of formation in the liquid state were calculated: for pentacyclo[6.3.1.1^3.6.0^2.7.0^9.11]tridecene, –7713.9±3.8 and 25.8±3.8 kJ mol^–1; for 10-met hylpentacyclo[6.3.1.1^3.6.O^2,.7.0^9.11 It ridecene, -8348.8±3.9 and -18.7+-3.9 k] mol^–1; and for 11-methylpentacyclo[6.4.1.1^3.6.0^2.7.0^9.12]tetradecene-10-spirocyclopropane,clopropane, –10157.9±3.4 and 38.1±3.8 kJ mol^–1. The thermochemical I parameters obtained agree with calculated values as well as with experimental and calculated enthalpies of formation of some hydrocarbons that contain the same fragments as the compounds studied.The authors thank Academician O. M. Nefedov and Yu. V. Tomilov for submitting the samples and for discussion of the results obtained.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2676–2678, November, 1996.     

Kinetics and mechanisms of the oxidation of the octacyanoniobate(III)ion by oxyanions in alkaline aqueous media

Summary The kinetics and mechanisms of the oxidation of Nb(CN) _inf8 ^sup5– by the oxyanions S₂O _inf8 ^sup2– , BrO _inf3 ^sup– , and IO _inf4 ^sup– have been investigated in alkaline aqueous media (pH 12). The second-order rate constant for the electron transfer reaction between Nb(CN) _inf8 ^sup5– and S₂O _inf8 ^sup2– at 25.0 °C, I = 0.36m (K⁺), is 11.1± 0.3 m ^–1 s ^–1 with H = 30 ± 2kJmol^–1 and S = - 125 + 7JK^–1 mol^–1. The rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by BrO _inf3 ^sup– at 25.0 °C, I = 0.20m (Na⁺), is 2.39 ± 0.08m ^–1 s ^–1 with H = 28 ± 2kJmol^–1 and S = -139 ± 7JK^–1mol^–1. The oxidation of Nb(CN) _inf8 ^sup5– by IO _inf4 ^sup– proceeds by two parallel pathways involving the monomeric IO _inf4 ^sup– ion and the hydrated dimer H₂I₂O _inf10 ^sup4– . The second-order rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by monomeric IO _inf4 ^sup– at 5.0 °C, I = 0.050m (Na⁺), is (3.3 ± 0.6) × 10³ m ^–1 s ^–1 with H = 75 ± 6 kJ mol^–1 and S = 94 ± 15 J K^–1 mol^–1, while the rate constant for the oxidation by H₂I₂O _inf10 ^sup4– is (1.8 ± 0.1) × 10³ m ^–1 s ^–1 with H = 97 ± 5 kJ mol^–1 and S = 166 ± 16 J K^–1 mol^–1 under the same reaction conditions. The rate constants for each of the oxidants employed display specific cation catalysis with the order of increasing rate constants: Li⁺ < Na+ < NH _inf4 ^sup+ < K⁺ < Rb⁺ < Cs⁺, in the same direction as the electronic polarizability of the cations. The results are discussed in terms of the outer-sphere electron-transfer processes and compared with the corresponding data and mechanisms reported for other metal-cyano reductants.     

The mean enthalpy of the lanthanide-oxygen bond in 2,2,6,6-tetramethyl-3,5-heptanedione chelates of praseodimium and holmium

The general thermochemical reaction LnCl₃·6H₂O(c)+3Hthd(1)+73.92H₂O(1) = Ln(thd)₃(c) +3HCl·26.64H₂O(aq); rHm (Ln = Pr, Ho and thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) was employed to determine through solution-reaction calorimetry at 298.15 K the standard molar enthalpies of formation of crystalline chelates, –2434.3±11.5 (Pr) and –2384.8±11.5 (Ho) kJ mol^–1. These values and the corresponding molar enthalpies of sublimation enabled the determination of the standard molar enthalpies of chelates in the gaseous phase. From these values the mean enthalpies of the lanthanide-oxygen bond, 265±10 (Pr) and 253±10 (Ho) kJ mol^–1 were calculated.     

Determination of the Thermal Degradation Kinetic Parameters of Carbon Fibre Reinforced Epoxy Using TG

In this work, a kinetic study on the thermal degradation of carbon fibre reinforced epoxy is presented. The degradation is investigated by means of dynamic thermogravimetric analysis (TG) in air and inert atmosphere at heating rates from 0.5 to 20°C min⁻¹ . Curves obtained by TG in air are quite different from those obtained in nitrogen. A three-step loss is observed during dynamic TG in air while mass loss proceeded as a two step process in nitrogen at fast heating rate. To elucidate this difference, a kinetic analysis is carried on. A kinetic model described by the Kissinger method or by the Ozawa method gives the kinetic parameters of the composite decomposition. Apparent activation energy calculated by Kissinger method in oxidative atmosphere for each step is between 40–50 kJ mol⁻¹ upper than E _a calculated in inert atmosphere. The thermo-oxidative degradation illustrated by Ozawa method shows a stable apparent activation energy (E _a ≈130 kJ mol⁻¹ ) even though the thermal degradation in nitrogen flow presents a maximum E _a for 15% mass loss (E _a ≈60 kJ mol⁻¹ ). This revised version was published online in July 2006 with corrections to the Cover Date.     

The kinetics of vapor-phase nitration of cellulose 2. Nitration with nitric acid under static conditions

The regularities of vapor-phase nitration of cellulose with HNO₃ under conditions of natural convection and hindered heat removal in the absence of air were studied using the nonisothermal kinetic method. It was established that the nitration rate at the depth of conversion of 0.08 to 0.7 is described by the kinetic law d/dt =k ₁ p/(1+), wherek ₁ = 10^4.49±0.6 exp(–A/RT) s^–1 atm^–1, = 10^{–35.5±15.7}exp(B/RT),A = 36.6±3.8 kl mol^–1, andB = 203±88 kJ mol^–1. The diffusion mechanism of vapor-phase nitration of cellulose, which explains the high value of activation energies, is discussed. The effective diffusion coefficient of HNO₃ in cellulose at 25 °3.7 · 10^–7 cm² s^–1) and the activation energy of diffusion (38.3±4.2 kJ mol^–1) were estimated.For Part 1, see Ref. l.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1981–1985, August, 1996.     

Standard Enthalpies of Formation of 2,6-Di-tert-butyl4-methylphenol and 3,5-Di-tert-butylphenol and Their Phenoxy Radicals

The standard (p^o = 0.1 MPa) enthalpies of formation of 2,6-di-tert-butyl-4-methylphenol and 3,5-di-tert-butylphenol in the gaseous phase, –315.5 ± 4.4 kJ mol^–1 and –312.7 ± 4.6 kJ mol^–1, respectively, were derived from the standard enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry, and from the standard enthalpies of sublimation, at 298.15 K, measured by Calvet microcalorimetry. The O—H bond dissociation enthalpies in those compounds were determined in benzene by photoacoustic calorimetry, leading to the standard enthalpies of formation of the gaseous phenoxy radicals: –189 ± 8 kJ mol^–1 and –154 ± 6 kJ mol^–1, respectively. These results were used to calculate enthalpies of substituent redistribution reactions, which are proposed as a method to estimate new data for substituted phenols.     

Thermochemistry of the solid complex Gd(Et<Subscript>2</Subscript>dtc)<Subscript>3</Subscript>(phen)

Summary A ternary solid complex Gd(Et₂dtc)₃(phen) has been obtained from reactions of sodium diethyldithiocarbamate (NaEt₂dtc), 1,10-phenanthroline (phen) and hydrated gadolinium chloride in absolute ethanol. The title complex was described by chemical and elemental analyses, TG-DTG and IR spectrum. The enthalpy change of liquid-phase reaction of formation of the complex, Δ_rH^Θ_m(l), was determined as (-11.628±0.0204) kJ mol^-1 at 298.15 K by a RD-496 III heat conduction microcalorimeter. The enthalpy change of the solid-phase reaction of formation of the complex, Δ_rH^Θ_m(s), was calculated as (145.306±0.519) kJ mol^-1 on the basis of a designed thermochemical cycle. The thermodynamics of reaction of formation of the complex was investigated by changing the temperature of liquid-phase reaction. Fundamental parameters, the apparent reaction rate constant (k), the apparent activation energy (E), the pre-exponential constant (A), the reaction order (n), the activation enthalpy (Δ_rH^Θ_≠), the activation entropy (Δ_rS^Θ_≠), the activation free energy (Δ_rG^Θ_≠) and the enthalpy (Δ_rH^Θ_≠), were obtained by combination of the thermodynamic and kinetic equations for the reaction with the data of thermokinetic experiments. The constant-volume combustion energy of the complex, Δ_cU, was determined as (-18673.71±8.15) kJ mol^-1 by a RBC-II rotating-bomb calorimeter at 298.15 K. Its standard enthalpy of combustion, Δ_cH^Θ_m, and standard enthalpy of formation, Δ_fH^Θ_m, were calculated to be (-18692.92±8.15) kJ mol^-1 and (-51.28±9.17) kJ mol^-1, respectively.