Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following halogens does not turn starch iodine paper blue? (a) \(\mathrm{I}_{2}\) (b) \(\mathrm{F}_{2}\) (c) \(\mathrm{Cl}_{2}^{2}\) (d) \(\mathrm{Br}_{2}\)

Step-by-Step Solution

Answer

Fluorine ( \mathrm{F}_{2}\) does not turn starch iodine paper blue.

Step 1: Understand the test

The starch iodine test is used to identify the presence of iodine. When \( ext{I}_2\) is present, it reacts with starch to form a blue-black complex.

Step 2: Consider the options

We are given the halogens \( ext{I}_2\), \( ext{F}_2\), \( ext{Cl}_2\), and \( ext{Br}_2\). We need to determine which does not cause a color change on starch iodine paper.

Step 3: Evaluate iodine (I2)

Iodine \( ext{I}_2\) in the presence of starch forms a blue-black complex. Therefore, \( ext{I}_2\) will turn the starch iodine paper blue.

Step 4: Evaluate chlorine (Cl2)

Chlorine \( ext{Cl}_2\) can oxidize \( ext{I}^-\) ions to \( ext{I}_2\), which then reacts with starch to form a blue-black complex. Thus, chlorine indirectly causes the paper to turn blue.

Step 5: Evaluate bromine (Br2)

Bromine \( ext{Br}_2\) can also oxidize \( ext{I}^-\) ions to \( ext{I}_2\), which then reacts with the starch to form a blue-black complex, turning the paper blue.

Step 6: Evaluate fluorine (F2)

Fluorine \( ext{F}_2\) cannot oxidize \( ext{I}^-\) ions to form \( ext{I}_2\) under normal conditions in aqueous solution. Therefore, it will not cause a color change on starch iodine paper.

Key Concepts

Halogens

Halogens are a group of elements found in Group 17 of the periodic table. They include familiar substances like chlorine, fluorine, bromine, and iodine. These elements are known for their high reactivity, especially with metals and hydrogen. One of the distinctive features of halogens is that they can exist in molecular form, such as

• Fluorine (\( \text{F}_2 \))
• Chlorine (\( \text{Cl}_2 \))
• Bromine (\( \text{Br}_2 \))
• Iodine (\( \text{I}_2 \))

Each halogen has its own characteristics, but they typically exhibit similar chemical behaviors because they all strive to gain one electron to achieve a stable electron configuration. This high reactivity makes them useful in various applications, including sterilization and the formation of complex molecules. Because of their reactivity, halogens can easily participate in chemical reactions, often altering their physical states or color as they form new compounds.

Oxidation Reactions

Oxidation reactions are chemical processes where an atom, ion, or molecule loses electrons. In simpler terms, it's about gaining oxygen or losing hydrogen. This type of reaction is central to the behavior of halogens. For instance, in the starch iodine test, halogens like chlorine and bromine partake in oxidation reactions.These halogens oxidize iodide ions (\( \text{I}^- \)) to iodine (\( \text{I}_2 \)), which then reacts with starch to form a blue-black complex. Despite their ability, not all halogens behave the same in these reactions:

• Chlorine (\( \text{Cl}_2 \))
Cl_2 oxidizes iodide ions readily, leading to a color change.
• Bromine (\( \text{Br}_2 \))
Like chlorine, Br_2 can also oxidize iodide ions.However, fluorine (\( \text{F}_2 \)) does not effectively oxidize iodide under similar conditions, mainly because it reacts so vigorously with water and other substances before it can oxidize iodide ions. Therefore, oxidation ability significantly influences a halogen's ability to cause a starch iodine color change. Understanding these reactions provides insight into various analytical and industrial processes involving halogens.

Color Change

Color changes in chemistry indicate a reaction has occurred or a new substance has formed. In the context of the starch iodine test, color change is a key indicator in determining the presence of iodine.

• Blue-Black Complex:
When iodine (\( \text{I}_2 \)) is present, it interacts with starch to produce a distinct blue-black color. This color change is due to the iodine fitting into the helical structure of the starch molecule, forming a stable complex.For other halogens like \( \text{Cl}_2 \) and \( \text{Br}_2 \), the color change is indirect. They cause the oxidation of \( \text{I}^- \) ions to \( \text{I}_2 \), which then manifests the blue-black color upon reaction with starch. However, \( \text{F}_2 \) does not cause this color change because it cannot convert iodide ions to iodine under usual conditions.Observing color changes is a simple, yet powerful, method to detect certain reactions and substances in chemical experiments. This makes it an excellent qualitative test for educational and laboratory settings to demonstrate chemical concepts in action.