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) \(\mathrm{Cl}_{2}\) (d) \(\mathrm{Br}_{2}\)

Step-by-Step Solution

Answer

Fluorine ( F 2) does not turn starch iodine paper blue.

Step 1: Understanding the Reaction

Starch iodine paper turns blue in the presence of iodine, specifically when iodine ions are present. This reaction occurs because iodine acts as a complexing agent with starch, forming a blue-black complex.

Step 2: Identifying the Options

The halogens given in the options are iodine ( 12 2), fluorine ( F 2), chlorine ( Cl 2), and bromine ( Br 2). We need to identify if any of these halogens does not cause the paper to turn blue.

Step 3: Analyzing the Interaction with Iodine

12 ) naturally complexes with starch to produce the blue color. So, 12 2 will turn the paper blue.

Step 4: Fluorine's Interaction

Fluorine ( F 2) is the most electronegative element and a strong oxidizing agent, but it does not form 12 2 ions in solution since it doesn't react to create iodine molecules; hence it won't turn the paper blue.

Step 5: Chlorine's Interaction

Chlorine ( Cl 2) can oxidize iodide ions to form iodine, which would turn the paper blue when in sufficient contact.

Step 6: Bromine's Interaction

Bromine ( Br 2) can also oxidize iodide ions to form iodine, leading to a blue color if the conditions allow 12 2 formation.

Step 7: Conclusion

Given that 12 2 directly turns the paper blue, Cl 2 and Br 2 can cause the formation of iodine that would turn the paper blue as well. F 2, however, does not participate in reactions that result in the formation of 12 2, hence it will not turn the paper blue.

Key Concepts

Starch-Iodine Test

The starch-iodine test is a classic method in chemistry that is used to detect the presence of iodine. When iodine comes into contact with starch, a blue-black colored complex forms. This hue arises because iodine molecules fit neatly into the helical structure of the starch, fundamentally changing its optical properties. It's crucial to note that for the formation of this complex, free iodine (\(\text{I}_2\)) is required, not just iodine ions (\(\text{I}^-\)). Therefore, this test is popular for visual confirmation of iodine presence, as the color change is quite pronounced.

Oxidizing Agents

Oxidizing agents are substances that can accept electrons from other species in a reaction, effectively oxidizing them. Halogens like chlorine (\(\text{Cl}_2\)) and bromine (\(\text{Br}_2\)) are well-known oxidizing agents. They can oxidize iodide ions to free iodine (\(\text{I}_2\)), enabling the starch-iodine test to confirm their presence. Oxidizing agents play crucial roles in various chemical processes, including those that involve halogen reactions.

They often have high electronegativity and can pull electrons towards themselves easily. It's interesting to note, however, that while fluorine (\(\text{F}_2\)) is a strong oxidizing agent, it does not result in the formation of free iodine, making it unique among the halogens.

Halogen Chemistry

Halogens are a group of highly reactive elements, consisting of fluorine (\(\text{F}_2\)), chlorine (\(\text{Cl}_2\)), bromine (\(\text{Br}_2\)), iodine (\(\text{I}_2\)), and astatine. These elements are known for their ability to form salts with metals and their propensity to participate in oxidation reactions.

• Fluorine is the most reactive of the halogens, but it doesn't follow typical halogen behavior in the context of starch-iodine testing.
• Chlorine and bromine readily react with iodide ions to form iodine, which turns starch blue.
• Iodine naturally forms its blue complex with starch without prior reaction.

Understanding the reactivity and unique properties of each halogen is key to predicting their behavior in chemical reactions.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons within a bond. In the periodic table, electronegativity generally increases across a period and decreases down a group. The halogens are highly electronegative, with fluorine being the most electronegative element of all. This high electronegativity makes them excellent oxidizing agents, as they have a strong tendency to attract electrons from other atoms and molecules.

While fluorine's high electronegativity makes it extremely reactive, it behaves differently from other halogens under certain conditions. For instance, in a starch-iodine test, its inability to convert iodide ions into free iodine is a direct result of its unique chemical properties. Recognizing these nuances in electronegativity and reactivity helps in understanding the differences in halogen behavior.