NEET 2022 Chemistry Standard Electrode Potential MCQ Question

Chapter: Electrochemistry

Class: 12 | Topic: Standard Electrode Potential | Difficulty: 🟡 Moderate

✅ Ans: C — Yes, $E^\circ_{\text{cell}}=+0.287\,V$

$$MnO_4^-+8H^++5e^-\rightarrow Mn^{2+}+4H_2O,\quad E^\circ=+1.51V$$ $$\frac12O_2+2H^++2e^-\rightarrow H_2O,\quad E^\circ=+1.23V$$

For $O_2$ liberation, water is oxidised, so the second reaction is reversed:

$$E^\circ_{\text{oxidation}}=-1.23V$$ $$E^\circ_{\text{cell}}=1.51+(-1.23) =\boxed{+0.287V}$$

Since $E^\circ_{\text{cell}}>0$, liberation of $O_2$ is feasible.

❌ Why other options are wrong?

• A. $+2.733V$ ❌ — Incorrectly adds $1.51+1.23$.
• B. $-2.733V$ ❌ — Wrong magnitude and sign.
• D. $-0.287V$ ❌ — Magnitude is correct, but sign is wrong.

📌 NCERT: $E^\circ_{\text{cell}}=E^\circ_{\text{cathode}}-E^\circ_{\text{anode}}$. A positive $E^\circ_{\text{cell}}$ indicates a spontaneous reaction.

🧠 NEET Trick: Reverse a half-reaction → reverse the sign of $E^\circ$.