Practice Problem 2:

Classify each of the following as either a metathesis or an oxidation-reduction reaction.  Note that mercury usually exists in one of three oxidation states: mercury metal, Hg₂²⁺ ions, or Hg²⁺ ions.

(a) Hg₂²⁺(aq) + 2 OH^-(aq)  Hg₂O(s) + H₂O(l)

(b) Hg₂²⁺(aq)  + Sn²⁺(aq)  2 Hg(l) + Sn⁴⁺(aq)

(c) Hg₂²⁺(aq) + H₂S(aq)  Hg(l) + HgS(s) + 2 H⁺(aq)

(d) Hg₂CrO₄(s) + 2 OH^-(aq)  Hg₂O(s) + CrO₄^2-(aq) + H₂O(l)

Solution

(a) Metathesis.  Mercury is in the +1 oxidation state in both the Hg₂²⁺ ion and in [Hg₂²⁺][O^2-].

(b) Oxidation-reduction.  Mercury is reduced from the +1 to the 0 oxidation state, while tin is oxidized from +2 to +4.

(c) Oxidation-reduction.  This is a disproportionation reaction (see chapter 10) in which mercury is simultaneously reduced from +1 to 0 and oxidized from +1 to +2.

(d) Metathesis.  Mercury is in the +1 oxidation state in both [Hg₂²⁺][CrO4^2-] and [Hg₂²⁺][O^2-].