Question

When FeCr₂O₄ is fused with Na₂CO₃ in the presence of air it gives a yellow solution of compound (А). Compound (A) on acidification gives compound (B). Compound (B) on reaction with KCl forms an orange coloured (C). An acidified solution of compound (C) oxidises Na₂SO₃ to (D). Identify (A), (B), (C) and (D).

Answer 1

I. When chromite ore (FeCr₂O₄) is fused with sodium carbonate in the presence of excess air, it produces sodium chromate and iron oxide.

\[\ce{\underset{Ore}{\underset{Chromite}{4FeCr2O4}} + \underset{carbonate}{\underset{Sodium}{8Na2CO3}} + 7O2 ->\underset{(A)}{\underset{chromate}{\underset{Sodium}{8Na2CrO4}}} + \underset{Iron(III) oxide}{2Fe2O3} + 8CO2}\]

II. The yellow sodium chromate solution is filtered and then acidified with sulphuric acid to obtain solution (B), from which orange sodium dichromate is formed.

Na₂Cr₂O.2H₂O can be crystallised.

\[\ce{2Na2CrO4 + H2SO4 ->\underset{(B)}{\underset{Sodium dichromate}{Na2Cr2O7}} + Na2SO4 + H2O}\]

III. The orange compound potassium dichromate (K₂Cr₂O₇) is created when sodium dichromate (Na₂Cr₂O₇) and potassium chloride (KCl) combine.

\[\ce{Na2Cr2O7 + 2KCl ->\underset{(C)}{\underset{Potassium dichromate}{K2Cr2O7}} + 2NaCl}\]

IV. Potassium dichromate acts as a strong oxidising agent and oxidises sodium sulphite to sodium sulphate (Na₂SO₄) in an acidic medium.

\[\ce{K2Cr2O7 + 3Na2SO3 + 4H2SO4 -> \underset{(D)}{\underset{Sodium sulphate}{3Na2SO4}} + K2SO4 + Cr2(SO4)3 + 4H2O}\]