Why does thiosulfate oxidize to sulfate in an acidic medium?

1 Answer
Feb 9, 2017

Answer:

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.