Examkin

Question

Difficulty: MediumOxidizing and Reducing Agents and Tests

Match each chemical testing reagent or reaction system on the left with its corresponding diagnostic observation and redox transformation on the right.

  • Acidified potassium dichromate(VI) (K2Cr2O7K_2Cr_2O_7) solution exposed to a reducing agentTurns from orange to green as Cr2O72−Cr_2O_7^{2-} is reduced to Cr3+Cr^{3+}
  • Moistened starch-potassium iodide paper exposed to an oxidizing agentTurns blue-black as I−I^- is oxidized to I2I_2, which complexes with starch
  • Iron(III) chloride (FeCl3FeCl_3) solution when hydrogen sulfide (H2SH_2S) gas is bubbled through itTurns from reddish-brown to pale green as Fe3+Fe^{3+} is reduced to Fe2+Fe^{2+} with yellow precipitate formation
  • Acidified potassium tetraoxomanganate(VII) (KMnO4KMnO_4) solution exposed to a reducing agentTurns from purple to colorless as MnO4−MnO_4^- is reduced to Mn2+Mn^{2+}

Answer

Acidified potassium dichromate(VI) solution turns from orange to green (Cr2O72−→Cr3+Cr_2O_7^{2-} \rightarrow Cr^{3+}); starch-potassium iodide paper turns blue-black (I−→I2I^- \rightarrow I_2); iron(III) chloride solution turns from reddish-brown to pale green with yellow sulfur precipitate (Fe3+→Fe2+Fe^{3+} \rightarrow Fe^{2+}); acidified potassium tetraoxomanganate(VII) turns from purple to colorless (MnO4−→Mn2+MnO_4^- \rightarrow Mn^{2+}).
Each laboratory reagent exhibits a specific color change reflecting its underlying redox reaction: potassium dichromate(VI) changes from orange to green (Cr2O72−→Cr3+Cr_2O_7^{2-} \rightarrow Cr^{3+}); starch-potassium iodide paper turns blue-black due to iodine liberation (I−→I2I^- \rightarrow I_2); iron(III) chloride changes from reddish-brown to pale green with yellow sulfur precipitation (Fe3+→Fe2+Fe^{3+} \rightarrow Fe^{2+}); and potassium tetraoxomanganate(VII) changes from purple to colorless (MnO4−→Mn2+MnO_4^- \rightarrow Mn^{2+}).

Step-by-Step Solution

1
Determine the diagnostic color change for acidified potassium dichromate(VI).
Orange Cr2O72−Cr_2O_7^{2-} ions are reduced to green Cr3+Cr^{3+} ions when reacting with a reducing agent.
Chromium undergoes a reduction in oxidation state from +6+6 to +3+3.
2
Analyze the chemistry of the starch-potassium iodide test paper.
Oxidizing agents convert I−I^- (iodide) into I2I_2 (iodine), producing a characteristic blue-black complex with starch.
Iodide ions act as the reducing agent in the indicator paper and undergo oxidation.
3
Identify the reaction between iron(III) ions and hydrogen sulfide.
Reddish-brown Fe3+Fe^{3+} is reduced to pale green Fe2+Fe^{2+}, while sulfide (S2−S^{2-}) is oxidized to free yellow sulfur (SS).
H2SH_2S is a classic laboratory reducing agent for iron(III) salts.
4
Determine the diagnostic observation for acidified potassium tetraoxomanganate(VII).
Purple MnO4−MnO_4^- ions are reduced to virtually colorless Mn2+Mn^{2+} ions in acid medium.
Manganese undergoes reduction from +7+7 to +2+2 oxidation state.

Key Concept

Qualitative laboratory tests and color changes associated with common oxidizing and reducing agents.
Rate this question

Topics

Oxidizing and Reducing Agents and Tests

More questions from this topic