Question

If standard reduction potential of four electrodes A, B, C, D are \(+2.5\,\text{V},+3.0\,\text{V},-2.0\,\text{V}\) and \(-1.5\,\text{V}\) respectively. When is the standard emf of cell maximum?

• A. 'A' is anode and 'B' is cathode
• B. 'B' is anode and 'D' is cathode
• C. 'C' is anode and 'B' is cathode
• D. 'B' is anode and 'C' is cathode

Hint:

Maximum emf needs the highest reduction potential as cathode and the lowest as anode.

Answer 1

The Correct Option is C

Step 1: Spot the extremes:
The largest reduction potential is B at $+3.0$ V, and the smallest is C at $-2.0$ V. The spread between them is $3.0 - (-2.0) = 5.0$ V, the biggest possible difference among the four.

Step 2: Orient the cell correctly:
A positive emf needs reduction at the high-potential electrode. So B must be the cathode and C the anode.

Step 3: Check the options:
Options (B) and (D) use B as the anode, which gives negative emf. Option (A) pairs B with A and gives only $0.5$ V. Only option (C) uses the extreme pair in the correct direction.

Final Answer:
Option (C), with $E^{\circ}_{cell} = 5.0$ V. \[ \boxed{\text{C anode, B cathode (C)}} \]