Describe the preparation of potassium permanganate. How does the acidified potassium permanganate react with : (i) `SO_(2)` (ii) oxalic acid (iii) `FeSO_(4)`. Write the ionic equations for these reactions.

Explanation

Preparation of potassium permanganate. `KMnO_(4)` is prepared on a large scale from pyrolusite. It is fused with KOH in the presence of air forming a green mass of potassium manganate.
`2MnO_(2) + 4KOH + O_(2) to underset("Pot. manganate")(2K_(2)Mnoverset(Delta)O_(4))+ 2H_(2)O`
Potassium manganate is oxidised electrochemically to permanganate ion in the cathode compartment.
At anode :
`MnO_(4)^(2-) to MnO_(4)^(-)+ e^(-)`
At cathode :
`2H^(+)+ 2e^(-) to 2H`
`2H to H_(2)`
Properties. The important properties of `KMnO_(4)` are discussed below :
1. It is a dark violet crystalline solid having a metallic lustre
2. Its melting point is 523 K.
3. It is slightly soluble in water.
4. When heated strongly, it decomposes to give oxygen
`2KMnO_(4) to K_(2)MnO_(4)+ MnO_(2)+ O_(2)`
5. Oxidising character. Potassium permanganate acts as a powerful oxidising agent in neutral, alkaline or acidic solution because it liberales nascent oxygen as
Neutral solution,
`2KmnO_(4) +H_(2)O to 2KOH+ 2MnO_(2)+ 3O`
or `MnO_(4)^(-)+ 2H_(2)O+ 3e^(-) to MnO_(2)+ 4OH^(-)`
Alkaline solution
` 2KmnO_(4)+2KOH to 2K_(2)MnO_(4)+ H_(2)O+O`
or `MNO_(4)^(-)+ e^(-) to MNO_(4)^(2-)`
Acidic solution
`2KmnO_(4)+ 3H_(2)SO_(4) to K_(2)SO_(4) + 2MnSO_(4)+ 3H_(2)O+ 5O`
or `MnO_(4)^(-)+ 8H^(+)+ 5e^(-) to Mn^(2+)+ 4H_(2)O`
However, it is frequently used in acidic medium in the laboratory. The important oxidation reactions of acidified KMnO4 in the laboratory are :However, it is frequently used in acidic medium in the laboratory. The important oxidation reactions of acidified KMnO4 in the laboratory are :
(i) It oxidises ferrous salts to ferric salis.
`2FeSO_(4) + H_2SO_4 +O to Fe_(2) (SO_(4))_(3) + H_(2)O`
or `5Fe^(2+)+ MnO_(4)^(-) + 8H^(+) to Mn^(2+) + 4H_(2)O+ 5Fe^(3+)`
(ii) It oxidises oxalates or oxulic acid to carbon dioxide
`underset(COOH)underset(|)COOH+O overset(50-60^(@)C)to 2CO_(2)+ H_(2)O`
`underset(COO)underset(|)COO+ 2MNO_(4)^(-)+ 16H^(+)overset(50-60^(@)C)to 2MN^(2+)+ 8H_(2)O+ 10CO_(2)`
(iii) It oxidises potassium iodide to iodine.
`2KI+H_(2)O+O to 2KOH+I_(2)`
or `10I^(-)+ 2MNO_(4)^(-)+ 16H^(+) to 2MN^(2+)+ 8H_(2)O+ 5I_(2)`
(iv) oxidises sulphur dioxide to sulphuric acid.
`SO_(2)+ O + H_(2)O to H_(2)SO_(4)`
`5SO_(2)+ 2MnO_(4)^(-)+ 2H_(2)O to 2Mn^(2+) + 5SO_(4)^(2-)+4H^(+)`
Uses of `KMnO_(4)`. It is used:
(i) as an oxidising agent in the laboratory and in industry.
(ii) as a disinfectant, germicide and deodorant.
(iii) in qualitative and quantitative analysis. (iv) as alkaline solution in organic reactions under the name Baeyer.s reagent.
(v) for purification of water.