Problem 19 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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

Step-by-Step Solution

Answer

\(\mathrm{HCl}\) is the most stable to heat.

Step 1: Understanding the problem

We need to identify which of the given hydrogen halides or related compounds is most stable when exposed to heat. The options are (a) \(\mathrm{HCl}\), (b) \(\mathrm{HOCl}\), (c) \(\mathrm{HBr}\), and (d) \(\mathrm{HI}\).

Step 2: Concept of Thermal Stability

Thermal stability refers to the resistance of a compound to decomposition at high temperatures. For hydrogen halides, this stability generally decreases down the group in the periodic table. This means \(\mathrm{HF}\) is most stable, followed by \(\mathrm{HCl}\), then \(\mathrm{HBr}\), and finally \(\mathrm{HI}\), which is least stable.

Step 3: Applying Thermal Stability to the Options

Among the options listed, (b) \(\mathrm{HOCl}\) is not a hydrogen halide, so it is not considered in the periodic trend. Comparing the stability of the actual hydrogen halides: \(\mathrm{HCl}\), \(\mathrm{HBr}\), and \(\mathrm{HI}\), according to the trend, \(\mathrm{HCl}\) should be the most thermally stable.

Step 4: Conclusion

Given our understanding of thermal stability in hydrogen halides and disregarding \(\mathrm{HOCl}\) as it is not a pure hydrogen halide, \(\mathrm{HCl}\) (option a) is the most stable to heat.

Key Concepts

Hydrogen Halides

Hydrogen halides are a group of diatomic molecules where hydrogen is bonded to a halogen. The common halogens include fluorine, chlorine, bromine, and iodine, leading to hydrogen halides such as

• HF (Hydrogen fluoride)
• HCl (Hydrogen chloride)
• HBr (Hydrogen bromide)
• HI (Hydrogen iodide)

Each of these compounds has unique properties that influence their behavior in different situations. One of these key properties is their thermal stability, which determines how they react when exposed to heat.

Hydrogen halides are known for their ability to dissociate and form ions in solution, leading to acidity. Their acidity and chemical properties make them useful in various industrial applications, such as

• production of acids for chemical reactions
• cleaning agents due to their reactive nature
• in some synthesis processes to introduce halogen atoms

Periodic Table Trends

The periodic table trends play a significant role in understanding the thermal stability of hydrogen halides. As we move down the group of halogens in the periodic table, the atomic size increases. This trend affects the bond strength between hydrogen and the halogen.

Generally, bond strength decreases as you move down the group:

• HF has the strongest hydrogen-halogen bond
• HI has the weakest bond
• HCl and HBr have intermediate bond strengths

These differences occur because larger atoms form weaker bonds due to increased atomic radius and less effective overlap of orbitals.

Consequently, hydrogen halides show a trend of decreasing thermal stability down a group. This means

• HF is most stable due to strong bond strength
• HI is least stable as its bonds are weak

Decomposition Resistance

Decomposition resistance is a measure of how well a compound can withstand heat without breaking down into simpler substances. For hydrogen halides, this resistance diminishes as we proceed down the group in the periodic table. The explanation lies in the bond enthalpy, which is less for molecules with weaker bonds.

The lower the bond enthalpy, the more susceptible the substance is to breaking apart when heated. For instance,

• HCl has a higher decomposition resistance compared to HBr and HI because of its stronger H-Cl bond.
• HCl's relative stability is useful in various chemical processes where heat stability is required.

Understanding these trends helps predict how different hydrogen halides will behave under heat stress, which can be crucial in industrial applications where temperature resistance is vital.

Overall, recognizing the factors that influence decomposition resistance enhances our ability to choose the right hydrogen halide for specific tasks or reactions in scientific and industrial fields.