A: `Cu^(2+)` is the only ion `(M^2+)` which has positive `E_(red)^(@) (M^(2+)//M))` in 3d series.
R: Cu has lower hydration enthalpy as comparision 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 given assertion and reason question, we will analyze both the assertion (A) and the reason (R) step by step.

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

- **Explanation**: The standard reduction potential indicates how easily a species can be reduced. A positive value means that the ion is more likely to gain electrons and be reduced to its elemental form. In the case of \( \text{Cu}^{2+} \), it has a positive standard reduction potential, which is unique among the 3d transition metals.

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

- **Explanation**: Hydration enthalpy refers to the energy released when ions are solvated by water. For \( \text{Cu}^{2+} \), the hydration enthalpy is relatively low. Ionization enthalpy is the energy required to remove electrons from an atom, and atomization enthalpy is the energy required to convert a substance into its gaseous atoms. The relationship between these enthalpies affects the overall energy change when \( \text{Cu}^{2+} \) is formed from elemental copper.

### Step 3: Analyzing the Relationship
To determine if the assertion is true and if the reason correctly explains it, we need to consider the following:

1. **Hydration Enthalpy**: The hydration enthalpy of \( \text{Cu}^{2+} \) is negative (exothermic process), meaning energy is released when the ion is hydrated.
2. **Ionization Enthalpy**: The first and second ionization enthalpies of copper are positive and relatively high because removing electrons from a positively charged ion requires more energy.
3. **Atomization Enthalpy**: The atomization enthalpy is also positive, as energy is required to convert solid copper into gaseous atoms.

### Step 4: Conclusion
The overall energy change when forming \( \text{Cu}^{2+} \) from elemental copper involves the balance of these enthalpies. Since the hydration enthalpy is lower than the combined ionization and atomization enthalpies, the overall reaction can yield a positive standard reduction potential for \( \text{Cu}^{2+} \).

Thus, both the assertion and the reason are correct, and the reason provides a valid explanation for the assertion.

### Final Answer
Both the assertion (A) and the reason (R) are true, and R is the correct explanation of A.

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