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 option (b): \(\mathrm{C}-\mathrm{Cl}>\mathrm{C}-\mathrm{Br}>\mathrm{C}-\mathrm{I}\).

Step 1: Understand Bond Energy Concept

Bond energy is the measure of bond strength in a chemical bond. It is defined as the amount of energy required to break one mole of bonds in gaseous molecules under standard conditions. The larger the bond energy, the stronger the bond.

Step 2: Compare Atomic Sizes

The bond energy generally decreases as the size of the atoms involved increases. Larger atoms, such as iodine (I), have weaker bonds due to the increased bond length compared to smaller atoms like chlorine (Cl).

Step 3: Analyze Periodic Table Trends

Halogens (group 17 elements: Cl, Br, and I) show a trend in bond energies. As we move down the group from chlorine to iodine, the atomic size increases, and thus the bond energy with carbon decreases. Consequently, the bond energy order will be based on this trend: Cl > Br > I.

Step 4: Determine the Correct Bond Energy Order

Using the trend that bond energy decreases from Cl to Br to I due to increasing atomic size, the bond energy order is: \(\mathrm{C}-\mathrm{Cl}>\mathrm{C}-\mathrm{Br}>\mathrm{C}-\mathrm{I}\).

Step 5: Verify the Correct Option

Based on the comparison of bond energies analyzed in the previous steps, the correct order is \(\mathrm{C}-\mathrm{Cl}>\mathrm{C}-\mathrm{Br}>\mathrm{C}-\mathrm{I}\). This matches option (b).

Key Concepts

Bond Strength

Bond strength is a key concept in chemistry, as it determines how tightly two atoms are held together in a molecule. This strength is quantified by bond energy, the amount of energy needed to break the bond in gaseous molecules under standard conditions. Imagine bond strength as the adhesive force between two bricks in a building. The stronger the adhesive, the more difficult it is to separate the bricks. Similarly, in chemical bonds, stronger bonds require more energy to break.

• Strong bonds have high bond energies.
• Weak bonds have low bond energies.

For carbon-halogen bonds, bond strength is affected by the size of the halogen atom. Smaller atoms, like chlorine, form stronger bonds with carbon due to the shorter distance between the nuclei of the two bonded atoms. This short distance means stronger interactions and, therefore, higher bond energy. Given these insights, understanding the nature of bond strength helps in predicting reactivity and stability of molecules.

Halogens

Halogens are fascinating elements belonging to group 17 of the periodic table. These elements include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They are known for their high reactivity, mainly due to their desire to gain one electron to achieve a full outer electron shell.

• Halogens have seven valence electrons, needing just one more to fulfill the octet rule.
• This high electronegativity makes them eager participants in forming covalent or ionic bonds.

In the context of bond strength, the size of the halogen greatly influences the bond energy. As we move down the group from fluorine to iodine, each halogen atom gets larger. Larger atoms like iodine will have weaker bonds to carbon due to increased atomic size and subsequently longer bond length. In everyday applications, compounds containing halogens are vital in creating disinfectants, medications, and even in the field of photography. Understanding halogens can provide insight into why they form such a diverse array of useful, reactive compounds across different industries.

Periodic Table Trends

Periodic table trends are essential for making predictions about the properties of elements. One important trend is that as you proceed down a group in the periodic table, the atomic radius increases. This increase is due to the addition of electron shells, which outweighs the effect of increasing nuclear charge. The impact of increased atomic size on bond energy includes:

• Longer bond lengths, which generally lead to weaker bonds.
• Lower bond energies due to increased separation between bonded nuclei.

In the case of carbon-halogen bonds, the trend is evident. Chlorine, being higher up in the group, forms a stronger bond with carbon as compared to bromine and iodine. This is because chlorine's smaller atomic radius allows for a shorter bond length, translating to stronger interatomic forces and higher bond energy. Recognizing these periodic trends is crucial for understanding patterns in elements' reactions and bonding behaviors, explaining why certain elements behave similarly while others differ dramatically.