Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Bond dissociation energies of \(\mathrm{HF}, \mathrm{HCl}\), HBr follows the order \(\ldots \ldots .\) (a) \(\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}\) (b) \(\mathrm{HCl}>\mathrm{HBr}>\mathrm{HF}\) (c) \(\mathrm{HF}>\mathrm{HBr}>\mathrm{HCl}\) (d) \(\mathrm{HF}>\mathrm{HCl}>\mathrm{HBr}\)

Step-by-Step Solution

Answer

(d) HF > HCl > HBr.

Step 1: Understanding Bond Dissociation Energy

Bond dissociation energy is the energy required to break a specific bond in a molecule. It indicates the strength of the bond; higher bond dissociation energy means a stronger bond.

Step 2: Consider Bond Strength Factors

The bond strength between hydrogen and a halogen atom depends primarily on the size and electronegativity of the halogen. Smaller size and higher electronegativity usually result in stronger bonds.

Step 3: Assess the Halogen Trends

Fluorine is smaller and more electronegative than chlorine or bromine. Consequently, the H-F bond is expected to be stronger than H-Cl or H-Br.

Step 4: Sequence the Elements by Bond Strength

Given that H-F is expected to be the strongest due to fluorine's high electronegativity and small size, compare H-Cl and H-Br next. As chlorine is more electronegative than bromine, H-Cl should be stronger than H-Br. Thus, the bond dissociation energy order is HF > HCl > HBr.

Key Concepts

Bond Strength

Understanding "bond strength" is essential to comprehend how molecules behave and react. Bond strength, in simple terms, conveys how firmly two atoms are held together in a chemical bond. It boils down to how much energy it would take to break a bond.
A stronger bond means more energy is required to dissociate the atoms, translating to higher bond dissociation energies. Bond strength is crucial because:

• It affects the stability of a molecule.
• It influences the reactivity of the compound.
• Predicts how the molecule might interact with others in chemical reactions.

In molecules like \( \text{HF} \text{, } \text{HCl} \text{, and } \text{HBr}\), bond strength differences help explain why they behave differently and react in varied ways under the same conditions.

Electronegativity

Electronegativity, in chemistry, is a measure of an atom's ability to attract and hold onto electrons. It's a critical factor in determining how atoms come together to form chemical bonds.
In the context of hydrogen halides like \( \text{HF} \text{, } \text{HCl} \text{, and } \text{HBr}\), electronegativity dictates bond polarity and, in part, bond strength.

• Fluorine, being the most electronegative element, pulls electrons towards itself more strongly compared to chlorine or bromine.
• This results in the \( \text{H-F}\) bond being not only highly polar but also very strong since the shared electrons are tightly held.
• The difference in electronegativity between hydrogen and each halogen also explains why \( \text{H-F}\) has the highest bond dissociation energy followed by \( \text{H-Cl}\) and then \( \text{H-Br}\).

Understanding electronegativity provides insight into why some responses to chemical stimuli differ within similar compounds.

Halogen Trends

"Halogen trends" refer to the patterns in properties as you move down the halogen group on the periodic table: fluorine, chlorine, bromine, and so on.
These trends include changes in:

• Atomic and ionic size
• Electronegativity
• Bond energies

As we delve deeper:

• Fluorine, being highest in the group, has the smallest size and highest electronegativity, leading to stronger and shorter bonds compared to chlorine and bromine.
• As we move down the group, each element becomes larger and less electronegative. This results in weaker bonds as seen in the decreasing bond dissociation energies from \( \text{HF}\) to \( \text{HBr}\).

These trends help predict how a halogen will behave in various chemical environments, providing valid predictions when working with halogen-containing compounds.

Chemical Bonds

At the heart of chemistry are "chemical bonds" which describe how atoms connect to form compounds. This concept forms the cornerstone of chemistry, since the type, number, and nature of bonds greatly affect the resultant molecule's properties.

• Covalent bonds, like those in \( \text{HF} \text{, } \text{HCl} \text{, and } \text{HBr}\), involve sharing electrons between atoms.
• The strength of these bonds, often measured by bond dissociation energy, is determined by factors such as the participating atoms' electronegativities and sizes.
• Understanding chemical bonds allows us to predict the behaviors of substances, including their reactivity and stability.

By grasping the essence of chemical bonds, especially in common compounds like hydrogen halides, you unlock a deeper understanding of the molecular world, influencing countless applications in science and technology.