Decomposition energetics

Mechanism of Nonoxidative Thermal Dehydrochlorination. This subject is still very controversial, with various workers being in favor of radical, ionic, or molecular (concerted) paths. Recent evidence for a radical mechanism has been provided by studies of decomposition energetics (52), the degradation behavior of PVC-polystyrene (53) or PVC-polypropylene (54) mixtures, and the effects of radical traps (54). Evidence for an ionic mechanism comes from solvent effects (55) and studies of the solution decomposition behavior of a model allylic chloride (56). Theoretical considerations (57,58) also suggest that an ionic (El) path is not unreasonable. Other model compound decompositions have been interpreted in terms of a concerted process (59), but differences in solvent effects led the authors to conclude that PVC degrades via a different route (59). 

The fact that different metal azides exhibit sizable differences in sensitivity and pseudostability is associated with molecular and solid-state properties and with the decomposition energetics of the individual substances. A great deal of this information is now available for the azides, as discussed in different chapters of this book. Work on isoelectronic substances, reviewed recently by Iqbal [3], has been substantial but not very detailed. However, a qualitative general correlation... 

Most metals react exothermically with oxygen to form an oxide. Figure 3.4 shows how the value of AG for this process varies with temperature for a number of metals (and for carbon), and it can be seen that in all cases AG becomes less negative as the temperature is increased. However, the decomposition of these metal oxides into the metal and oxygen is an endothermic process, and Figure 3.4 shows that this process does not become even energetically feasible for the majority of metals until very high temperatures are reached. 

Furthermore, an actual or conceptual decomposition is useful because it can lead to a better appreciation of factors underlying the binding energetics. We consider here the following four components. 

Ions can be induced to fragment by increasing an electric potential known as a cone voltage, which speeds them. Accelerating the ions causes them to collide more energetically with neutral molecules, a process that causes them to fragment (collision-induced decomposition). 

The preparation of semiconductors by thermal decomposition would appear to be impossible because of the high amount of energy required to break all of the metal-carbon bonds before the atomic species could be formed. However, the thermal method is successful because the reaction to form free methyl radicals, which combine to form ethane, lowers the energetic requirements for the formahon of gallium, for example, according to the equation... 

The collision theory considers the rate to be governed by the number of energetic collisions between the reactants. The transition state theory considers the reaction rate to be governed by the rate of the decomposition of intermediate. Tlie formation rate of tlie intermediate is assumed to be rapid because it is present in equilibrium concentrations. 

Characterization of Energetic Materials, Their Decomposition Products and Their Residues — Progress in the Identification and Detection of Explosives , PATM 2136 (Mar 1974) 69)... 

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

The nitration of N,N -diethylurea gives nitrated products which are precursors for a new energetic plasticizer N,N -dialkyl-N,N -dinitrourea (DNDA). For macroscopic batch processing, this reaction is characterized by a lack of selectivity owing to mononitro derivative formation and thermal decomposition of the dinitro product due to increasing temperature during the course of reaction [37, 38]. 

The enthalpy of decomposition is now replaced by the enthalpy of reaction to analyse the potential danger. Since the danger of a chemical reaction is usually related to a modification in its procedure, which makes it uncontrollable and causes destruction of the molecular groups, it seems to make more sense to write down the most energetic reaction possible. The risk will indicate the maximum potential danger considering the stoichiometry chosen. This approach may be... 

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