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Question #44196

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The Question

The standard reduction potentials ( $E^\circ_{M^{3+}/M}$ ) of lanthanides generally exhibit a slight, irregular decrease in negative value across the series from $\ce{La}$ to $\ce{Lu}$ . However, certain elements show marked deviations from this trend, acting as exceptions to the expected smooth transition. At $298\text{ K}$ , which of the following lanthanides possesses the least negative $E^\circ_{M^{3+}/M}$ value, and what is the dominant thermodynamic factor responsible for this anomaly?

Options

A.
$\ce{Eu}$ ; The anomalously high third ionization enthalpy ( $IE_3$ ) required to disrupt the stable $4f^7$ configuration of $\ce{Eu^{2+}}$ is not sufficiently compensated by its hydration enthalpy.
B.
$\ce{Yb}$ ; The exceptionally low enthalpy of atomization ( $\Delta H_{atm}$ ) caused by the fully filled $4f^{14}$ subshell minimizes the thermodynamic driving force for oxidation.
C.
$\ce{Gd}$ ; The exchange energy stabilization of the $4f^7$ configuration in $\ce{Gd^{3+}}$ drastically reduces its hydration enthalpy ( $\Delta H_{hyd}$ ) in aqueous media.
D.
$\ce{Lu}$ ; The cumulative effect of the lanthanide contraction maximizes the sum of the first three ionization enthalpies ( $\sum_{i=1}^{3} IE_i$ ), making oxidation highly endothermic.

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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

Periodic Properties of Lanthanides|Metal Activity Series Logic|Third Law Application

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to deconstruct the standard reduction potential into its thermodynamic Born-Haber cycle components.

The 'trap' lies in assuming that elements forming stable $+3$ ions (like Gd with $4f^7$ ) will have anomalous potentials, or that the lanthanide contraction (Lu) dominates the trend.

The hidden variable is the massive $IE_3$ penalty for breaking the half-filled $4f^7$ core of $Eu^{2+}$ , which overrides the expected periodic trend and hydration enthalpy, making Eu the least reactive lanthanide towards forming $+3$ ions.

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