Dissociation, endothermic

Decomposition and Detonation Hazard. Ammonium nitrate is considered a very stable salt, even though ammonium salts of strong acids generally lose ammonia and become slightly acidic on storage. For ammonium nitrate, endothermic dissociation from lowering pH occurs above 169°C. 

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). 

It should be emphasized that the theoretical and real flame temperatures will be significantly different. The real flame temperature will be lower than the theoretical flame temperature because, in practice, heat is lost from the flame (mainly due to radiation). Also, part of the heat released provides heat for a variety of endothermic dissociation reactions, which occur at high temperatures, such as ... 

Larger differences are observed when comparing the enthalpy of formation of the different halides of a given alkali metal. The enthalpy of formation of gaseous halide ions is exothermic since the exothermic electron gain enthalpy in absolute value is larger than the endothermic dissociation enthalpy. Furthermore, the enthalpy of formation of gaseous halide ions becomes less favourable with... 

Low chlorinity zones were coincident with zones of anomalously low recovered core temperatures on the ship catwalk. For example, while some of the background core temperatures were at 10-12°C, cores in suspected hydrate regions had temperatures as low as 1°C, perhaps caused by endothermic dissociation of hydrate. The extrapolated geothermal gradient of 33.5°C/km yielded a temperature of 18.3°C at the BSR (440 mbsf), well within the temperature stability field of methane hydrate. 

By contrast, the decomposition reaction involves an endothermic dissociation into hydrogen and sulphur,... 

Values for transition-state frequencies were assumed following the arguments by Armentrout et al. [143,144] and Squires et al. [145]. For an endothermic dissociation of gas-phase ions, a loose transition state can be assumed. Accordingly, the product frequencies can be used where available. For the five normal modes that correspond, at the asymptotic limit of dissociation, to relative rotations and translations of the departing fragments, further estimates had to be made. Numerical estimates for these five frequencies were taken from computations of analogous oxidative addition, reductive elimination, and a-bond metathesis transition states. The deconvoluted threshold, with the methyl... 

More recently, higher temperature processes have been considered (at T > 2000 K), such as two-step thermal chemical cycles using metal oxide reactions.2 The first step is solar the endothermic dissociation of the metal oxide to the metal or the lower-valence metal oxide. The second step is non-solar, and is the exothermic hydrolysis of the metal to form FI2 and the corresponding metal oxide. The net reaction is H2O = H2 + 0.5 O2, but since FI2 and O2 are formed in different steps, the need for high-temperature gas separation is thereby eliminated ... 

Hydrogen is not exceptionally reactive, in part due to the highly endothermic dissociation ... 

Ammonium nitrate undergoes several decomposition processes. At 442.8 K, ammonimn nitrate starts to melt and at 443.2K endothermic dissociation takes place (AH = -1-175 kJmol ) ... 

Molecular dynamics studies of diatomic model detonations were first carried out by Karo and Hardy in 1977 [14]. They were soon followed by other groups [15, 16]. These early studies employed predissociative potentials, in which the reactant dimer molecules are metastable and can dissociate exothermically. More realistic models, combining an endothermic dissociation of reactants with an exothermic formation of product molecules, were introduced by White and colleagues at the Naval Research Laboratory and U.S. Naval Academy, first using a LEPS (London-Eyring-Polanyi-Sato) three-body potential for nitric oxide [17], and later a Tersoff-type bond-order potential [18] for a generic AB model, loosely based on NO [19, 20]. 

Ammonium Nitrate. Like AP, the decomposition of AN occurs via complex decomposition mechanisms. Studies performed by Oxley and coworkers indicate two modes of decomposition.[52-54] In the temperature range 200-300 °C, decomposition starts through an endothermic dissociation to ammonia and nitric acid and the formation of the nitronium ion is the rate-determining step (See Scheme II). 

A number of endothermic dissociative charge transfers have also been studied by several investigators. Such studies have proved to be very important since they provide a powerful means for determining bond energies, in addition to their shedding light on the problem of energy transfer in chemical reactions. 

Fig. 22. Comparison of values and energy dependence of cross-sections for endothermic dissociative charge transfer with those for exothermic dissociative charge transfer. (From ref. 180.)...

Vance and Bailey [94] studied an endothermic dissociative charge transfer with a non-rare-gas reactant... 

As to another important aspect of the study of endothermic dissociative charge transfer, i.e. its application to the determination of bond dissociation energies of Vcirious molecules, we do not have enough space to describe it, and the reader is referred to the original papers [176, 181] for detailed discussion on individual molecules. 

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. 

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