When pure AN is heated to a temperature range of 169o to approximately 200<sup>o</sup>C, essentially no decomposition occurs. When heated
further, to the range of 200-250C, the following exothermic reaction (heat is released) primarily occurs:
NH<sub>4</sub>NO<sub>3</sub> => N<sub>2</sub>O + 2 H<sub>2</sub>O, ΔH= -8.8 kcal/mol
From the above equation, for 100 grams of AN, 45 grams of H<sub>2</sub>O is produced, with the remaining 55 grams being gaseous nitrous
oxide. This reaction is exothermic, with the release of 110 calories/gram.
Simultaneous to this reaction, a dissociating reaction occurs endothermically (heat is absorbed) whereby the AN breaks down into ammonia and nitric
acid.:
NH<sub>4</sub>NO<sub>3</sub> => NH<sub>3</sub> + HNO<sub>3</sub>, ΔH= +44.6 kcal/mol
The combination of these two effects results in a steady-state, or self-limiting temperature, provided the decomposition process is carried out with
the gaseous reaction products allowed to freely escape (in particular the HNO<sub>3</sub> . As such, if pure AN is heated at a moderate rate in the open air with no confinement, the temperature cannot rise
appreciably beyond its melting point.
Under steady-state conditions, the endothermic dissociation of AN into gaseous NH<sub>3</sub> and HNO<sub>3</sub> absorbs all
the heat available from decomposition. Thus, when heat is added to AN at atmospheric pressure and even from a very hot source, the temperature of the
AN is limited by its own dissociation to values at which decomposition rate is comparatively moderate. At elevated pressures, however, the
dissociation reaction is repressed and the rate of decomposition accelerates.
|