We know that the maximum possible oxidation state of sulphur is $+6$. Whereas the oxidation states of sulphur in sodium metabisulphite are $+3$ and $+5$. Can't these oxidation states reduces to a lower oxidation state? If it is possible can you give me a reaction for better clarity. Because my book is saying that it cannot act as an oxidising agent. Is this true?
Thanks in advance
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$\begingroup$ The sulfur could be reduced, actually, as in thermal decomposition yielding some elemental sulfur. It's just that normal laboratory conditions do not favor such reactions. $\endgroup$– Oscar LanziCommented Oct 14, 2021 at 11:07
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$\begingroup$ Okay thats fine! Can't we use sodium metabisulphite as an oxidising agent in labouratory like potassium permanganate or potassium dichromate. $\endgroup$– InfiniteCommented Oct 14, 2021 at 11:14
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6$\begingroup$ A brick is a poor substitute for paper. I mean, you kinda can write on it, but that's just not very useful. Same thing here. Metabisulphite can act as an oxidizing agent, but it is nowhere near permanganate or dichromate. $\endgroup$– Ivan NeretinCommented Oct 14, 2021 at 11:27
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1 Answer
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Metabisulphite can slowly oxidize the zinc powder, according to the following reaction : $$\ce{2 Na2S2O5 + Zn -> ZnS2O4 + 2 Na2SO3}$$ This produces the zinc dithionite $\ce{ZnS2O4}$, where the two sulfur atoms have the oxidation number +$3$.
Ref.: R. E. Morcom, Inorganic Chemistry, Teach Yourself Books, St. Paul's House, London EC4P 4AH, Second Edition 1973, p. 163.
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1$\begingroup$ Says who? Answers like this are more likely to draw upvotes if a reference is given. $\endgroup$ Commented Oct 15, 2021 at 1:51
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4$\begingroup$ @OscarLanzi I agree about references. But OTOH all chemists ( applies to common life as well ) with some life experience collected a lot of knowledge based on studying and empirical experience without remembering or being aware of the source. Such source may still exist, but may be difficult to impossible to track it down. $\endgroup$– PoutnikCommented Oct 15, 2021 at 7:08
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$\begingroup$ (+1) Or just replace zinc with cesium: if the cesium gets oxidized, and it will, then we know the oxidizing agent, if not the equation! $\endgroup$– Ed VCommented Oct 15, 2021 at 12:01