For M²⁺/M and M³⁺/M²⁺systems, the values for some metals are as follows:

Cr²⁺/Cr −0.9V

Cr³/Cr²⁺ −0.4 V

Mn²⁺/Mn −1.2V

Mn³⁺ /Mn²⁺ +1.5 V

Fe²⁺ /Fe −0.4V

Fe³⁺/Fe²⁺ +0.8 V

Use this data to comment upon:

(i) The stability of Fe³⁺ in acid solution as compared to that of Cr³⁺ or Mn³⁺and

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

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Solution

(i) The value for Fe³⁺/Fe²⁺ is higher than that for Cr³⁺/Cr²⁺ and lower than that for Mn³⁺/Mn²⁺. So, the reduction of Fe³⁺ to Fe²⁺ is easier than the reduction of Mn³⁺ to Mn²⁺, but not as easy as the reduction of Cr³⁺to Cr²⁺. Hence, Fe³⁺ is more stable than Mn³⁺, but less stable than Cr³⁺. These metal ions can be arranged in the increasing order of their stability as: Mn³⁺ < Fe³⁺ < Cr³⁺

(ii) The reduction potentials for the given pairs increase in the following order.

Mn²⁺/ Mn < Cr²⁺/ Cr < Fe²⁺/Fe

So, the oxidation of Fe to Fe²⁺ is not as easy as the oxidation of Cr to Cr²⁺ and the oxidation of Mn to Mn²⁺. Thus, these metals can be arranged in the increasing order of their ability to get oxidised as: Fe < Cr < Mn

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Similar questions

Q. For

M^{2 +} / M

and

M^{3 +} / M^{2 +}

systems the

E^{⊖}

values for some metals are as follows:

$C r^{2 +} / C r$	$- 0.9 V$ $	$C r^{3} / C r^{2 +}$	$- 0.4 V$
$M n^{2 +} / M n$	$- 1.2 V$	$M n^{3 +} / M n^{2 +}$	$+ 1.5 V$
$F e^{2 +} / F e$	$- 0.4 V$	$F e^{3 +} / F e^{2 +}$	$+ 0.8 V$

Use this data to comment upon:
(i) the stability of

F e^{3 +}

in acid solution as compared to that of

C r^{3 +}

M n^{3 +}

and
(ii) the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal

For $\frac{M^{2 +}}{M}$ and $\frac{M^{3 +}}{M^{2 +}}$ systems the $E^{\circ}$ values for some metals are as follows:
$\frac{C r^{2 +}}{C r - 0.9 V}; \frac{C r^{3 +}}{C r^{2 +} - 0.4 V} \frac{M n^{2 +}}{M n - 1.2 V}; \frac{M n^{3 +}}{M n^{2 +}} + 1.5 V \frac{F e^{2 +}}{F e - 0.4 V}; \frac{F e^{3 +}}{F e^{2 +}} + 0.8 V$
Use this data to comment upon
(i) The stability of $F e^{3 +}$ in acid solution as compared to that of $C r^{3 +}$ or $M n^{3 +}$

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

Q. For

M^{2 +} / M

and

M^{3 +} / M^{2 +}

systems the

E^{\circ}

values for some metals are as follows:

$C r^{2 +} / C r$	$- 0.9 V$	$C r^{3 +} / C r^{2 +}$	$- 0.4 V$
$M n^{2 +} / M n$	$- 1.2 V$	$M n^{3 +} / M n^{2 +}$	$+ 1.5 V$
$F e^{2 +} / F e$	$- 0.4 V$	$F e^{3 +} / F e^{2 +}$	$+ 0.8 V$