Problem 39 - A Complete Resource Book in Chemistry for JEE Main

Question

The C-X bond energy order for carbon tetrahalide is (a) \(\mathrm{CI}_{4}>\mathrm{CBr}_{4}>\mathrm{CCl}_{4}>\mathrm{CF}_{4}\) (b) \(\mathrm{CCl}_{4}>\mathrm{CBr}_{4}>\mathrm{CI}_{4}>\mathrm{CF}_{4}\) (c) \(\mathrm{CF}_{4}>\mathrm{CCl}_{4}>\mathrm{CBr}_{4}>\mathrm{CI}_{4}\) (d) \(\mathrm{CCl}_{4}>\mathrm{CI}_{4}>\mathrm{CBr}_{4}>\mathrm{CF}_{4}\)

Step-by-Step Solution

Answer

The correct order is option (c) \\(CF_4 > CCl_4 > CBr_4 > CI_4\\).

Step 1: Understand the Carbon-Halogen Bond

The C-X bond energy depends on multiple factors such as bond length and halogen size. Generally, smaller halogens allow for stronger bonds due to shorter bond lengths. The halogens in this exercise are fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

Step 2: Order of Halogens by Atomic Size

Fluorine is the smallest halogen, followed by chlorine, bromine, and iodine in increasing order of size. Smaller halogens like fluorine form stronger and shorter bonds with carbon compared to larger halogens.

Step 3: Predicting Bond Energy Based on Halogen Size

Since fluorine is the smallest, it forms the strongest bond with carbon, so \C-F\ bond energy is expected to be the highest. Conversely, iodine is the largest, so \C-I\ bond energy is expected to be the lowest. Thus, the order for bond energy from strongest to weakest is expected to be \(CF_4 > CCl_4 > CBr_4 > CI_4\).

Step 4: Verify Against Provided Options

Compare \(CF_4 > CCl_4 > CBr_4 > CI_4\) with the given options: \(c) CF_4 > CCl_4 > CBr_4 > CI_4\) matches our expectation.

Key Concepts

Halogen Atomic Size

Halogens are elements found in Group 17 of the periodic table. Their size varies significantly as you move down the group from fluorine (F) to iodine (I). The atomic size of halogens plays a critical role in determining the strength and length of bonds they form with other elements, such as carbon.
Fluorine is the smallest halogen, which means it can get closer to the carbon atom. This smaller size translates to a higher bond strength because the atoms are closer together, allowing for greater overlap of orbitals.
In contrast, iodine is the largest halogen, so it cannot get as close to the carbon atom, leading to weaker bonds. Understanding the trend of increasing atomic size from fluorine to iodine helps predict how halogens will bond with carbon and their resulting bond energies.

Bond Length

Bond length is the distance between the nuclei of two bonded atoms. In the context of carbon-halogen bonds, bond length is influenced by the size of the halogen atom.
Smaller halogen atoms, like fluorine, lead to shorter bond lengths because the atomic nuclei can be much closer. This proximity results in stronger interactions between the overlapping electron clouds, yielding a stronger chemical bond.

• Fluorine, being the smallest, has the shortest bond length when bonded to carbon.
• Iodine has the longest bond length with carbon due to its larger atomic size.

Shorter bonds are typically stronger and require more energy to break. This principle helps explain why the bond energy decreases as the bonded halogen size increases, influencing the overall chemical reactivity.

Chemical Bond Strength

Chemical bond strength refers to the energy required to break a bond between two atoms, and it is measured in terms of bond dissociation energy.
In carbon-halogen bonds, bond strength varies based on the halogen involved. The relationship between halogen atomic size, bond length, and bond energy gives key insights into bond strength.
Typically, stronger bonds are indicated by higher bond energies.

• The C-F bond is the strongest among carbon-halogen bonds, owing to fluorine's small size and the resulting short bond length.
• The C-I bond is the weakest due to iodine's large atomic size and the resultant long bond length.

This understanding allows students to predict bond energies, as seen in the ordered bond energy trend: \(CF_4 > CCl_4 > CBr_4 > CI_4\). Recognizing these patterns is crucial for fields such as organic chemistry and material science, where bond strength influences molecular stability and reactivity.