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# For M^2+/ M and M^2+/ M^2+ systems the E^ominus values for some metals are as follows: the stability of Fe^3+ in acid solution as compared to that of Cr^3+ or Mn^3+

8.17 For $M^{2+}/M$and $M^{3+}/M^{2+}$ systems the $E^{\ominus }$ values for some metals are as follows:

$\\Cr^{2+}/Cr\; \; \; \; \; \; \; \; \; \; \; \; -0.9V\\Mn^{2+}/Mn\; \; \; \; \; \; \; \;\; \; -1.2V\\Fe^{2+}/Fe\; \; \; \; \; \; \; \; \; \; \; \; \; -0.4V$

$\\Cr^{3+}/Cr^{2+}\; \; \; \; \; \; \; \; \; \; \; \; -0.4V\\Mn^{3+}/Mn^{2+}\; \; \; \; \; \; \; \; +1.5V\\Fe^{3+}/Fe^{2+}\; \; \; \; \; \; \; \; \; \; \; +0.8V$

Use this data to comment upon:
(i) the stability of $Fe^{3+}$ in acid solution as compared to that of $Cr^{3+}$ or $Mn^{3+}$

Views

The $E^{\Theta }$ value of $Fe^{3+}/Fe^{2+}$ is higher than that of $Cr^{3+}/Cr^{2+}$ but less than that of $Mn^{3+}/Mn^{2+}$. So, the reduction of ferric ion ($Fe^{3+}$) to ferrous ion($Fe^{2+}$) is easier than $Mn^{3+}/Mn^{2+}$ but as not easy as $Cr^{3+}/Cr^{2+}$. Hence ferric ion is more stable than manganese ion($Mn^{3+}$), but less stable than chromium ion($Cr^{3+}$).

Order of relative stablities of different ions is-

$Mn^{3+}

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