Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is most stable? (a) \(\mathrm{HI}\) (b) \(\mathrm{HCl}\) (c) \(\mathrm{HBr}\) (d) \(\mathrm{HF}\)

Step-by-Step Solution

Answer

The most stable molecule is (d) HF.

Step 1: Understanding Stability in Terms of Bond Strength

Molecular stability in diatomic molecules like HI, HCl, HBr, and HF is closely linked to the bond strength between the hydrogen atom (H) and the halogen atom (Iodine, Chlorine, Bromine, or Fluorine). A stronger bond generally indicates a more stable molecule.

Step 2: Analyzing Bond Strengths Based on Electronegativity and Size

The strength of a chemical bond can be influenced by the electronegativity of the atoms as well as their size. Fluorine is highly electronegative and forms very strong bonds. Due to its small atom size, HF has a very strong bond compared to HI, HCl, and HBr.

Step 3: Comparing the Halogen-Hydrogen Bonds

The bond dissociation energies (a measure of bond strength) are highest for HF and decrease in the order HF > HCl > HBr > HI. This is because the bond strength decreases as the halogen atom grows larger, lessening the overlap between orbitals.

Step 4: Concluding with the Most Stable Molecule

Since HF has the highest bond dissociation energy among the given options, it is the most stable compound. Its strong bond is due to the high electronegativity and small size of fluorine, enabling effective overlap with hydrogen's 1s orbital.

Key Concepts

Bond Strength

In chemistry, bond strength is a crucial factor that determines the stability of a molecule. When we talk about bond strength, we are referring to the energy required to break a chemical bond between two atoms. In general, stronger bonds imply more stable molecules. For diatomic molecules such as HI, HCl, HBr, and HF, the bond strength is related to the interaction between hydrogen and a halogen atom.
The stronger the bond, the more energy it takes to separate the atoms, making the compound less likely to dissociate under normal conditions. This characteristic is often measured using bond dissociation energy, which is the energy needed to break one mole of bonds in gaseous molecules.

• HF has the strongest bond due to its high bond dissociation energy.
• The order of bond strength in these molecules is: HF > HCl > HBr > HI.

This means that HF is the most stable, as more energy is required to break its hydrogen-fluorine bond than the bonds in the other molecules.

Electronegativity

Electronegativity is the ability of an atom to attract electrons towards itself in a chemical bond. It plays a significant role in determining the bond strength in molecules. Fluorine, which is the most electronegative element, forms very strong bonds with hydrogen.
Electronegativity can influence bond strength in the following ways:

• A higher electronegativity means that the atom can attract bonding electrons more strongly. This leads to stronger bonds.
• In our list of molecules, fluorine's high electronegativity results in a very strong H-F bond.

Thus, HF stands out again as the most stable of the options, largely due to fluorine's ability to pull the shared electrons tightly towards itself, enhancing bond strength.

Halogen-Hydrogen Bonds

Halogen-hydrogen bonds, like those in HI, HCl, HBr, and HF, are a prime example of covalent bonding where hydrogen shares electrons with a halogen atom. The stability of these molecules decreases as you move down the group in the periodic table.
Here's why:

• The size of the halogen atoms increases from fluorine to iodine.
• Larger atoms have a more diffused electron cloud, leading to weaker bonds because the overlap between hydrogen's small 1s orbital and the larger halogen orbitals becomes less effective.

This can be seen in bond dissociation energies, which rank as follows: HF (strongest) > HCl > HBr > HI (weakest). This decrease in energy reflects the reduced bond stability as you move to larger halogens. Therefore, HF is the most stable halogen-hydrogen bond due to the small size and high electronegativity of fluorine, making the overlap with hydrogen's orbital particularly effective.