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NEET CHEMISTRYOrganic Chemistry – Some Basic Principles and Techniques Medium

Question

Which one of the following orders is correct for the bond dissociation enthalpy of halogen molecules?

A

Cl2>Br2>F2>I2\text{Cl}_2 > \text{Br}_2 > \text{F}_2 > \text{I}_2

B

Br2>I2>F2>Cl2\text{Br}_2 > \text{I}_2 > \text{F}_2 > \text{Cl}_2

C

F2>Cl2>Br2>I2\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2

D

I2>Br2>Cl2>F2\text{I}_2 > \text{Br}_2 > \text{Cl}_2 > \text{F}_2

Step-by-Step Solution

Generally, as we move down the halogen group, the atomic size increases, which leads to longer and weaker bonds, so bond dissociation enthalpy should decrease . However, fluorine (F2\text{F}_2) is anomalous. Because of the extremely small size of the fluorine atom, the lone pairs of electrons on the two adjacent bonded atoms experience strong interelectronic repulsions , making the F-F\text{F-F} bond weaker than expected. As a result, the bond dissociation enthalpy of F2\text{F}_2 is lower than that of both Cl2\text{Cl}_2 and Br2\text{Br}_2. According to the standard values , the bond dissociation enthalpies are roughly Cl2\text{Cl}_2 (243\approx 243 kJ/mol) > Br2\text{Br}_2 (192\approx 192 kJ/mol) > F2\text{F}_2 (155\approx 155 kJ/mol) > I2\text{I}_2 (151\approx 151 kJ/mol). Therefore, the correct order is Cl2>Br2>F2>I2\text{Cl}_2 > \text{Br}_2 > \text{F}_2 > \text{I}_2.

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Organic Chemistry – Some Basic Principles and Techniques . Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

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