Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the \(\mathrm{C}-\mathrm{X}\) bond (where \(\mathrm{X}=\mathrm{Cl}, \mathrm{Br}, \mathrm{I})\) the correct bond energy order is (a) \(\mathrm{C}-\mathrm{I}>\mathrm{C}-\mathrm{Cl}>\mathrm{C}-\mathrm{Br}\) (b) \(\mathrm{C}-\mathrm{Cl}>\mathrm{C}-\mathrm{Br}>\mathrm{C}-\mathrm{I}\) (c) \(\mathrm{C}-\mathrm{Br}>\mathrm{C}-\mathrm{Cl}>\mathrm{C}-\mathrm{I}\) (d) \(\mathrm{C}-\mathrm{Cl}>\mathrm{C}-\mathrm{I}>\mathrm{C}-\mathrm{Br}\)

Step-by-Step Solution

Answer

The correct bond energy order is (b) \(\mathrm{C}-\mathrm{Cl} > \mathrm{C}-\mathrm{Br} > \mathrm{C}-\mathrm{I}\).

Step 1: Understanding Bond Energy

Bond energy is the measure of a bond's stability and is defined as the energy required to break one mole of bonds in a molecule in the gaseous state. It is influenced by the size of the atoms involved; smaller atoms tend to form stronger bonds.

Step 2: Comparing Atomic Sizes

Chlorine (Cl), Bromine (Br), and Iodine (I) increase in atomic size respectively. This means iodine is the largest and chlorine is the smallest among the given elements. As atomic size increases, bond length increases and bond energy decreases.

Step 3: Evaluating Bond Energies

Given the relationship between atomic size and bond energy, we deduce that the bonds are strongest with the smallest atom. Therefore, the bond energy order should be from the strongest \(C-Cl\) to weakest \(C-I\).

Step 4: Conclusion

Arrange the bonds in terms of decreasing bond energy. The correct order is \(C-Cl > C-Br > C-I\) because chlorine, being the smallest atom, forms the strongest bond with carbon.

Key Concepts

Understanding Atomic Size

Atomic size refers to the average distance from the center of the nucleus of an atom to its valence shell, where the outermost electrons are located. This size can have a profound impact on a molecule's properties, including bond energy.

As you move down a group in the periodic table, the atomic size increases. This happens because each succeeding element has an additional electron shell, increasing the distance between the nucleus and the outermost electrons.

Here's how it applies:

• Chlorine (Cl) is smaller than Bromine (Br).
• Bromine (Br) is smaller than Iodine (I).
• Thus, Cl < Br < I in terms of atomic size.

The smaller the atom, the closer its electrons are to the nucleus, allowing it to form stronger and shorter bonds with other atoms, such as carbon.

Exploring Bond Stability

Bond stability is a concept central to understanding chemical reactions and properties of molecules. It is determined by the energy required to break a chemical bond in a molecule.

Stronger bonds require more energy to break, indicating higher bond stability. This is heavily influenced by atomic size as well as other factors like atomic charge and electronegativity.

In the case of carbon-halogen bonds:

• Smaller atoms such as chlorine can create a tighter, more stable bond due to their smaller size and closer electron-nucleus proximity.
• As atomic size increases, bond lengthens, reducing stability since it becomes easier to break.

Therefore, the bond stability of carbon-halogen bonds is strongest with chlorine and weakest with iodine, following an order of strength: - C-Cl > C-Br > C-I.

Carbon-Halogen Bonds

Carbon-halogen bonds are a type of covalent bond fundamental in organic chemistry, where carbon is bonded to a halogen, such as chlorine (Cl), bromine (Br), or iodine (I).

The nature of these bonds can significantly impact the properties of a compound:

• Bond Energy: Carbon forms stronger bonds with halogens that are smaller in size, because smaller halogen atoms allow for a more compact and stable electron sharing arrangement.
• Reactivity: Less stable bonds, such as C-I, are more reactive, while more stable bonds, like C-Cl, are less prone to undergoing chemical reactions.

The order of bond strength is crucial for predicting chemical behavior and reactivity in organic compounds, where C-Cl bonds are strongest, followed by C-Br, and finally C-I being the weakest.