Question

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

Cr2+/Cr	−0.9 V
Mn2+/Mn	−1.2 V
Fe2+/Fe	−0.4 V
Cr3/Cr2+	−0.4 V
Mn3+/Mn2+	+1.5 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⁺ 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. The value of E° for Cr³⁺/Cr²⁺ is negative. Hence, Cr³⁺ is stable and cannot be reduced to Cr²⁺. The value of E° for Mn³⁺/Mn²⁺ is more positive; hence, Mn³⁺ is not very stable and can easily be reduced to Mn²⁺. The value of E° for Fe³⁺/Fe²⁺ is less positive but smaller. Hence, Fe³⁺ is more stable than Mn³⁺, but it is less stable than Cr²⁺.
2. The oxidation potentials for Fe, Cr, and Mn are +0.4 V, +0.9 V, and +1.2 V, respectively. Hence, the order of ease of oxidation of these is Mn > Cr > Fe.