Question

Explain why E°(Mn⁺³/Mn⁺²) couple is more positive than for E°(Fe⁺³/Fe⁺²) (At.no.of Mn = 25; Fe = 26).

Answer 1

The electronic configuration of Mn⁺² is Mn⁺² = [Ar] 3d⁵; Mn⁺³ is 3d⁴ and that of Fe⁺² = [Ar] 3d⁶; Fe⁺³ is [Ar] 3d⁵.

Thus Mn⁺² has a more stable configuration than Mn⁺³ while Fe⁺³ has a more stable configuration than Fe⁺². Consequently, large third ionisation enthalpy is required to change Mn⁺² to Mn⁺³. As E° values is the sum of the enthalpy of atomisation, ionisation enthalpy and hydration enthalpy, therefore E° for Mn⁺³/ Mn⁺² couple is more positive than Fe⁺³/Fe⁺².

Note: The large positive E° for Mn⁺³/ Mn⁺² means that Mn⁺³ can be easily reduced to Mn⁺² (Mn⁺³ + e → Mn⁺²) i.e., Mn⁺³ is less stable, i.e., Fe⁺³ can be reduced to Fe⁺² (Fe⁺³ + e → Fe⁺² but ion easily. Thus, Fe⁺³ is more stable than Mn⁺³.