question_answer 27)

For M2+/M and M3 + /M2+ systems, the values of some metals are given : Use this data to comment upon: (a) The stability of Fe3+ in acid solution as compared to that of Cr3+ or Mn3+. (b) The ease with which iron can be oxidised as compared to the similar process for either chromium or manganese metal.  

Answer:

                (a) As E°Cr3+/Cr2+ is negative (–0.4 V), this means that Cr3+ ions in solution cannot be reduced to Cr2+ ions i.e., Cr3+ ions are very stable. As further comparison of E° values show that Mn3+ ions more readily than Fe3+ ions which means that Mn3+ is least stable. Thus, is the light of this, the order of relative stabilities of different ions is: Mn3+< Fe3+< Cr3+. (b) From the E° values, the order of oxidation c the metal to the divalent cation is: Mn> Cr > Fe.