1. Tardigrade
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  3. Chemistry
  4. 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 )

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Q. 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. $Cr = 24, Mn = 25, Fe = 26,Co = 27$ )

AIPMTAIPMT 2011The d-and f-Block Elements

Solution:

This can be understood on the basis of $E^{\circ}$ values for $M^{2+} / M$.
image
$E^{\circ}$ value for $Mn$ is more negative than expected from general trend due to extra stability of half-filled $Mn ^{2+}$ ion.
Thus, the correct order should be
$Mn > Cr > Fe >\text { Co }$
An examination of $E^{\circ}$ values for redox couple $M ^{3+} / M ^{2+}$ shows that $Cr ^{2+}$ is strong reducing agent $\left(E_{M^{3+} / M^{2+}}^{\circ}=0.41 V \right)$ and liberates $H _{2}$ from dilute acids.
$2 Cr ^{2+}(a q)+2 H ^{+}(a q) \longrightarrow 2 Cr ^{3+}(a q)+ H _{2} \uparrow(g)$
$\therefore$ The correct order is $Mn > Fe > Cr > Co$.