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Difficulty: MediumSulfur Allotropes, Hydrogen Sulfide, and Sulfur(IV) Oxide

When sulfur(IV) oxide (SO2SO_2) gas is bubbled into an acidified solution of potassium tetraoxomanganate(VII) (KMnO4KMnO_4), a distinct chemical change occurs. Which statement correctly describes the chemical role of sulfur(IV) oxide and the observed color change?

  1. Sulfur(IV) oxide acts as a reducing agent, decolorizing the purple solution.Answer
  2. B
    Sulfur(IV) oxide acts as an oxidizing agent, changing the purple solution to green.
  3. C
    Sulfur(IV) oxide acts as a dehydrating agent, forming a dark brown precipitate.
  4. D
    Sulfur(IV) oxide acts as an allotropic catalyst, depositing yellow solid sulfur.

Answer

Sulfur(IV) oxide acts as a reducing agent, decolorizing the purple potassium tetraoxomanganate(VII) solution.
In the reaction with acidified potassium tetraoxomanganate(VII), sulfur(IV) oxide (SO2SO_2) is oxidized to tetraoxosulfate(VI) ions (SO42−SO_4^{2-}), with sulfur's oxidation state increasing from +4 to +6. Because it undergoes oxidation, SO2SO_2 acts as a reducing agent. Consequently, the purple tetraoxomanganate(VII) ions (MnO4−MnO_4^-) are reduced to colorless manganese(II) ions (Mn2+Mn^{2+}).

Step-by-Step Solution

1
Determine the oxidation state of sulfur in SO2SO_2
Sulfur has an oxidation state of +4 in SO2SO_2.
Since oxygen has an oxidation state of -2, solving S+2(−2)=0S + 2(-2) = 0 gives S=+4S = +4.
2
Analyze the redox reaction between SO2SO_2 and acidified KMnO4KMnO_4
SO2SO_2 is oxidized to SO42−SO_4^{2-} (oxidation state of S increases from +4 to +6), while MnO4−MnO_4^- is reduced to Mn2+Mn^{2+}.
A substance that undergoes oxidation by losing electrons functions as a reducing agent.
3
Identify the visual observation associated with the reduction of MnO4−MnO_4^- to Mn2+Mn^{2+}
The intense purple color of the solution turns colorless.
Tetraoxomanganate(VII) ions (MnO4−MnO_4^-) impart a purple color to the solution, whereas reduced manganese(II) ions (Mn2+Mn^{2+}) are colorless.

Key Concept

Reducing action of sulfur(IV) oxide on acidified potassium tetraoxomanganate(VII)
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Topics

Sulfur Allotropes, Hydrogen Sulfide, and Sulfur(IV) Oxide

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