Autocatalysis explosive

The preceding situation is an example of thermal autocatalysis, and its consequence is fiequently a thermal explosion. 

Cook (Ref 1), in describing thermal decomposition of some HE s conducted in the quartz spring apparatus (described in Ref 1, p 175 and shown there in Figs 8.1a 8.1b), stated that PETN, RDX, Tetryl and to a small extent TNT decomposed autocatalyti-cally. EDNA followed the first-order decomposition law only until about 5% of the explosive had decomposed and then the reaction stabilized. The term autostabilization was applied here on the supposition that one of the condensed decomposition products of EDNA which accumulated in the explosive apparently tended to stabilize the bulk of expl and thus slow down the decomposition. After about 10% of the expl had decompd, however, the "autocatalysis developed. 

Syllabus (1957-1958) - (No discussion on "autocatalysis , but on p 140 is mentioned the "activated complex , which is an important agent in catalysis) 3) Andreev Belyaev (I960), pp 57-61 (Avtokataliticheskii Vzryv) (Autocatalytic Explosion)... 

If decomposition proceeds at the same rate over entire range until practically no sample remains (like with(AN),it is said that the explosive exhibits (ideal) first-order decomposition, and that no autocatalyzation takes place as in the decompn of.PETN,Tetryl or RDX. EDNA followed the first-order decomposition law only until ca 5% of the expl had decomposed. This was followed by autostabilization, the term applied here on the supposition that one of the condensed decompn products of EDNA which accumulated in the sample apparently tended to stabilize it, thus slowing down the decompn. After ca 10% of the expl had decomposed, however, autocatalysis developed... 

The isothermal method for such expls as PETN, RDX, NG and Tetryl is complicated by autocatalysis to such an extent that one cannot determine the intrinsic (pure explosive) decompn rate from the logw vs t curves and their change with temperature. Hence, the results obtd by the adiabatic(sensitivity) methods may be more reliable from this viewpoint (Ref 8, p 177)... 

For every 10 °C increase in temperature, the rate of decomposition is approximately doubled, but may increase as much as 50 times if the explosive is in the molten state. The rates of decomposition depend on the condition of storage and the presence of impurities which may act as catalysts. For example, nitroglycerine and nitrocellulose decompose at an accelerated rate due to autocatalysis, whereas the decomposition rate of TNT, picric acid and tetryl can be reduced by removing the impurities which are usually less stable than the explosive itself. With many of the explosives the presence of moisture increases the rate of decomposition. 

Over the last 10-15 years, interest has grown significantly in the kinetics of combustion and explosion reactions, which are characterized by the presence of some mechanism of acceleration of the reaction. This acceleration, which leads to ignition, may be related either to the accumulation of active products which catalyse the reaction, the chain carriers (autocatalysis, chain explosion), or to an increase in the temperature of the mixture due to heat release in an exothermic chemical reaction (thermal explosion). 

It is in fact because of the autocatalytic clniracter of the H and Cl intermediates that a fast reaction between Ho and CI2 is observed. But such chain reactions are normally not explosions. In what sense, then, can autocatalysis by chain carriers lead to an explosion What is required, if catalysis by chain carriers is to lead to an explosion, is that the mechanism of the chain cause an increase in the concentration of chain carriers beyond that present in the normal reaction. 

Autocatalysis is a distinctive phenomenon while in ordinary catalysis the catalyst re-appears from the reaction apparently untouched, additional amounts of catalyst are actively produced in an autocatalytic cycle. As atoms are not interconverted during chemical reactions, this requires (all) the (elementary or otherwise essential) components of autocatalysts to be extracted from some external reservoir. After all this matter was extracted, some share of it is not introduced in and released as a product but rather retained, thereafter supporting and speeding up the reaction(s) steadily as amounts and possibly also concentrations of autocatalysts increase. At first glance, such a system may appear doomed to undergo runaway dynamics ( explosion ), but, apart from the limited speeds and rates of autocatalyst resupply from the environment there are also other mechanisms which usually limit kinetics even though non-linear behavior (bistability, oscillations) may not be precluded ... 

The autocatalytic nature of the reaction, described by Hinshelwood and Williamson [1], is in sharp contrast with the effect of water on the surface reaction at lower temperatures, which is poisoned by steam, and also with the inhibiting effect of water vapour on the second limit explosions. The autocatalysis has been studied in some detail by Chirkov [36], who used a reaction vessel of Durobax glass with diameter 5 cm and volume 200- 250 cm. For hydrogen oxygen ratios of about 2 1 at 550 torr initial pressure and 524 °C, he found the reaction rate w (torr sec ) to be given in terms of the initial pressure p and the amount of gases reacted x by... 

The next milestone appeared in the 1950s in the context of the development of asymmetric reactions. Various stereochemical reactions induced by facial discrimination of the carbonyl group have always been pivotal in this field. Cram s rule inspired an explosion of studies on diastereoselective reactions followed by enan-tioselective versions. The recent outstanding progress in the non-linear effect of chirality or asymmetric autocatalysis heavily relies on the carbonyl addition reactions. Thanks to these achievements, natural products chemistry has enjoyed extensive advancement in the synthesis of complex molecules. It is no exaggeration to say that we are now in a position to be able to make any molecules in as highly selective a manner as we want. 

The process of lead nucleation is thus favored by the conditions, which in turn increase the rate of free-radical formation and the onset of autocatalysis, resulting in the observation of shorter times to explosion. 

How to progress The best lead to the hardest part of the problem - the forward problem - is the hypothesis that life evolved, somehow, from autocatalytic reactions (that is, reactions whose products are themselves catalysts for the reactions that produce them). We know something about autocatalytic reactions flames are autocatalytic, and so are explosions (and one speaks, sometimes, of the explosion of life). We also know other reactions that are autocatalytic, although the subject of autocatalysis has not been a particular preoccupation of chemistry or biochemistry. Autocatalysis offers, I believe, a plausible trail into the wilderness. 

Situations of this type are grouped under the term autocatalysis, which means that the system produces its catalyst as it goes along. As in the growth of populations, the acceleration can become so intense that the word explosion properly describes the catastrophic event. The explosion may fizzle if reactants are exhausted before the situation has gone out of hand — the rate... 

No steady-state reaction is possible. The reaction rate, which is proportional to (X) also increases exponentially with time the autocatalysis reaches catastrophic proportions and explosion takes place. It must be stressed that the primary cause of the explosion is not the accumulation of heat in the system, as occurs in thermal explosions (see Chapter 7). The self-acceleration of the rate can take place isothermally. Naturally, as the reaction rate becomes very high, self-heating of the system may also take place and contribute to the explodon. 

A further distinction may be made between explosive autocatalysis, in which the concentration of A, the precursor of X in reaction (5.19), is held constant, and nonexplosive or self-limiting autocatalysis, in which (A) is a variable that will naturally decrease as the autocatalytic reaction proceeds. Self-limiting autocatalysis can serve as a positive feedback on one species (X) and a negative feedback on another (A) simultaneously. 

Reaction (5.27) constitutes the positive feedback, explosive autocatalysis, since A is assumed to be constant. Although reaction (5.28) provides some negative feedback, analysis shows that it is insufficient to bring the system back to a point where the cycle can start again. The combination of steps (5.29) and (5.26) provides the necessary limitation on X, as Z generates enough Y (bromide ion) to compete successfully with the autocatalysis, at least if the stoichiometric factor/is... 

A second type of negative feedback is double autocatalysis. The essence of this pattern is that a species generated autocatalytically, often explosively, in the primary autocatalysis is consumed by a second autocatalytic reaction in which it serves as the precursor. As the primary autocatalyst builds up, the second autocatalytic reaction accelerates, consuming the primary species at an ever-faster rate and ultimately terminating the explosion. Franck (1985) constructed a modified version of the original Lotka-Volterra model that nicely illustrates double autocatalysis ... 

Autocatalysis, with n = 1, characterizes biological population growth, for example, since the number of offspring born is proportional to the number of individuals in the population. It leads to the Malthusian population explosion. In chemical systems, where autocatalysis is less common, it can also result in explosion, since the solution to Eq. (2.1) is an exponentially growing concentration. Of relevance to polymer systems is the fact that any exothermic reaction is inherently autocatalytic, since an increase in the product concentration corresponds to production of heat, which leads to an increase in the rate constant of the reaction via the Arrhenius factor. If the reaction in question is lengthening a polymer chain by addition of the monomer, the rate should increase as the chain grows if the heat produced is not rapidly removed from the system. 

Much of this Volume deals with the transition phenomena observed in isothermal or temperature dependent reaction sequences, involving appropriate cooperative interactions like autocatalysis, and functioning far from equilibrium. Classical bifurcation phenomena involving the loss of stability of a uniform steady state and the evolution to a limit cycle or a space pattern, abrupt overshoots associated to ignition and explosion, or transition to chaotic dynamics are some characteristic examples. 

Consider next a system in which explosion occurs primarly because of chemical kinetics. Real world systems of this type involve multiple steps and competition between various pathways, many of which contain autocatalytic or inhibitory effects associated with the appearance of chain reactions and free radicals [4]. Instead of developping the analysis of such a system however, we present hereafter a mathematical model which captures the essence of the phenomenon while still allowing a fairly complete mathematical treatment. The specific example we choose is the autocatalytic mechanism suggested by Schlbgl [6]. Further comments on the role of autocatalysis are to be found in the Chapters by P. Gray and S.K. Scott and by I. Epstein. 

Autocatalysis can lead to explosive behavior. This has been observed in the reaction between NO and CO on the surfaces of platinum and rhodium. The overall reaction is... 

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