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Question #44888

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The Question

Consider the pH-dependent equilibrium between the yellow oxoanion $\ce{CrO4^2-}$ and the orange binuclear oxoanion $\ce{Cr2O7^2-}$ in aqueous solution at 298 K. Let $\Delta E_1$ and $\Delta E_2$ be the energies of the lowest-energy allowed electronic transitions responsible for the colors of $\ce{CrO4^2-}$ and $\ce{Cr2O7^2-}$ , respectively. Identify the correct set of parameters for this system in the following order: [Origin of Color, Energy Relationship, Geometry of Cr in $\ce{Cr2O7^2-}$ , $\ce{Cr-O-Cr}$ bond angle nature].

Options

A.
LMCT, $\Delta E_1 > \Delta E_2$ , Tetrahedral, Bent
B.
d-d transition, $\Delta E_1 < \Delta E_2$ , Octahedral, Linear
C.
LMCT, $\Delta E_1 < \Delta E_2$ , Tetrahedral, Linear
D.
MLCT, $\Delta E_1 > \Delta E_2$ , Square Planar, Bent

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Correct Answer

Step-by-Step Solution

Explanation

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

Dichromate-Chromate Equilibrium|Ligand Denticity Variations|Color and d-d Transitions

Why This Question Is Tricky

Common Mistakes

This question tests the 'Hidden Variable' of color origin in transition metal complexes.

Students frequently rote-memorize that transition metal colors are due to d-d transitions, failing to account for the $d^0$ configuration of $\ce{Cr^{6+}}$ which necessitates an LMCT mechanism.

Furthermore, it traps students who assume that dimerization into a larger, bridged structure (Dichromate) increases the energy gap.

By requiring the correlation of transmitted color to absorbed photon energy (complementary colors), it proves that the LMCT energy gap actually decreases ( $\Delta E_1 > \Delta E_2$ ).

Finally, it tests the structural exception that despite being a bridge, the $\ce{Cr-O-Cr}$ linkage is bent, not linear, while maintaining tetrahedral geometry at both metal centers.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	D and F Block Elements - Class 12
Concept	Dichromate-Chromate Equilibrium
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free