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Q4.17(i):

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

(i) the stability of \(Fe^{3+}\) in acid solution as compared to that of \(Cr^{3+}\) or \(Mn^{3+}\)

Solution :

1. Analysis of Standard Electrode Potentials ($E^o$)

To evaluate the stability of $M^{3+}$ ions in an aqueous acidic solution, we examine the standard electrode potential values ($E^o$) for the reduction process $M^{3+} + e^- \rightarrow M^{2+}$. A more positive (or less negative) value of $E^o(M^{3+}/M^{2+})$ indicates a higher tendency for the $M^{3+}$ ion to gain an electron, implying lower stability of the $3+$ oxidation state relative to the $2+$ state.

Based on standard electrochemical data for the transition series:

System ($M^{3+}/M^{2+}$) $E^o$ value (V)
$Cr^{3+}/Cr^{2+}$ $-0.41$
$Mn^{3+}/Mn^{2+}$ $+1.57$
$Fe^{3+}/Fe^{2+}$ $+0.77$

2. Comparative Stability Analysis

  • For $Mn^{3+}$: The $E^o$ value is $+1.57\text{ V}$. [Per the high positive value], $Mn^{3+}$ is an extremely strong oxidizing agent and is highly unstable in aqueous solution. It readily reduces to $Mn^{2+}$ ($d^5$ configuration, which is exceptionally stable due to half-filled $t_{2g}$ and $e_g$ orbitals).
  • For $Fe^{3+}$: The $E^o$ value is $+0.77\text{ V}$. [Per standard electrochemical trends], this value indicates moderate ease of reduction to $Fe^{2+}$. While $Fe^{3+}$ ($d^5$) is stable, it is less stable than the $Mn^{3+}$ system's reduction potential would suggest, but it remains significantly more stable than $Mn^{3+}$ in acid.
  • For $Cr^{3+}$: The $E^o$ value is $-0.41\text{ V}$. [Per the negative value], the reduction of $Cr^{3+}$ to $Cr^{2+}$ is thermodynamically non-spontaneous. Thus, $Cr^{3+}$ is the most stable state among the three in an acidic medium. The stability of $Cr^{3+}$ ($d^3$) arises from the stable $t_{2g}^3$ configuration in an octahedral field [Crystal Field Theory].

3. Visualizing Electronic Configurations and Stability

Cr³⁺ (d³): t₂g³ (Stable, inert) Fe³⁺ (d⁵): Half-filled (Relatively stable) Mn³⁺ (d⁴): Unstable (Reduces to d⁵ Mn²⁺)

4. Logical Conclusion of Stability Order

Comparing the potentials, we observe the trend in $E^o(M^{3+}/M^{2+})$ is: $Mn^{3+} > Fe^{3+} > Cr^{3+}$. Since stability is inversely proportional to the ease of reduction ($E^o$):

  1. $Cr^{3+}$ is the most stable because its reduction to $Cr^{2+}$ is non-spontaneous ($E^o = -0.41\text{ V}$).
  2. $Fe^{3+}$ has intermediate stability, as the value is moderately positive ($+0.77\text{ V}$).
  3. $Mn^{3+}$ is the least stable because its reduction to the highly stable $Mn^{2+}$ ($d^5$) is strongly favored ($E^o = +1.57\text{ V}$).

Final Solution: The stability order of $M^{3+}$ ions in acidic solution is $Cr^{3+} > Fe^{3+} > Mn^{3+}$. Therefore, $Fe^{3+}$ is more stable than $Mn^{3+}$ but less stable than $Cr^{3+}$.


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