Class 12 Chemistry D and F Block Elements Preparation of K2Cr2O7

Preparation of K2Cr2O7

Potassium dichromate, (K2Cr2O7) is an orange-ish inorganic chemical reagent. In different laboratory or industry it is basically used as an oxidizing agent usually for alcohols.

It can be prepared through the following process:

  • At first the fusion of chromite ore FeCr2O4 with sodium or potassium carbonate in the presence of access of air.

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

  • Solution of sodium chromate is first filtered and then acidified with a solution of sulfuric acid which results in an orange sodium dichromate solution Na2Cr2O7 2H2O can be crystallized.

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

  • Sodium dichromate is more soluble than potassium dichromate and therefore it is fused with KCl that leads to the formation of orange crystals of potassium dichromate.

Na2Cr2O7 + 2KCl --> K2Cr2O7 + 2NaCl

 

  • At pH equal to 4 the dichromates and chromates exists in equilibrium and can be inter convertible.

2CrO4 2- + 2H2+ --> Cr2O72- + H2O  

Cr2O7 2- + 2OH- --> 2CrO42- + H2O

 

  • The yellow colour of chromate changes to orange coloured dichromate in the presence of acidic medium whereas the dichromate changes into chromate in the presence of basic medium.

2CrO42- + 2H+ --> 2HCrO4- (Hydrogen chromate)

2HCrO4-  --> Cr2O72- + H2O  Dichromate (orange)

  • The chromate ion is tetrahedral and the dichromate ion consists of two tetrahedral sharing at one corner, with Cr-O-Cr bond angle 126 degree.

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