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) \(\mathrm{Hl}>\mathrm{HCl}>\mathrm{HF}>\mathrm{HBr}\) (c) \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}\) (d) \(\mathrm{HF}>\mathrm{HCl}>\mathrm{HBr}>\mathrm{HI}\)

Step-by-Step Solution

Answer

The order of thermal stability is (d) HF > HCl > HBr > HI.

Step 1: Understand Thermal Stability

Thermal stability refers to the resistance of a compound to decomposition at high temperatures. For hydrogen halides, 7as we move down the group in the periodic table from fluorine to iodine, the bond strength between hydrogen and the halogen atom decreases, leading to a decrease in thermal stability.

Step 2: Analyze Bond Strength

The bond strength in hydrogen halides depends on the size of the halogen. As the size of the halogen atom increases from F to I, the bond length increases, and thus the H-X bond becomes weaker. This means that HF has the strongest bond, followed by HCl, HBr, and HI, which has the weakest bond.

Step 3: Determine the Correct Order of Stability

Based on the bond strength information, the thermal stability decreases in the order of strongest to weakest bond. HF, with the strongest bond, is the most thermally stable, followed by HCl, HBr, and finally HI, which is the least stable.

Step 4: Match with Given Options

Compare the determined order of thermal stability (HF > HCl > HBr > HI) with the given options. The correct answer matches option (d).

Key Concepts

Hydrogen Halides

Hydrogen halides are a group of chemical compounds where hydrogen is bonded to a halogen atom. These halogens include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). The general formula for hydrogen halides is HX, where X represents the halogen.

• They are polar molecules due to the electronegativity difference between hydrogen and the halogen.
• These compounds are gaseous at room temperature, except for hydrogen fluoride (HF) which can be liquid due to hydrogen bonding.
• In water, they dissociate to form acids such as hydrochloric acid (HCl), making them important in various chemical reactions.

Understanding the nuances of each hydrogen halide is crucial as their properties, including thermal stability, vary significantly based on the halogen involved.

Bond Strength

In hydrogen halides, bond strength refers to the energy required to break the chemical bond between hydrogen and the halogen atom. This is a critical factor that impacts their thermal stability.

• As you move down the periodic table from fluorine to iodine, the halogen atoms get larger. This increases the bond length as the outer electrons are further from the nuclear pull.
• A longer bond length typically results in a weaker bond, meaning less energy is required to break the bond.
• In this series, HF has the strongest bond due to the small size and high electronegativity of fluorine. Conversely, HI, with iodine being the largest halogen, has the weakest bond.

This inverse relationship between bond length and bond strength helps explain the trend in thermal stability among the hydrogen halides.

Periodic Trends

Periodic trends provide valuable insight into the properties of elements and their compounds as you move across or down the periodic table.

• In the case of hydrogen halides, one essential periodic trend is the change in halogen atom size.
• As you go from top to bottom in the group, there is an increase in atomic size. This results from the addition of electron shells, which increases the distance between the nucleus and the valence electrons.
• This increase in size influences both bond strength and thermal stability, with a weaker bond and lower stability observed as you move down from HF to HI.

Understanding these periodic trends is critical for predicting the behavior and reactivity of hydrogen halides in various chemical processes.

Decomposition Resistance

Decomposition resistance is the ability of a compound to resist breaking down at elevated temperatures. For hydrogen halides, this is directly connected to bond strength.

• Strongest bonds, like in HF, confer higher resistance, making it the most thermally stable among the hydrogen halides.
• In contrast, HI, with its weakest bond, has the lowest decomposition resistance and, therefore, the least thermal stability.
• This property is crucial in determining the practical applications and storage conditions for each hydrogen halide.

By analyzing decomposition resistance, we can better understand the preferred use cases for these compounds in industrial and laboratory settings.