I would like to ask about the dissolution of HgS in brom-hydrochloric acid. The products are (HgBr4)2- and SO42- But I don't know what is the actual mechanism that would answer why brom-HCl is neaded if Cl- is apparently not taking part in the redox reaction, and the H+ (acidity) provided by the reaction seems to make the presence of HCl also unnecessary. My idea for the reaction is:
1 Hg(II)S + 4 Br2 + 4 H2O = 1 [Hg(II)Br4]2- + 4 Br- + 1 SO42- + 8 H+
In general, why is brom-HCl superior to Bromine as an oxydative agent? The only reason I could make up was either as producing H+ for acidity or Cl- for complex-forming with the product to "push" the reaction toward the products-side of the equation.
My ideas for the above reaction are:
1) HCl "pushes" out S2- from the HgS solution because HCl is stronger acid, and the removed S2- is protonated to H2S that will immediately be oxydized into sulfate. I think the protonation of sulfide of the very water-insoluble HgS will promote its dissociation and the consequent redox reaction.
2) HCl forms BrCl interhalogenide with the Br2 which will be a more potent oxydative agent than Br2 would be. If this is the case, I would be happy to know the mechanism how BrCl reacts.
I would very much appreciate an answer.