For $M ^{2+} / M$ and $M ^{3+} / M ^{2+}$ systems, the ${E^\Theta }$ values for some metals are as follows:
$Cr ^{2+} / Cr \,-0.9 \,V$
$Cr ^{3} / Cr ^{2+}\,-0.4 \,V$
$Mn ^{2+} / Mn \,-1.2\, V$
$Mn ^{3+} / Mn ^{2+}\,+1.5 \,V$
$Fe ^{2+} / Fe\, -0.4 \,V$
$Fe ^{3+} / Fe ^{2+}\,+0.8\, V$
Use this data to comment upon:
$(i)$ The stability of $Fe ^{3+}$ in acid solution as compared to that of $Cr ^{3+}$ or $Mn ^{3+}$ and
$(ii)$ The ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.
$(i)$ The ${E^\Theta }$ value for $Fe ^{3+} / Fe ^{2+}$ is higher than that for $Cr ^{3+} / Cr ^{2+}$ and lower than that for $Mn ^{3+} / Mn ^{2+} . So ,$ the reduction of $Fe ^{3+}$ to $Fe ^{2+}$ is easier than the reduction of $Mn ^{3+}$ to $Mn ^{2+},$ but not as easy as the reduction of $Cr ^{3+}$ to $Cr ^{2+}$. Hence, $Fe ^{3+}$ is more stable than $Mn ^{3+},$ but less stable than $Cr ^{3+}$. These metal ions can be arranged in the increasing order of their stability as: $Mn ^{3+}\,< \,Fe ^{3+}\,<\, Cr ^{3+}$
$(ii)$ The reduction potentials for the given pairs increase in the following order.
$Mn ^{2+} / Mn \,<\, Cr ^{2+} / Cr \,<\, Fe ^{2+} / Fe$
So, the oxidation of $Fe$ to $Fe ^{2+}$ is not as easy as the oxidation of $Cr$ to $Cr ^{2+}$ and the oxidation of $Mn$ to $Mn ^{2+}$. Thus, these metals can be arranged in the increasing order of their ability to get oxidised as: $Fe \,<\, Cr \,<\, Mn$
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