COMEDK · Chemistry · 23. d and f Block Elements

Arrange the following ions in the increasing order of their \(\Delta H_{(\text {hydration})}\) values.
\(\mathrm{Cr}^{2+}, \mathrm{Co}^{2+}, \mathrm{Mn}^{2+}, \mathrm{Ni}^{2+}\)

A \(\mathrm{Cr}^{2+}<\mathrm{Mn}^{2+}<\mathrm{Co}^{2+}<\mathrm{Ni}^{2+}\)
B \(\mathrm{Co}^{2+}<\mathrm{Ni}^{2+}<\mathrm{Cr}^{2+}<\mathrm{Mn}^{2+}\)
C \(\mathrm{Mn}^{2+}<\mathrm{Cr}^{2+}<\mathrm{Co}^{2+}<\mathrm{Ni}^{2+}\)
D \(\mathrm{Ni}^{2+}<\mathrm{Co}^{2+}<\mathrm{Mn}^{2+}<\mathrm{Cr}^{2+}\)

Verified Solution

Answer & Solution

Correct Answer

(C) \(\mathrm{Mn}^{2+}<\mathrm{Cr}^{2+}<\mathrm{Co}^{2+}<\mathrm{Ni}^{2+}\)

Step-by-step Solution

Detailed explanation

The enthalpy of hydration (\(\Delta H_{\text{hydration}}\)) for transition metal ions is primarily determined by the charge-to-size ratio, which is inversely proportional to the ionic radius. As the atomic number increases across the 3d series for ions of the same charge…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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Arrange the following ions in the increasing order of their \(\Delta H_{(\text {hydration})}\) values.\(\mathrm{Cr}^{2+}, \mathrm{Co}^{2+}, \mathrm{Mn}^{2+}, \mathrm{Ni}^{2+}\)

Step-by-step Solution

Arrange the following ions in the increasing order of their \(\Delta H_{(\text {hydration})}\) values.
\(\mathrm{Cr}^{2+}, \mathrm{Co}^{2+}, \mathrm{Mn}^{2+}, \mathrm{Ni}^{2+}\)