Question

Give the preparation and properties (acidic, basic, neutral medium) of

{KMnO}_{4}

.

Accepted Answer

We will study the answer in two parts.

(1) Preparation of potassium permanganate:

Step 1-The fusion of pyrolusite with potassium hydroxide and an oxidizing agent like potassium nitrate/atmospheric oxygen to form potassium manganate (green mass), which disproportionate in a neutral or acidic solution to form permanganate. Step-2 The Product is extracted with water and then oxidized by passing ozone/chlorine into the solution or electrolytically.

$2 {MnO}_{2} + 4 KOH + O_{2} \to 2 K_{2} {MnO}_{4} + 2 H_{2} O$

Electrolytic oxidation:

$K_{2} {MnO}_{4} \to 2 K^{+} + MnO 4^{2_}$

$H_{2} O \to H^{+} + {OH}^{-}$

At Anode, manganite ions are oxidized to permanganate ions.

$MnO 4^{2 -} \to MnO 4^{-} + e^{-} (Green) (purple) The purple coloured solution is concentrated by evaporation and forms crystals of potassium permanganate on cooling .$

At Cathode, hydrogen is evolved.

$2 H^{+} + 2 e^{-} \to H_{2} ↑$

(2) Properties of Potassium permanganate.

(a) It exists in the form of dark purple crystals which melts at $513 K$ .

(b) It is sparingly soluble in cold water but fairly soluble in hot water.

(c) It is affected by heat and is thermally unstable. $2 {KMnO}_{4} \to 2 K_{2} {MnO}_{4} + {MnO}_{2} + O_{2}$

(d) With cold concentrated sulphuric acid the reaction is violent and product decomposes explosively

$2 {KMnO}_{4} + 2 H_{2} {SO}_{4} (cold) \to {Mn}_{2} O_{7} + 2 {KHSO}_{4} + H_{2} O {Mn}_{2} O 7 \to Δ \to 2 {MnO}_{2} + 3 O_{2}$

with hot concentrated sulphuric acid the product is colorless solution.

$4 {KMnO}_{4} + 6 H_{2} {SO}_{4} (hot) \to 4 {MnSO}_{4} + 2 K_{2} {SO}_{4} + 6 H_{2} O + 5 O_{2}$

(e) Potassium permanganate is an excellent oxidising agent but behaves differently in the medium that is acidic, basic and neutral medium. Equation for reduction of ${KMnO}_{4}$
$In acidic medium : MnO 4 ¯ + 8 H^{+} + 5 e^{-} \to {Mn}^{2 +} + 4 H_{2} O In neutral medium : 2 H_{2} O + {MnO}_{4}^{-} + 3 e^{-} \to {MnO}_{2} + 4 {OH}^{-} In basic medium : MnO 4^{-} + e^{-} \to MnO 4^{- 2}$

Note:-Oxidizing effect of ${KMnO}_{4}$ is maximum in acidic medium and least in basic medium as in acidic medium the reduction in oxidation state of $Mn$ is maximum while it is the least in basic medium.

Hence, the acidic medium is used when you want strong oxidization and basic when very mild oxidization is required.

${KMnO}_{4} + KI = {MnO}_{2} + {KIO}_{3} (in basic medium) {Mn O_{4}}^{-} + I^{-} = {Mn}^{2 +} + I_{2} (in acidic medium) .$

Give the preparation and properties (acidic, basic, neutral medium) of ${KMnO}_{4}$ .

Important Questions on d- and f-Block Elements

Learn and Prepare for any exam you want !

Give the preparation and properties (acidic, basic, neutral medium) of KMnO4.

Important Questions on d- and f-Block Elements

Learn and Prepare for any exam you want !

Give the preparation and properties (acidic, basic, neutral medium) of ${KMnO}_{4}$ .