Question

Why is the Evalue for the Mn* /Mncouple much more positive
than that for Cr**/Cr? or Fe**/Fe2'? Explain.
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Answer 1

Answer:

The E° value for the Mn3+/Mn2+ couple much more positive than that for Cr3+/Cr2+ or Fe3+/Fe2+.

Much more third ionisation energy of Mn (where the required change is d5 to d4) is mainly responsible for this. This also explains why the +3 state of Mn is of little importance.

I hope that this answer will help you

Answer 2

Because Mn3+ has the outer electronic configuration of 3d4 and Mn2+ has the outer electronic configuration of 3d5. Thus, the conversion of Mn3+ to Mn2+ will be a favourable reaction since 3d5 is a very stable configuration as it is half filled configuration. Hence, Eo value for Mn3+ / Mn2+ couple is positive.

Cr3+ to Cr2+ undergoes a change in outer electronic configuration from 3d3 to 3d4. Fe3+ to Fe2+ undergoes a change in outer electronic configuration from 3d5 to 3d6. Both these configurations of the resultant are not stable and hence have a lower Eo value.