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Question #44283

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The Question

The standard reduction potential $E^\circ(\ce{M^{2+}/M})$ for the 3d transition series is governed by the enthalpy sum of the Born-Haber cycle: $\Delta_r H^\circ = \Delta_{\text{sub}}H^\circ + IE_1 + IE_2 + \Delta_{\text{hyd}}H^\circ$ . While $E^\circ$ generally becomes less negative from \ce{Ti} to \ce{Cu} due to the steady increase in effective nuclear charge ( $Z_{\text{eff}}$ ), significant anomalies are observed at \ce{Mn}, \ce{Cu}, and \ce{Zn}. Which of the following statements correctly identifies the dominant thermodynamic and electronic factors responsible for these anomalies?

Options

A.
The $E^\circ(\ce{Cu^{2+}/Cu})$ is positive because its high $\Delta_{\text{sub}}H^\circ$ and exceptionally high $IE_2$ (due to $d^{10}$ disruption) are not compensated by its $\Delta_{\text{hyd}}H^\circ$ . \ce{Mn} and \ce{Zn} exhibit anomalously more negative $E^\circ$ values primarily because their stable $3d^5$ and $3d^{10}$ configurations restrict d-electron participation in metallic bonding, resulting in exceptionally low $\Delta_{\text{sub}}H^\circ$ .
B.
The $E^\circ(\ce{Cu^{2+}/Cu})$ is positive due to its low $\Delta_{\text{hyd}}H^\circ$ caused by the absence of Crystal Field Stabilization Energy (CFSE) in the $d^9$ configuration. \ce{Mn} and \ce{Zn} have more negative $E^\circ$ values because their high $Z_{\text{eff}}$ drastically increases their $\Delta_{\text{hyd}}H^\circ$ , overriding their high $\Delta_{\text{sub}}H^\circ$ .
C.
The general trend of $E^\circ$ is dictated by the steady increase in $\Delta_{\text{hyd}}H^\circ$ . \ce{Cu} is an exception because its $4s^1$ configuration leads to a low $\Delta_{\text{sub}}H^\circ$ , while \ce{Mn} and \ce{Zn} deviate because their stable $d^5$ and $d^{10}$ states make their $(IE_1 + IE_2)$ exceptionally high, preventing oxidation.
D.
The $E^\circ(\ce{Cu^{2+}/Cu})$ is positive because its $\Delta_{\text{sub}}H^\circ$ is the highest in the 3d series. \ce{Mn} and \ce{Zn} exhibit more negative $E^\circ$ values because their fully paired 4s electrons shield the 3d electrons completely, reducing $Z_{\text{eff}}$ and minimizing $(IE_1 + IE_2)$ .

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Concepts Behind This Question

Effective Nuclear Charge (Zeff)|E° (M²⁺/M) Anomalies|Third Law Application

Why This Question Is Tricky

Common Mistakes

The question tests the 'Chemical Exception' where students over-rely on the periodic trend of Effective Nuclear Charge ( $Z_{\text{eff}}$ ) to predict standard reduction potentials.

While $Z_{\text{eff}}$ smoothly increases ionization energies across the 3d series, it fails to account for the non-periodic nature of metallic bonding strength ( $\Delta_{\text{sub}}H^\circ$ ).

Students often overlook that exactly half-filled ( $d^5$ ) and fully-filled ( $d^{10}$ ) configurations localize electrons, drastically weakening metallic bonds and lowering sublimation energy.

This thermodynamic 'hidden variable' is the true cause of the $E^\circ$ anomalies for Mn and Zn, overriding the expected $Z_{\text{eff}}$ trend.

Furthermore, it tests the specific reason for Copper's positive $E^\circ$ , which is driven by the massive $IE_2$ required to break its $d^{10}$ core, not a lack of hydration energy.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	Effective Nuclear Charge (Zeff)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free