Write the reaction mechanism for the following process: #Na_2S_2O_3+2HCl=2NaCl+S+SO_2+H_2O# Justify , based on the mechanism of the reaction , the role of HCl as a catalyst(?).

1 Answer
anor277
Jan 14, 2018

This is a disproportionation reaction....and it is an excellent 2nd year practical experiment....

Explanation:

I like to think of #S_2O_3^(2-)# as the analogue of #SO_4^(2-)#, where one sulfide ion has replaced an oxide ion, and assumed its oxidation state, i.e. we got #S^(VI+)# and #S^(-II)#, #S_"average oxidation state"=(VI-II)/2=+II#...

Anyway one sulfur in thiosulfate is oxidized to sulfate....and one sulfur is REDUCED to elemental sulfur.

#S_2O_3^(2-) +5H_2O(l) rarr 2SO_4^(2-)+8e^(-)+10H^+# #(i)#

#S_2O_3^(2-) +6H^+ + 4e^(-)rarr2S +3H_2O(l)# #(ii)#

And we takes #(i)+2xx(ii)#....

#3S_2O_3^(2-) +12H^+ +5H_2O(l) + 8e^(-)rarr4S +6H_2O(l)+2SO_4^(2-)+8e^(-)+10H^+#

...to give...

#3S_2O_3^(2-) +2H^+ rarr4S +2SO_4^(2-)+H_2O(l)#

Which I think is balanced with respect to mass and charge... The hydronium ion is used here as a reagent...in the form of #HCl#. And what would we see in this reaction? We would see a precipitate of white, finely divided sulfur deposit in the reaction vessel.

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