SELECT * FROM question_mgmt as q WHERE id=2102 AND status=1 SELECT id,question_no,question,chapter FROM question_mgmt as q WHERE courseId=3 AND subId=9 AND chapterId=63 and ex_no='2' AND status=1 ORDER BY CAST(question_no AS UNSIGNED) CBSE Free NCERT Solution of 12th chemistry The d-and f-Block Elements describe the preparation of potassium dichromate f

Question:

Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?

Answer:

Potassium dichromate is prepared from chromite ore (FeCr2O4) in the following steps.

Step (1):

Preparation of sodium chromate

4FeCr2O4     +   16NaOH   +   7O2    →      8NaCrO4    +   2Fe2O3  +  8H2O

 

Step (2):

Conversion of sodium chromate into sodium dichromate

2Na2CrO4 + conc.H2SO4    →   Na2Cr2O7  +  Na2SO4  +  H2O

 

Step(3): Conversion of sodium dichromate to potassium dichromate

Na2Cr2O7    +   2KCl       →       K2Cr2O7   +   2NaCl

 

Potassium dichromate being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filtration.

The dichromate (Cr2O2-7) ion exists in equilibrium with chromate (CrO2-4) ion at pH 4. However, by changing the pH, they can be interconverted.


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