Question

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:

1. the stability of Fe³⁺ in acid solution as compared to that Cr³⁺ or Mn³⁺ and
2. the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.

Answer 1

1. For Cr³⁺ and Cr²⁺, the E° value is negative. Consequently, it is impossible to decrease Cr³⁺ to Cr²⁺ since it is stable. Mn³⁺/Mn²⁺ has a strong positive E° value. As a result, Mn³⁺ is rapidly reduced to Mn²⁺ and is not very stable. Fe³⁺/Fe²⁺ has a positive but modest E° value. Fe³⁺ is therefore more stable than Mn³⁺. It is not as stable as Cr³⁺, though.
2. Fe, Cr, and Mn have oxidation potentials of +0.4 V, +0.9 V, and +1.2 V, respectively. As a result, the sequence of ease of oxidation will be Mn > Cr > Fe.