Chemistry · Electrochemistry

JEE Main 2025 — 3 April, Evening Shift — Question 10

The standard cell potential ( $E^\circ_{\text{cell}}$ ) of a fuel cell based on the oxidation of methanol in air is measured as 1.21 V. The standard half-cell reduction potential for $\text{O}_2$ ( $E^\circ_{\text{O}_2/\text{H}_2\text{O}}$ ) is 1.229 V. Choose the correct statement:

Option A:
The standard half-cell reduction potential for the reduction of $\text{CO}_2$ ( $E^\circ_{\text{CO}_2/\text{CH}_3\text{OH}}$ ) is 19 mV
Correct
Option B:
Reduction of methanol takes place at the cathode
Option C:
Reactants are fed at one go to each electrode
Option D:
Oxygen is formed at the anode

Answer: A

Step-by-step solution

Overall reaction for the methanol fuel cell:

\text{CH}_3\text{OH} + \tfrac{3}{2}\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}

For fuel cells:

E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}} \quad (\text{all \;as\; reductions})

Given:

E^\circ_{\text{cell}} = 1.21\ \text{V}, \quad E^\circ_{\text{cathode}} = 1.229\ \text{V}

Therefore,

E^\circ_{\text{anode}} = 1.229\ \text{V} - 1.21\ \text{V} = 0.019\ \text{V}

Thus the standard reduction potential for $\text{CO}_2 \rightarrow \text{CH}_3\text{OH}$ is $19\;\ \text{mV}$ .

Thus statement A is true. Methanol is oxidized at the anode, so B is false. Fuel cells require continuous supply of reactants, so C is false. Oxygen is reduced at the cathode, so D is false.

Thus, the correct answer is A.

Answer key and solution verified before publishing.

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Exam: JEE Main 2025
Paper: 3 April, Evening Shift
Subject: Chemistry
Chapter: Electrochemistry
Topic: Batteries and Corrosion