Secondary explosives activation energy

Table 6.3 Values for the activation energy E and collision factor A for some primary and secondary explosive substances...

Primary explosives have low values for the activation energy and collision factor compared with secondary explosives. Therefore, it takes less energy to initiate primary explosives and makes them more sensitive to an external stimulus, i.e. impact, friction, etc., whereas secondary explosives have higher values for the activation energy and collision factor, and are therefore more difficult to initiate and less sensitive to external stimulus. 

PROBLEM 12.19 Would you expect a higher activation energy for the decomposition of a primary or a secondary explosive Explain. 

Explosives can be divided into two categories primary explosives and secondary explosives. Primary explosives are very sensitive to shock or heat, and they detonate very easily. That is, the energy of activation for the explosion is very small and easily overcome. In contrast, secondary explosives have a larger activation energy and are therefore more stable. Secondary explosives are often detonated with a very small amount of primary explosive. 

Recently, a chain type of reaction lias been postulated for the oxidation of ethylene.20 In highly exothermic vapor phase reactions such as the homogeneous oxidation of olefinic hydrocarbons conditions are particularly favorable for chain type reactions. The initial reaction in such transformations results in the formation of a molecule with excess energy and capable of readily reacting further with another molecule of the hydrocarbon to continue the cycle. Secondary processes such as absorption of the excess energy by a wall or surface, or destruction of the molecule by further oxidation serve to break the chain of reactions. The relative rates of formation and destruction of the active centers determine the speed of the reaction. Obviously, if the number of chains started is more than the number broken, the reaction becomes very rapid and explosion results. If the two rates are very nearly equal, the reaction proceeds with a measurable rate, the speed of which depends upon chain length. In general, the concentration of the reactants in the mixture determines the rates of formation and destruction of the chains and, hence, determines the rate of the reaction.27 The first step in the chain would be the formation of an unstable peroxide which could then react with another molecule of ethylene to form an unstable hydroxylated molecule to continue the chain reaction. Further oxidation of the peroxide first formed, however, would end the chain. Supplementary data and additional evidence that this type of reaction occurs have been supplied by Spence and Taylor.241... 

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