For M2+/M and M3+/M2+ systems, the \(E^\ominus\) values for some metals are as follows:
Cr2+/Cr -0.9V
Cr3 /Cr2+ -0.4 V
Mn2+/Mn -1.2V
Mn3+/Mn2+ +1.5 V
Fe2+/Fe -0.4V
Fe3+ /Fe2+ +0.8 V
Use this data to comment upon:
(i) The stability of Fe3+ in acid solution as compared to that of Cr3+ or Mn3+ and
(ii) The ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.
For M2+/M and M3+/M2+ systems, the \(E^\ominus\) values for some metals are as follows:
Cr2+/Cr -0.9V
Cr3 /Cr2+ -0.4 V
Mn2+/Mn -1.2V
Mn3+/Mn2+ +1.5 V
Fe2+/Fe -0.4V
Fe3+ /Fe2+ +0.8 V
Use this data to comment upon:
(i) The stability of Fe3+ in acid solution as compared to that of Cr3+ or Mn3+ and
(ii) The ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.
Solution and Explanation
(i) The \(E^\ominus\) value for Fe3+/Fe2+ is higher than that for Cr3+/Cr2+ and lower than that for Mn3+/Mn2+ . So, the reduction of Fe3+ to Fe2+ is easier than the reduction of Mn3+ to Mn2+ , but not as easy as the reduction of Cr3+ to Cr2+ . Hence, Fe3+ is more stable than Mn3+ , but less stable than Cr3+. These metal ions can be arranged in the increasing order of their stability as : Mn3+< Fe3+< Cr3+
(ii) The reduction potentials for the given pairs increase in the following order.
Mn2+/Mn < Cr2+/Cr < Fe2+/Fe
So, the oxidation of Fe to Fe2+ is not as easy as the oxidation of Cr to Cr2+ and the oxidation of Mn to Mn2+. Thus, these metals can be arranged in the increasing order of their ability to get oxidised as: Fe<Cr<Mn
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Concepts Used:
D and F Block Elements
The d-block elements are placed in groups 3-12 and F-block elements with 4f and 5f orbital filled progressively. The general electronic configuration of d block elements and f- block elements are (n-1) d 1-10 ns 1-2 and (n-2) f 1-14 (n-1) d1 ns2 respectively. They are commonly known as transition elements because they exhibit multiple oxidation states because of the d-d transition which is possible by the availability of vacant d orbitals in these elements.
They have variable Oxidation States as well as are good catalysts because they provide a large surface area for the absorption of reaction. They show variable oxidation states to form intermediate with reactants easily. They are mostly lanthanoids and show lanthanoid contraction. Since differentiating electrons enter in an anti-penultimate f subshell. Therefore, these elements are also called inner transition elements.
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