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: Understanding the Concept:
The standard emf of a cell is the difference between the reduction potential of the cathode and that of the anode. In a working cell the cathode has the higher reduction potential.

Step 2: Key Formula or Approach:
\[ E^{\circ}_{\text{cell}} = E^{\circ}_{\text{cathode}} - E^{\circ}_{\text{anode}} \]
To make this as large as possible, choose the highest \(E^{\circ}\) as cathode and the lowest as anode.

Step 3: Detailed Explanation:
The potentials are A = +2.5 V, B = +3.0 V, C = -2.0 V, D = -1.5 V.
(A) A anode, B cathode: \(3.0 - 2.5 = 0.5\) V.
(B) B anode, D cathode: \(-1.5 - 3.0 = -4.5\) V, which is negative, so not a working cell.
(C) C anode, B cathode: \(3.0 - (-2.0) = 5.0\) V.
(D) B anode, C cathode: \(-2.0 - 3.0 = -5.0\) V, also negative.
The largest positive value is 5.0 V in option (C), which uses the highest and the lowest potentials.

Final Answer:
The cell emf is maximum when C is the anode and B is the cathode, option (C). \[ \boxed{\text{C anode, B cathode (C)}} \]