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) HF

Step-by-Step Solution

Answer

HF is the most stable compound.

Step 1: Understand Stability

Stability of a compound often depends on the bond strength between atoms. In the case of hydrogen halides like HI, HCl, HBr, and HF, stability is related to the bond energy of the hydrogen-halogen bond.

Step 2: Analyze Bond Energies

The bond energy is the energy required to break a bond. Generally, the greater the bond energy, the more stable the compound. The order of bond energies for hydrogen halides is: HF > HCl > HBr > HI.

Step 3: Determine Most Stable Based on Bond Energy

Since the stability is directly proportional to bond strength, and HF has the greatest bond energy among the given choices, HF is the most stable compound.

Key Concepts

Bond Energy

Bond energy represents how much energy is needed to break a chemical bond between two atoms. In essence, it's a measure of bond strength.
For hydrogen halides like HI, HCl, HBr, and HF, this is particularly important because the stability of these compounds is heavily influenced by their bond energies.
When a bond is strong, it means more energy is needed to break it, signifying a higher bond energy.

• Bond energy is an indicator of bond stability.
• A higher bond energy results in greater compound stability.
• Molecules with high bond energies are less reactive and more stable.

In the context of hydrogen halides, the bond energy progressively decreases from HF to HI, making HF the most stable due to its high bond energy.

Hydrogen-Halogen Bond

The hydrogen-halogen bond is the single covalent bond formed between a hydrogen atom and a halogen atom. Each hydrogen-halogen bond varies based on the specific halogen involved, influencing the overall stability of the compound.
Halogens include fluorine, chlorine, bromine, and iodine. Each element forms a different bond strength with hydrogen, primarily because of the atom's size and electronegativity.

• Hydrogen-fluorine (HF): The strongest bond due to fluorine's high electronegativity and small atomic size, enabling closer interaction with hydrogen.
• Hydrogen-chlorine (HCl): Strong but weaker than HF, as chlorine's larger atomic size reduces bond strength.
• Hydrogen-bromine (HBr): Weaker still because bromine is larger and less electronegative.
• Hydrogen-iodine (HI): The weakest bond with the largest halogen atom, leading to less interaction strength.

As the atomic size of halogens increases, the distance between hydrogen and halogen atoms in a bond also increases, leading to a weaker bond.

Compound Stability

Compound stability is a key factor in predicting a compound's behavior and reactivity. It often relies on the strength of the bonds connecting the elements within the molecule.
In hydrogen halides, more stable compounds have higher bond energies, meaning they require more energy to break apart.
This is why HF is considered the most stable hydrogen halide. Due to its high bond energy, it is less likely to react and degrade.

• Stability is crucial for determining a compound's lifespan in chemical reactions.
• Stable compounds like HF have low reactivity due to their strong molecular structure.
• Bond strength and stability are directly related; higher bond energies correlate with higher stability.

Understanding compound stability can help predict how molecules will behave under different chemical environments, ensuring better comprehension of chemical reactions and processes.