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