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

Options

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

Explanation

To determine which of the four successive members of the first series of transition metals has a positive standard electrode potential `(E_(M^(2+)//M)^(@))`, we can analyze the standard reduction potentials of each metal. The metals in question are cobalt (Co), nickel (Ni), copper (Cu), and iron (Fe).

### Step-by-Step Solution:

1. **Identify the Metals and Their Oxidation States**:
- The metals we are considering are:
- Cobalt (Co)
- Nickel (Ni)
- Copper (Cu)
- Iron (Fe)

2. **Look Up Standard Reduction Potentials**:
- We need to find the standard reduction potential values for the half-reactions:
- Co²⁺ + 2e⁻ → Co
- Ni²⁺ + 2e⁻ → Ni
- Cu²⁺ + 2e⁻ → Cu
- Fe²⁺ + 2e⁻ → Fe

3. **Standard Reduction Potentials**:
- The standard reduction potentials (E°) for these half-reactions are approximately:
- E°(Co²⁺/Co) = -0.28 V
- E°(Ni²⁺/Ni) = -0.26 V
- E°(Cu²⁺/Cu) = +0.34 V
- E°(Fe²⁺/Fe) = -0.45 V

4. **Analyze the Values**:
- From the values above, we can see:
- Cobalt: -0.28 V (negative)
- Nickel: -0.26 V (negative)
- Copper: +0.34 V (positive)
- Iron: -0.45 V (negative)

5. **Conclusion**:
- The only metal with a positive standard reduction potential is **Copper (Cu)**. Therefore, the standard potential `(E_(M^(2+)//M)^(@))` for copper is positive.

### Final Answer:
The metal for which the standard potential `(E_(M^(2+)//M)^(@))` value has a positive sign is **Copper (Cu)**.