A Daniell Cell is an electrochemical cell that converts chemical energy into electrical energy through a spontaneous redox reaction. It is also referred as a galvanic or voltaic cell. It consists of two half-cells that are connected by a salt bridge to maintain electrical neutrality through the flow of ions. The first half-cell consists of a Zn rod dipped in a ZnSO4 solution, while the other half-cell consists of a Cu rod immersed in a CuSO4 solution. The two rods are connected by a wire to complete the circuit through which the electron flows, and a voltmeter may be connected in the circuit to measure the electrode potential. If the circuit in a Daniell cell represented by the cell notation, Zn | Zn2+ || Cu2+ | Cu, is closed, then it is observed that as time passes the concentration of Zn2+ keeps on increasing, while the concentration of Cu2+ keeps on decreasing. At the same time, the voltage of the cell as read on the voltmeter keeps on decreasing. After some time, the voltmeter shows zero reading.
A zero reading in the voltmeter indicates
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The equilibrium has been attained, and no change in the concentration of Cu2+ and Zn2+ ions is observed.
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 Ecell keeps decreasing. It reaches zero when the ratio \( \ce{[Zn^{2+}]} \)/\( \ce{[Cu^{2+}]} \) becomes equal to the equilibrium constant Kc. 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 entire solid Zn has been consumed and converted to Zn2+ ions. — Incorrect. The reading falls to zero because the ion ratio has reached Kc, not because the zinc rod has been used up; zinc metal can still be present at equilibrium.
Option (2) All the Cu2+ ions have been converted to solid Cu. — Incorrect. At equilibrium some \( \ce{Cu^{2+}} \) ions are still left in the solution; the reaction stops at the equilibrium ratio, not at complete conversion.
Option (4) Ecell has attained a negative value. — Incorrect. The cell potential falls to zero and stays there at equilibrium. It does not turn negative on its own; the reaction runs in the reverse direction only when an external opposing potential greater than 1.1 V is applied.
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:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 07-06 S2 29-05 S2 | – | – | – | – |