Mercury cells as “button” batteries are very common and consist of HgO, ZnO, and
ID: 957348 • Letter: M
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) eo = 0.0977 V
Zn(OH)42-(aq) + 2e = Zn(s) + 4OH-(aq) eo = -1.285 V
Zn(OH)42-(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.
Explanation / Answer
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