Question

Mercury cells as “button” batteries are very common and consist of HgO, ZnO, and Zn(OH)₂ in a KOH paste. Useful half-cell reactions:

            HgO(s) + H₂O(l) + 2e = Hg(l) + 2OH^-(aq)              e^o = 0.0977 V

            Zn(OH)₄^2-(aq) + 2e = Zn(s) + 4OH^-(aq)                 e^o = -1.285 V

            Zn(OH)₄^2-(aq) = ZnO(s) + H₂O(l) + 2OH^-(aq)

A)What is the redox reaction in such batteries.

B)What is the standard cell potential?

C)Such batteries have a + sign marked on them. Which half-reaction is occurring at the electrode so marked?

D)Draw a schematic of a mercury button battery showing the materials making up the anode and cathode.

Answer 1

A)

The reduction half-cell reaction: HgO(s) + H₂O(l) + 2e = Hg(l) + 2OH^-(aq)

The oxidtion half-cell reaction: Zn + 2OH⁻ = ZnO + H₂O + 2e

The redox reaction : reduction reaction + oxidation reaction

Zn + HgO = ZnO + Hg

B)

Standard cell potential = standard reduction potential of reducion - standard reduction potential of oxidation

                                    =(0.0977 V)-(-1.285 V)

                                    = 1.38 V