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The d & f Block Elements — Chemistry STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceChemistryThe d & f Block Elements5 Marks
Question
Answer the following questions: For $M^{2+}/M$ and $M^{3+}/M^{2+}$^ systems, $E^\circ $ values for some metals are as follows:
$Cr^{2+}/Cr = - 0.9 V$ $Cr^{3+}/Cr^{2+} = - 0.4 V$
$Mn^{2+}/Mn = 0 1.2 V$ $Mn^{3+}/Mn^{2+} = +1.5 V$
$Fe^{2+}/Fe = - 0.4 V$ $Fe^{3+}/Fe^{2+} = +0.8 V$
Use this data to comment upon:
  1. The stability of $Fe^{3+}$​​​​​​​^ in acid solution as compared to that of $Cr^{3+}$​​​​​​​^ and $Mn^{3+}.$
  2. The ease with which iron can be oxidised as compared to the similar process for either Cr or Mn metals.
✓

Answer

a. Higher the reduction potential of a species, greater is the ease with which it undergo reduction. Among these pairs, $Mn ^{3+} / Mn ^{2+}$ has largest positive reduction potential. Hence $Mn ^{3+}$ can be easily reduced to $Mn ^{2+}$ i.e., $Mn ^{3+}$ is least stable. $Cr ^{3+} / Cr ^{2+}$ has a negative $E ^{\circ}$ value, therefore, $Cr ^{3+}$ is most stable. $Fe ^{3+} / Fe ^{2+}$ has a positive value but small. Hence, $Fe ^{3+}$ is more stable than $Mn ^{3+}$ but less stable than $Cr ^{3+}$.
b. Lower the reduction potential or higher the oxidation potential of a species, greater is the ease with which it undergo oxidation. Among these pairs, $Mn ^{2+} / Mn$ has the most negative reduction potential or most positive oxidation potential. Therefore, it will be most easily oxidised. Thus, the decreasing order of their ease of oxidation is $Mn > Cr > Fe$.

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