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Question #44334

Difficulty: 8/10 | 🟢 Free

The Question

An aqueous solution of \ce{FeCl3} is highly acidic, with the dominant acidic species being the hexaaqua complex \ce{[Fe(H2O)6]^{3+}} ( $pK_a \approx 2.2$ ). In contrast, the $pK_a$ of \ce{[Al(H2O)6]^{3+}} is $\approx 5.0$ , despite \ce{Al^{3+}} ( $r \approx 53.5$ pm) having a smaller ionic radius than high-spin \ce{Fe^{3+}} ( $r \approx 64.5$ pm). Upon the addition of excess \ce{KCN} to the \ce{FeCl3} solution, a stable complex $Z$ is formed.

Identify the correct set of parameters: [Primary reason for the lower $pK_a$ of \ce{Fe^{3+}}], [Formula of Complex $Z$ ], [Magnetic moment of $Z$ ], [Hybridization of $Z$ ].

Options

A.
Higher $Z_{eff}$ due to poor $3d$ shielding, \ce{[Fe(CN)6]^{3-}}, $1.73$ B.M., $d^2sp^3$
B.
Higher CFSE of the $d^5$ configuration, \ce{[Fe(CN)6]^{3-}}, $0$ B.M., $sp^3d^2$
C.
Higher ionic potential ( $Z/r$ ) of \ce{Fe^{3+}}, \ce{[Fe(CN)6]^{4-}}, $0$ B.M., $d^2sp^3$
D.
Formation of insoluble \ce{Fe(OH)3}, \ce{[Fe(CN)6]^{3-}}, $5.92$ B.M., $sp^3d^2$

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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

Fajans' Rule Polarization Power|Salt Hydrolysis Logic|Complex Ion Oxidation States

Why This Question Is Tricky

Common Mistakes

The question tests the 'Chemical Exception' to the simple electrostatic model of salt hydrolysis.

Students often rely solely on the ionic potential ( $\phi = Z/r$ ) to predict polarizing power, which incorrectly suggests \ce{AlCl3} is more acidic than \ce{FeCl3}.

The hidden variable is the electron core configuration: the poor shielding of the $3d$ electrons in \ce{Fe^{3+}} leads to a significantly higher effective nuclear charge ( $Z_{eff}$ ) compared to the noble gas core of \ce{Al^{3+}}.

This results in a stronger inductive withdrawal of electron density from the coordinated water ligands, making \ce{[Fe(H2O)6]^{3+}} a much stronger Brønsted acid.

Furthermore, it tests the correct application of Crystal Field Theory for the resulting cyano complex, avoiding traps regarding auto-reduction or incorrect spin states.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	Fajans' Rule Polarization Power
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free