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The d-and f-Block Elements — Chemistry STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceChemistryThe d-and f-Block Elements3 Marks
Question
Explain chemical properties of $\ce{KMnO _4}$
✓

Answer

$\rightarrow \ce{KMnO} _4$ act as strong oxidising agent in neutral, alkaline and in acidic medium.
$(1)$ In Acidic Solution
$(a)$ Iodine is liberated from potassium iodide:
$\begin{array}{l}2 MnO _4^{-}+16 H ^{+}+10 e ^{-} \rightarrow 2 \ce{Mn ^{2+}x+8 H _2 O} \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \  10 I ^{-} \rightarrow 5 I _2+10 e ^{-} \\ \hline 2 \ce{MnO _4}^{-}+16 H ^{+}+10 I^{-} \rightarrow 2 \ce{Mn ^{2+}+5 I _2+8 H _2 O} \\\end{array}$
$(B) Fe ^{2+}$ ion (green) is converted to $Fe ^{3+} ($yellow$)$ :
$\begin{aligned} \ce{MnO _4^{-}+8 H ^{+}+5 e ^{-}} & \rightarrow \ce{Mn ^{2+}+4 H _2 O} \\
5 \ce{Fe ^{2+}} & \rightarrow 5 \ce{Fe ^{3+}+4 e ^{-}} \\ \hline
5 \ce{Fe ^{2+}+ MnO _4^{-}+8 H ^{+}} & \rightarrow \ce{Mn ^{2+}+4 H _2 O +5 Fe ^{3+}}
\end{aligned}$
$(c)$ Oxalate ion or oxalic acid is oxidised at $333 K.$
$\begin{array}{c}2 \ce{MnO _4^{-}+16 H ^{+}+10 e ^{-}} \rightarrow 2 \ce{Mn ^{2+}+8 H _2 O} \\ 5 \ce{C _2 O _4^{2-} \rightarrow 10 CO _2+10 e ^{-}} \\ \hline 5 \ce{C _2 O _4^{2-}+2 MnO _4^{-}+16 H ^{+}} \rightarrow 2 \ce{Mn ^{2+}+10 CO _2+8 H _4}\end{array}$
$(d)$ Hydrogen sulphide is oxidised, sulphur being precipitated:
$\begin{array}{l} \ce{H _2 S \rightarrow 2 H ^{+}+ S ^{2-} \\ 5 S^{2-}+2 MnO _4^{-}+16 H ^{+}} \rightarrow 2 \ce{Mn ^{2+}+8 H _2 O +5 S} \end{array}$
$(e)$ Sulphurous acid or sulphite is oxidised to sulphate or sulphuric acid:
$\begin{array}{r}5 \ce{SO _3^{2-}+2 MnO _4^{-}+6 H ^{+}} \rightarrow 2 \ce{Mn ^{2+}+3 H _2 O
+5 SO_4} ^{2-}\end{array}$
$(f)$ Nitrite is oxidised to nitrate:
$5 \ce{NO _2^{-}+2 MnO _4^{-}+6 H ^{+} \rightarrow 2 Mn ^{2+}+5 NO _3^{-}+3 HO _2}$
$(2)$ In neutral or faintly alkaline solutions:
$(a)$ A notable reaction is the oxidation of iodide to iodate:
$2 \ce{MnO _4^{-}+ H _2 O +\Gamma \rightarrow 2 MnO _2+2 OH ^{-}+ IO _3}$
$(b) $Thiosulphate is oxidised almost quantitatively to sulphate:
$8 \ce{MnO _4^{-}+3 S_2 O _3^{2-}+ H _2 O \rightarrow 8 MnO _2+6 SO _4^{2-}+2 OH^{-}}$
$(c)$ Manganous salt is oxidised to $\ce{MnO_2};$ the presence of zinc sulphate or zinc oxide catalyses the oxidation :
$2 \ce{MnO _4^{-}+3 Mn ^{2+}+2 H _2 O \rightarrow 5 MnO _2+4 H ^{+}}$

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