Question

Potassium dichromate is an orange coloured crystal and is an important compound used as an oxidant in many reactions. 

a) How do you prepare K₂Cr₂O₇ from chromite ore?

b) How will you account for the colour of potassium dichromate crystals?

Answer 1

a) The chromite ore is fused with sodium carbonate in free access of airto get sodium chromate.

4FeCr₂O₄+8Na₂CO₃+7O₂⟶8Na₂CrO₄+2Fe₂O₃+8CO₂

The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to get sodium dichromate.

2Na₂CrO₄+2H⁺→Na₂Cr₂O₄+2Na⁺+H₂O

The sodium dichromate solution is treated with potassium chloride to get potassium dichromate.

Na₂Cr₂O₇+2KCl→K₂Cr₂O₇+2NaCl

Orange crystals of potassium dichromate crystallise out.

b) This is due to charge transfer spectra i.e., Chromium being a transition element has vacant d – orbitals.