Question

How many Faradays are required to reduce 1 mol of \( Cr_2O_7^{2-} \) to \( Cr^{3+} \) in acidic medium?

• A. 2
• B. 3
• C. 6
• D. 4

Hint:

Number of Faradays = \( n \)-factor \(\times\) number of moles. Always calculate the total change in oxidation state for all atoms in the molecule.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The number of Faradays required for a reduction process is equal to the total number of moles of electrons transferred per mole of the substance being reduced.
Step 2: Detailed Explanation:
In the dichromate ion \( Cr_2O_7^{2-} \), the oxidation state of \( Cr \) is \( +6 \).
In the product \( Cr^{3+} \), the oxidation state is \( +3 \).
Change in oxidation state per \( Cr \) atom = \( 6 - 3 = 3 \).
Since there are 2 \( Cr \) atoms in one mole of \( Cr_2O_7^{2-} \), the total number of moles of electrons required is \( 2 \times 3 = 6 \) moles of electrons.
The charge of 1 mole of electrons is 1 Faraday (F).
Therefore, 6 Faradays are required.
Step 3: Final Answer:
Total 6 Faradays are required to reduce 1 mol of \( Cr_2O_7^{2-} \).