Question

Indicate the steps in the preparation of:

1. K2Cr2O7 from chromite ore.
2. KMnO4 from pyrolusite ore.

Answer 1

(i)
Potassium dichromate (K2Cr2O7) is prepared from chromite ore (FeCr2O4) in the following steps.
Step (1) :Preparation of sodium chromate
4FeCr2O4+16NaOH+7O2 $\rightarrow$ 8Na2CrO4+2Fe2O3+8H2O
Step (2) :Conversion of sodium chromate into sodium dichromate
2Na2CrO4+conc.H2SO4 $\rightarrow$ Na2Cr2O7+Na2SO4+H2O
Step(3) :Conversion of sodium dichromate to potassium dichromate
Na2Cr2O7+2KCl $\rightarrow$ 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 (Cr2$O^{2-}_7$) exists in equilibrium with chromate ($CrO^{2-}_4$) ion at pH4. However, by changing the pH, they can be interconverted.
$2Cr^{2-}_4 \underleftrightarrow{Acid \,Alkali}\; 2HCrO^-_4 \underleftrightarrow{Acid \,Alkali}\;Cr_2O^{2-}_7$

Chromate Hydrogen Dichromate
(Yellow) Chromate (Orange)

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(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 $\underrightarrow{heat}$ 2K2MnO4+2H2O
(green)
The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution.
Electrolytic oxidation
K2MnO4 $\leftrightarrow$ 2K++$MnO^{2-}_4$
H2O $\leftrightarrow$ H++OH-
At anode, manganate ions are oxidized to permanganate ions
MnO^{2-}_4$$\leftrightarrow $MnO^{-}_4$+e-
Green Purple
Oxidation by chlorine
2K2MnO4+Cl2 $\rightarrow$ 2KMnO4+2KCl
2$MnO^{2-}_4$+Cl2 $\rightarrow$ 2$MnO^{-}_4$+2Cl
Oxidation by ozone
2K2MnO4+O3+H2O $\rightarrow$ 2KMnO4+2KOH+O2
2$MnO^{2-}_4$+O3+H2O $\rightarrow$ 2$MnO^{2-}_4$+2OH-+O2