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Question #44194

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The Question

The hydration enthalpies ( $\Delta H_{hyd}$ ) of $M^{3+}$ ions in the 3d transition series exhibit a characteristic 'double-hump' curve due to significant Crystal Field Stabilization Energy (CFSE). For the Lanthanide series ( $\ce{La^{3+}}$ to $\ce{Lu^{3+}}$ ) at 298 K, the lanthanide contraction causes a steady decrease in ionic radius from $1.03 \text{ \AA}$ to $0.86 \text{ \AA}$ , significantly increasing the charge density. Considering the electronic structure of $\ce{Ln^{3+}}$ ions, what is the approximate magnitude of the CFSE contribution to their total hydration enthalpy, and what is the resulting shape of the $\Delta H_{hyd}$ versus Atomic Number curve?

Options

A.
$\sim 1 - 5 \text{ kJ/mol}$ , Smooth monotonic curve
B.
$\sim 100 - 150 \text{ kJ/mol}$ , Double-hump curve
C.
$\sim 100 - 150 \text{ kJ/mol}$ , Smooth monotonic curve
D.
$\sim 1 - 5 \text{ kJ/mol}$ , Double-hump curve

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Periodic Properties of Lanthanides|Crystal Field Splitting (Octahedral)|Coordination Compounds

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to differentiate between 3d and 4f coordination chemistry.

The trap is assuming that the lanthanide contraction (which increases charge density and theoretical ligand field strength) leads to a high CFSE and a 'double-hump' thermodynamic curve, similar to 3d metals.

The hidden variable is the deep shielding of 4f orbitals by $5s^2$ and $5p^6$ electrons, which renders ligand-field interactions (and thus CFSE) negligible.

This results in a smooth, monotonically increasing hydration enthalpy curve driven purely by electrostatics ( $Z^2/r$ ).

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	Periodic Properties of Lanthanides
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free