Answer the question on the basis of passage given below:

All transition elements display typical metallic properties like high melting point, boiling point, tensile strength, high malleability and ductility, luster, etc. Their metallic character increases down the group and elements of 4d and 5d series exhibit higher metallic characters. They show variable oxidation states and forms a number of coloured compounds due to the presence of unpaired electrons which shows d-d transitions and makes them paramagnetic.

$E^º_{Ni^{2+}/Ni}$ which is more negative than the expected value, because-

Ni is having the highest negative enthalpy of hydration

The correct answer is Option (2) → Ni is having the highest negative enthalpy of hydration.

Let us look at the concept around the standard electrode potential for the \(\text{Ni}^{2+}/\text{Ni}\) couple and why the option stating that "Ni is having the highest negative enthalpy of hydration" is considered the correct answer.

Understanding the Role of Enthalpy of Hydration

Enthalpy of Hydration: When ions are solvated by water, they release energy, known as the enthalpy of hydration. A more negative enthalpy of hydration means that the ions are stabilized significantly when they are dissolved in water.

Impact on Electrode Potential: A higher (more negative) enthalpy of hydration for \(\text{Ni}^{2+}\) indicates that when nickel ions are hydrated, they are stabilized significantly. This stabilization makes it energetically favorable for nickel to exist in the ionic form \((\text{Ni}^{2+})\) in solution, rather than as solid nickel.

The stabilization of the \(\text{Ni}^{2+}\) ions makes the process of reduction to metallic nickel less favorable compared to what might be expected based on ionization energies alone.

Evaluation of the Options

1. High \(\Delta_iH^\circ_2\):

While high ionization energy indicates that it is not easy to remove electrons, the effect of hydration can outweigh this, especially if the hydration energy is particularly high.

2. Highest Negative Enthalpy of Hydration:

If \(\text{Ni}^{2+}\) has a very high negative enthalpy of hydration, it means that the release of energy upon hydration greatly stabilizes the ion, making it energetically more favorable for nickel to remain as \(\text{Ni}^{2+}\) in solution rather than being reduced to \(\text{Ni}\). Thus, this leads to a more negative \(E^\circ\).

3. Incompletely Filled \(3d\) Subshell:

While having an incompletely filled subshell can influence reactivity, it does not directly correlate to the standard electrode potential being more negative.

4. Low Sublimation Enthalpy:

A low sublimation enthalpy generally means that the transition from solid to gas is easier, but it does not directly explain the negative electrode potential of the nickel couple.

Conclusion

The correct interpretation is : Ni is having the highest negative enthalpy of hydration.

This substantial stabilization of the \(\text{Ni}^{2+}\) ion due to high hydration energy effectively makes the reduction process \((\text{Ni}^{2+} + 2e^- \rightarrow \text{Ni})\) less favorable, resulting in a more negative \(E^\circ\) than anticipated