For the four successive transition elements (Cr, Mn, Fe and Co), the stability of + 2 oxidation stat

Question

For the four successive transition elements (Cr, Mn, Fe and Co), the stability of + 2 oxidation state will be there in which of the following order ? (At. no. Cr = 24, Mn = 25, Fe = 26, Co = 27)

Accepted Answer

This can be understood on the basis of E^o values for M²⁺/M.
E^o / V             Cr           Mn             Fe             Co
M²⁺/M       - 0.90       - 1.18       - 0.44       - 0.28
E^o values for Mn is more negative than expected from general trend due to extra stability of half-filled Mn²⁺ ion.
THus the correct order should be
          Mn > Cr > Fe > Co
An examination of E^o values for redox couple M³⁺/M²⁺ shows that Cr²⁺ is strong reducing agent (E^o_{M³⁺/M²⁺} = 0.41 V) and liberates H₂ from dilute acids.
2Cr²⁺ (aq) + 2H⁺ (aq) → 2Cr³⁺ (aq) + H₂ ↑ (g)
∴ The correct order is Mn > Fe > Cr > Co.

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