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

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

Grade:9

1 Answers

Sunil Kumar FP
askIITians Faculty 183 Points
6 years ago
(1)Potassium dichromate (K2Cr2O7) is prepared from chromite ore FeCr2O4. The chromite ore is fused with sodium or potassium carbonate in free access of air.

4FeCr2O4 + 8Na2CO3 + 7O2 -------> 8Na2CrO4 + 2FeO3 + 8CO2

The yellow solution of sodium chromate is filtered and acidified with sulfuric acid to give a solution from which orange sodium dichromate , Na2Cr2O7 2H2O can be crystallized.

2Na2CrO4 + 2H+ ---------> Na2Cr2O7 + 2Na + H2O

Sodium dichromate is more soluble than potassium dichromate. Hence sodium dichromate when fused with KCl forms orange crystals of potassium dichromate.

Na2Cr2O7 + 2KCl --------> K2Cr2O7 + 2NaCl

(2)Potassium permanganate is manufactured on a large scale due to its manifold uses in the laboratory. In the first stage, pyrolusite, which is the mineral form of manganese dioxide, is fused with potassium hydroxide and heated in air or with potassium nitrate (a source of oxygen). This leads to the formation of potassium manganate, which on electrolytic oxidation in alkaline solution gives potassium permanganate.

MnO2 + 2OH− + O2 → MnO42− + H2O
MnO42− + Cl2 → MnO4− + 2Cl−

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