Question

A green chromium compound (A) on fusion with alkali gives a yellow compound (B) which on acidification gives an orange coloured compound (C). ‘C’ on treatment with NH₄Cl gives an orange coloured product (D), which on heating decomposes to give back (A). Identify A, B, C and D. Write equations for reactions.

Answer 1

The green chromium compound (A) is chromium(III) oxide, Cr₂O₃.
On fusion with alkali like sodium hydroxide in the presence of air or oxygen, it gives a yellow coloured compound (B), sodium chromate, Na₂CrO₄.
Acidification of sodium chromate gives an orange coloured compound (C), sodium dichromate, Na₂Cr₂O₇.
This reacts with ammonium chloride to give ammonium dichromate, (NH₄)2Cr₂O₇, which is orange in colour and is the compound (D).
On heating, D decomposes to form green Cr₂O₃ again, hence regenerating compound A.

A = Cr₂O₃ (Chromium(III) oxide, green)

B = Na₂CrO₄ (Sodium chromate, yellow)

C = Na₂Cr₂O₇ (Sodium dichromate, orange)

D = (NH₄)₂Cr₂O₇ (Ammonium dichromate, orange)

Formation of B from A (fusion with alkali in the presence of oxygen):

\[\ce{Cr2O3 + 4NaOH + 3O2 -> 2Na2CrO4 + 2H2O}\]

Formation of C from B (acidification of sodium chromate):

\[\ce{2Na2CrO4 + H2SO4 -> Na2Cr2O7 + Na2SO4 + H2O}\]

Formation of D from C (reaction with ammonium chloride):

\[\ce{Na2Cr2O7 + 2NH4Cl -> (NH4)2Cr2O7 + 2NaCl}\]

Decomposition of D to regenerate A:

\[\ce{(NH4)2Cr2O7 ->[Heat] Cr2O3 + N2 + 4H2O}\]