[learner_greg]

I would like to ask about the dissolution of HgS in brom-hydrochloric acid. The products are (HgBr₄)^2- and SO₄^2- 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 SO₄^2- + 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 S^2- from the HgS solution because HCl is stronger acid, and the removed S^2- 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.

[Borek]

Protonation of S^2- would be my first guess as well.

Doesn't mean it is right.