woelen
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Violent reaction between Mg and P
I did an experiment by mixing red P and Mg (appr. equal amounts) and then heating this mix (assuring that it does not burn in air).
I was really surprised about the violence of the reaction. The mix is not very easily ignited, but once it is going, it is going very violently, with
a lot of fire and a little amount of smoke (looks like white P, contaminated with red P).
When I add water to the cooled down remaining grey/brown mix, then a vigorous reaction occurs, with production of a lot of gas. I assume this is PH3.
When the gas is ignited, then it burns with a grey flame, and a lot of white smoke is produced.
Now I am wondering, is this what you would expect, that the reaction is so violent? Both Mg and P are quite strong reductors, I did not expect the
reaction between to be that violent.
How pure is the PH3, obtained from that 'burnt' mix and water? Is there a risk of formation of P2H4? I'm afraid of that, because it is said to ignite
in contact with air, even at room temperature. In my experiment I did not have spontaneous ignition, but that could be good luck, instead of reliably
repeatable behavior.
Are there any other people with experience of making phosphides? I just want to experiment a little with PH3 and I regard this as a suitable route to
making it. However, I do not like the very violent reaction between Mg and P. Is there a way to moderate the reaction somewhat?
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guy
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I looked up some properties of PH3 in an Inorganic chemistry book.
When burned it produces H3PO4.
It reduces Ag+ and Cu2+ to their respective metals.
It decomposes at high temperatures to P and H2 (not sure how high).
I think its possible to do this reaction on a larger scale if you put the reactants in some metal can and close the lid. Surely it would hold
assuming its not as violent as thermite.
As to the feasibilty: maybe for you since red phosphorus is obtainable but its not in the US. I was wondering, since phosphates are common, if
magnesium would reduce that to a phosphide. Or even aluminum mixed with a hydroxide and phospate. It's possible thermodynamically.
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garage chemist
chemical wizard
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Yes, magnesium reduces phosphates to phosphides. Thats exactly what I am proposing all the time as a route to elemental Phophorus! PH3 decomposes into
the elements above 350°C.
The PH3 from the reaction of magnesium phosphide with water can spontaneously inflame in air due to P2H4 content. If it didnt do that, then it was
pure luck.
I have made PH3 once by heating white phosphorus with NaOH solution, and the PH3 bubbles always caught fire on bursting.
The PH3 from this method seems to contain the most P2H4 and always catches fire in air.
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guy
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Phosphides of Group II seem to be more prone to P2H4 formation since there is more P-P interaction. Aluminum phosphide maybe less prone to have P2H4
formation since the it is a 1:1 ratio of Al to P.
Wikipedia-
"Calcium phosphide (nominally Ca3P2) produces more P2H4 than other phosphides because of the preponderance of P-P bonds in the starting material."
[Edited on 2/14/2007 by guy]
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12AX7
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What's the melting point on Mg3P2? Probably a pretty good ceramic, if not for hydrolysis.
Tim
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