Question

How do you prepare K₂Cr₂O₇ from chromite ore?

Hint:

Remember the color changes as a guide for the steps: Chromite Ore (Brown/Black) $\to$ Chromate (Yellow) $\to$ Dichromate (Orange).
Also, note that the conversion of chromate to dichromate is a reversible pH-dependent process: chromate is stable in basic medium, while dichromate is stable in acidic medium.

Answer 1

Concept:
The industrial preparation involves three main steps:
1. Preparation of Sodium Chromate.
2. Conversion of Sodium Chromate to Sodium Dichromate.
3. Conversion of Sodium Dichromate to Potassium Dichromate.

Step 1: Preparation of Sodium Chromate.
Chromite ore (FeCr₂O₄) is fused with sodium carbonate in free access of air:
4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂

Step 2: Conversion to Sodium Dichromate.
The yellow solution of sodium chromate is filtered and acidified with sulphuric acid:
2 Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O

Step 3: Conversion to Potassium Dichromate.
Sodium dichromate is treated with potassium chloride. Potassium dichromate, being less soluble, crystallizes out:
Na₂Cr₂O₇ + 2 KCl → K₂Cr₂O₇ + 2 NaCl