Question

When \( \text{FeCr}_2\text{O}_4 \) is fused with \( \text{Na}_2\text{CO}_3 \) in the presence of air, it gives a yellow solution of compound (A). Compound (A) on acidification gives compound (B). Compound (B) on reaction with KCl forms an orange-colored compound (C). An acidified solution of compound (C) oxidizes \( \text{Na}_2\text{SO}_3 \) to (D). Identify (A), (B), (C), and (D).

Hint:

Chromates and dichromates are strong oxidizing agents. Chromates are yellow in alkaline conditions, while dichromates are orange in acidic conditions.

Answer 1

Step 1: Identifying Compound (A). When \( \text{FeCr}_2\text{O}_4 \) is fused with \( \text{Na}_2\text{CO}_3 \) in the presence of air, it forms a yellow solution of sodium chromate (\( \text{Na}_2\text{CrO}_4 \)). Thus, \( A = \text{Na}_2\text{CrO}_4 \). 

Step 2: Identifying Compound (B). Upon acidification, \( \text{Na}_2\text{CrO}_4 \) is converted to potassium dichromate (\( \text{K}_2\text{Cr}_2\text{O}_7 \)), which is orange in color. Thus, \( B = \text{K}_2\text{Cr}_2\text{O}_7 \).

 Step 3: Identifying Compound (C). The reaction of \( \text{K}_2\text{Cr}_2\text{O}_7 \) with KCl leads to the formation of orange-colored \( \text{Cr}_2\text{O}_7^{2-} \) ions. Thus, \( C \) is an orange-colored chromium compound. Thus, \( C = \text{K}_2\text{Cr}_2\text{O}_7 \). \bigskip 

Step 4: Identifying Compound (D). An acidified solution of \( C \) oxidizes sodium sulfite (\( \text{Na}_2\text{SO}_3 \)) to sodium sulfate (\( \text{Na}_2\text{SO}_4 \)). This is a typical redox reaction where chromium compounds act as oxidizing agents. Thus, \( D = \text{Na}_2\text{SO}_4 \). So, the identities are:
\( A = \text{Na}_2\text{CrO}_4 \)
\( B = \text{K}_2\text{Cr}_2\text{O}_7 \)
\( C = \text{K}_2\text{Cr}_2\text{O}_7 \)
\( D = \text{Na}_2\text{SO}_4 \)