Eddygp
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SeO2 and Mg thermite
I have decided to make this experiment, because Mg burns in solid CO2 and some other like SO2. This makes possible reactions with SiO2, TeO2 and SeO2.
My question is: how should I prepare this thermite, and would it be especially exothermic? Will part of the selenium go out as a (toxic) gas?
Thank you.
-Eddygp-
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blogfast25
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Firstly, if you really are going to try this, aluminium would be a better choice than magnesium. But Se has one important drawback for aluminothermic
(or magnesiothermic) reduction of its dioxide: Se’s boiling point is only 685 C! There is every reason to believe that the reaction would proceed
well but that your Se would boil off very quickly as the reaction proceeds.
CAUTION: many of its compounds are very toxic!
Se can prepared by reducing aqueous selenous acid with SO2.
[Edited on 13-11-2012 by blogfast25]
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elementcollector1
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If someone could give me an OTC or cheap source for a selenium compound, I would love to try some of these preps...
Elements Collected:52/87
Latest Acquired: Cl
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UnintentionalChaos
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Quote: Originally posted by blogfast25  | Firstly, if you really are going to try this, aluminium would be a better choice than magnesium. But Se has one important drawback for aluminothermic
(or magnesiothermic) reduction of its dioxide: Se’s boiling point is only 685 C! There is every reason to believe that the reaction would proceed
well but that your Se would boil off very quickly.
Se can prepared by reducing aqueous selenous acid with SO2.
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Not only would it boil off, it would probably burn immediately the end result being oxidation of the Mg, and dispersal of the SeO2 as a smoke into the
surrounding area. Selenium dioxide is easily reduced using aqueous methods.
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kristofvagyok
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| Quote: Originally posted by Eddygp | | My question is: how should I prepare this thermite, and would it be especially exothermic? Will part of the selenium go out as a (toxic) gas?
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I didn't calculate the plus energy of the reaction, but based on that SeO2 is used as an oxidant and reacts well with a lot thing I think that it will
work perfectly.
But do it at a place where you can't get poisoned by Se vapors. E.g.: granny's garden, because the low BP of the Se could cause serious problems.
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Eddygp
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| Quote: Originally posted by kristofvagyok | | Quote: Originally posted by Eddygp | | My question is: how should I prepare this thermite, and would it be especially exothermic? Will part of the selenium go out as a (toxic) gas?
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I didn't calculate the plus energy of the reaction, but based on that SeO2 is used as an oxidant and reacts well with a lot thing I think that it will
work perfectly.
But do it at a place where you can't get poisoned by Se vapors. E.g.: granny's garden, because the low BP of the Se could cause serious problems.
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OK, I just wanted to be sure. By the way, is SeO2 as toxic as selenium or less? And I would like to perform this thermite mostly because of curiosity,
and I am thinking of many other experiments to do with selenium oxide.
Would it be possible to re-distill the selenium vapors?
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Eddygp
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Apparently, SeO2 is more toxic. Curiously, it has a very low sublimation point: around 340ºC. I think the thermite will change SeO2 to a gas... Hmph.
Anyway, would it be possible to re-distill Se (from its gas state) or SeO2 (same, the gas state)?
[Edited on 14-11-2012 by Eddygp]
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blogfast25
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You'd have to capture them first. Unless the vapours are protected from air oxygen they are likely to burn back to SeO2, as UC rightly suggested.
But even if they don't immediately reoxidise: the reaction is likely to run very hot and you'll get a lot of vapour, not so easy to successfully cool
and condense!
Alumino or magnesiothermy simply isn't a great way to prepare selenium...
[Edited on 14-11-2012 by blogfast25]
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Eddygp
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I know, it was just because I thought it would be interesting to make this thermite other people have not made too much.
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blogfast25
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| Quote: Originally posted by Eddygp | | I know, it was just because I thought it would be interesting to make this thermite other people have not made too much. |
My advice? Don't do it: SeO2 is too precious to waste on this.
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Eddygp
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OK. Do you know some really good organic or inorganic experiment with SeO2?
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blogfast25
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Why not attempt to reduce a watery solution of SeO2 ('selenous acid', H2SeO3) with a mild reduction agent like SO2 (e.g. sulphites or metasulphites)
to elemental Se? Or try and synth some selenites (MSeO3) compounds?
[Edited on 15-11-2012 by blogfast25]
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Eddygp
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Good idea. I'll certainly try them.
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zed
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My buddy Dr. Crazyfingers reported to me the results of his incinerating a Selenium containing semiconductor device, in a burning trash barrel. Well,
it produced clouds of red smoke, and he went screaming off in terror.
Selenium gas or smoke, is bad...real bad. If Dr. Crazyfingers was afraid, he probably had good reason to be. Dr. C was a very intrepid guy. I'm
going to check on the Se toxicity.
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woelen
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To me, making a thermite with SeO2 seems like a really stupid thing to do. It is VERY toxic and very expensive. You need a lot of it to get a decent
thermite reaction (thermites don't scale down well to hundreds of milligrams).
A nice experiment with SeO2 is reduction of it by SO2 and observing the influence of the presence of chloride ions on the speed of this reaction.
Another very nice experiment is adding some SeO2 to a highly concentrated and acidified solution of KBr or NaBr. You'll get a beautiful red compound,
probably SeBr4 or some complex ion of this. If you add water to this red solution (e.g. dilute it), then the color disappears again, due to hydrolysis
and formation of HBr and H2SeO3.
Try the same with adding SeO2 to conc. HCl and to a solution containing iodide.
If you add zinc to a solution of SeO2 in some dilute hydrochloric acid, then you see inhibition of the reaction of zinc with the acid. Probably this
is due to formation of either a thin layer of selenide or of elemental selenium. This effect also can be a nice basis for some experiments.
Yet another very nice experiment is to dissolve some SeO2 in water, add a few drops of sulphuric acid as well and perform electrolysis of this
solution. Use a graphite cathode and graphite anode. Watch what happens at the cathode!
As you see, there are enough nice experiments to do with SeO2. With any experiment, be careful. Selenium and especially its compounds are very toxic.
[Edited on 21-11-12 by woelen]
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weiming1998
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Also, reaction of SeO2 with Mg or Al will most likely cause the resulting selenium (if any formed) to oxidize the Mg/Al, forming selenides.
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blogfast25
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| Quote: Originally posted by weiming1998 | | Also, reaction of SeO2 with Mg or Al will most likely cause the resulting selenium (if any formed) to oxidize the Mg/Al, forming selenides.
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Unless these selenides have high Heats of Formation that is in fact unlikely to happen: the equilibria are what they are. Formation of selenides would
be more likely if a great excess of Al or Mg is present.
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Valentine
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I have prepared a lot of compounds from Se (SeCl4, Se2Cl2, SeOCl2, SeO2, even carbon
diselenide, which was the worst of all), and after using these substances in syntheses, I always recovered the precipitated selenium. To prepare pure
Se,I added the Se into hot 65% HNO3 in little portions (it makes a lot of NO2 and heat). After filtering the hot solution, added
some dimethylformamide to reduce the Se(IV). It's a very exothermic, but not violent reaction. I broke a few 2 l Erlenmeyer flasks with this step,
because the temperature of the reaction mixture jumped from 100 °C to 160 °C in a matter of seconds.
Making thermite from SeO2 and Mg to prepare Se is not a good idea, but doing it for fun is a good idea  It's just a bit dangerous (the SeO2 vapors are very toxic, they cause a
very bad headache and nausea, I can tell you that) and smelly (it's like a ton of rotten radish).
If you have SeO2 and you want elemental Se, just add it to acetone and boil it with a reflux condenser for some time. The pure selenium
will deposit as a dark grey powder, you just wash it with some water. If you have formamide or dimethylformamide, then heat the amide above 100°C and
add the SeO2 into the flask in small portions. After heating the amide above 100°C, you can turn the heating off, you can maintain or rise
the temperature with the adding of SeO2.
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