Sensitized Explosions Branching Chains

The simple model of a thermal explosion which we examined in the last section was based on the view that, for an exothermic reaction, heat played the role of an autocatalytic product. It is now interesting to ask if it is possible for chemical species produced as intermediates, or even final 

If we consider the mechanism of any typical chain reaction, we see that the atoms or free radicals produced are, by virtue of the existence of the chain, autocatalytic agents for the reaction. Thus in the chain reaction H2 + CI2 — 2HC1 both H and Cl arc chain carriers, the chain mechanism being 

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. 

Now if we examine again the H2 + CI2 chain system [Eq. (XIV.4.1)], we see that this typical two-center chain cannot change the total concentration of chain centers. All the chain does is change the identity of c.hain carriers (that is, II to Cl or Cl to H) but not their total concentration. Such chain systems cannot thus provide for an increase in the concentration of chain carrici s. Explosions, if they occur in such systems, must be thermal in character. 

however, possible to induce explosions in these systems by the use of additives which are frequently referred to as sensitizers. Thus Ashmore has shown that the addition of 0.5 mm Ilg of NO to 50 mm Ilg of an equimolar mixture of H2 + CI2 lowers the critical explosion temperature from 400 to 270°C. The explosion in this case is still, however, a thermal explosion, and it has been shown that the lowering of the explosion temperature was produced by an increase in the concentration of Cl atoms, not by a change in the chain mechanism. This increase in concentration of Cl atoms was produced by the replacement of the slow, high-activation-energy initiation reaction, M + CI2 2C1 + M(E 57 Real), by the much-lower-activation-energy reaction, NO + CI2 NOCl + C1(jE = 22 Real). 

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The feature which is unique to the chain-branching system is the paradoxical, upper, or second explosion limit. Plere one observes that a reaction proceeding with explosive speed at pressures below the limit is effectively (picnched on raising the pressure. In addition, the pressure limit increases if the temperature increases, just opposite to the behavior at the first and third limits. It is the existence of this limit that is the real evidence of the branching chain. It is observed that the limit is much less sensitive to surface-volume effects than is the first limit, while added inert gases always tend here to lower the limit (i.e., quench the explosion). 

Straight and branched chain reactions almost invariably have complex rate expressions, as shown by -d[H2]/df = A [H2]° [02]° [N2]° for the H2 + O2 reaction in aged boric-acid-coated vessels at 500 Torr and 773 K [6]. Change of pressure can have striking effects even when achieved by addition of an inert gas such as N2. Hydrocarbon combustion reactions proceed through the formation of many intermediates, both radical and molecular, prior to formation of the final products CO2 and H2O. Another striking feature is the sensitivity of chain reactions to traces of impurities and to changes in surface properties. This is particularly pronounced in the case of some explosion boundaries or limits where parts per million quantities of an impurity may completely subdue the explosion. 

The second explosion limit must be explained by gas-phase production and destruction of radicals. This limit is found to be independent of vessel diameter. For it to exist, the most effective chain branching reaction (3.17) must be overridden by another reaction step. When a system at a fixed temperature moves from a lower to higher pressure, the system goes from an explosive to a steady reaction condition, so the reaction step that overrides the chain branching step must be more pressure-sensitive. This reasoning leads one to propose a third-order reaction in which the species involved are in large concentration [2], The accepted reaction that satisfies these prerequisites is... 

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