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.
The standard Gibbs energy for the reaction mentioned will be
[Given, F = 96487 C mol−1]
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → −212.3 kJ mol−1
A galvanic cell does electrical work at the cost of the reaction’s Gibbs energy. For n moles of electrons passing through a cell of standard emf E°cell:
\( \displaystyle \Delta_rG^{\circ}=-nFE^{\circ}_{cell} \)
A positive E°cell gives a negative ΔG°, which means the cell reaction is spontaneous.
For the Daniell cell:
\( \displaystyle \ce{Zn(s) + Cu^2+(aq) -> Zn^2+(aq) + Cu(s)} \)
Anode: \( \ce{Zn(s) -> Zn^2+(aq) + 2e^-} \); cathode: \( \ce{Cu^2+(aq) + 2e^- -> Cu(s)} \); so n = 2.
E°cell = E°cathode − E°anode = 0.34 − (−0.76) = 1.10 V.
\( \displaystyle \Delta_rG^{\circ}=-2\times96487\ \mathrm{C\,mol^{-1}}\times1.10\ \mathrm{V} \)
\( \displaystyle \Delta_rG^{\circ}=-212271\ \mathrm{J\,mol^{-1}} \)
\( \displaystyle \Delta_rG^{\circ}=-212.3\ \mathrm{kJ\,mol^{-1}} \)
Shortcut: 2 × 96487 × 1.1 ≈ 212000 J, which is about 212 kJ, with a negative sign because the cell reaction is spontaneous.
Note 1: In the book’s working (extract below), the value in joules is printed as −21227 J mol−1; it should be −212270 J mol−1, which is the −212.27 kJ mol−1 given on the next line.
Note 2: This question is from NCERT (2026-27). NTA sets numerical questions from NCERT too, so do not ignore NCERT while practising numericals.
NCERT (2026-27 edition):
Electrochemical Cell and Gibbs Energy of the Reaction – …
Note 3: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 30-05 S1 29-05 S2 29-05 S1 11-05 S1 | 29-05 S1 26-05 S2 | 19-07 S1 | 22-06 S3 12-06 S3 | – |