Problem 62 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has the least bond energy? (a) \(\mathrm{H}_{2}\) (b) \(\mathrm{F}_{2}\) (c) \(\mathrm{O}_{2}\) (d) \(\mathrm{N}_{2}\)

Step-by-Step Solution

Answer

The molecule with the least bond energy is (b) \( \mathrm{F}_{2} \).

Step 1: Understanding Bond Energy

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. Typically, stronger bonds have higher bond energies.

Step 2: Analyze Each Molecule's Bond Type

Consider the type of bond present in each molecule: - \( \mathrm{H}_2 \) involves a single bond.- \( \mathrm{F}_2 \) involves a single bond.- \( \mathrm{O}_2 \) involves a double bond.- \( \mathrm{N}_2 \) involves a triple bond.

Step 3: Compare Bond Energies

In general, the order of bond strength from weakest to strongest is single < double < triple due to increased electron sharing. Therefore, molecules with single bonds typically have lower bond energies than those with double or triple bonds.

Step 4: Identify the Molecule with the Weakest Bond

Among the given options, both \( \mathrm{H}_2 \) and \( \mathrm{F}_2 \) have single bonds. However, \( \mathrm{F}_2 \) has a lower bond dissociation energy due to its longer bond length and weaker bond. This makes it the molecule with the least bond energy.

Key Concepts

Chemical Bonding

Chemical bonding refers to the process by which atoms combine to form molecules. These connections are essential for the stability of most compounds. Atoms bond to achieve a full outer shell of electrons, typically following the octet rule, which states that atoms are more stable when they have eight electrons in their valence shell. There are several types of chemical bonds, including ionic, covalent, and metallic bonds, with covalent bonds being the focus here.

• Ionic bonds occur when electrons are transferred from one atom to another, creating ions that attract each other.
• Covalent bonds involve the sharing of electrons between atoms to fill their outer shells.
• Metallic bonds are characterized by a "sea of electrons" that are free to move around a lattice of positive ions, providing conductive properties.

Understanding chemical bonding is crucial for determining a compound's properties, including its stability and reactivity.

Single bonds vs double/triple bonds

In chemical compounds, bonds vary in their number of shared electron pairs. The more electron pairs shared between atoms, the stronger and shorter the bond tends to be. Let's explore the differences among these bonds.

• Single Bonds: This type occurs when two atoms share one pair of electrons. Single bonds are generally weaker and longer than double or triple bonds. They allow for rotation around the bond axis, providing flexibility in molecular geometry.
• Double Bonds: Formed when two atoms share two pairs of electrons. These bonds are shorter and stronger than single bonds. Their rigidity prevents rotation around the bond axis, impacting molecular shapes and properties.
• Triple Bonds: These form between atoms sharing three pairs of electrons, making them the strongest and shortest of the bonds. Like double bonds, they do not allow rotation and significantly affect the molecule's properties.

The presence of these bonds can greatly influence the chemical and physical properties of molecules, including bond energy.

Bond Dissociation Energy

Bond dissociation energy is a crucial concept in understanding chemical reactions and bond strength. It represents the energy required to break a chemical bond in a molecule, resulting in the separation of atoms in their gaseous state. This energy is directly related to a bond's stability.

The higher the bond dissociation energy, the stronger the bond. It means that more energy is required to break the bond. Typically:

• Single bonds have lower dissociation energy compared to double and triple bonds.
• This property is important for predicting reaction mechanisms and products in chemical reactions.
• In the given exercise, the molecule with the least bond dissociation energy, due to its single bond and weaker interaction, is \( \mathrm{F}_2 \).

Understanding bond dissociation energy helps chemists analyze and predict the reactivity of different substances, guiding the study and application of chemical interactions.