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Author: Subject: Explosives and Entropy
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[*] posted on 21-5-2013 at 16:15
Explosives and Entropy

Hi everyone. I've been wondering about something silly lately that maybe someone can answer for me...

Why does modern explosives research move toward compounds with more nitrogen atoms?

If one examines the heat of formation of compounds, we know that species containing only one type of atom have no enthalpy of formation, N2 for example. In terms of pressure generated, nitrogen has a little over half the molar mass of carbon dioxide; however, the huge amount of energy released when CO2 is formed should easily make CO2 the preferential reaction product in terms of pressure. Is this because CO2 is more easily liquefied/solidified under high pressure, and it's the difference in vapor pressure that makes explosives containing N2 more brisant rather than outright energetic? What about explosives designed to form H2 instead? I would expect hydrogen to have a much higher vapor pressure, and its low density would only serve to increase pressure.

So why is there so much hype over maximizing nitrogen? Theoretically, wouldn't an all-nitrogen compound only absorb energy on its solid-gas conversion, except for maybe bond strain or something?

Clearly I am missing something because these compounds are thought to be extremely energetic. Can anyone point me in the right direction?



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[*] posted on 21-5-2013 at 16:24

nitrogen in compound, upon combinaison with another nitrogen generate a very good deal of energy because of triple bond formation.

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[*] posted on 22-5-2013 at 08:41

Quote: Originally posted by DougTheMapper  

If one examines the heat of formation of compounds, we know that species containing only one type of atom have no enthalpy of formation, N2 for example.

Here's your mistake. N2(g) has an enthalpy of formation of zero, because N2(g) is the standard state of nitrogen (and enthalpy of formation is defined as the molar enthalpy change for the formation of a substance from the elements in their standard states. The enthalpy change for N2(g) sitting around and staying as itself is obviously zero by definition). Other substances that contain only nitrogen do not have formation enthalpies of zero- N(g) has an enthalpy of formation of 472.7 kJ/mol (according to the appendix of Petrucci's General Chemistry). N2(l) and N2(s) will also have non-zero formation enthalpies (differing from zero by the enthalpies of vaporization and sublimation).

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[*] posted on 22-5-2013 at 12:32

Energy is only part of the story, as you found out. Burning a mole of gasoline yields more energy than burning a mole of TNT.

The energy has to be released quickly. In chemical terms: you want the decomposition of explosives to have a low activation energy. Explosives tend to have one or more bonds that are easily broken upon heating or when subjected to mechanical shock.

N-N and N-O bonds have such a low energy of dissociation.

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[*] posted on 22-5-2013 at 12:36

The triple bond of the N2 releases a great quantity of energy but in a very fast way, which is what is wanted in explosives. Nitrogen triiodide, sodium azide, silver fulminate, TNT, nitroglycerin, etc. all contain nitrogen because it forms N2, and that change is very rapid and "fierce".

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[*] posted on 22-5-2013 at 12:38

maybe off topic but a couple years ago my head researcher told me that there is a ratio of
N+Cl+O : C

That above 1.5 in a given compound usually means its explosive. While something like 2 is dangerous, and something like 3 is don't touch/don't synthesize it.

Is this just a rule of thumb, or is there a way to actually calculate such a ratio for "explosiveness"?

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[*] posted on 23-5-2013 at 18:44

The interest in high nitrogen content compounds is because they are
highly endothermic and have comparatively greater density than high
carbon content based compounds. Many are unreactive , reasonably
stable and relatively insensitive materials. The end of the line has been
attained with Nitro carbons and they have been found wanting in these
desired properties. Hydrogen is effective as a detonation product but
requires combustion of elemental Aluminum or Boron to be formed
uncombined with oxygen. The products of detonation are ceramic
powders that generate higher energy and temperature but are not gas
so the performance is overall quite less and disappointing in every
respect except only if high blast effect is desired.

There is computer simulation of prospective compounds , but selection
of candidates remains an art directed by intuition founded on experience.
I don't know that there is a general method for establishing the explosive
character of an arbitrarily chosen compound. The literature most relevant
in this regard cautions safety for the handling of uninvestigated materials
of a known type or family such as peroxides. The known explosophores
( functional groups characteristically exhibiting explosivness ) are more prone
to explosion the less negative the oxygen balance becomes. Compounds
that are highly endothermic generally exhibit more pronounced explosive
character with increasing atomic weight. The nitrogen halides are an example.

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[*] posted on 24-5-2013 at 05:57

An interesting compound that is explosive and doesn't contain nitrogen is copper acetylide. It forms with copper and ethyne under pressure. It explodes violently and only produces solids : copper powder and carbon powder. This reaction has destroyed a few ethyne producing factories in the past.
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