Question

Why emn 3 + changes to mn 2 + couple is more positive then e fe3 + changes to fe 2 +?

Answer 1

The reason to this is:
#half filled configuration of Mn+2:
Mn2+ exists in 3d5 configuration which is half filled configuration and such half filled configurations provide extra stability to the Metal Ion. While Mn3+ will exist in 3d4 configuration which is less stable than 3d5.
Hence the conversion from 3+ to 2+ OS is very feasible. Hence E° value is +very depicting that ∆G° being -ve. And we know when ∆G° becomes -ve the reaction is feasible.
 
#half filled t2g configuration of Cr+3:
Because of 3d3 half filled d orbital extra stability is attained by Cr+3 than Cr+2. hence Cr+3------->Cr+2 is less feasible. Hence reaction bing against the stability has less E° Value.

Answer 2

$\mathcal{\red{\huge{Heya\:mate\:!!}}}$

$\mathtt{Here\:is\:ur\:@nswer}$ :

⎇.half filled configuration of Mn+2:

Mn2+ exists in 3d5 configuration which is half filled configuration and such half filled configurations provide extra stability to the Metal Ion. While Mn3+ will exist in 3d4 configuration which is less stable than 3d5.

Hence the conversion from 3+ to 2+ OS is very feasible. Hence E° value is +very depicting that ∆G° being -ve. And we know when ∆G° becomes -ve the reaction is feasible.

 

#half filled t2g configuration of Cr+3:

Because of 3d3 half filled d orbital extra stability is attained by Cr+3 than Cr+2. hence, Cr+3------->Cr+2

is less feasible. Hence reaction bing against the stability has less E° Value.