Arrhenius parameters, decompositions

A and E refer to the desorption, dissociation, decomposition or other surface reactions by which the reactant or reactants represented by M are converted into products. If [M] is constant within the temperature interval studied, then the values of A and E measured refer to this process. Alternatively, if the effective magnitude of [M] varies with temperature, the apparent Arrhenius parameters do not specifically refer to the product evolution step. This is demonstrated quantitatively by the following example [36]. When E = 100 kJmole-1 andA [M] = 3.2 X 1030 molecules sec-1, then rate coefficients at 400 and 500 K are 2.4 X 1017 and 1.0 X 1020 molecules sec-1, respectively. If, however, E is again 100 kJ mole-1 and A [M] varies between 3.2 X 1030 molecules sec-1 at 500 K and z X 3.2 X 1030 molecules sec-1 at 400 K, the measured values of A and E vary significantly, as shown in Fig. 7, when z ranges from 10-3 to 103. Thus, the measured value of E is not necessarily identifiable with the rate-limiting step if a concentration of a participant is temperature-dependent. This... 

Fig. 16. Graphical representation of Arrhenius parameters for the low temperature decomposition of ammonium perchlorate (pelleted, orthorhombic, o, and cubic, , forms). Compensation behaviour is observed. Data from Jacobs and Ng [452]. N = nucleation, B = branching, G = growth processes.

Powell and Searcy [1288], in a study of CaMg(C03)2 decomposition at 750—900 K by the torsion—effusion and torsion—Langmuir techniques, conclude that dolomite and C02 are in equilibrium with a glassy phase having a free energy of formation of (73 600 — 36.8T)J from 0.5 CaO + 0.5 MgO. The apparent Arrhenius parameters for the decomposition are calculated as E = 194 kJ mole-1 and activation entropy = 93 JK-1 (mole C02)-1. 

From the tabulated half time and decomposition of tetrahydrofuran (JACS 68 reaction and the Arrhenius parameters. 

The decomposition of nitrogen dioxide, 2N02 = 2N0 + 02, has a second order rate equation. Data at different temperatures are tabulated. Find the Arrhenius parameters. 

The homogeneous thermal decomposition of HC1 has only been studied in shock tubes. Fishbume51 investigated the shock pyrolysis of HC1 diluted with Ar in the temperature region 3300-5400 °K and obtained Arrhenius parameters for... 

Arrhenius parameters calculated from the data in Tables 1-4 are shown in Table 5. The pre-exponential factors are all within the range expected for uni-molecular decompositions, with the exception of Co2(CO)6C2H2. The low value for its decomposition has been attributed to formation of a CO bridge in the transition state24. 

ARRHENIUS PARAMETERS FOR THE DECOMPOSITION OF COBALT CARBONYL COMPLEXES... 

The Arrhenius parameters and the thermochemical sum of the phenyl-carbon and phenyl-halogen bond dissociation energies are shown in Table 8. The extent of the diphenyl mercury decomposition was determined from the weight of mercury produced. It is the present author s opinion that in calculating the Arrhenius parameters for this compound Carter et al.81 gave too great a statistical... 

TABLE 12.2. Arrhenius parameters for the decomposition of catechol and hydroquinone in the presence and absence of iron oxide, assuming pseudo first-order kinetics... 

The Arrhenius parameters obtained for the reaction of t-BuO radical with EtsSiH are log /M s = 8.5 and = 8.8kJ/mol [23]. The Arrhenius parameters for MesSiH in the gas phase are also available and were obtained by competition with the tcrt-butoxyl radical decomposition, i.e., log /M s = 8.7 and /fa = 10.9kJ/mol [28]. These preexponential factors lie in the expected range and, therefore, the activation energy is expected to be the major factor... 

Roenigk, K. F., Jensen, K. F., and Carr, R. W., Rice-Rampsperger-Kassel-Marcus theoretical prediction of high-pressure Arrhenius parameters by nonlinear regression Application to silane and disilane decomposition, J. Phys. Chem. 91, 5732 (1987). 

TABLE 12. Arrhenius parameters and rate constants at 373 K for radical decomposition of A,A-dimethoxy-4-substituted benzamides (194a-d) ... 

The chemistry of phenoxy (CeHsO ) radical has been of interest for several decades. Benson et al. proposed the first rate coefficient for its unimolecular decomposition, while Lin and Lin provided information on the Arrhenius parameters for the reaction. Experimental and theoretical studies have examined phenoxy radical s elec-... 

Table 6.21 gives the Arrhenius parameters for the decomposition of various PANs and the rate constants and corresponding lifetimes for PAN. The decomposition is strongly temperature dependent, with long lifetimes, of the order of a year or more at low temperatures of 215 K, and very short lifetimes, < 1 h, at the higher temperatures around 298 K. 

The slow combustion reactions of acetone, methyl ethyl ketone, and diethyl ketone possess most of the features of hydrocarbon oxidation, but their mechanisms are simpler since the confusing effects of olefin formation are unimportant. Specifically, the low temperature combustion of acetone is simpler than that of propane, and the intermediate responsible for degenerate chain branching is methyl hydroperoxide. The Arrhenius parameters for its unimolecular decomposition can be derived by the theory previously developed by Knox. Analytical studies of the slow combustion of methyl ethyl ketone and diethyl ketone show many similarities to that of acetone. The reactions of methyl radicals with oxygen are considered in relation to their thermochemistry. Competition between them provides a simple explanation of the negative temperature coefficient and of cool flames. 

Other groups of Arrhenius parameters obtained for reactions on two or more metals and which have been shown to exhibit compensation effects include para-hydrogen conversion (on Cu, Ag and Au) (144), hydrogenation of methylacetylene (7 8) and of allene (245), and the inverse compensation behavior in decomposition of acetylene on metal films (26). 

Compensation behavior occurs in the decomposition of hydrogen peroxide on Ag-Au alloys (25) and, unlike most other alloy systems, there is a systematic change in the Arrhenius parameters with proportions of metals present. This behavior is ascribed to the progressive transformation, with alloy composition, of the reaction mechanism from that characteristic of one metal to that which occurs on the other. In contrast, decomposition of hydrogen peroxide on Pd-Au alloys (27) does not correlate with ratios of metals present in the catalyst, and kinetic parameters are sensitive to surface pretreatment. 

Trillo et al. (47,137) have reported compensation behavior in oxide-catalyzed decomposition of formic acid and the Arrhenius parameters for the same reactions on cobalt and nickel metals are close to the same line, Table V, K. Since the values of E for the dehydration of this reactant on titania and on chromia were not influenced by doping or sintering, it was concluded (47) that the rate-limiting step here was not controlled by the semiconducting properties of the oxide. In contrast, the compensation effect found for the dehydrogenation reaction was ascribed to a dependence of the Arrhenius parameters on the ease of transfer of the electrons to the solid. The possibility that the compensation behavior arises through changes in the mobility of surface intermediates is also mentioned (137). 

Krupay and Ross (272b), in a study of the decomposition of formic acid on manganese (II) oxide, demonstrate that manganese (II) formate is produced during reaction and discuss the probable role of this participant in the catalytic process. The reported Arrhenius parameters (log A, E) for the dehydration and dehydrogenation reactions were (28.7, 132) and (24.9, 87), respectively both points were close to the compensation line (Table V, K) characteristic of the breakdown of formic acid on oxides. 

For addition of Cl atoms, the dissociation energy of the radicals AX has been estimated at about 20-22 kcal./mole. At room temperature k ifki(A) should be well below 10-3 and mechanism (C) should be obeyed and has indeed been frequently observed. At higher temperatures (about 225°C.) k 2/k2(A) 10-3 and a change to mechanism (B) should occur. This has been confirmed experimentally by Adam et al. (1) in a study of the photochlorination of tetrachlorethylene. They observed a maximum in the rate and a change in mechanism at about 180°C., as a result of the increased importance of the radical decomposition reaction (—2). From their data they were able to deduce the Arrhenius parameters for this reaction. In extensions of this work Goldfinger and his collaborators have carried out competitive experiments with a number of hydrocarbons and chlorinated hydrocarbons. 

This fragmentation mode is not altered for silacyclobutanes bearing a vinyl group at the silicon17, as the same Arrhenius parameters are found for the decomposition of 1 and of 1 -methyl-1-vinylsilacyclobutane 3 (logA = 15.64 s 1, E = 62.6 kcalmol-1), in sharp contrast to the pyrolysis of cyclobutanes where a vinyl group accelerates the pyrolysis by a factor of nearly 60018. 2-Silabuta-l,3-diene 4 was produced in a laser-photosensitized (SFg) decomposition (LPD) of 1-methyl-1-vinylsilacyclobutane 3... 

The second reason is closely related to the time scale of the experiment. This merits some kinetic considerations. Arrhenius activation energy of the homolytic decomposition of AIBN in toluene is l43kJmol 1 and log A is 17.33 (32). The Arrhenius parameters are not very sensitive to the solvent (33). Not all radicals produced by AIBN decomposition yield ROO radicals, because of reaction (17)... 

Decomposition rate constants are measured over as wide a temperature range as possible. Only the first one third to one half of the decomposition can be analyzed before it becomes severely autocatalytic. With the rate constants, an Arrhenius plot can be constructed and activation parameters calculated. Activation energies and pre-exponential factors correlate the decomposition rates with temperature. In addition, the magnitude of the activation energy may shed light on the key step in the decomposition process, and Arrhenius parameters are necessary in many explosive code calculations. Our procedure is to input the activation parameters into the Frank-Kamentskii equation [145] and use it to predict critical temperature of a reasonable size (e.g. 1 kilogram) of the energetic material ... 

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