What caused this explosion?

[Justonium]

I was synthesizing magnesium nitride in a small clay crucible. I lit a small chunk of magnesium with a blowtorch (causing it to have a lot of carbon on it as it reduced the carbon in the carbon dioxide from the blowtorch flame). After it was burning bright, I dropped it into the crucible and put the lid on. The limited oxygen supply inside the crucible is supposed to cause the magnesium to start burning in the nitrogen, creating magnesium nitride. After a while, thinking the magnesium was spent, I took the lid off the crucible only to hear a pop and have a fireball spit out of it. This indicates that it may have been filled with a flammable gas which could not ignite without oxygen, but was so hot that it spontaneously ignited when exposed to air, but I cannot think of how such a gas could be created. I put the lid back on and took it off again 10 seconds later, to the same effect. (Yes, I know I really screwed up my yield of nitride, but I was curious!) Both times, when I took the lid off and the fireball was released, the magnesium exposed was just glowing red hot, but quickly began to glow brighter in the air. Later, after all of the remaining magnesium burned, I tested the remains chemically and they contained magnesium nitride. There wasn't much, but it was enough to make an area in the middle of the magnesium remains have a slightly yellow look as well. OK, so those are all my observations, now can anybody tell me why there were small explosions every time I let air into the red hot crucible?

 

I do have one guess, but I still am not sure. The carbon mixed with the burning magnesium could also have burned in the hot crucible, and as the oxygen supply was limited, could have formed carbon monoxide, a flammable gas. But it seems like the magnesium would reduce the carbon back out of the carbon monoxide. The crucible was closed for a while, so I wouldn't think much carbon monoxide could remain in the crucible, being the favorable reactant over nitrogen.

 

PS: Don't comment about my safety, there are many precautions I took which I don't feel like wasting time describing.

Edited September 25, 2009 by Justonium
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[Theophrastus]

To be honest I have no clue exactly what occured, but I think you really ought to use a pure nitrogen (or nitrogen + inert gas (ie helium)) environment next time, as I'm pretty sure that the fireballs (pretty neat ) were the result of aerial impurities.

 

My first thought was that magnesium nitride reacts with moisture in the air to produce ammonia gas. While ammonia by nature, is not very flamable in absence of a catalyst, and I'ld be pretty surprised if that were true, as you stated that you had a rather low oxygen supply. Of course, given the heat, as you opened the lid, it is possible that the still- hot ammonia might burn. Hot ammonia gas, is also known to burn when reacting with magnesium metal: another possibility.

 

[ce] Mg3N2 + 6H2O -> 3Mg(OH)2 + 2NH3 [/ce]

 

[ce] 4NH3 + 3O2 -> N2 + 6H2O [/ce]

 

[ce] 2NH3 + 3Mg -> Mg3N2 + 3H2 [/ce]

 

In the third reaction, in absence of oxygen, the hydrogen might later burn when exposed to air (via the opening of the lid).

 

I also thought about the results of small quantities of hydrazine (which as you know burns very violently), given the production of ammonia, however, ammonia is usually converted to hydrazine in presence of a strong oxidiser, and thus would be very unfavourable or nonexistant. (If ammonia were the only reactant, and temperature the only influence)

 

[ce] 2NH3 + H2O2 -> H2N-NH2 + 2H2O [/ce]

 

I'm not sure how favourable such a reaction would be, as magnesium hydride decomposes (or reacts exothermically, in presence of water or oxygen) at 300 degrees celsius, to hydrogen and magnesium. (given a pressure of 1 bar) I also thought that under certain conditions, magnesium may only partially reduce the ammonia, to produce hydrazine. (however this is purely hypothetical)

 

Like I said I'm not sure, but what I've described above, may be a possibility.

Edited September 26, 2009 by Theophrastus

[UC]

At temperatures where magnesium is molten or burning, it's not unreasonable to expect hot, magnesium vapor to fill the crucible rapidly. I suspect that with the sudden influx of oxygen, it burned very rapidly.

[Theophrastus]

(sigh) Lo and behold, a far simpler, more obvious, coherent, (not to mention, more likely correct) answer from UC!

[Justonium]

The ammonia theories just aren't possible, as there would not be enough water available to cause any noticeable amount of ammonia or hydrazine or any other substance containing hydrogen. The only reactants that we can consider are carbon, magnesium, oxygen, and nitrogen.

 

It does seem likely that instead of the cause for the fireballs being a flammable gas like carbon monoxide (the only flammable gas I could even consider being created), it was, like UC says, actually magnesium vapor, which would have been hot enough to spontaneously combust when exposed to the air. The magnesium was well past its melting point, so I see no problems with this theory. Because all other theories considered have flaws that make them very hard to believe, and I see nothing left to consider when looking at just carbon, magnesium, oxygen, and nitrogen, I will assume that magnesium vapor was the cause of the explosions. Thanks for giving me you're time, guys.

 

LOL, by the way, the spell check on here doesn't recognize the word "combust."