subsecret
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H2SO4, NaOH, and S - Unknown Product
Recently, I decided to prepare some very, very crude (actually, the amount that was prepared was probably undetectable) sodium sulfide by letting
elemental sulfur react with concentrated aqueous sodium hydroxide. The mixture was left for several days to complete whatever reaction may take place.
After this time, the sulfur settled to the bottom, and the water had taken a slight red-orange tint. A drop of this mixture was placed on a watch
glass, and a drop of Rooto sulfuric acid (~16-18 molar) was added. The reaction took place instantly, creating a quick "psst" sound, and turning the
residue white. The room smelled of hydrogen sulfide.
Later, some of the sulfur and NaOH was dripped into several milliliters of tap water, and then several drops of Rooto sulfuric acid were added. An
off-white precipitate formed, and the test tube became warm.\
I believe that the precipitate was elemental sulfur, produced according to the reaction:
Na2S + 3 H2SO4 -> 2 NaHSO4 + SO2 + S + 2 H2O
Can anyone confirm this? Any advice is appreciated.
Fear is what you get when caution wasn't enough.
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Galinstan
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without any further evidence it would seem a reasnable first guess as the oxidation states of the sulphur all make sense. but i think u can also get
polysulphides from sulphur and NaOH so the reaction is probably more varied and complicated than portrayed above.
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Oscilllator
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Your precipitate was sulfur, but I dont think your reaction was correct, since the reaction of elemental sulfur with NaOH produces sodium
thiosulphate. I posted a thread on this somewhere, but I cant seem to find it. In the thread I established that I produced sodium thiosulphate, so
that's probably what happened here.
Edit: Galinstan is correct, the reaction does produce sulfur polymers which in my case remained in solution, so the reaction mixture was a deep red
slightly viscous fluid
[Edited on 7-4-2014 by Oscilllator]
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blogfast25
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Quote: Originally posted by Oscilllator  | Your precipitate was sulfur, but I dont think your reaction was correct, since the reaction of elemental sulfur with NaOH produces sodium
thiosulphate.
[Edited on 7-4-2014 by Oscilllator] |
Not sure why that would be relevant.
The OP is right about one thing: if sulphide (or polysulphides) is oxidised to sulphur then something else has to be reduced. He's assumed that that
something was sulphate to sulphurous oxide. But the other species that can be reduced in that mix quite easily is
H<sub>3</sub>O<sup>+</sup>, to hydrogen and water. Much more easily reduced than sulphate, in fact...
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woelen
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All of this can be explained quite well.
Sulphur reacts with NaOH in a disproportionation reaction to sulfide and thiosulfate, vaguely similar to the disproportionation of chlorine to
chloride and hypochlorite:
4 S + 6 OH(-) --> 2 S(2-) + S2O3(2-) + 3 H2O
If excess sulphur is used, then this sulphur reacts with sulfide ion to get yellow/orange polysulfides.
When acid is added, then the reverse reaction occurs. H2S2O3 and H2S are formed. H2S2O3 quickly decomposes to H2SO3 and elemental S. H2SO3 in turn
reacts to H2O and SO2, but also reacts with H2S very quickly to form H2O and S. A nice experiment is to dissolve some Na2S in acid. You get a clear
solution of H2S in water. If you add a solution of SO2 in water to this (which can be made from sulfite, dissolved in dilute acid), then you get
elemental sulphur quite quickly.
[Edited on 7-4-14 by woelen]
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