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The d- and f-Block Elements — Chemistry STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceChemistryThe d- and f-Block Elements3 Marks
Question
For M2+/M and M3+/M2+ systems the $E ^{\circ}$ values for some metals are as follows : 
$\begin{array}{llll} Cr ^{2+} / Cr & -0.9 V & Cr ^3 / Cr ^{2+} & -0.4 V \\ Mn ^{2+} / Mn & -1.2 V & Mn ^{3+} / Mn ^{2+} & +1.5 V \\ Fe ^{2+} / Fe & -0.4 V & Fe ^{3+} / Fe ^{2+} & +0.8 V\end{array}$
Use this data to comment upon :
(i) the stability of Fe3+ in acid solution are compared to that of Cr3+or Mn3+ and
(ii) the case with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.
✓

Answer

(i) When the reduction potential (electrode potential) of a species is high, then its tendency to be reduced is high. Reduction potential of Mn+3 is maximum hence it gets easily reduced to Mn2+ hence Mn+3 is less stable than Fe+3 But Cr+3 is much lower than the reduction potential of Fe+3.
(ii) When the value of electrode potential (reduction potential) of a metal ion is low, then the tendency of that metal atom to be oxidized will the more, hence Mn will have the highest tendency to be oxidized to Mn+2 and Fe will have the highest tendency to be oxidized to Fe+2. The tendency for oxidation will be minimum in +2. Therefore the order of their oxidation is as follows - $Mn > Cr > Fe$.

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