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Indicate the steps in the preparation of: (i) K2Cr2O7 from chromite ore. (ii) KMnO4 from pyrolusite ore.

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Indicate the steps in the preparation of:

(i) K2Cr2O7 from chromite ore.

(ii) KMnO4 from pyrolusite ore.

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(i) Potassium dichromate (K2Cr2O7) is prepared from chromite ore (FeCr2O4) in the following steps.

Step (1): Preparation of sodium chromate

FeCr2O4 + 16NaOH + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8H2O

Step (2): Conversion of sodium chromate into sodium dichromate

2Na2CrO4 + conc.H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

Step(3): Conversion of sodium dichromate to potassium dichromate

Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl

Potassium chloride being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filtration.

The dichromate ion (Cr2O72-) exists in equilibrium with chromate (CrO42-) ion at pH 4. However, by changing the pH, they can be interconverted.

CrO42- \(\underset{Acid}{\stackrel{Alkali}{\longrightarrow}}\) 2HCrO4-\(\underset{Acid}{\stackrel{Alkali}{\longrightarrow}}\)Cr2O72-

(ii) Potassium permanganate (KMnO4) can be prepared from pyrolusite (MnO2). The ore is fused with KOH in the presence of either atmospheric oxygen or an oxidising agent, such as KNO3 or KClO4, to give K2MnO4.

2MnO2 + 4KOH + O2 [/latex]\overset{heat}\longrightarrow[/latex] 2K2MnO4 + 2H2O

The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution.

Electrolytic oxidation

K2MnO4 ↔️ 2K+ + MnO42-

H2O ↔️ H+ + OH-

At anode, manganate ions are oxidized to permanganate ions.

MnO42- ↔️ MnO4+ e-

Green       Purple

Oxidation by chlorine

2K2MnO4 + Cl2 → 2KMnO4 + 2KCl

2MnO42- + Cl2 → 2MnO42-+ 2Cl-

Oxidation by ozone

2K2MnO4 + O3 + H2O → 2K2MnO4 + 2KOH + O2

2MnO42- + O3 + H2O → 2MnO42- + 2OH- + O2

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