A: `Cu^(2+)` is the only jon `(M^2+)` which has positive`E_(red)^(@) (M^(2+)//M))` in 3d series.
R: Cu has lower hydration enthalpy ascomparision to its I.E. and `triangleH_(atm)`

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question regarding the assertion and reason about the copper ion \( Cu^{2+} \), we will analyze both statements step by step.

### Step 1: Understanding the Assertion
The assertion states that \( Cu^{2+} \) is the only ion in the \( M^{2+} \) category of the 3d series that has a positive standard reduction potential \( E^{\circ} (M^{2+} // M) \).

- **Explanation**: The standard reduction potential \( E^{\circ} \) indicates the tendency of a species to gain electrons and be reduced. A positive \( E^{\circ} \) means that the reduction is favorable. In the 3d series, most transition metal ions have negative standard reduction potentials, but \( Cu^{2+} \) is an exception.

### Step 2: Understanding the Reason
The reason given is that copper has a lower hydration enthalpy compared to its ionization energy and atomization enthalpy.

- **Explanation**: Hydration enthalpy is the energy released when ions are surrounded by water molecules. For \( Cu^{2+} \), the hydration enthalpy is significant, but when comparing it to the ionization energy (the energy required to remove electrons from the atom) and the atomization enthalpy (the energy required to convert solid copper into gaseous atoms), it is lower. This balance of energies influences the overall standard reduction potential.

### Step 3: Analyzing the Relationship
To understand why \( Cu^{2+} \) has a positive \( E^{\circ} \), we need to consider the interplay of these energies:

- **Ionization Energy**: The energy required to remove electrons from copper atoms. For \( Cu \), the first and second ionization energies are relatively high, which contributes positively to the overall energy balance.

- **Atomization Enthalpy**: The energy required to convert solid copper into gaseous atoms. This is also a positive value.

- **Hydration Enthalpy**: The energy released when \( Cu^{2+} \) ions are solvated by water. This is a negative value, as energy is released during hydration.

### Step 4: Conclusion
The assertion is true because \( Cu^{2+} \) indeed has a positive standard reduction potential, making it unique among the 3d transition metals. The reason is also true; the lower hydration enthalpy relative to the ionization and atomization energies contributes to this positive potential.

### Final Answer
Both the assertion (A) and the reason (R) are correct, and R correctly explains A.

---