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Question #44853

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The Question

Consider the thermodynamic parameters governing the standard reduction potentials ( $E^\circ_{M^{3+}/M}$ ) of Group 6 transition metals (\ce{Cr}, \ce{Mo}, \ce{W}) at $298\text{ K}$ . A bimetallic corrosion cell is constructed by coupling a Tungsten (\ce{W}) component with an Iron (\ce{Fe}) fitting ( $E^\circ_{Fe^{2+}/Fe} = -0.44\text{ V}$ ) in an aerated aqueous environment at $\text{pH} = 7$ . Given that the first ionization energy of \ce{W} is comparable to that of \ce{Cr}, which of the following correctly identifies the trend in $E^\circ_{M^{3+}/M}$ , the metal acting as the anode in the corrosion cell, and the dominant thermodynamic factor responsible for \ce{W}'s relative nobility?

Options

A.
Trend: $E^\circ_{Cr^{3+}/Cr} < E^\circ_{Mo^{3+}/Mo} < E^\circ_{W^{3+}/W}$ | Anode: \ce{Fe} | Factor: Exceptionally high $\Delta_{atom}H^\circ$ of \ce{W} due to lanthanoid contraction and strong metallic bonding.
B.
Trend: $E^\circ_{W^{3+}/W} < E^\circ_{Mo^{3+}/Mo} < E^\circ_{Cr^{3+}/Cr}$ | Anode: \ce{W} | Factor: Lower $\sum IE$ of \ce{W} due to the increased principal quantum number and shielding.
C.
Trend: $E^\circ_{Cr^{3+}/Cr} < E^\circ_{Mo^{3+}/Mo} < E^\circ_{W^{3+}/W}$ | Anode: \ce{Fe} | Factor: Highly exothermic $\Delta_{hyd}H^\circ$ of $\ce{W^{3+}}$ due to its high effective nuclear charge ( $Z_{eff}$ ).
D.
Trend: $E^\circ_{W^{3+}/W} < E^\circ_{Mo^{3+}/Mo} < E^\circ_{Cr^{3+}/Cr}$ | Anode: \ce{W} | Factor: Poor shielding of 4f electrons leading to a highly exothermic electron gain enthalpy for \ce{W}.

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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

d-orbital contraction|Electrode Potential Dependencies|Corrosion as Electrochemical Process

Why This Question Is Tricky

Common Mistakes

This question tests a critical exception to standard periodic trends.

Students often rely on the s- and p-block trend where ionization energy decreases down a group, incorrectly predicting that heavier metals (5d) are more easily oxidized (lower reduction potentials).

This trap ignores the Born-Haber cycle for aqueous electrode potentials.

The 'Hidden Variable' is the exceptionally high enthalpy of atomization of 5d metals caused by strong metallic bonding and lanthanoid contraction, which imposes a massive thermodynamic penalty on dissolution, making 5d metals unexpectedly noble.

Quick Facts About This Question

Question Information

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