Question

Complete and balance the following equations:
(i) $2Na_2CrO_4 + 2H^+ \longrightarrow$
(ii) $5S^{2-} + 2MnO_4^- + 16H^+ \longrightarrow$

Hint:

Chromate ($CrO_4^{2-}$) and Dichromate ($Cr_2O_7^{2-}$) are interconvertible depending strictly on pH.

Answer 1

Step 1: Concept
Redox reactions of d-block transition metal compounds in acidic medium.

Step 2: Meaning
(i) Chromate to dichromate conversion. (ii) Oxidation of sulfide by permanganate.

Step 3: Analysis
(i) In acidic medium, yellow chromate ions convert to orange dichromate ions.
$2Na_2CrO_4 + 2H^+ \longrightarrow Na_2Cr_2O_7 + 2Na^+ + H_2O$.
(ii) Permanganate reduces to $Mn^{2+}$ in acidic medium, oxidizing sulfide to elemental sulfur.
$5S^{2-} + 2MnO_4^- + 16H^+ \longrightarrow 2Mn^{2+} + 5S + 8H_2O$.

Step 4: Conclusion
The balanced equations follow the conservation of mass and charge.

Final Answer:
(i) $2Na_2CrO_4 + 2H^+ \longrightarrow Na_2Cr_2O_7 + 2Na^+ + H_2O$
(ii) $5S^{2-} + 2MnO_4^- + 16H^+ \longrightarrow 2Mn^{2+} + 5S + 8H_2O$