d & f Block Elements Part - 4 | Class 12th

 ü  Preparation of Potassium dichromate (K₂Cr₂O₇): It is prepared by the reaction of chromate ore (FeCr₂O₄) with sodium carbonate in excess of air.

(i)    Conversion of chromite ore to sodium chromate

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

(ii)  Acidification of sodium chromate to sodium dichromate

2Na₂CrO₄ + 2 H⁺ → Na₂Cr₂O₇ + 2 Na⁺ + H₂O

(iii)  Conversion of sodium dichromate to potassium dichromate

Na₂Cr₂O₇ + 2 KCl → K₂Cr₂O₇ + 2 NaCl

Ø  Properties

·        Physical properties:

(i)     It forms orange red crystals which melt at 396 ⁰C.

(ii)   It is moderately soluble in cold water but freely soluble in hot water.

·        Chemical properties:

The chromate and dichromate are inter-convertible in aqueous solution depending upon pH of the solution. Chromate on acidification gives dichromate and the dichromate on treating with alkali gives chromate.

2 CrO₄^2– + 2H⁺ → Cr₂O₇^2– + H₂O

Cr₂O₇^2– + 2OH^- → 2 CrO₄^2– + H₂O

The oxidation state of chromium in chromate and dichromate is +6.

   The structures of chromate ion, CrO₄^2– and the dichromate ion, Cr₂O₇^2– are shown  

   below.

Sodium and potassium dichromates are strong oxidising agents. The sodium salt has a greater solubility in water and is extensively used as an oxidising agent in organic chemistry.

o   Action of Alkalies: When an alkali is added to an orange red solution of dichromate, a yellow solution results due to the formation of chromate.

K₂Cr₂O₇ + KOH → K₂CrO₄ + H₂O

 

o   Oxidising Properties: K₂Cr₂O₇ is a good oxidising agent in acidic medium. Its oxidising action can be represented as follows:

Cr₂O₇^2– + 14H⁺ + 6e^– → 2Cr³⁺ + 7H₂O

Thus, acidified potassium dichromate will oxidise

 

1)    Iodides to iodine

6 I ^– → 3I₂ + 6 e^-

Cr₂O₇^2– + 14H⁺ + 6e^– → 2Cr³⁺ + 7H₂O

6I ^– + Cr₂O₇^2– + 14H⁺  →  3I₂ + 2Cr³⁺ + 7H₂O        Overall Reaction

 

2)    Sulphides to sulphur

3S^2- →3 S + 6e^-

Cr₂O₇^2– + 14H⁺ + 6e^– → 2Cr³⁺ + 7H₂O

3S^2- + Cr₂O₇^2– + 14H⁺ → 3S + 2Cr³⁺ + 7H₂O            Overall Reaction

 

3)    Tin(II) to tin(IV)

3 Sn²⁺ → 3Sn⁴⁺ + 6 e^-

Cr₂O₇^2– + 14H⁺ + 6e^– → 2Cr³⁺ + 7H₂O

3Sn²⁺ + Cr₂O₇^2– + 14H⁺ → 3 Sn⁴⁺ + 2Cr³⁺ + 7H₂O        Overall Reaction

4)    Iron(II) (ferrous) to iron(III) (ferric)

6 Fe²⁺ → 6Fe³⁺ + 6 e^-

Cr₂O₇^2– + 14H⁺ + 6e^– → 2Cr³⁺ + 7H₂O

Cr₂O₇^2– + 14 H⁺ + 6 Fe²⁺ → 2 Cr³⁺ + 6 Fe³⁺ + 7 H₂O        Overall Reaction