For a general electrochemical reaction of the type:
\(aA+bB\overset{ne^-}{\longrightarrow}cC+dD\) Nernst equation can be written as:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(E_{cell}=E^\circ_{cell}-\frac{RT}{nF}\ln\frac{[C]^c[D]^d}{[A]^a[B]^b}\)
For aA + bB → cC + dD involving n electrons, the cell potential depends on the reaction quotient Q:
Ecell = E°cell − (RT/nF) ln Q, Q = [C]c[D]d / [A]a[B]b
Q has products in the numerator and reactants in the denominator, and it is the cell potential Ecell (not the standard potential) that is calculated from E°cell.
Why the other options are incorrect
Option (1) — Incorrect. E°cell and Ecell are interchanged; E°cell is a constant for the reaction and is not obtained from the concentrations.
Option (2) — Incorrect. E°cell and Ecell are interchanged and the quotient is inverted.
Option (4) — Incorrect. The quotient is inverted (reactants over products); with this form the potential would wrongly increase as products build up.
NCERT (2026-27 edition):
Nernst Equation – “… For a general electrochemical reaction of the type:
aA + bB → cC + dD (ne–)
Nernst equation can be written as:
E(cell) = E⊖(cell) – (RT/nF) ln Q = E⊖(cell) – (RT/nF) ln ([C]c[D]d/[A]a[B]b)”