When an electrochemical cell’s voltage (E) has dropped to zero it ind…

The correct answer is Option (3) → The cell reaction has reached equilibrium

The potential of the Daniell cell depends on the ion concentrations through the Nernst equation (at 298 K, n = 2):

    \( \displaystyle E_{cell}=E^{\circ}_{cell}-\frac{0.059}{2}\log\frac{[\ce{Zn^2+}]}{[\ce{Cu^2+}]} \)

As the cell works, \( \ce{[Zn^{2+}]} \) rises and \( \ce{[Cu^{2+}]} \) falls, so the log term grows and E_cell keeps decreasing. It reaches zero when the ratio \( \ce{[Zn^{2+}]} \)/\( \ce{[Cu^{2+}]} \) becomes equal to the equilibrium constant K_c. At that point the forward and backward reactions go at the same rate: the reaction is at equilibrium, no net current flows, and the concentrations stop changing.

So a zero reading means that equilibrium has been attained.

Why the other options are incorrect

• Option (1) The concentration of the reactants has increased — Incorrect. The reactant ion \( \ce{Cu^{2+}} \) is used up while the cell works, so [Cu²⁺] decreases.

• Option (2) The concentration of the products has decreased — Incorrect. The product ion \( \ce{Zn^{2+}} \) is formed while the cell works, so [Zn²⁺] increases.

• Option (4) The concentration of ions at both electrodes will start increasing. — Incorrect. At equilibrium there is no net reaction, so the concentrations stay constant.

NCERT (2026-27 edition):
Equilibrium Constant from Nernst Equation

Note: The same/similar question has been asked in different years by NTA as per the details given below: