The standard electrode potential () for the couple is highly positive (), whereas it is heavily nega — d and f Block Elements Chemistry Question
Question
The standard electrode potential ($E^\circ$) for the $Mn^{3+}/Mn^{2+}$ couple is highly positive ($+1.57\text{ V}$), whereas it is heavily negative for the $Cr^{3+}/Cr^{2+}$ couple ($-0.41\text{ V}$). What is the root theoretical cause of this massive discrepancy?
Answer: A
💡 Solution & Explanation
$Mn^{3+}$ ($d^4$) is a strong oxidizing agent because gaining an electron yields the exceptionally stable half-filled $d^5$ state. Conversely, $Cr^{3+}$ already possesses a highly stable $t_{2g}^3$ state in an aqueous octahedral field, so it resists gaining an electron to become $Cr^{2+}$ ($d^4$).
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