Standard electrode potentials of redox couples `A^(2+)//A, B^(2+)//B, C//C^(2+)" and "D^(2+)//D` are 0.3 V, -0.5 V, -0.75 V and 0.9 V respectively. Which of these is best oxidising agent and reducing agent respectively?

Options

A. `D^(2+)//D" and "B^(2+)//B`
B. `B^(2+)//B" and "D^(2+)//D`
C. `D^(2+)//D" and "C^(2+)//C`
D. `C^(2+)//C" and "D^(2+)//D`

Answer

To determine the best oxidizing and reducing agents from the given standard electrode potentials of the redox couples, we will follow these steps:

### Step-by-Step Solution:

1. **List the Given Electrode Potentials**:
- A²⁺/A: +0.3 V
- B²⁺/B: -0.5 V
- C/C²⁺: -0.75 V (Note: This is an oxidation reaction)
- D²⁺/D: +0.9 V

2. **Identify Reduction and Oxidation Reactions**:
- For A²⁺/A: A²⁺ + 2e⁻ → A (Reduction)
- For B²⁺/B: B²⁺ + 2e⁻ → B (Reduction)
- For C/C²⁺: C → C²⁺ + 2e⁻ (Oxidation)
- For D²⁺/D: D²⁺ + 2e⁻ → D (Reduction)

3. **Convert Oxidation Potential to Reduction Potential**:
- The oxidation potential for C is -0.75 V. To find the reduction potential, we take the negative of this value:
- Reduction potential for C/C²⁺ = +0.75 V

4. **Compile All Reduction Potentials**:
- A²⁺/A: +0.3 V
- B²⁺/B: -0.5 V
- C/C²⁺: +0.75 V
- D²⁺/D: +0.9 V

5. **Order the Reduction Potentials**:
- B²⁺/B: -0.5 V (Lowest)
- A²⁺/A: +0.3 V
- C/C²⁺: +0.75 V
- D²⁺/D: +0.9 V (Highest)

6. **Determine the Best Oxidizing Agent**:
- The best oxidizing agent is the species with the highest reduction potential. Thus, D²⁺/D (+0.9 V) is the best oxidizing agent.

7. **Determine the Best Reducing Agent**:
- The best reducing agent is the species with the lowest reduction potential (or highest oxidation potential). Thus, B²⁺/B (-0.5 V) is the best reducing agent.

### Final Answer:
- **Best Oxidizing Agent**: D²⁺/D
- **Best Reducing Agent**: B²⁺/B