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Difficulty: Very hardOxidizing and Reducing Agents and Tests

An aqueous solution of a sodium salt XX completely decolorizes acidified potassium tetraoxomanganate(VII) solution. Subsequent addition of barium chloride solution to the resulting reaction mixture yields a white precipitate that is insoluble in dilute hydrochloric acid. What role does salt XX play in the redox reaction, and what is the chemical formula of the sulfur-containing anion in XX?

  1. Salt XX acts as a reducing agent, and the anion is SO32−SO_3^{2-}.Answer
  2. B
    Salt XX acts as an oxidizing agent, and the anion is SO32−SO_3^{2-}.
  3. C
    Salt XX acts as a reducing agent, and the anion is SO42−SO_4^{2-}.
  4. D
    Salt XX acts as an oxidizing agent, and the anion is SO42−SO_4^{2-}.

Answer

Salt XX acts as a reducing agent, and the anion is SO32−SO_3^{2-}.
In the reaction with acidified potassium tetraoxomanganate(VII), the manganate(VII) ion (MnO4−MnO_4^-) is reduced from oxidation state +7 to +2, causing the purple solution to become colorless. The species that causes this reduction must be a reducing agent. Trioxosulfate(IV) ions (SO32−SO_3^{2-}) contain sulfur in the +4 oxidation state and are oxidized by KMnO4KMnO_4 to tetraoxosulfate(VI) ions (SO42−SO_4^{2-}). The formed SO42−SO_4^{2-} ions then react with Ba2+Ba^{2+} from barium chloride to form a white insoluble precipitate of BaSO4BaSO_4. Thus, salt XX is a reducing agent containing SO32−SO_3^{2-}.

Step-by-Step Solution

1
Analyze the color change of acidified potassium tetraoxomanganate(VII).
The change from purple (MnO4−MnO_4^-) to colorless (Mn2+Mn^{2+}) indicates that MnO4−MnO_4^- is reduced.
Manganese is reduced from oxidation state +7 to +2 by gaining electrons from a reducing agent.
2
Determine the role of salt XX.
Salt XX provides electrons to reduce MnO4−MnO_4^-, meaning salt XX is oxidized and acts as a reducing agent.
A substance that undergoes oxidation and causes another species to be reduced is a reducing agent.
3
Identify the sulfur-containing anion from the precipitation test.
The oxidation product of salt XX reacts with Ba2+Ba^{2+} to form insoluble BaSO4BaSO_4. Therefore, the oxidized species formed in solution is SO42−SO_4^{2-}.
Trioxosulfate(IV) ion (SO32−SO_3^{2-}) has sulfur in oxidation state +4 and is oxidized by acidified KMnO4KMnO_4 to tetraoxosulfate(VI) ion (SO42−SO_4^{2-}), which gives a white precipitate of BaSO4BaSO_4 insoluble in HCl.

Key Concept

Oxidizing and Reducing Agents and Qualitative Redox Tests
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Topics

Oxidizing and Reducing Agents and Tests

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