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Difficulty: EasyOxidizing and Reducing Agents and Tests

Match each redox testing reagent or indicator on the left with its characteristic diagnostic observation on the right when reacting with an oxidizing or reducing agent.

  • Acidified KMnO4\text{KMnO}_4 solutionTurns from purple to colorless in the presence of a reducing agent
  • Moist starch-iodide paperTurns blue-black in the presence of a strong oxidizing gas
  • Acidified K2Cr2O7\text{K}_2\text{Cr}_2\text{O}_7 solutionChanges from orange to green in the presence of a reducing agent
  • Freshly prepared FeSO4\text{FeSO}_4 solutionChanges from pale green to reddish-brown when treated with an oxidizing agent

Answer

Acidified potassium tetraoxomanganate(VII) turns from purple to colorless with reducing agents; starch-iodide paper turns blue-black with oxidizing agents; acidified potassium heptaoxodichromate(VI) turns from orange to green with reducing agents; and freshly prepared iron(II) sulfate changes from pale green to reddish-brown with oxidizing agents.
Each testing reagent displays a distinct diagnostic color change depending on whether it reacts with an oxidizing or reducing agent. Acidified KMnO4\text{KMnO}_4 changes from purple to colorless in the presence of a reducing agent due to reduction of MnO4−\text{MnO}_4^- to Mn2+\text{Mn}^{2+}. Starch-iodide paper turns blue-black in the presence of an oxidizing agent as iodide is oxidized to iodine. Acidified K2Cr2O7\text{K}_2\text{Cr}_2\text{O}_7 changes from orange to green in the presence of a reducing agent as Cr2O72−\text{Cr}_2\text{O}_7^{2-} is reduced to Cr3+\text{Cr}^{3+}. Freshly prepared FeSO4\text{FeSO}_4 changes from pale green to reddish-brown when an oxidizing agent oxidizes Fe2+\text{Fe}^{2+} to Fe3+\text{Fe}^{3+}.

Step-by-Step Solution

1
Identify the role of acidified KMnO4\text{KMnO}_4 solution in redox testing.
Acidified KMnO4\text{KMnO}_4 contains manganese in the +7+7 oxidation state (purple). When it oxidizes a reducing agent, manganese is reduced to Mn2+\text{Mn}^{2+} (colorless).
This is the classic quantitative and qualitative test for reducing agents.
2
Determine the response of moist starch-iodide paper to oxidizing gases.
Oxidizing agents liberate free iodine (I2I_2) from iodide ions (I−I^-). Free iodine reacts with starch to yield a distinctive blue-black color.
This tests specifically for oxidizing agents such as chlorine or ozone.
3
Identify the color change associated with acidified K2Cr2O7\text{K}_2\text{Cr}_2\text{O}_7.
Dichromate ions (orange, chromium oxidation state +6+6) are reduced to Cr3+\text{Cr}^{3+} ions (green, oxidation state +3+3) by reducing agents.
The reduction of dichromate(VI) to chromium(III) causes the orange-to-green color change.
4
Determine the oxidation behavior of iron(II) sulfate solution.
Iron(II) ions (pale green) act as a reducing agent and are oxidized to iron(III) ions (reddish-brown/yellow) by oxidizing agents.
The oxidation of Fe2+\text{Fe}^{2+} to Fe3+\text{Fe}^{3+} shifts the solution color from green to brown/yellow.

Key Concept

Laboratory Diagnostic Tests for Oxidizing and Reducing Agents
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Topics

Oxidizing and Reducing Agents and Tests

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