Question

HF has highest boiling point among hydrogen halides, because it is:
A) Lowest dissociation enthalpy
B) Strongest van der Waals interaction
C) Strongest hydrogen bonding
D) Lowest ionic character

Answer 1

Hint: Recall the factors on which boiling point of a substance depends. The boiling point of a system depends upon the intermolecular interactions. Higher the strength of these interactions, higher is the value of boiling point.

Complete step by step solution:
Let us discuss the molecule HF called hydrogen fluoride. In HF, there is electronegativity difference between H atom and F atom. Fluorine being more electronegative will have partial negative charge and hydrogen being less electronegative will have partial positive charge. A more electronegative has greater tendency to attract the electrons that’s why a more electronegative element has partial negative charge. It can shown as:

Now, to check the boiling point of HF, let us consider a lattice containing HF molecules. We know that there will not be only a single HF molecule in any lattice. There are always clusters of atoms or molecules present in a lattice. So, the H atom of a HF molecule having partial positive charge will be attracted towards the F atom of another molecule having partial negative charge. Also, we know that hydrogen bonding results when a hydrogen atom, which is covalently bonded to a very electronegative atom, gets attracted towards another high electronegative atom. Thus, the same attraction observed in HF molecules hence, this attraction results in strong hydrogen bonding among the HF molecules. Hydrogen bonding is the strongest intermolecular force. Due to strong intermolecular forces, the molecules of HF will be tightly packed in a lattice. Thus, HF will have the highest boiling point since the boiling point of a system depends upon the intermolecular interactions.

Thus, option C is correct.

Note: Hydrogen halides are diatomic molecules with the molecular formula HX, where X is any halogen. Hydrogen halides are colourless gases at STP except hydrogen fluoride (HF). From HCl to HI, boiling point increases but among the all hydrogen halides, only HF exhibits strong hydrogen bonding, and therefore has the highest boiling point as well as melting point.