Question

Identify the case(s) where there is change in oxidation number

• A. Acidified solution of CrO$_4^{2-}$
• B. SO$_2$ gas bubbled through an acidic solution of Cr$_2$O$_7^{2-}$
• C. Alkaline solution of Cr$_2$O$_7^{2-}$
• D. Ammoniacal solution of CrO$_4^{2-}$
• E. Aqueous solution of CrO$_2$Cl$_2$ in NaOH

Hint:

Chromate–dichromate interconversion is not a redox reaction.

Answer 1

The Correct Option is B

Concept:
Only redox reactions involve change in oxidation number.

Step 1: Analyze interconversions.
\[ 2CrO_4^{2-} + 2H^+ \leftrightarrow Cr_2O_7^{2-} + H_2O \] No oxidation change.

Step 2: Check option (B).
\[ Cr_2O_7^{2-} + SO_2 + H^+ \rightarrow Cr^{3+} + SO_4^{2-} \] Cr: +6 → +3 (reduction)
S: +4 → +6 (oxidation)

Step 3: Other options.
Only involve equilibrium or hydrolysis, not redox. Final Conclusion:
Only option (B) shows change in oxidation number.