Question

Consider the given standard potential (in volts) in 1 M acidic solution:

Fe²⁺/Fe : −0.4 V, Fe³⁺/Fe²⁺ : +0.8 V,

Mn²⁺/Mn : −2.1 V, Mn³⁺/Mn²⁺ : +1.5 V

1. Comment on the relative stabilities of +2 and +3 oxidation states of iron and manganese.
2. Which of the two metals may be more easily oxidised to +2 state?

Answer 1

i. Relative stabilities of +2 and +3 oxidation states:

• The E° value of Fe³⁺/Fe²⁺ is +0.8 V, which is positive but small. This indicates that Fe³⁺ is relatively more stable than Mn³⁺ but less stable than Cr³⁺.
• The E° value of Mn³⁺/Mn²⁺ is +1.5 V, which is large and positive. This shows that Mn³⁺ is unstable and can be easily reduced to Mn²⁺.
• Hence, among the two, Fe³⁺ is more stable than Mn³⁺ in aqueous solution.

ii. The oxidation potential (standard reduction potential of M²⁺/M) is:

Mn²⁺/Mn = −2.1 V,

Mn³⁺/Mn²⁺ = +1.5 V

Since Mn has a more negative E° value, it means Mn is more easily oxidised to Mn²⁺ than Fe is to Fe²⁺.