Question

How is potassium dichromate prepared from a sample of chromite ore? Give balanced equations for the chemical reactions involved.

Answer 1

Preparation of potassium dichromate from chromite ore involves the following steps:

(i) Conversion of chromite ore to sodium chromate: It is achieved by roasting finely powdered chromite ore with sodium carbonate and quick lime in excess of air.

\[\ce{\underset{\text{Chromite ore}}{4FeCr2O4} + \underset{\text{Sodium carbonate}}{8Na2CO3} + 7O2 ->[Cao + air]\underset{\text{Ferric oxide}}{2Fe2O3} + \underset{\text{Sodium chromate}}{8Na2CrO4} + 8CO2}\]

(ii) Conversion of sodium chromate to sodium dichromate: It is achieved by the action of conc. H₂SO₄ on sodium chromate solution.

\[\ce{2Na2CrO4 + \underset{\text{conc.}}{H2SO4} ->\underset{\text{Sodium dichromate}}{Na2Cr2O7} + Na2SO4 + H2O}\]

(iii) Conversion of sodium dichromate to potassium dichromate: It is achieved by treating hot conc. solution of sodium dichromate with KCl.

\[\ce{Na2Cr2O7 + 2KCl -> \underset{\text{Potassium dichromate}}{K2Cr2O7} + 2NaCl}\]

The resulting solution on cooling yields orange crystals of potassium dichromate.