Endothermic decomposition/dissociation

The remainder of this section is devoted to a survey of recent studies of the primary photochemistry of these two molecules. Studies on molecules of this complexity are becoming increasingly commonplace as our understanding of model polyatomic systems improves and both experimental and theoretical methods continue to advance. In the case of methanol, classical photochemistry combined with end-product analyses [173] had established that the dissociation channel given by Eq. 4 dominates following photoexcitation within the long-wavelength absorption band, despite the possible availability of no less than three alternative, less endothermic decomposition routes. 

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). 

There have been few attempts to classify decomposition reactions of solids. Gamer [64] made only the broad distinction between endothermic processes (which are often reversible and include dissociation of crystalline hydrates and carbonates) and exothermic processes (which are usually... 

Even with these possible modifications, the mechanism used by Chan and McIntosh21 may be unsatisfactory. As is abnormally low for a unimolecular decomposition. Es Z)[(CO)3Ni-CO] = 19.1 kcal.mole-1 also seems low in view of the fact that the mean Ni-CO bond dissociation energy20 in Ni(CO)4 is 35.2 kcal.mole-1. It is reasonable to assume that As > A7. For reaction (5) to be ratecontrolling the maximum value of E7 would therefore be approximately 17 kcal.mole-1. Assuming all back reactions have zero activation energy would then give a minimum value D[(CO)Ni-CO]+D(Ni-CO) = 105 kcal.mole-1. This would eliminate the sequence of reactions (10)—(13) and would render reaction (9) unlikely because it would now be expected to be at least 50 kcal.mole-1 endothermic. Therefore, if the value of E5 reported is correct, no logical sequence of reactions has been found that would yield the observed overall process Ni(CO)4 = Ni+4 CO. 

However, (11) is approximately 180 kcal.mole-1 endothermic. If it occurs it must be a stepwise process. Partial stepwise decomposition may take place but complete dissociation to liberate Si atoms would be inconsistent with the observation that the carbon content of the solid product under all decomposition conditions is approximately half that of the parent alkyl (low decomposition — grey solid, 25.3 % C high decomposition — black solid, 28.3 % C Si(CH3)4 = 54 % C)130. 

Ammonium perchlorate is a monopropellant which is exothermic in its decomposition to the extent of 270 cal./gram (1, 30, 54), based on measured decomposition products, and has a measured adiabatic flame temperature of 950°C. (1, 54) (see Table I). The dissociative sublimation step is zero order and is endothermic to the extent of 500 cal./gram... 

The surface decomposition process of AP propellants is net exothermic, with heat absorbed at the AP and fuel surfaces by endothermic pyrolysis but with more heat liberated in the gas phase close to the AP crystals. In detail, decomposition at the AP surface consists of endothermic pressure-independent solid-to-gas phase dissociative sublimation, followed by exothermic pressure-dependent gas-phase oxidation. The over-all reaction for the AP decomposition is pressure dependent. 

In the last reaction the decompn is endothermic, and if the gaseous mixt of products of decompn is heated, these react exothermically with explosive effect (See also Heat of Decomposition, Heat of Dissociation and Heat of Explosion of AN)... 

Equilibrium concentrations of carbon or ammonia are not found in short combustion chambers used in rocket motors. The reason for this non-equilibrium situation is that the rate of formation of soot is very slow and carbon does not have time to form. Similarly the dissociation of NH3 is very slow. Thus in ethylene oxide monopropellant rocket motors one finds very little carbon, whereas equilibrium considerations predict carbon as a predominant product and in hydrazine decomposition chambers one finds an excess of NH3 over that predicted by equilibrium considerations. In ethylene oxide motors carbon forms from the decomposition of methane, not the reaction represented above, thus both non-equilibrium situations give higher performance than expected, since the endothermic reactions do not have time to take place. Of course, carbon also could form in cool reactions which take place in boundary layers along the walls where velocities are slow. 

Metastable loss of H2 is only observed for the chloro and bromo compounds. For X = F, the exothermicity of the protonation-dissociation sequence (7 kcalmol-1) is probably not sufficient to exceed the energy barrier for H2 loss, which for the 1,2-H2 elimination is predicted by theoretical calculations to be 18 kcalmol1 (thermochemical data for a 1,1 -H2 elimination are not available)106. For X = I, the protonation-dissociation reaction is exothermic by 41 kcal mol-1 and the absence of a metastable peak for this process is probably due to a very high activation barrier. The existence of an energy barrier for the H2 (HD) elimination from protonated chloro- and bromohalomethane, and their deuterated isotopomers was indeed revealed by the large KER values (140 eV) associated with their metastable decomposition. An intensity ratio of about 50 was measured for metastable peaks due to H2 and HX losses in accordance with the known thermochemical data which establish that H2 loss is less endothermic than HX loss by 10 kcalmol-1 and 28 kcalmol-1 for X = Cl and Br, respectively109. 

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

The dissociation pressure of calcite reaches 0.101 kPa (1 atm) at 894°C (S20) and the decarbonation reaction is highly endothermic (Section 3.1.4). The rate of decarbonation becomes significant at 500-600°C if a sufficiently low partial pressure of COj is maintained or if the calcite is intimately mixed with materials, such as quartz or clay mineral decomposition products, that react with the calcium oxide. Even in a precalciner, such mixing occurs, aided by agglomeration caused by the presence of low-temperature sulphate melts. 

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 ) ... 

The decomposition products are LiH and MgH2 which could be further decomposed to the elements at higher temperatures. The first step between 100 and 130 °C is exothermic which makes the materials not applicable for hydrogen storage. The second step between 150 and 180 °C is endothermic. The intermediate LiMgAlHg contains 9.4 wt.% H2 of which 4.8 wt.% are released in the second step. When heating to about 250 °C, another 3.6 wt.% can be released. From the peak areas of DSC measurements dissociation enthalpies of about —15kJ mol for the first decomposition step (exothermic) and 13 kj mol for the second step (endothermic) were calculated [166]. 

Many polymerizations are carried out at temperatures between 0 and 100°C. Initiation at the required rates under these conditions is confined to compounds with activation energies for thermal homolysis in the range 1(X)-165 kJ/mol. If the decomposition process is endothermic, the activation energy can be considered to be approximately equal to the dissociation energy of the bond which is being split. It can be expected, then, that useful initiators will contain a relatively weak bond. (The normal C—C sigma bond dissociation energy is of the order of 350 kJ/mol, and alkanes must be heated to 3(X)-500°C to yield radicals at the rates required in free-radical polymerizations.)... 

MgCOj decomposes at substantially lower temperatures the dissociation pressure reaches atmospheric pressure at 400—480 C. The reaction enthalpy is 121 kJ (28.9 kcal) per mole at 25 °C. MgC03 bound in dolomite decomposes at a temperature somewhat higher than pure MgC03. The dissociation pressures are plotted in Fig. 27. The decomposition of dolomite proceeds in two stages as illustrated by differential thermal analysis curves, showing two distinctly separate endothermal peaks for MgCOa and CaCOj respectively. The DTA curves for various minerals are shown in Fig. 28 (Ivanova et al., 1974). 

On heating, metal carbonates undergo dissociation of the anion (CO, -> O + COj) in reactions that are endothermic and usually reversible. Complete uptake of the volatile product, to restore the original composition of the stoichiometric reactant, does not always occur, however, in any reasonable time interval. Calculated values of , are often close to the reaction enthalpy. Not all reactions proceed to completion in a single kinetic step. These reactions show a general pattern of similarity [1] with the dehydrations of crystalline hydrates, surveyed in Chapter 7, and also were amongst the earliest reactants used in kinetic and mechanistic investigations of solid state decompositions. 

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). 

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