Problem 165 - 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 of thermal stability is (d) \(\mathrm{HF} > \mathrm{HCl} > \mathrm{HBr} > \mathrm{HI}\).

Step 1: Understanding Thermal Stability

Thermal stability of a compound is determined by the strength of the bond between its atoms. For hydrogen halides \((\mathrm{H}-\mathrm{X})\), thermal stability depends on the bond strength of the \(\mathrm{H}-\mathrm{X}\) bond.

Step 2: Comparing Bond Lengths and Strengths

The bond strength between hydrogen and the halogens (F, Cl, Br, I) decreases as the size of the halogen increases. This is because a larger atomic radius leads to a longer and weaker bond.

Step 3: Order of Atomic Size

The order of atomic size for halogens is \(\mathrm{I} > \mathrm{Br} > \mathrm{Cl} > \mathrm{F}\). As atomic size increases, bond length increases and bond strength decreases.

Step 4: Determine the Stability Order

In hydrogen halides, \(\mathrm{HI}\) has the weakest bond due to iodine's large size, making it the least thermally stable. Conversely, \(\mathrm{HF}\) has the strongest bond due to fluorine's small size, making it the most thermally stable.

Step 5: Finalizing the Order

Considering the bond strength, the order of thermal stability of hydrogen halides is \(\mathrm{HF} > \mathrm{HCl} > \mathrm{HBr} > \mathrm{HI}\). This matches option (d).

Key Concepts

Bond Strength

Bond strength is crucial in understanding the thermal stability of hydrogen halides. A bond involves the attractive force between atoms, and its strength can determine how easily a compound is broken down or preserved when subjected to heat. The bond strength in hydrogen halides such as HF, HCl, HBr, and HI is determined by the attractions between the hydrogen atom and a halogen. The stronger the bond, the more energy is required to break it. In the case of hydrogen halides, the bond strength decreases as the atomic size of the halogens increases. This is primarily due to the increase in bond length that accompanies larger atomic sizes. Once the bond length increases, the attractive force between atoms diminishes, weakening the bond.

Atomic Size of Halogens

The atomic size of halogens significantly affects the bond strength in hydrogen halides. When we refer to atomic size, we typically talk about the radius of the atom, which increases from top to bottom in the halogen group. Thus, the order in atomic size among halogens is iodine (I) being the largest, followed by bromine (Br), chlorine (Cl), and fluorine (F) being the smallest.
Larger atomic sizes result in longer bonds since the larger halogen atoms extend the distance between the hydrogen atom and the electron clouds surrounding the halogen. This extended distance weakens the bond, as there is less overlap of electron clouds where bonds form. It’s essential to realize the relationship between atomic size and bond strength to predict the thermal stability of compounds.

Hydrogen Halides

Hydrogen halides are compounds consisting of hydrogen bonded to a halogen from the group consisting of fluorine, chlorine, bromine, and iodine. In these compounds, the hydrogen-halogen bond plays a pivotal role in determining thermal stability. It is observed that fluorine, due to its small atomic size, forms the strongest bond with hydrogen in HF. Conversely, iodine, with its large atomic size in hydrogen iodide (HI), forms the weakest bond.
The stability of these bonds inversely influences how easily the compound can decompose when exposed to heat: stronger bonds such as in HF require more energy to decompose, resulting in greater thermal stability, whereas weaker bonds like in HI break more easily, making it thermally less stable. Understanding these principles can help predict and explain behaviors of hydrogen halides in various chemical environments.