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Justonium

Why did my ammonium nitrate go boom?

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Yes, I know about the danger of a thermal runaway with ammonium nitrate, and I was heating only .5g of the stuff.

 

OK, I was heating some ammonium nitrate in a test tube over a Bunsen burner, and it was decomposing pretty vigorously, until all of the sudden, I heard a pop, and then a bunch of crackling inside of the test tube. When it popped, there was a flash of yellow light. What happened? This reaction looked remarkably like what happened when somebody on Youtube dropped a gummy bear into molten potassium perchlorate, but there was nothing else in the test tube to react with my ammonium nitrate. The only explanation I can possibly think of is that a spider fell into my test tube from the ceiling, and what is the probability of THAT? Yes the ammonium nitrate was very pure. If there is no other explanation, then I guess I have to say, that was one unlucky and very ill fortuned spider.

 

I think I figured out what happened. I repeated this only I didn't take the test tube off the flame when it started going crazy, and maintained a yellow crackling glow above the ammonium nitrate. I believe that the high temperature I created allowed for a reaction between N2O and air, and I would never have thought of this, but I noticed a brown gas forming in the test tube, and smelled NO2. So N2O must react with oxygen! That is expected, but to go all the way to forming NO2? Wow. You'd think it would just make O2 and N2, and maybe it does, but some NO2 was formed as well. I guess the high temperature reaction between N2O and O2 promotes the formation of NO2, which is more stable at higher temperatures.

 

OK, I now realize that oxygen from the air was not a major factor in this experiment, because gas was being pushed OUT of the test tube, and the crackling yellow pops were coming from deeper within the tube, close to the molten ammonium nitrate. So I really don't know what happened after all. NO2 was definitely made though, because I just know what that stuff looks and smells like. I mean I know the only possible result is that instead of ultimately getting just water and laughing gas, I've made NO2, and O2 as well. I read something about NO being formed unintentionally in laughing gas plants, but I don't know exactly what is going on. NO makes sense to react and then make NO2, but why would NO form if its enthalpy is so high compared to N2O?

Edited by Justonium
updating

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I sincerely doubt that 0.5g in a test tube went "boom"!

AN has a critical diameter of at least 10cms even then, with the best of intentions DDT would be a low order detonation!

 

you`de likely got impurities in there such as dust that was burning much like as you mentioned the jelly bean in chlorate, AN is a fairly good oxidiser when hot like that.

 

it Must be mentioned however that heating AN to those sort of temps is just plain dumb and won`t yield a good result for N2O production, you`v simply wasted some good chemical.

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It didn't all suddenly decompose spontaneously, there was a vigorous reaction taking place just above the ammonium nitrate, indicating a completely gaseous reaction.

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at those temps you`ll get all sort of side rxns taking place, as well as those provided by impurities.

the temp gradients available in a test tube will make for all manner of "ideals" that will support/favor certain rxns.

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you mentioned you could smell the NO2, which suggests that either you weren't using a fume hood or your fume hood isn't good enough, either way, not smart.

 

Don't mess with that stuff, it can easily lead to pulmonary oedema.

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i've read in a book (unified approach to inorganic chemistry by w potterfield to be exact) that although ammonium nitrate is a powerful explosive (if i remember correctly united states gives granuals of it coated with wax like substance to farmers so that terrorists cant use it) but its very hard to detonate hence its unlikely that in the test tube it was kind of explosion. rather it should be rapid decomposition. nitrates are first choice as explosives as after reaction they form gases loke N2, O2, H2O which are very stable hence highly exothermic. so cant there be formation of N2O in small quantities along with N2 etc which went undetected? hence the small pop...

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i've read in a book (unified approach to inorganic chemistry by w potterfield to be exact) that although ammonium nitrate is a powerful explosive (if i remember correctly united states gives granuals of it coated with wax like substance to farmers so that terrorists cant use it) but its very hard to detonate hence its unlikely that in the test tube it was kind of explosion. rather it should be rapid decomposition. nitrates are first choice as explosives as after reaction they form gases loke N2, O2, H2O which are very stable hence highly exothermic. so cant there be formation of N2O in small quantities along with N2 etc which went undetected? hence the small pop...

 

Deeeelicious misonformation. Ammonium nitrate is a crappy explosive. It is extremely hard to detonate pure, and even mixtures like ANFO need a large initiating charge. Some fertilizers come with coatings on the prills, though this is usually to aid wetting (as in sulfur, which normally repels water) or keep the pellets intact during transport. Waxy coatings are nothing more than a fuel if you were to try to detonate the stuff and would probably make it work slightly better.

 

Also, ammonium nitrate prills intended for ANFO blasting (still used for mining operations) are not as compact as fertilizer prills (as dense as possible for cheaper shipping). They are slightly porous, which allows them to collapse as the detonation front propagates through the mixture. Fertilizer prills are harder to detonate because they are solid.

 

When heated to the point of decomposition (and especially above that, as would be reached in a bunsen flame), it is much more sensitive to detonation. Using careful temperature control, N2O production is still done industrially by the same process. The sideproducts NO and NO2 are then thoroughly scrubbed from the mixture. I suspect that these would be formed in much higher proportions the hotter the reaction was. Wikipedia lists phosphates as catalysts for the formation of purer N2O at lower temperatures.

Edited by UC

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There seem to be any number of ideas about am. nitrate.

The stuff is slightly hygroscopic. It's a useful (and fairly cheap) nitrogen source so it gts used as a fertiliser.

 

Unfortunately, as a powder, it gets damp then forms into solid lumps that are difficult to handle.

So someone came up with the idea of coating prilled AN with wax. The wax stops the grains sticking together.

Even more unfortunately this was found to accidentally generate a fairly good explosive (albeit, one that's quite hard to detonate). Further refinements of this gave rise to the low density prill based ANFO used as an explosive.

 

Since the decomposition of AN is exothermic, it's easy to see how the reaction could get out of control.

In particyular, the reaction is catalysed by impurities so, as the AN decomposes and leaves the tube (as H2O/N2O) any impurities left behind get more amd more concentrated. This would increase the effective reaction rate.

That raises the temperature.

I guess you can all see the punchline now.

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i dont think ammonium nitrate is a crappy explosive. its was used in oklahoma city bombings, and and that waxy coatings are indeed to prevent caking of AN. But while googling it i found this link

http://www.worldofcoalash.org/2009/ashpdf/a078-taulbee2009.pdf

here they are using by products of coal ash to prevent explosion (thats what i have understood from it:embarass:)

also on wikipedia its written :"Ammonium nitrate decomposes into gases including oxygen when heated (non-explosive reaction); however, ammonium nitrate can be induced to decompose explosively by detonation."

 

so dont u think "pop" could have been because of that decomposition and oxygen might have to play a role in it??

 

also i read about the coating many years ago in newspaper there was also mentioned that those granuals can be crushed using the coffe bean grounding machines and then can be used as an explosive.

Edited by amit

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i dont think ammonium nitrate is a crappy explosive. its was used in oklahoma city bombings.

 

No, it was ANFO that was used in the Oklahoma city bombing. By itself, ammonium nitrate is a horrible explosive. Even with fuel oil added to improve the oxygen balance, it still needs a very large initiator charge. ANFO is not something you'd want to use on the battlefield, but it is good for blasting because it's cheap and effective. Good explosives by comparison are glyceryl trinitrate, trinitrotoluene, pentaerythritol tetranitrate, etc.

 

The newspaper is generally not a reliable source, especially for anything science related.

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of all the things that would explode in a lab, this is one of the last on my list, it takes a massive shock wave to detonate, takes quite a bit on a large pile/bar/sphere to detonate, and heat isn't a major detonation factor as far as i know

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