Four successive members of the first series of transition metals are listed below. For which one of the of standard potential `(E_(M^(2+)//M)^(@))` value has a positive sign ?

Options

A. `Co (Z = 27)`
B. `Cu (Z = 29)`
C. `Fe (Z = 26)`
D. `Ni (Z= 28)`

Answer

To determine which of the four successive members of the first series of transition metals has a positive standard reduction potential value, we will analyze each of the given elements: cobalt (Co), copper (Cu), iron (Fe), and nickel (Ni).

### Step-by-Step Solution:

1. **Identify the Elements and Their Atomic Numbers**:
- Cobalt (Co) - Atomic Number 27
- Copper (Cu) - Atomic Number 29
- Iron (Fe) - Atomic Number 26
- Nickel (Ni) - Atomic Number 28

2. **Understand Standard Reduction Potential**:
- The standard reduction potential (E°) indicates the tendency of a species to gain electrons and be reduced. A positive value means a greater tendency to be reduced.

3. **Refer to the Electrochemical Series**:
- In the electrochemical series, elements are arranged based on their standard reduction potentials. Hydrogen is used as a reference point with a standard potential of 0 V.
- Elements below hydrogen have positive reduction potentials, while those above have negative values.

4. **Check the Standard Reduction Potentials**:
- For Copper (Cu²⁺ + 2e⁻ → Cu): E° = +0.34 V (Positive)
- For Nickel (Ni²⁺ + 2e⁻ → Ni): E° = -0.25 V (Negative)
- For Cobalt (Co²⁺ + 2e⁻ → Co): E° = -0.28 V (Negative)
- For Iron (Fe²⁺ + 2e⁻ → Fe): E° = -0.44 V (Negative)

5. **Conclusion**:
- Among the four elements, only copper has a positive standard reduction potential (+0.34 V). Therefore, the correct answer is **Option B: Copper (Cu)**.