Problem 205 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of the thermal stability of hydrogen halides \((\mathrm{H}-\mathrm{X})\) is (a) \(\mathrm{HCl}>\mathrm{HF}>\mathrm{HBr}>\mathrm{HI}\) (b) \(\mathrm{HI}>\mathrm{HCl}>\mathrm{HF}>\mathrm{HBr}\) (c) \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}\) (d) \(\mathrm{HF}>\mathrm{HCl}>\mathrm{HBr}>\mathrm{HI}\)

Step-by-Step Solution

Answer

The correct order is \( \text{HF} > \text{HCl} > \text{HBr} > \text{HI} \), option (d).

Step 1: Understand Thermal Stability

Thermal stability refers to the compound's ability to resist decomposition at higher temperatures. The thermal stability of hydrogen halides is influenced by the bond strength between hydrogen and the halide.

Step 2: Analyze Bond Strengths

The bond strength decreases down the halogen group in the periodic table. That means HI has the weakest bond and HF has the strongest bond due to the small size of fluorine, allowing stronger overlap and a stronger bond with hydrogen.

Step 3: Order the Compounds by Bond Strength

Given that HF has the strongest bond and HI has the weakest, the order of bond strength is: \( \text{HF} > \text{HCl} > \text{HBr} > \text{HI} \).

Step 4: Determine Thermal Stability Order

Since thermal stability is directly related to bond strength, the order of thermal stability is the same as that of bond strength. Therefore, the order is \( \text{HF} > \text{HCl} > \text{HBr} > \text{HI} \).

Step 5: Match with Given Options

Compare the determined order \( \text{HF} > \text{HCl} > \text{HBr} > \text{HI} \) to the provided options and find that option (d) is correct.

Key Concepts

Bond Strength

Bond strength is a key factor in determining the thermal stability of hydrogen halides. When we talk about bond strength, we're referring to the amount of energy that's needed to break a bond between two atoms. The stronger a bond, the higher the energy required to break it.
For hydrogen halides, the bond strength varies significantly as we move down the periodic table. Fluorine in HF forms the strongest bond with hydrogen because fluorine is small and allows a closer and stronger overlap of orbitals, resulting in a strong bond. Meanwhile, iodine in HI, being larger, forms the weakest bond due to less efficient orbital overlap.

• Stronger bonds mean more energy is needed to break them, hence higher thermal stability.
• Weaker bonds require less energy to break, leading to lower thermal stability.

This explains why HF, with the strongest bond, also exhibits the highest thermal stability.

Hydrogen Halides

Hydrogen halides are a group of binary compounds consisting of hydrogen and halogens. These include HF, HCl, HBr, and HI. The properties of these compounds are largely influenced by the nature of the halogen atom. Each hydrogen halide has its unique characteristics based on the halogen it contains.
These differences in characteristics are due to varying bond strengths and atomic radii of the halogens.

• Hydrogen fluoride (HF) has a strong hydrogen-fluorine bond, making it highly stable.
• Hydrogen chloride (HCl) is moderately stable, with a less strong bond compared to HF.
• Hydrogen bromide (HBr) features a weaker bond than both HF and HCl, lowering its stability.
• Hydrogen iodide (HI) is the least stable due to its weakest bond.

Understanding the traits of hydrogen halides is important for predicting their reactivities and thermal behaviors.

Periodic Table Trends

Periodic table trends are essential for understanding the properties of elements like halogens in hydrogen halides. These trends help predict how properties such as atomic size, electronegativity, and bond strength change across and down the periodic table.
As you move down the halogen group (from F to I), atomic size increases. Larger atoms lead to longer bonds, which are typically weaker.

• Fluorine's small size enables it to form strong bonds, accounting for HF's high stability.
• As we descend the group, bond distances increase, leading to weaker bonds with each subsequent halogen.

This pattern explains why hydrogen halides become less stable with increasing atomic number of the halogen. Predicting the stability and chemical behavior of hydrogen halides is thus straightforward when periodic trends are understood.