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Difficulty: MediumOxidizing and Reducing Agents and Tests

Complete the following statement regarding the chemical test for an oxidizing agent using moist starch-iodide paper.

Answer:When chlorine gas (Cl2Cl_2) is brought into contact with moist starch-iodide paper, the paper turns 【blue-black】 because iodide ions (I−I^-) undergo 【oxidation】 to form free iodine (I2I_2).

Answer

The starch-iodide paper turns blue-black (or blue) because iodide ions (I−I^-) undergo oxidation to form elemental iodine (I2I_2).
Moist starch-iodide paper is used to test for oxidizing agents such as chlorine (Cl2Cl_2). Chlorine oxidizes iodide ions (I−I^-) to elemental iodine (I2I_2) through the loss of electrons (oxidation). The liberated iodine subsequently reacts with starch on the test paper to form a characteristic blue-black complex.

Step-by-Step Solution

1
Identify the chemical role of chlorine gas (Cl2Cl_2) reacting with iodide ions (I−I^-)
Chlorine acts as an oxidizing agent by accepting electrons from iodide ions: Cl2+2I−→2Cl−+I2Cl_2 + 2I^- \rightarrow 2Cl^- + I_2.
Chlorine has a stronger electron affinity and higher standard reduction potential than iodine.
2
Determine the type of redox process occurring at the iodide ion
The oxidation number of iodine increases from −1-1 in I−I^- to 00 in I2I_2, which represents oxidation (loss of electrons).
Oxidation is defined as the loss of electrons or an increase in oxidation state.
3
Identify the visual observation produced on the indicator paper
The liberated elemental iodine (I2I_2) reacts with the starch embedded in the paper to form a deep blue-black starch-iodine complex.
Starch serves as a specific indicator that yields a blue-black coloration in the presence of free molecular iodine.

Key Concept

Laboratory test for oxidizing agents using moist starch-iodide paper
Estimated Time:1m 0s
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Topics

Oxidizing and Reducing Agents and Tests

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