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Nitrogen triiodide, on slightest physical contact, explodes to evolve dense purple fumes of iodine. Why does this phenomenon occur?

video link - https://www.youtube.com/watch?v=2KlAf936E90

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  • $\begingroup$ Big badda boom! $\endgroup$ – bodacydo Jul 20 '15 at 2:12
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Wikipedia states

The instability of $\ce{NI3}$ and $\ce{NI3 · NH3}$ can be attributed to the large steric strain caused by the three large iodine atoms being held in close proximity to each other around the relatively tiny nitrogen atom. This results in a very low activation energy for its decomposition, a reaction made even more favorable due to the great stability of $\ce{N2}$.

The steric strain between the atoms can be better visualized by the representations here.

The decomposition reaction is $$\ce{2NI3\to N2 + 3I2}$$ Note that there is an increase in the number of moles (4 versus 2), which means that the reaction is favored by entropy, because there are more possible microstates.

More specifically, the enthalpy of formation is 154.4 kJ/mol, which is incredibly favorable for a reaction.

Reference: UC Davis ChemWiki

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    $\begingroup$ You might want to add the equation for decomposition. Since only elements are formed upon decomposition - and they have a heat of formation of zero - the full 154 kJ/mol is released upon decomposition. Further, 2 moles of product are formed per mole reactant, so the reaction is also entropically driven. $\endgroup$ – ron Jul 19 '15 at 17:03
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    $\begingroup$ @ron I should have included the latter information, as I saw it in the link. I'll also include the reaction. $\endgroup$ – HDE 226868 Jul 19 '15 at 17:03
  • $\begingroup$ Also - depending on the particle size of the iodine and how long it is allowed to react with the ammonium hydroxide before washing the filtrate can result in less than a 100% yield, so some elemental iodine can be left. Upon detonation of the nitrogen tri-iodide the elemental iodine can be vaporized. $\endgroup$ – docscience Jul 22 '15 at 3:08
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HDE 226868's answer covers pretty much about the thermodynamic property of nitogen triiodide, my answer just points out another point, its composition.

Quoting from Weller, M.; Overton, T.; Rourke, J.; Armstrong, F. Inorganic Chemistry, 6th ed:-

Although the formula of nitrogen triiodide is usually written as $\ce{NI3}$, it would be more accurate to write $\ce{I3N}$, as the compound is thought to consist of $\ce{I+}$ and $\ce{N^3-}$ ions and its sensitivity to shock is due to redox instability of these ions. The instability is also due to the combination of a large anion and a small cation.

So, in general if a cation is small and anion is big, lattice enthalpy increases and decomposition is more likely to be favorable. Detail explanation of the mechanism of contact explosion is given in this answer.

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