Question

The E° value for the Mn³⁺/Mn²⁺ couple is more positive than that of Cr³⁺/Cr²⁺ or Fe³⁺/Fe²⁺ due to change of

• A. d⁵ to d⁴ configuration
• B. d⁵ to d² configuration
• C. d⁴ to d⁵ configuration
• D. d³ to d⁵ configuration

Answer 1

The Correct Option is C

The more positive E° value for the Mn³⁺/Mn²⁺ couple compared to Cr³⁺/Cr²⁺ or Fe³⁺/Fe²⁺ is due to electronic configuration changes. This change highlights the enhanced stability of the half-filled d-subshell in Mn²⁺. Let's examine the electronic configuration changes for these transitions:

Mn³⁺ (d⁴) to Mn²⁺ (d⁵): Manganese transitions from a d⁴ to a d⁵ configuration, achieving a half-filled 3d subshell. This state is notably stable due to symmetrical electron distribution.

Cr³⁺ (d³) to Cr²⁺ (d⁴): No significant stability is gained, as the d⁴ state is less stable than a half-filled or full d-subshell.

Fe³⁺ (d⁵) to Fe²⁺ (d⁶): Iron shifts from a half-filled d-subshell to a state that is more than half-filled, which does not confer additional stability.

This analysis indicates that the Mn³⁺ to Mn²⁺ transition uniquely benefits from the stability of the half-filled d⁵ configuration, leading to a more positive E° value. Consequently, the electronic configuration change from d⁴ to d⁵ is responsible for this increased E° value.