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Question #44182

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The Question

Three hydrate isomers of Chromium(III) chloride hexahydrate are known: the violet isomer (I), the light green isomer (II), and the dark green isomer (III). These isomers exhibit distinct thermodynamic and structural properties due to the differential distribution of water and chloride between the coordination and ionization spheres. Based on the principles of lattice thermodynamics and coordination chemistry, which of the following statements is \textbf{INCORRECT}?

Options

A.
The magnitude of the ionic lattice enthalpy ( $|U|$ ) strictly follows the order $\text{I} > \text{II} > \text{III}$ , primarily driven by the decreasing charge product of the ions and the reduction in the number of ions per formula unit ( $\nu$ ).
B.
The transition from isomer I to III involves an expansion of the unit cell due to the incorporation of interstitial lattice water, which decreases the purely Coulombic ion-ion interactions but provides compensatory stabilization via extensive hydrogen-bonding networks.
C.
As chloride ligands successively replace coordinated water molecules from isomer I to III, the effective ionic radius of the complex cation decreases, which partially offsets the sharp decline in lattice energy caused by the reduced cation charge.
D.
Upon dissolution in water at $298\text{ K}$ , the total hydration enthalpy of the gaseous ions produced per mole of the solid is most exothermic for the violet isomer (I) due to the high charge density of the $\ce{[Cr(H2O)6]^{3+}}$ cation.

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How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Lattice Energy Factors|Hydrate and Ionization Isomerism

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of coordination chemistry and solid-state thermodynamics.

The 'Lethal Collision' occurs when students apply the Kapustinskii equation but incorrectly deduce the trend in the effective ionic radius of the complex cation.

A common misconception is that a lower-charge complex or one containing anionic ligands might be smaller due to electrostatic neutralization.

In reality, the larger size of the chloride ligand compared to water, combined with the reduced contraction of the coordination sphere at lower charges, makes the complex cation larger.

This expansion synergizes with the reduced charge to cause a massive drop in purely ionic lattice energy, highlighting the critical compensatory role of the hydrogen-bonding network provided by the lattice water.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	Lattice Energy Factors
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free