Four successive members of the first row transition elements are listed below with atomic numbers. Which one of them is expected to have the highest `E_(M^(3+)//M^(2+))^(0)` value

Options

A. `Co(Z= 27)`
B. `Cr(Z= 24)`
C. `Mn(Z=25)`
D. `Fe(Z= 26)`

Explanation

Highest `E_(M^(3+) //M^(2+))^(@)` value means highest reduction potential increases from left to right along the period in d-block but there are some exceptions. `Cr^(3+)` is more stable than `Cr^(2+)`. `Fe^(3+)(3d^(5))` is more stable than `Fe^(3+)` but `Mn^(2+)(3d^(5))` is more stable than `Mn^(3+)` but `Co^(2+)` ionsare still more stable than `Co^(3+)` in aqueous solution as`Co^(3+)` are easily reduced to`Co^(2+)` by `H_(2)O` . Hence, `Co^(3+) // Co^(2+)` has highest reduction potential
`(E_(Cr^(3+)//Cr^(2+))^(@)= -0.41V, E_(Mn^(3+ ) //Mn^(2+))^(@)= + 1.57V,E_(Fe^(3+) //Fe^(2+))^(@)= + 0.77V, E_(Co^(3+) //Co^(2+))^(@)= 1.97V)`