Na2S2O3 + 2HCl → S + SO2 + 2NaCl + H2O

Question

I would really appreciate an detailed explanation of how the thiosulfate in the reaction can act as both a reductant and an oxidiser. Thank you!

Answer 1

This is referred to as a disproportionation reaction where an element in a compound is both oxidized and reduced. This takes place in two separate half reactions. The details of such reactions are in debate, so a detailed explanation is difficult. Suffice it to say that the S atom in S₂O₃^2- is both oxidized (SO2) and reduced (S). The mechanism of the overall reaction would help to explain how this occurs in separate reaction steps, but I'm not familiar with this particular mechanism. Sorry I can't give a more detailed explanation.