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The d & f Block Elements — Chemistry STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceChemistryThe d & f Block Elements5 Marks
Question
Answer the following questions:

For M2+/M and M3+/M2+ systems, E° values for some metals are as follows:

Cr2+/Cr = - 0.9 V Cr3+/Cr2+ = - 0.4 V
Mn2+/Mn = 0 1.2 V Mn3+/Mn2+ = +1.5 V
Fe2+/Fe = - 0.4 V Fe3+/Fe2+ = +0.8 V

Use this data to comment upon:

  1. The stability of Fe3+ in acid solution as compared to that of Cr3+ and Mn3+.
  2. The ease with which iron can be oxidised as compared to the similar process for either Cr or Mn metals.
✓

Answer

  1. Higher the reduction potential of a species, greater is the ease with which it undergo reduction. Among these pairs, Mn3+/Mn2+ has largest positive reduction potential. Hence Mn3+ can be easily reduced to Mn2+ i.e., Mn3+ is least stable. Cr3+/Cr2+ has a negative E° value, therefore, Cr3+ is most stable. Fe3+/Fe2+ has a positive value but small. Hence, Fe3+ is more stable than Mn3+ but less stable than Cr3+.
  2. Lower the reduction potential or higher the oxidation potential of a species, greater is the ease with which it undergo oxidation. Among these pairs, Mn2+/Mn has the most negative reduction potential or most positive oxidation potential. Therefore, it will be most easily oxidised. Thus, the decreasing order of their ease of oxidation is Mn > Cr > Fe.

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