When chromite ore FeCr2O4 is fused with NaOH in presence of air, a yellow-coloured compound (A) is obtained, which on acidification with dilute sulphuric acid gives a compound (B). Compound (B) on reaction with KCl forms an orange coloured crystalline compound (C).
(i) Write the formulae of the compounds (A), (B) and C.
(ii) Write one use of compound (C).
Solution
Chromite ore, on fusion with NaOH, forms sodium chromate (yellow-coloured compound):
4 FeCr2O4 + 16 NaOH + 7 O2 →8 Na2CrO4 + 2 Fe2O3 + 8 H2O
(A) = Na2CrO4
Sodium chromate, on acidification, gives sodium dichromate (B): 2 Na2CrO4 + H2SO4 →Na2Cr2O7 + Na2SO4 + H2O
(B) = Na2Cr2O7
Sodium dichromate, on reaction with KCl, forms potassium dichromate (orange-coloured crytalline compound):
Na2Cr2O7 + 2 KCl →K2Cr2O7 + 2 NaCl
(C) = K2Cr2O7
One use of potassium dichromate is that it is a powerful oxidising agent. It is used in preparation of azo compounds. It is also used in leather industry.
Disclaimer: Students can write any one use of potassium dichromate.
