Question

For M²⁺/M and M³⁺/M²⁺ systems, the E^Θ 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:

The stability of Fe³⁺ in acid solution as compared to that of Cr³⁺ or Mn³⁺.

Answer 1

The value of E^Θ for Cr³⁺/Cr²⁺ is negative. Hence Cr³⁺ is stable and cannot be reduced to Cr²⁺.

The value of E^Θ for Mn³⁺/Mn²⁺ is more positive; hence, Mn³⁺ is not very stable and can easily be reduced to Mn²⁺. The value of E^Θ for Fe³⁺/Fe²⁺ is less positive but smaller. Hence, Fe³⁺ is more stable than Mn³⁺, but it is less stable than Cr²⁺.