Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following order is correct for the bond energies of halogen molecules? (a) \(\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\) (b) \(\mathrm{Br}_{2}>\mathrm{Cl}_{2}>\mathrm{I}_{2}\) (c) \(\mathrm{I}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}\) (d) \(\mathrm{I}_{2}>\mathrm{Br}_{2}>\mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\)

Step 1: Understand Bond Energies

Bond energy refers to the amount of energy required to break one mole of chemical bonds in gaseous molecules. Typically, the bond energy is directly related to the bond strength: the stronger the bond, the higher the bond energy.

Step 2: Recall the Trends in Halogens

Halogens are group 17 elements. Their bond energies usually decrease going down the group. This is because the overlap between orbitals becomes less effective as the atomic size increases, leading to weaker bonds.

Step 3: Compare Cl-Cl, Br-Br, and I-I Bonds

The Cl-Cl bond is expected to be stronger than the Br-Br bond because chlorines are smaller than bromines, allowing for better overlap. Similarly, the Br-Br bond is expected to be stronger than the I-I bond.

Step 4: Identify Correct Order

Now, let's arrange the molecules based on decreasing bond energy: - \(\text{Cl}_2\) has the highest bond energy due to the strong Cl-Cl bond.- \(\text{Br}_2\) comes next with a medium bond energy.- \(\text{I}_2\) has the lowest bond energy.

Step 5: Select the Correct Option

The correct order based on the comparison is Cl2 > Br2 > I2, which corresponds to option (a).

Key Concepts

Bond Energy

Bond energy is a fundamental concept in chemistry, especially when discussing molecules. It refers to how much energy is needed to break one mole of chemical bonds in gaseous molecules. The amount of energy reflects the strength of the bonds. Stronger bonds need more energy to break. This energy gives scientists and students clues about the stability of a molecule. In simple terms:

• High bond energy means a strong bond.
• Low bond energy means a weaker bond.

Bond energy is crucial because it helps us understand reactions and predict the behavior of different substances under various conditions. When a bond is broken, it's a chance to observe how much energy is involved. This is why bond energy is a key topic in studying the behavior of chemical compounds.

Halogen Molecules

Halogen molecules are part of a fascinating group in the periodic table. These molecules consist of atoms from the halogens group, which includes elements like fluorine, chlorine, bromine, and iodine. Each of these forms diatomic molecules, meaning they exist naturally as two atoms bonded together like ( ext{F}_2, ext{Cl}_2, ext{Br}_2, ext{I}_2"). Halogens are known for their high reactivity, making them very interesting in chemical reactions. Important traits of halogen molecules include:

• They are all non-metals.
• They have similar properties and chemical behavior.
• Despite their similarities, their reactivity and bond energy can vary significantly.

The variation in bond energies among halogen molecules is primarily due to the differences in atomic size and orbital overlap, which are crucial when considering the effectiveness of their bonds. This makes studying halogens important for understanding chemical bonding and reactions.

Group 17 Elements

Group 17 on the periodic table is home to the halogen elements. Members of this group include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are distinct because they have seven valence electrons, which makes them highly reactive as they seek to gain one more electron to achieve a stable octet configuration. Some key properties of Group 17 elements are:

• High electronegativity and strong oxidizing abilities.
• Non-metallic and typically form salts when they react with metals.
• Exhibit colored vapors and are mostly poor conductors of electricity.

Moving down the group, elements get heavier, and atomic sizes increase. As a result, the bond energy decreases as you go down the group, due to less effective orbital overlap and increased atomic size impacting their chemical reactions and bonds. Understanding Group 17 is essential for students learning about chemical bonds and molecular interactions.

Chemical Bonds

Chemical bonds form the basis of molecules and compounds in chemistry. They arise from atoms sharing or transferring electrons to achieve more stable electronic configurations. Different types of chemical bonds exist, including covalent, ionic, and metallic bonds. Each type has distinct characteristics depending largely on how electrons are handled between the atoms. Here's a closer look:

• Covalent Bonds: Atoms share pairs of electrons. They are common in organic compounds and non-metal molecules like water or hydrogen gas.
• Ionic Bonds: Form when atoms transfer electrons. Typically seen in compounds formed between metals and non-metals, like table salt (NaCl).
• Metallic Bonds: Involve a free flow of electrons around metals, giving them conductivity and malleability.

Studying chemical bonds is fundamental to chemistry; understanding these interactions explains how substances form and change. It helps in understanding the properties of different materials and their potential reactions, a critical aspect of learning about elements like halogens and their molecular forms.