Question

$KMn{O}_4$ acts as an oxidizing agent in:

• A. acidic medium only
• B. neutral and acidic media
• C. neutral and alkaline media
• D. neutral, acidic and alkaline media

Answer 1

The correct option is D neutral, acidic and alkaline media

$KMn{O}_4$ is a strong oxidizing agent with $Mn$ in $+7$ state. In acidic medium there is a change of 5 electrons whereas in neutral and basic medium there is a change of 5 and 1 electron repectively.
$\underset{v.f.=1(7-2)=5}{Mn{O}_4^{-}}\to M{n}^{2+}\text{(acid)}$
$\underset{v.f.=1(7-4)=3}{Mn{O}_4^{-}}\to Mn{O}_2\text{(basic/neutral)}$
$\underset{v.f.=1(7-6)=1}{Mn{O}_4^{-}}\to Mn{O}_4^{2-}\text{(highly basic)}$

The following reactions take place in acidic medium:
1) Hydrogen sulphide is oxidised, sulphur being precipitated.
$3H_{2}S+2KMn{O}_4\to 3S+2Mn{O}_2$

2) Sulphurous acid or sulphite is oxidised to a sulphate or sulphuric acid.
$2Mn{O}_4^{-}+5S{O}_3^{2-}+6H^{+}\to 2M{n}^{2+}+5S{O}_4^{2-}+3H_{2}O$

In neutral or faintly alkaline solutions:
(a) A notable reaction is the oxidation of iodide to iodate:
$2Mn{O}_4^{–}+H_{2}O+I^{–}\to 2Mn{O}_2+2O{H}^{–}+I{O}_3^{–}$
(b) Thiosulphate is oxidised almost quantitatively to sulphate:
$8Mn{O}_4^{–}+3S_2{O}_3^{2–}+H_{2}O——>8Mn{O}_2+6S{O}_4^{2–}+2O{H}^{–}$