Electrochemical cells
We can construct innumerable number of galvanic cells on the pattern of Daniell cell by taking combinations of different half-cells. Each half-cell consists of a metallic electrode dipped into an electrolyte. The two half-cells are connected by a metallic wire through a voltmeter and a switch externally. The electrolytes of the two half-cells are connected internally through a salt bridge. Sometimes, both the electrodes dip in the same electrolyte solution and in such cases we do not require a salt bridge.
For the cell
Ni(s) | Ni2+(aq) || Ag+(aq) | Ag
The cell reaction is Ni(s) + 2Ag+ (aq) → Ni2+(aq) + 2Ag(s).
Nernst equation relates the emf of the cell with standard emf and the concentration of reduced and oxidized species.
The standard electrode potential for the cell
Ni(s) + 2Ag+ (aq) → Ni2+(aq) + 2Ag(s)
is 0.80 V. The standard Gibbs energy for the reaction is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → -154.379 kJ mol−1
In a galvanic cell, the decrease in Gibbs energy equals the electrical work the cell can do. That work is the charge that flows (nF) multiplied by the potential (E):
\( \displaystyle \Delta_rG^{\circ}=-nFE^{\circ}_{cell} \)
Here n is the number of electrons transferred in the balanced reaction, and F = 96487 C mol−1.
The half reactions show that 2 electrons are transferred:
\( \displaystyle \ce{Ni(s) -> Ni^2+(aq) + 2e^-} \)
\( \displaystyle \ce{2Ag^+(aq) + 2e^- -> 2Ag(s)} \)
So n = 2 and E°cell = 0.80 V.
\( \displaystyle \Delta_rG^{\circ}=-2\times96487\ \mathrm{C\,mol^{-1}}\times0.80\ \mathrm{V} \)
\( \displaystyle \Delta_rG^{\circ}=-154379.2\ \mathrm{J\,mol^{-1}} \)
\( \displaystyle \Delta_rG^{\circ}=-154.379\ \mathrm{kJ\,mol^{-1}} \)
Shortcut: E°cell is positive, so the reaction is spontaneous and ΔrG° must be negative, which rules out Options (2) and (3). Then 2 × 96.5 × 0.8 ≈ 154, which gives Option (1).
Note: NTA picked this question from an NCERT (2026-27) example and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27 edition):
Electrochemical Cell and Gibbs Energy of the Reaction – “… The standard electrode potential for Daniell cell is 1.1V. Calculate the standard Gibbs energy for the reaction:
\( \ce{Zn(s) + Cu^2+(aq) -> Zn^2+(aq) + Cu(s)} \)
\( \Delta_rG^{\circ} = -nFE^{\circ}_{(cell)} \)
n in the above equation is 2, F = 96487 C mol−1 and E°(cell) = 1.1 V
\( \text{Therefore, } \Delta_rG^{\circ} = -2 \times 1.1\,\mathrm{V} \times 96487\,\mathrm{C\,mol^{-1}} = -21227\,\mathrm{J\,mol^{-1}} = -212.27\,\mathrm{kJ\,mol^{-1}} \)”