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What are the products of the reaction: $$\ce{K + SO2->}$$ Here, $\ce{S}$ is in $+4$ oxidation state. Hence, $\ce{SO2}$ has both oxidizing and reducing character. $\ce{K}$ is a stronger reducing agent and hence, it will reduce $\ce{SO2}$.

So, $$\ce{K + SO2->KO2 + S}$$ is a possibility.

But the answer given is $$\ce{K + SO2->K2SO3 + K2S2O3}$$

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$\ce{KO2}$ is definitely out of question. True, it does form when $\ce{K}$ reacts with $\ce{O2}$, but that's another story. It is a peroxide (in a broader sense), so it contains oxygen in oxidation states other than -2. $\ce{SO2}$ contains oxygen in -2. Who is going to oxidize oxygen?

Turning down peroxides, we resort to the more realistic option of $\ce{K2O}$. Now, that's a basic oxide, and $\ce{SO2}$ is an acidic oxide. Surely some chemistry is going to happen between them.

As to where the reduced sulfur would go, it is not quite clear and depends on many things. With excess of potassium one might expect ending up with $\ce{K2O}$ and $\ce{K2S}$. Obviously that's not our case. Then we must consider a reaction between $\ce S$ and $\ce{K2SO3}$. Such reaction is known (though probably under different conditions than we have here) and indeed produces thiosulfate.

That being said, partially reduced compounds of sulfur are numerous and I see no reason to favor one above the other without some deep investigation. Have you heard about dithionites, for example? They are known to appear when reducing $\ce{SO2}$ with certain metals; why not here?

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  • $\begingroup$ I am hearing it for the first time. I have learnt reactions of sulphur dioxide with metals iron and magnesium. In both case, oxide and sulphide of the metal is formed just like you have specified. $\endgroup$ – Aditya Dev Oct 26 '15 at 15:31
  • $\begingroup$ It is much more complicated than that. $\ce{Zn}$ in water solutions gives dithionite. Without water (and with some heating), it probably would end up in oxide and sulphide as well. $\endgroup$ – Ivan Neretin Oct 26 '15 at 15:37
  • $\begingroup$ Where can I read about this? (I am using JD Lee and Shriver,Atkins book for inorganic chemistry) $\endgroup$ – Aditya Dev Oct 26 '15 at 15:38
  • $\begingroup$ Are you writing "peroxide (in a broader sense)" instead of superoxide? $\endgroup$ – Mithoron Oct 26 '15 at 17:39
  • $\begingroup$ 2 Aditya Dev: I'm no expert when it comes to literature. I learned this all many years ago, in a different language. 2 Mithoron: yes, exactly. $\endgroup$ – Ivan Neretin Oct 26 '15 at 18:00

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