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

Question

Bond dissociation energies of HF, HCl, HBr follows the order ....... (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 is the energy required to break a bond between two atoms in a molecule. The higher the bond dissociation energy, the stronger the bond.

Step 2: Analyze Bond Strength in Halogen Acids

The bond strength in hydrogen halides (HF, HCl, HBr) depends on the size of the halogen atoms. As we move down the halogen group in the periodic table, the size of the halogen atoms increases, and the bond length increases, making the bond weaker.

Step 3: Determine Order of Halogen Size

Fluorine is the smallest, followed by chlorine, and then bromine. Therefore, the hydrogen bonds decrease in strength from HF to HBr due to increasing atomic size.

Step 4: Arrive at Correct Order of Dissociation Energies

Based on the bond strength analysis, the bond dissociation energies should be highest for HF, followed by HCl, and then HBr. Hence, the correct order is \(\mathrm{HF} > \mathrm{HCl} > \mathrm{HBr}\).

Key Concepts

Hydrogen Halides

Hydrogen halides are a group of binary compounds, consisting of hydrogen and a halogen. They are known for their strong acidic properties and are fundamental in various chemical reactions. These compounds include

• Hydrogen Fluoride (HF)
• Hydrogen Chloride (HCl)
• Hydrogen Bromide (HBr)

Each halide has distinct characteristics determined by the halogen atom. As a result, the bond energies in these molecules vary. These variations are crucial in understanding bond strengths and the chemical behavior of hydrogen halides in reactions.

Periodic Table Trends

Periodic table trends help us predict the behavior of elements, especially when forming compounds like hydrogen halides. As you move down the halogen group in the periodic table, you’ll notice a trend: increasing atomic size. This influences several properties:

• The size of fluorine (at the top) to iodine (at the bottom) increases.
• This increase results in longer bond lengths when these elements form compounds.
• Longer bond lengths often correlate to weaker chemical bonds.

Understanding these trends allows chemists to predict bond strengths and the ease of breaking bonds, which is crucial when analyzing substances like hydrogen halides.

Halogen Size and Bond Strength

The size of halogen atoms influences the bond strength they form with hydrogen. Fluorine, being the smallest halogen, forms the strongest bond with hydrogen. As halogens increase in size, their bond strength with hydrogen decreases:

• Fluorine's small size means a shorter and stronger bond in HF.
• Chlorine is larger than fluorine, hence HCl has slightly weaker bond strength than HF.
• Bromine, even larger, results in the weakest bond strength among HF, HCl, and HBr.

The trend reflects the influence of atomic size on bond strength, which is pivotal for understanding compounds' stability and reactivity like those exhibited by hydrogen halides.

Chemical Bonding in Molecules

Chemical bonding in molecules involves understanding how atoms connect to form compounds. In hydrogen halides:

• Atoms share electrons to achieve stability through covalent bonding.
• The nature of the bonds dictates the physical and chemical properties of these molecules.
• Strong bonds like in HF mean high bond dissociation energies and more energy required to break the bond.

Grasping these concepts allows students to better predict and understand molecular behavior. The bond dissociation energy pattern observed in hydrogen halides emphasizes these principles, illustrating why HF has the highest dissociation energy compared to HCl and HBr.