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NCERT Class XII Chemistry
Chapter - The d- and f-Block Elements
Questions with Solutions

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Question : 17
Total: 49
For M2+∕M and M3+∕M2+ systems the E° values for some metals are as follows
 Cr2+∕Cr  –0.9 V  Cr3+∕Cr2+  –0.4 V
 Mn2+∕Mn  –1.2 V  Mn3+∕Mn2+  +1.5 V
 Fe2+∕Fe  –0.4 V  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:  
(i) From the E° values for Mn3+∕Mn2+ for Cr, Mn and Fe, it is veryclear that Fe3+ is more stable than Mn3+ in an acidic medium, whereas lessstable than Cr3+ because of the higher reduction potential in comparison toCr3+∕Cr2+ and lower reduction potential than Mn3+∕Mn2+.
(ii) Reduction potential for Mn3+∕Mn is most negative and therefore,it will be most easily oxidised and ease of getting oxidised will beMn>Cr>Fe.
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