Assertion (A): The cell potential of the mercury cell remains constant, at about 1.35 V, during its life.
Reason (R): The overall reaction of the mercury cell does not involve any ion in solution whose concentration can change during its life.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
The mercury cell is a primary cell used in low current devices like hearing aids and watches. Its anode is zinc–mercury amalgam, its cathode a paste of HgO and carbon, and its electrolyte a paste of KOH and ZnO.
Anode: \( \displaystyle \ce{Zn(Hg) + 2OH^- -> ZnO(s) + H2O + 2e^-} \)
Cathode: \( \displaystyle \ce{HgO + H2O + 2e^- -> Hg(l) + 2OH^-} \)
Cell reaction: \( \displaystyle \ce{Zn(Hg) + HgO(s) -> ZnO(s) + Hg(l)} \)
Assertion (A): The cell potential of the mercury cell remains constant, at about 1.35 V, during its life. — True. The potential is approximately 1.35 V and does not fall as the cell is used.
Reason (R): The overall reaction of the mercury cell does not involve any ion in solution whose concentration can change during its life. — True. The OH− used at the anode is given back at the cathode, so it cancels out; the cell reaction has only solids and liquid mercury.
Why R explains A?
By the Nernst equation, the potential of a cell depends on the concentrations of the ions taking part in its reaction.
The cell reaction of the mercury cell has no ion in solution, so there is nothing whose concentration can change, and the potential stays at about 1.35 V. R is therefore the cause of A.
NCERT (2026-27 edition):
2.6.1 Primary Batteries – “The overall reaction is represented by Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)”
“The cell potential is approximately 1.35 V and remains constant during its life as the overall reaction does not involve any ion in solution whose concentration can change during its life time.”
Summary – “…Concentration dependence of the potentials of the electrodes and the cells are given by Nernst equation.”