Why does thiosulfate oxidize to sulfate in an acidic medium?

1 Answer
anor277
Feb 9, 2017

Presumably because oxidation to sulfate (S(VI+)) is the thermodynamic product under these conditions.

Explanation:

Sodium thiosulfate is known to undergo disproportionation in acidified aqueous solution:

"Oxidation:"
S_2O_3^(2-) + H_2O(l) rarr 2SO_2(g)+2H^(+) + 4e^-

"Reduction:"
S_2O_3^(2-) +6H^(+) + 4e^(-)rarr 2S(s)darr+3H_2O

"Overall:"
2S_2O_3^(2-) + 4H^(+) rarr 2SO_2(g) + 2S(s) + 2H_2O

Now this is a balanced chemical equation, but the equation should conform to reality, not vice versa. Under these conditions, I presume, that oxidation to "sulfate ion", would be the preferred outcome, i.e.

"Oxidation:"
S_2O_3^(2-) + 5H_2O(l) rarr 2SO_4^(2-)+10H^(+) + 8e^-

Note that I presume that sulphur dioxide (S(IV+)) is fully oxidized to S(VI+) under the given conditions.

"Overall:"
3S_2O_3^(2-) + 4H^(+) rarr 2SO_4^(2-) + 2S(s) + 2H_2O

So I would presume that the answer your question is that under these conditions oxidation of sulfur to sulfate is the thermodynamic outcome.

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