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Question
For M2+/M and M3+/M2+ systems, the EΘ 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+.
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Solution
The value of EΘ for Cr3+/Cr2+ is negative. Hence Cr3+ is stable and cannot be reduced to Cr2+.
The value of EΘ for Mn3+/Mn2+ is more positive; hence, Mn3+ is not very stable and can easily be reduced to Mn2+. The value of EΘ for Fe3+/Fe2+ is less positive but smaller. Hence, Fe3+ is more stable than Mn3+, but it is less stable than Cr2+.
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