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 iodide to (D). Identify (A), (B), (C) and (D).

Answer 1

I. Yellow sodium chromate (Na₂CrO₄) is produced when fused chromite (FeCr₂O₄) and sodium carbonate (Na₂CO₃) react in the presence of air.

\[\ce{4FeCr2O4 + 8Na2CO3 + 7O2 ->\underset{(A)}{\underset{(Sodium Chromate)}{8Na2CrO4}} + 2Fe2O3 + 8CO2}\]

II. Sodium dichromate (Na₂Cr₂O₇) is produced when sodium chromate (Na₂Cr₂O₄) is acidified.

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

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 (K₂Cr₂O₇) solution is acidic. It oxidizes iodide (I⁻) to iodine (I₂), acting as an oxidizing agent.

\[\ce{CrO^{2-}_7 + 6I- + 14H+ ->2Cr^{3+} + \underset{(D)}{\underset{Iodine}{3I2}} + 7H2O}\]