Question

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(?).

Answer 1

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.