Problem 100 - A Complete Resource Book in Chemistry for JEE Main

Question

Bond dissociation energies of HF, 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) \( \mathrm{HF} > \mathrm{HCl} > \mathrm{HBr} \)

Step 1: Understand Bond Dissociation Energy

Bond dissociation energy measures the strength of a chemical bond and is defined as the energy required to break one mole of bonds in a chemical compound into its constituent atoms.

Step 2: Analyze the Periodic Trends

Bond dissociation energies are generally affected by factors such as bond length and bond strength. As you move down the group in the periodic table, bond length increases while bond strength decreases.

Step 3: Apply to Halogens

For hydrogen halides like HF, HCl, and HBr, the bond length increases as you move from F to Cl to Br. This is because F, being the smallest atom, forms the shortest and strongest bond with hydrogen.

Step 4: Determine the Order

As bond strength decreases from HF to HCl to HBr, the bond dissociation energy also decreases. So the order of bond dissociation energies is HF > HCl > HBr.

Key Concepts

Periodic Trends

Periodic trends help us understand how elements behave in the periodic table based on their positions. When exploring the concept of bond dissociation energy, periodic trends offer valuable insights. In general, as you move down a group in the periodic table, several observable patterns emerge in bonding characteristics:

• Bond Length: The bond length tends to increase as you move down a group. This is due to atoms increasing in size, which leads to a greater distance between the bonded atoms.
• Bond Strength: Conversely, bond strength typically decreases down a group. Larger atoms have more distant electron clouds, which weakens the bond between atoms.

Understanding these periodic trends helps explain why certain chemical properties, including bond dissociation energy, vary among elements. This knowledge is fundamental when analyzing the bond dissociation energies of halogens bonded with hydrogen.

Halogens

The halogens are a group of elements located in Group 17 of the periodic table. This group consists of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Halogens are known for their high reactivity and are frequently involved in chemical bonding with hydrogen to form hydrogen halides. For hydrogen halides such as HF, HCl, and HBr, several factors influence their chemical behaviors:

• Atomic Size: Halogens increase in atomic size from fluorine to iodine. This affects the length of the bond they form with hydrogen.
• Bonding with Hydrogen: Due to the increasing size of halogens, the bonds they form with hydrogen atoms become longer and weaker as you move from F to Cl to Br.

These properties make halogens excellent examples when studying bond characteristics and dissociation energies. Their ability to form bonds of varying strengths is a result of periodic trends observed across the periodic table.

Chemical Bond Strength

Chemical bond strength is an essential concept when discussing bond dissociation energy. It refers to how strongly atoms are held together within a molecule, and this is quantified by the bond dissociation energy. For the hydrogen halides, various factors influence bond strength:

• Bond Length and Electron Cloud Overlap: Stronger bonds generally have shorter bond lengths. Atomic size affects how much the electron clouds of atoms overlap, influencing bond strength.
• Electronegativity: Highly electronegative atoms, like fluorine, attract electrons more strongly. This creates a more stable and stronger bond with hydrogen.

When examining HF, HCl, and HBr, the strength of these bonds and their corresponding bond dissociation energies decrease in that order, from HF being the strongest and HBr the weakest. Thus, knowing these chemical bonding principles helps in predicting and understanding the reactivity and stability of molecules in chemical reactions.