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Difficulty: HardOxidizing and Reducing Agents and Tests

When a gas XX is bubbled into an acidified solution of potassium heptaoxodichromate(VI), K2Cr2O7K_2Cr_2O_7, the solution changes color from orange to green, accompanied by the formation of a yellow precipitate. Which of the following correctly identifies gas XX and describes its role in the reaction?

  1. Gas XX is hydrogen sulfide (H2SH_2S), and it acts as a reducing agent because the oxidation state of sulfur increases from −2-2 to 00.Answer
  2. B
    Gas XX is sulfur(IV) oxide (SO2SO_2), and it acts as an oxidizing agent because it causes chromium to be reduced.
  3. C
    Gas XX is chlorine (Cl2Cl_2), and it acts as a reducing agent because it is oxidized to chloride ions (Cl−Cl^-).
  4. D
    Gas XX is carbon(II) oxide (COCO), and it acts as an oxidizing agent by precipitating elemental sulfur from the acid solution.

Answer

Gas XX is hydrogen sulfide (H2SH_2S), acting as a reducing agent as sulfur is oxidized from an oxidation state of −2-2 to 00.
Hydrogen sulfide (H2SH_2S) acts as a reducing agent by donating electrons to dichromate ions (Cr2O72−Cr_2O_7^{2-}), reducing chromium from oxidation state +6+6 (orange) to +3+3 (green). Simultaneously, sulfide ions (S2−S^{2-}) in H2SH_2S are oxidized to elemental sulfur (S(s)S_{(s)}), which appears as a characteristic yellow precipitate.

Step-by-Step Solution

1
Analyze the color change of acidified potassium heptaoxodichromate(VI), K2Cr2O7K_2Cr_2O_7.
The orange color is due to Cr2O72−Cr_2O_7^{2-} ions (where Cr has an oxidation state of +6+6). The green color indicates reduction to Cr3+Cr^{3+} ions (oxidation state +3+3).
Potassium heptaoxodichromate(VI) acts as an oxidizing agent when reduced from +6+6 to +3+3.
2
Identify the role of gas XX and the origin of the yellow precipitate.
Gas XX causes the reduction of dichromate, so gas XX must be a reducing agent. The yellow precipitate formed is insoluble elemental sulfur (S(s)S_{(s)}).
A yellow colloidal precipitate in wet redox tests of gases is characteristic of sulfide oxidation to sulfur.
3
Determine the oxidation state change for hydrogen sulfide (H2SH_2S).
In H2SH_2S, sulfur has an oxidation state of −2-2. In elemental sulfur (SS), its oxidation state is 00.
An increase in oxidation state (from −2-2 to 00) represents oxidation; the species undergoing oxidation is the reducing agent.

Key Concept

Laboratory identification of reducing agents using acidified potassium heptaoxodichromate(VI)
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Topics

Oxidizing and Reducing Agents and Tests

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