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ExposeExciteIgniteMay2012

Why is Ammonia Nitrogen

Why is Ammonia Nitrogen Triiodide so extremely unstable? There are two major thermodynamic drivers in the chemical decomposition reaction: 8NI3.NH3 (s) ! 5N2 (g) + 6NH4I (s) + 9I2 (g) + energy Firstly. A major driver is the formation of products containing much less chemical potential energy than that of the reactants, leaving lots of energy to be released - a perfect exothermic reaction with a substantial decrease in enthalpy, !H 0 . Note the large value of !H 0 and small activation energy, Ea requirement in Figure 10.1. Figure 10.1 The products I2 and N2 both being pure elemental substances, present some of the lowest potential energy states, but it is the formation of nitrogen that is of much importance. Nitrogen is so stable because of its very strong covalent bonds. It takes lots of energy to break these bonds. The N2 triple bond is one of the strongest chemical bonds known and prevents nitrogen gas from being chemically active, rendering it “inert” (Table 10.1). Here is a thought: Even though nitrogen makes up the largest ratio of the earth’s atmosphere (78%), it is not abundant in the earth’s crust as it rarely reacts with other elements. Nitrogen is energetically in a pretty comfortable and happy state! 111

Bonding Energy kJmole -1 Nitrogen Hydrogen Iodine N2 945 H2 436 I2 148 Table 10.1 The second thermodynamic driver in this spontaneous process is the huge increase in entropy: A gain of 14 moles in gaseous products guarantees a huge entropy increase (!S 0 = pos.). Think: increase in randomness. This reasoning fits in nicely with the principles of thermodynamics for chemical reactions. See Appendix A for more information. Looking at commercial explosives, one finds that most contain high potential energy, nitrogen compounds with single or double bonds and have as a driving force the decomposition of these and the formation of the lower energy, more stable N2 compound (!H 0 = neg.). Because of these surplus energies, all explosives produce exothermic reactions. Furthermore. The increased entropy as more gaseous products form on decomposition, acts as another driver towards equilibrium and we come closer to understanding why explosives are as spontaneous and in some cases unstable as they are. The following molecular structures of commercial and military explosives show the heavy reliance on “high energy” nitro groups. They all release substantial energy when forming stable N2 molecules during decomposition. Figure 10.2 But, there is another important property, known as kinetics, that will determine if a thermodynamically unstable compound such as our triiodide, can earn the label of “explosive”. As we all know, not all combustible spontaneous processes cause explosions, eg. the burning of 112