ScienceGeek
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Burning Magnesium in crucible + water = Ammonia gas
Hi guys.
At school today I demonstrated the dangers of trying to extinguish a Magnesium fire with water (safety precautions were taken).
Magnesium filings were placed in a crucible and ignited with magnesium ribbon. After the Magnesium was left to burn for a little while, drops by drops
of water was added.
After quite a lot of water was added, the burning stopped and the crucible was filled with bubbling water (boiling and reacting).
Suddenly, strong ammonia odors was smelt, and confirmed by HCl.
Can someone please tell me what happened, as there is no Nitrogen in any reactant?
My personal theory is that the Magnesium burns hot enough to make some sort of compund with atmospheric Nitrogen, being hydrolysed by the addition of
water.
Any suggestions are appreciated 
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-jeffB
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Magnesium burns quite happily in pure nitrogen. In air, it burns to a mix of oxide and nitride.
Try burning it in CO2. It grabs the oxygen, and produces heavy carbon soot.
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JohnWW
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That ammonia must have been produced by reaction with the N2 in air. As well as in O2 and CO2, Mg (and other alkali and alkaline earth metals,
aluminium, and lanthanides) burns in N2 as well, producing the ionic nitride,, Mg3N2. This quickly hydrolyses on contact with water to produce firstly
amide anion, NH2-, then NH3. However, because of the large amount of energy required to firstly reduce Mg (or Ca etc) to the metal by electrolysis of
molten salts, this is a rather costly method of producing NH3, e.g. as salts for use in fertilizers, compared to the Born-Haber process.
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ScienceGeek
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So my theory was right, then. Quite some reductive power...
Very nice to get it confirmed, thank you guys!
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