Thermochemistry decomposition reactions

Laws of Thermochemistry. Lavoisier and Laplace (1780) found that the heat required to decompose a chemical compound into its elements was numerically equal to the heat generated in its formation under the same conditions of T and P. That is, AHj = -AHp where the subscript d refers to decomposition reaction [52, p. 24 61, p. 303]. 

Some of the decomposition reactions in Section V,A above bear upon the thermochemistry of Eq. (2) (Section II). Since a metal hydride has never been observed to be in equilibrium with a measurable quantity of a metal formyl (in Eq. (31), 19f of 25 would have been detectable) (42), AGrsn for Eq. (2) (forward direction) is commonly >3 kcal/mol (see, however, Addendum, p. 34). In contrast to Eq. (31), the acyl-alkyl equilibrium in Eq. (34) lies far to the left (42). 

See ASSESSMENT OF REACTIVE CHEMICAL HAZARDS, COMPUTATION OF REACTIVE CHEMICAL HAZARDS, EXOTHERMIC DECOMPOSITION REACTIONS, OXYGEN BALANCE, THERMOCHEMISTRY AND EXOTHERMIC DECOMPOSITION... 

Thermochemistry. Profiles of potential energy surfaces which are representative of hydrocarbon decomposition reactions and the associ-... 

The thermochemistry of a large number of transition-metal complexes of the type MLrtX2 have been investigated using DSC by Beech et al. (89-91). These results as well as others are discussed in a book by Mortimer and Ashcroft (93). The overall decomposition reactions of the complex ML4X2 are as follows ... 

A comparison of three different methods by which the enthalpy of decomposition reactions can be determined shows the third-law method to excel over the other two in all respects. It is superior in precision, accuracy, and output (productivity). This conclusion finds support in the opinion generally accepted in equilibrium thermochemistry [4]. The question then is why the third-law method has never, until the appearance of publication [5] in 2002, been invoked in studies of decomposition kinetics. Several possible reasons could be proposed. 

The decomposition reactions result in a number of unsaturated oxy-hydrocarbon intermediates and radical products, for which thermochemistry is not available. The Group Additivity method (GA) [2, 20] is a fast and reliable method to estimate or check the thermochemistry of unknown or large molecules. In this work we also develop a series of new groups (for use in group additivity (GA)) to aid our evaluation of enthalpies of formation for our system. 

Most azido compounds explode in their decomposition. But thermochemistry, photochemistry, and discharge methods can slow down the decomposition reaction. The products of explosive decomposition are the corresponding simple substances. Decomposition heat of one compound equals its standard formation heat. 

See entry thermochemistry and exothermic decomposition (reference 2) Two instances of used DMSO decomposing exothermally while being kept at 150°C prior to recovery by vacuum distillation were investigated. Traces of alkyl bromides lead to a delayed, vigorous and strongly exothermic reaction (Q =... 

THERMAL STABILITY OF REACTION MIXTURES AND SYSTEMS THERMOCHEMISTRY AND EXOTHERMIC DECOMPOSITION... 

The knowledge of thermochemistry is also important in early parts of mechanism development. For example, by examining the heats of reaction of competing elementary processes, unlikely reaction paths can be identified a priori and eliminated from further consideration. To illustrate this, consider the following simple unimolecular decomposition processes for CH3CI ... 

Following the determination of the geometry and the thermochemistry of transition states, the rate parameters for the two silane decomposition pathways can be obtained directly by the TST formulation presented earlier. These calculations have led to unimolecular rate constant expressions 10 exp(-91000/RT)s-S and 10 exp(-62000/l r)s" for Si-H bond scission and H2 elimination reactions, respectively. These results clearly... 

M,A.Cook et al, JChemPhys 24, 191-201 (1956) (Rate of reaction of TNT in detonation by direct pressure measurements) 22)Dunkle s Syllabus (1957-1958) (See Vol 4 of Encycl, p XLIX) p 126 (Reaction front in detonation) 135-42 (Thermal decomposition of solids) 23)M.A.Cook, "The Science of High Explosives , Reinhold NY(1958), pp 123-42 (Reaction rate in detonation) 174-87 (Thermal decomposition of soli ds) 386-89 (Thermochemistry of detonation and expltr) 24)F.A.Baum, K.P.Sranyukovich B.I.Shekhter "Fizika Vzryva , Moscow (1959), pp 81-108 (Thermochemistry of explosives) 25)K.K.An-dreev A. F. Belyaev, " Teoria Vzryvcha-rykh Veshchestv Moscow(1960), p 49-56 (Thermal expln in gases) p 56—61 (Thermal explosion in solids) 26) Encycl of Expls PATR 2700, Vol 1 (I960), p A501 (Atomic expins, chain reactions in) 27)F.M.Turner,... 

The thermochemistry of explosive compositions has been discussed in detail in Chapter 5. The Kistiakowsky-Wilson and the Springall Roberts rules both give an approximate estimate for the products of decomposition, which is independent of the temperature of explosion. The formulae and calculations for determining the heat of explosion also assume that the explosive reactions go to total completion. However, in practice the reactions do not go to completion and an equilibrium is set up between the reactants and the products. This equilibrium is also dependent upon the temperature of the explosion Te. 

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. 

Textile clothing static charges, 394 Theory without detailed thought, 394 Thermal explosions, 394 Thermal stability of reaction mixtures and systems, 394 Thermite reactions, 395 Thermochemistry and exothermic decomposition, 396 Thiatriazoles, 400 Thionoesters, 401 Thiophenoxides, 401 Thorium furnace residues, 401 Tollens reagent, 401 Toxic hazards, 402 Trialkylaluminiums, 402 Trialkylantimony halides, 403 Trialkylbismuths, 403... 

R. Shaw, F. E. Walker, Estimated Kinetics and Thermochemistry of Some Initial Unimolecular Reactions in the Thermal Decomposition of l,3,5,7-Tetranitro-l,3,5,7-tetraazacyclooctane in the Gas Phase J. Phys. Chem. 81 (1977) 2572-2576. 

Quantum calculations can make some selections in this case even though thermochemistry can not. The reverse of reaction (2) involves free radicals and the activation energy of free radicals is very low, 5,000 to 10,000 calories or less. The energy of activation for decomposition of acetone by reaction (2) then can not be much greater than 75,000 or at the most 80,000 calories and it can not be much less than 70,000. 

The thermochemistry (reaction enthalpies), indicated in the second to last column of Table 61 are those calculated from bond additivities assuming AHf (C2HsO ) °° = —4.8 kcaLmole". The later value comes from the kinetic results of Leggett and Thynne and the heat of formation of diethylperoxide. It is apparent that the reaction enthalpies, thus calculated, are in fair agreement with the various kinetic studies Probable errors in bond additivity relations in molecules are about 2 kcal.mole , therefore heats of formation of the alkoxy radicals calculated from the decomposition kinetics should be preferred. 

Gas phase kinetic results (Table 64) on hydroperoxide decompositions (methyl, ethyl, isopropyl and t-butyl hydroperoxide) are very poor. Since the thermochemistry is fairly well established for these reactions, and since observed activation energies are as much as 6 kcal.mole lower than the reaction enthalpies, it is apparent that the reported parameters cannot be those for the unimolecular hydro-peroxy bond fission processes. Surface catalysis was considerable in all experimental systems. It therefore seems likely that the true homogeneous reactions were never completely isolated. 

The thermal and photolytic decomposition of hydrazine and substituted hydrazines and azines provide a source of many nitrogen containing radicals . For this reason a considerable amount of thermochemical data is now available relating to the bond energies of the bonds involved in these reactions. Friswell and Gowen-lock" have recently reviewed the chemistry and thermochemistry of nitrogen containing radicals, and the salient features of their article will serve as an introduction to this section. 

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