For the reaction, Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s),
\( \displaystyle E^0\ cell=\frac{2.303\ RT}{2F}\log\frac{[\ce{Zn^2+}]}{[\ce{Cu^2+}]} \). This indicates that
A. The reaction is at equilibrium.
B. The reaction is proceeding in the forward direction.
C. Voltmeter gives a positive reading.
D. Voltmeter gives a zero reading.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A and D only
For the Daniell cell reaction, two electrons are transferred (n = 2), and the Nernst equation gives the cell potential at any concentration:
\( \displaystyle \ce{Zn(s) + Cu^2+(aq) -> Zn^2+(aq) + Cu(s)} \)
\( \displaystyle E_{cell}=E^0_{cell}-\frac{2.303\,RT}{2F}\log\frac{[\ce{Zn^2+}]}{[\ce{Cu^2+}]} \)
The question says that E0cell is equal to the log term. Putting this into the Nernst equation:
\( \displaystyle E_{cell}=\frac{2.303\,RT}{2F}\log\frac{[\ce{Zn^2+}]}{[\ce{Cu^2+}]}-\frac{2.303\,RT}{2F}\log\frac{[\ce{Zn^2+}]}{[\ce{Cu^2+}]} \)
\( \displaystyle E_{cell}=0 \)
A cell potential of zero means there is no driving force left: the reaction has reached equilibrium.
A. The reaction is at equilibrium. — Correct. The given relation is the condition Ecell = 0, which holds only at equilibrium.
B. The reaction is proceeding in the forward direction. — Incorrect. A net forward reaction needs Ecell > 0; at equilibrium the forward and backward rates are equal.
C. Voltmeter gives a positive reading. — Incorrect. A positive reading would mean Ecell > 0, but here Ecell = 0.
D. Voltmeter gives a zero reading. — Correct. With Ecell = 0 no current flows, so the voltmeter reads zero.
NCERT (2026-27 edition):
Equilibrium Constant from Nernst Equation
