Q16 / 75 · JEE Main 2026 (23 January Shift 1) · Chemistry
In the given electrochemical cell, $\mathrm{Ag}(\mathrm{s})|\mathrm{AgCl}(\mathrm{s})| \mathrm{FeCl}_{2}(\mathrm{aq}), \mathrm{FeCl}_{3}(\mathrm{aq}) \mid \mathrm{Pt}(\mathrm{s})$ at 298 K, the cell potential $\left(\mathrm{E}_{\text {cell }}\right)$ will increase when :
A. Concentration of $\mathrm{Fe}^{2+}$ is increased.
B. Concentration of $\mathrm{Fe}^{3+}$ is decreased.
C. Concentration of $\mathrm{Fe}^{2+}$ is decreased.
D. Concentration of $\mathrm{Fe}^{3+}$ is increased.
E. Concentration of $\mathrm{Cl}^{-}$is increased.
Choose the correct answer from the options given below :
A. Concentration of $\mathrm{Fe}^{2+}$ is increased.
B. Concentration of $\mathrm{Fe}^{3+}$ is decreased.
C. Concentration of $\mathrm{Fe}^{2+}$ is decreased.
D. Concentration of $\mathrm{Fe}^{3+}$ is increased.
E. Concentration of $\mathrm{Cl}^{-}$is increased.
Choose the correct answer from the options given below :
- AB Only
- BA and E Only
- Correct answerCC, D and E Only
- DA and B Only
Solution
The cell Ag(s)|AgCl(s)|FeCl₂(aq), FeCl₃(aq)|Pt(s) has the reaction:
Ag(s) + Cl⁻ + Fe³⁺ → AgCl(s) + Fe²⁺
Using Nernst equation:
$E_{cell} = E°_{cell} - \frac{0.059}{n}\log Q$ where $Q = \frac{[Fe^{2+}]}{[Fe^{3+}][Cl^-]}$
For $E_{cell}$ to increase, Q must decrease. Analyzing each condition:
A. Increase [Fe²⁺] increases Q, decreases $E_{cell}$ - false
B. Decrease [Fe³⁺] increases Q (smaller denominator), decreases $E_{cell}$ - false
C. Decrease [Fe²⁺] decreases Q, increases $E_{cell}$ - true
D. Increase [Fe³⁺] decreases Q, increases $E_{cell}$ - true
E. Increase [Cl⁻] decreases Q, increases $E_{cell}$ - true
Therefore C, D, and E cause $E_{cell}$ to increase.
Ag(s) + Cl⁻ + Fe³⁺ → AgCl(s) + Fe²⁺
Using Nernst equation:
$E_{cell} = E°_{cell} - \frac{0.059}{n}\log Q$ where $Q = \frac{[Fe^{2+}]}{[Fe^{3+}][Cl^-]}$
For $E_{cell}$ to increase, Q must decrease. Analyzing each condition:
A. Increase [Fe²⁺] increases Q, decreases $E_{cell}$ - false
B. Decrease [Fe³⁺] increases Q (smaller denominator), decreases $E_{cell}$ - false
C. Decrease [Fe²⁺] decreases Q, increases $E_{cell}$ - true
D. Increase [Fe³⁺] decreases Q, increases $E_{cell}$ - true
E. Increase [Cl⁻] decreases Q, increases $E_{cell}$ - true
Therefore C, D, and E cause $E_{cell}$ to increase.
Hence, the correct answer is option: 'C'.