Question

Which of the following statement is incorrect about mercury cell?

• A. Products of the electrode reactions are separated by thin membrane
• B. Anode reaction is
  $Zn\left(Hg\right)+2O{H}^{-}\to ZnO\left(s\right)+H_{2}O+2e^{-}$
• C. Cell reaction is
  $Zn\left(Hg\right)+HgO\left(s\right)+O{H}^{-}\to ZnO\left(s\right)+Hg\left(l\right)+H_{2}O$
• D. Used in low current devices such as electronic watches

Answer 1

The correct option is C Cell reaction is

$Zn\left(Hg\right)+HgO\left(s\right)+O{H}^{-}\to ZnO\left(s\right)+Hg\left(l\right)+H_{2}O$
In mercury cell,
Zinc – mercury amalgam will act as anode
Paste of $HgO$ and carbon will act as cathode
Paste of $KOH$ and $ZnO$ will be the electrolyte
In mercury cell, there will be a thin polymeric membrane which prevents the mixing of products formed at the electrodes. This membrane is called separator.
Reaction at electrodes:

At anode :
$Zn\left(Hg\right)+2O{H}^{-}\to ZnO\left(s\right)+H_{2}O+2e^{-}$

At cathode:
$HgO+H_{2}O+2e^{-}\to Hg\left(l\right)+2O{H}^{-}$

Overall cell reaction
$Zn\left(Hg\right)+HgO\left(s\right)\to ZnO\left(s\right)+Hg\left(l\right)$

$E_{cell}=1.35V$
The potential of the mercury cell remains constant due to absence of ions in the overall cell reaction.
These cells are generally suitable for low current devices such as electronic watches, hearing aids, microphones, cameras, etc.
Thus, statement (c) is incorrect.