Practice Problem 10

Predict whether the following oxidation-reduction reactions should occur as written:

(a) 2 Ag(s) + S(s) Ag₂S(s)

(b) 2 Ag(s) + Cu²⁺(aq) 2 Ag⁺(aq) + Cu(s)

(c) MnO₄^-(aq) + 3 Fe²⁺(aq) + 2 H₂O(l) MnO₂(s) + 3 Fe³⁺(aq) + 4 OH^-(aq)

(d) MnO₄^-(aq) + 5 Fe²⁺(aq)+ 8 H⁺(aq) Mn²⁺(aq) + 5 Fe³⁺(aq) + 4 H₂O(l)

Answer

(a) No.  The S^2- ion is a better reducing agent than Ag and the Ag⁺ ion is a better oxidizing agent than S.  Silver doesn't tarnish because it reduces sulfur; it tarnishes because it reacts with sulfur compounds in the presence of oxygen and the oxygen is reduced.

(b) No.  Cu is a better reducing agent than Ag and the Ag⁺ ion is a better oxidizing agent than Cu²⁺ ions.

(c) No.  MnO₄^- in base is not a strong enough oxidizing agent to oxidize Fe²⁺ to Fe³⁺.

(d) Yes.  MnO₄^- in acid is a strong enough oxidizing agent to oxidize Fe²⁺ to Fe³⁺.