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Question #44332

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The Question

The standard electrode potentials ( $E^\circ_{\ce{M^{2+}/M}}$ ) of the 3d transition series at 298 K exhibit an irregular trend, generally becoming less negative from Ti to Cu, with pronounced anomalies at Mn, Ni, and Zn. This macroscopic thermodynamic behavior is governed by the interplay of sublimation enthalpy ( $\Delta_{\text{sub}}H^\circ$ ), ionization enthalpies ( $IE_1 + IE_2$ ), and hydration enthalpy ( $\Delta_{\text{hyd}}H^\circ$ ).

Which of the following statements correctly identifies the thermodynamic parameter and electronic factor responsible for the specified trend or anomaly?

Options

A.
The general trend from Ti to Cu is driven by a steady decrease in the magnitude of $\Delta_{\text{hyd}}H^\circ$ caused by the poor shielding effect of 3d electrons.
B.
The anomaly of Mn ( $-1.18\text{ V}$ ) is due to the $3d^5$ exchange energy resulting in an exceptionally high negative $\Delta_{\text{hyd}}H^\circ$ for $\ce{[Mn(H2O)6]^{2+}}$ .
C.
The anomaly of Ni ( $-0.25\text{ V}$ ) is due to the high CFSE of the $3d^8$ configuration yielding the highest negative $\Delta_{\text{hyd}}H^\circ$ in the 3d series.
D.
The anomaly of Zn ( $-0.76\text{ V}$ ) is due to the completely filled $3d^{10}$ configuration giving it a higher negative $\Delta_{\text{hyd}}H^\circ$ than $\ce{Cu^{2+}}$ .

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Screening Effect in Inner Shells|Electrode Potential Dependencies|Standard Electrode Potential Sign Conventions

Why This Question Is Tricky

Common Mistakes

The question tests the 'Chemical Exception' regarding the standard electrode potentials of the 3d transition series.

Students often memorize that Mn and Zn have anomalous $E^\circ$ values due to half-filled and fully-filled d-orbitals, but they falsely attribute this stability to the hydration enthalpy rather than the ionization enthalpy.

Furthermore, they overlook the role of Crystal Field Stabilization Energy (CFSE) in determining the hydration enthalpy, which is the 'Hidden Variable' that makes Ni anomalous.

By forcing the student to evaluate the exact thermodynamic parameter (IE vs.

Hydration Energy) responsible for each anomaly, this question effectively traps those who rely on superficial heuristic rules.

Quick Facts About This Question

Question Information

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