Problem 75 - Chemistry and Chemical Reactivity

Question

Which halogen has the highest bond dissociation enthalpy? (a) \(\mathrm{F}_{2}\) (b) \(\mathrm{Cl}_{2}\) (c) \(\mathrm{Br}_{2}\) (d) \(\mathrm{I}_{2}\)

Step-by-Step Solution

Answer

(b) \( \mathrm{Cl}_2 \)

Step 1: Understand Bond Dissociation Enthalpy

Bond dissociation enthalpy is the energy required to break one mole of bonds in gaseous molecules. Higher bond dissociation enthalpy means stronger bonds between atoms.

Step 2: Analyze Halogen-Halogen Bond Strengths

The bond strength generally decreases down the group for halogens. However, \( \mathrm{F}_2 \) is an exception due to repulsion between lone pairs, resulting in weaker bonds.

Step 3: Compare Bond Dissociation Enthalpies

For halogens, \( \mathrm{Cl}_2 \) has the highest bond dissociation enthalpy, followed by \( \mathrm{Br}_2 \), and \( \mathrm{I}_2 \). \( \mathrm{F}_2 \) is lower than \( \mathrm{Cl}_2 \) due to lone pair repulsion.

Step 4: Determine the Answer

Based on bond dissociation enthalpies, \( \mathrm{Cl}_2 \) has the highest value compared to other halogens listed. Thus, \( \mathrm{Cl}_2 \) has the strongest bond among the given options.

Key Concepts

Halogen Bonds

Halogen bonds are quite unique in the realm of chemistry. They involve interactions between halogen atoms, such as fluorine, chlorine, bromine, and iodine. A halogen bond typically involves a halogen atom acting as an electron acceptor and interacting with an atom or group that has a lone pair of electrons to share.
These bonds are crucial for understanding many molecular interactions, especially in the context of bond dissociation enthalpy.

• Fluorine (F₂) tends to form weaker halogen bonds due to lone pair-lone pair repulsions.
• Chlorine (Cl₂), on the other hand, forms stronger halogen bonds as it has less repulsion and a larger atomic size compared to fluorine, allowing for a more stable bond structure.

Understanding the specific characteristics of halogen bonds helps to grasp why certain halogen halide bonds, like that in chlorine, display higher bond dissociation enthalpies.

Molecular Bond Strength

The concept of molecular bond strength is crucial in determining how much energy is needed to break a bond. Bond strength is generally inferred from the bond dissociation enthalpy, which is the energy required to break the bond. It can vary significantly among molecules depending on multiple factors.

• Size and charge distribution of atoms can influence bond strength. Larger atoms often form weaker bonds because they can't hold onto other atoms tightly.
• Lone pair repulsion, as seen with F₂, can weaken a bond making it easier to dissociate.
• The position of the elements in the periodic table helps predict these strengths as you observe trends like the decreasing bond strength in halogens as their atomic size increases.

These factors make Cl₂ have a stronger bond than F₂, for example, due to its advantageous size and lack of extensive lone pair repulsion.

Chemical Energy

Chemical energy is stored within the bonds of chemical compounds and is released during a chemical reaction. When breaking a bond, energy is absorbed, and upon forming a bond, energy is released. This is where bond dissociation enthalpy comes in as it represents the energy needed to break bonds.

• In molecules like halogens, chemical energy differences are more prominent due to variations in atomic size and bonding characteristics.
• The chemical energy of a molecule is directly related to its bond dissociation enthalpy; higher enthalpy implies more stored energy.
• In the evaluation of halogen-halogen bonds, the relatively high dissociation enthalpy of Cl₂ suggests that it stores more chemical energy compared to other halogens like F₂ or I₂.

Understanding chemical energy allows for appreciation of the energy transformations occurring in reactions, giving insights into why certain bonds are stronger and more stable, such as the bond in chlorine gas.