Question

The correct order of thermal stability of hydrogen halides \[\left( {H - X} \right)\] is:
A.$HI > HCl > HF > HBr$
B.$HCl < HF < HBr < HI$
C.$HF > HCl > HBr > HI$
D.$HI > HBr > HCl > HF$

Answer 1

Hint: The thermal stability is the stability of a water body and its resistance to mixing. It depends on the structure of the substance which in turn is dependent on the bonds between the atoms that hold that structure together. It decreases down the group.

Complete step by step answer:
-The hydrogen halides are diatomic organic compounds with formula HX, where X represents any halogen. Some of the halogens include iodine, chlorine, bromine and fluorine.
-Among the halogen fluorides chlorine and bromine fall under the category of volcanic gases. These gases consist of dissolved gases found in lava.
-At STP, the hydrogen halides are colorless gas. However, hydrogen fluoride is an exception case as it boils at ${19^ \circ }C$.
-Now, as we move down the group, the thermal stability of halides decreases due to decrease in the bond strength. Therefore, among the given options, the correct order is:
$HF > HCl > HBr > HI$
This is because hydrogen and fluorine have comparable size because of which they are more stable and can hold the electron pairs easily whereas when we move down the group the size of the central atom increases and hence the bond between the central atom and hydrogen becomes less stable.

Hence, option C is correct.

Note:
The bond strength basically describes how strongly each atom is joined to another atom, and how much energy is required to break the bond between the two atoms.. However, the strength of a bond between two atoms increases as the number of electron pairs in the bond increases.