JEE Main · 2022 · Shift-IIhardDNF-063

Which of the following 3d-metal ion will give the lowest enthalpy of hydration (hydH) when dissolved in water?

d & f-Block Elements · Class 12 · JEE Main Previous Year Question

Question

Which of the following 3d-metal ion will give the lowest enthalpy of hydration (ΔhydH\Delta_{\text{hyd}}H) when dissolved in water?

Options
  1. a

    Cr2+\text{Cr}^{2+}

  2. b

    Mn2+\text{Mn}^{2+}

    ✓
  3. c

    Fe2+\text{Fe}^{2+}

  4. d

    Co2+\text{Co}^{2+}

Correct Answerb

Mn2+\text{Mn}^{2+}

Detailed Solution

Concept: Enthalpy of hydration depends on charge density and CFSE

For M²⁺ ions of the same charge, hydration enthalpy is influenced by:

  1. Ionic radius — smaller radius → higher charge density → more exothermic hydration
  2. Crystal Field Stabilisation Energy (CFSE) — ions with CFSE gain extra stabilisation

Mn2+\text{Mn}^{2+} (3d53d^5, half-filled) has zero CFSE in any field (all orbitals singly occupied — no net stabilisation from crystal field splitting).

Other ions (Cr2+\text{Cr}^{2+}: 3d43d^4, Fe2+\text{Fe}^{2+}: 3d63d^6, Co2+\text{Co}^{2+}: 3d73d^7) all have non-zero CFSE, which contributes to their hydration enthalpy.

Mn2+\text{Mn}^{2+} has the lowest hydration enthalpy because it gains no CFSE stabilisation.

Answer: Option (2) — Mn2+\text{Mn}^{2+}

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