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Common Oxidation States

For M 2+/ M a...

Question

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.

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Solution

$E_{(\frac{C r^{3 +}}{C r^{2 +}})}^{\circ} i s n e g a t i v e (- 0.4 V)$ . It shows the stability of $C r^{3 +}$ ions, i.e. $C r^{3 +}$ in solution cannot be reduced to $C r^{2 +}$ ions. Further $M n^{3 +}$ has high positive $E^{\circ}$ value so it is easily converted to $M n^{2 +}$ as compared to the conversion of $F e^{3 +}$ into $F e^{2 +}$ . Thus, the order of relative stabilities of different ions is
$M n^{3 +} < F e^{3 +} < C r^{3 +}$

From the $E^{\circ}$ values, the order of oxidation of the metal to the divalent cation is:
$M n > C r > F e$

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Similar questions

M^{2 +} / M

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

E^{⊖}

F e^{3 +}

C r^{3 +}

M n^{3 +}

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.

M^{2 +} / M

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

E^{\circ}

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

E^{0}

C r^{2 +} (a q) + 2 e^{-} \to C r (s); E^{0} = - 0.91 V C r^{3 +} (a q) + e^{-} \to C r^{2 +} (a q); E^{0} = - 0.41 V

M n^{2 +} (a q) + 2 e^{-} \to M n (s); E^{0} = - 1.2 V M n^{3 +} (a q) + e^{-} \to M n^{2 +} (a q); E^{0} = + 1.51 V

F e^{2 +} (a q) + 2 e^{-} \to F e (s); E^{0} = - 0.44 V F e^{3 +} (a q) + e^{-} \to F e^{2 +} (a q); E^{0} = + 0.77 V

For the four successive transition elements (Cr, Mn, Fe and Co), what is the order of stability of their +2 states?

(At. Nos. Cr = 24, Mn = 25, Fe = 26, Co = 27)

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