Question

Which of the following hydrogen halides has the highest boiling point?
A. $HF$
B. $HCl$
C. $HBr$
D. $HI$

Answer 1

Hint: More strong bonds will require more heat or energy to break so, possess higher boiling point. In the presence of hydrogen bonding, boiling point increases as hydrogen bond makes a compound stable.

Complete Step by Step Answer:
The halogen hydride which can form hydrogen bonds has the highest boiling point.

The Conditions for making hydrogen bonds are as follows:
- The molecule must contain a highly electronegative atom that is attached to the hydrogen atom to form a covalent bond.
- Higher the electronegativity more is the polarisation of the molecule so that electrons of the covalent bonds are shifted towards the more electronegative atom and make partially positively charged hydrogen atoms.
- Among the halogen hydrides only hydrogen fluoride contains hydrogen bonding between electronegative fluorine and electropositive hydrogen atom. It has different properties from other halogen hydrides. That’s why hydrogen fluoride has the strongest bond and it has the highest boiling point.
- Thus we can write that $HF$ has the highest boiling point.
Thus the correct option is A.

Note: The temperature at which a solid or liquid starts to form gas is known as the boiling point of that solid or liquid. As we increase the temperature the atoms of a substance start vibrating so, motion of the atoms increases and boiling point also increases.