Question

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

$\begin{array}{l} {Cr}^{2 +} / Cr & - 0.9 V & {Cr}^{3 +} / {Cr}^{2 +} & - 0.4 V \\ {Mn}^{2 +} / Mn & - 1.2 V & {Mn}^{3 +} / {Mn}^{2 +} & + 1.5 V \\ {Fe}^{2 +} / Fe & - 0.4 V & {Fe}^{3 +} / {Fe}^{2 +} & + 0.8 V \end{array}$

Use this data to comment upon:

(a)The stability of ${Fe}^{3 +}$ in an acidic solution as compared to ${Cr}^{3 +}$ or ${Mn}^{3 +}$ .

(b) The ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.

Accepted Answer

(a)

The stability of $M^{3 +}$ can be predicted by their reduction potential.

The greater the value of the reduction potential of $M^{3 +} / M^{2 +}$ , the more is the tendency of $M^{3 +}$ to get reduced to $M^{2 +}$ and lesser is the stability of the metal in $+ 3$ oxidation state.

According to the given data for $M^{+ 3} / M^{+ 2}$ ,

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

Order of the reduction potential values: ${Mn}^{+ 3} > {Fe}^{+ 3} > {Cr}^{+ 3}$

So, ${Fe}^{3 +}$ is less stable than ${Cr}^{+ 3}$ but more stable than ${Mn}^{+ 3}$ .

(b)

The metal having a lesser reduction potential or a greater oxidation potential will be most easily oxidised.

According to the data for $M^{+ 2} / M$ ,

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

Order of reduction potential: ${Fe}^{2 +} > {Cr}^{2 +} > {Mn}^{2 +}$

So, ease of oxidation: $Mn > Cr > Fe$ .

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