Exothermic dissociation mechanism

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

For H to dissociate on a metal cluster two independent criteria must be satisfied. First, the reaction must be exothermic. We will discuss the conditions for exothermlcity at the end of this section and in the next section. Second, a dissociation mechanism must exist such that the barriers for dissociation will not be too high. During the past years we have studied the dissociation both for single transition metal atoms and complexes and for clusters as models for surfaces. Single iron, cobalt, nickel and copper atoms have been studied (5,6) as well as models of nickel and copper surfaces (7,8). 

The growth mechanism appears to be the same irrespective of type of hydrocarbon and whether it is the endothermic dissociation of methane or the exothermic dissociation of carbon monoxide (8). However, the resulting morphology and degree of graphitization depends on parameters such as type of hydrocarbon, metal, particle size, and temperature. Hence, there might not be a unique growth mechanism for the formation of carbon fibers and nanotubes. 

A considerable amount of research has been conducted on the decomposition and deflagration of ammonium perchlorate with and without additives. The normal thermal decomposition of pure ammonium perchlorate involves, simultaneously, an endothermic dissociative sublimation of the mosaic crystals to gaseous perchloric acid and ammonia and an exothermic solid-phase decomposition of the intermosaic material. Although not much is presently known about the nature of the solid-phase reactions, investigations at subatmospheric and atmospheric pressures have provided some information on possible mechanisms. When ammonium perchlorate is heated, there are three competing reactions which can be defined (1) the low-temperature reaction, (2) the high-temperature reaction, and (3) sublimation (B9). 

The burning mechanism of composite propellants differs from that described above. There is no exothermic reaction which can lead to a self-sustaining fizz zone. Instead, the first process appears to be the softening and breakdown of the organic binder/fuel which surrounds the ammonium perchlorate particles. Particles of propellant become detached and enter the flame. The binder is pyrolysed and the ammonium perchlorate broken down, initially to ammonia and perchloric acid. The main chemical reaction is thus in the gas phase, between the initial dissociation products. 

There are two mechanisms which allow a radical to react in situations where the parent molecule is inert. First, the reaction between a surface and parent molecule may be exothermic but require a large activation energy to proceed. (This is probably the case for methane which is very unreactive toward a large variety of clean surfaces .) When the activation energy is provided by dissociating the molecule while it is in the gas phase, then the resulting fragments spontaneously react with the surface. 

Another case for which ET could be expected as a viable alternative to the SN2 displacement mechanism concerns the reactions of CH3I and CC14 with the nitric oxide anion, NO-263. Because of the extremely low electron affinity of NO (0.024- 0.55 kcalmof1), an ET process to the halo-compounds would be exothermic. However, in neither case was the substrate radical anion observed, despite the fact that both have bound molecular anions. Both reactions yield only the halide ion, a product which can arise via dissociative ET (a) or S 2 (b) (Scheme 38). The mechanism could not be assigned. 

Reactions between hydrocarbons and elemental fluorine are extremely exothermic because of the high heats of formation of bonds from fluorine to carbon and hydrogen (approximately 456 and 560kJmol , respectively) [27, 111]. The value of A// for the dissociation of fluorine is very low ca. 157kJmol ), so it is frequently assumed that the preferred fluorination process proceeds by a radical chain mechanism (Figure 2.20), although this may not always be the case. 

The mechanism of conversion, which is exothermal in the direction o -> p, involves dissociation and recombination, during which the nuclear spins re-couple, parallel or anti-parallel, in equilibrium proportions. This occurs, for example, on collision (the high temperature mechanism) and probably in chemisorption (Eley and Rossington, 1957), when the atoms are separated by going into different lattice sites and subsequently recombine with nuclear spins oppositely coupled. 

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