ELECTROCHEMICAL PROCESSES
The standard electrode potentials are very important and we can extract a lot of useful information from them. If the standard electrode potential of an electrode is greater than zero then its reduced form is more stable compared to hydrogen gas. Similarly, if the standard electrode potential is negative then hydrogen gas is more stable than the reduced form of the species. Electrochemical cells are extensively used for determining the pH of solutions, solubility product, equilibrium constant, thermodynamic properties and for potentiometric titrations. The potential of individual half-cell cannot be measured. We can measure only the difference between the two half-cell potentials that gives the emf of the cell. If we arbitrarily choose the potential of one electrode (half-cell) then that of the other can be determined with respect to this. According to convention, a half-cell called the Standard Hydrogen Electrode (SHE) is assigned a zero potential at all temperatures corresponding to the reaction \( \mathrm{H^{+}_{(aq)}+e^{-}\rightarrow \frac{1}{2}H_2(g)} \)
A galvanic cell has electrical potential 1.1V, on applying external potential greater than 1.1 V the cell will
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → converts to an electrolytic cell.
Consider the Daniell cell, whose standard emf is 1.1 V:
\( \displaystyle \ce{Zn(s) + Cu^2+(aq) -> Zn^2+(aq) + Cu(s)}\quad E^{\circ}_{\text{cell}}=1.1\ \text{V} \)
If an opposing external potential greater than 1.1 V is applied, the current flows in the opposite direction and the reaction is reversed: zinc is deposited at the zinc electrode and copper dissolves at the copper electrode.
\( \displaystyle \text{Zn electrode (now the cathode): }\ce{Zn^2+(aq) + 2e^- -> Zn(s)} \)
\( \displaystyle \text{Cu electrode (now the anode): }\ce{Cu(s) -> Cu^2+(aq) + 2e^-} \)
\( \displaystyle \text{Cell reaction: }\ce{Zn^2+(aq) + Cu(s) -> Zn(s) + Cu^2+(aq)} \)
Now electrical energy from outside drives a non-spontaneous reaction. So the cell converts to an electrolytic cell.
Why the other options are incorrect
Option (1) stops working as no current flows through it — Incorrect. This happens only when the external potential is equal to 1.1 V.
Option (2) works longer as an electrochemical cell — Incorrect. Above 1.1 V the cell no longer works as a galvanic cell at all; its reaction is reversed.
Option (4) carry out spontaneous reaction at double rate — Incorrect. The original spontaneous reaction stops; the reaction that now runs is non-spontaneous and is driven by the external source.
NCERT (2026-27 edition):
Electrochemical Cells – …
