Question

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.

Answer 1

(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⁺

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(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