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Question
Account for the following:
E° value for the Mn3+/Mn2+ couple is highly positive (+1.57 V) as compare to Cr3+/Cr2+.
Solution
Mn2+ exists in half-filled d5 state which is very stable while Mn3+ is d4 which is not so stable. Conversion from d4 to d5 will be quick and have negative ∆G value. Hence, because of the stability factor the E° value is high for this process.
While, Cr3+ is d3 is half filled (t2g3) is stable in nature and Cr2+ is d4 has one extra electron which it would like to donate to attain the stable half filled (t2g3 ) configuration. Hence for the process Cr3+ to Cr2+ the value of E° is less.
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