8.17 For M^{2+}/Mand 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+}

Answers (1)
M manish

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+}<Fe^{3+}<Cr^{3+}

 

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