Energetics of Thermal Decomposition

The most energetically favorable mechanism for the formation of nitrogen gas in azide decomposition is through the bimolecular decomposition of azide 

The simpler unimolecular decompositions are either energetically unfavorable or forbidden by the quantum mechanical selection rules [8]. 

There is some evidence that the partial covalent character in the metal azides is associated with the asymmetry in the azide group, and, as discussed later (Section C.2) in this chapter, observations of lead azide suggest that decomposition takes place by the fission of the longest N-N bond [10]  

A complicating factor observed during the decomposition of the alkaline-earth azides is the reaction between nitrogen and the metal nuclei to produce nitride [11]. The reaction is temperature dependent, and its extent apparently depends upon the size of sample used [12]  

For Cu, Ag, n, and Pb azides, the correspondence between activation energy for decomposition and the thermal energy of the first exciton level is sufficiently close to suggest that the rate-determining step in their decomposition is the promotion of the electron from an azide ion to this level. 

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