Despite hydrogen fluoride (HF) possessing stronger individual hydrogen bonds than water (H… — JAMB UTME

Despite hydrogen fluoride ( $HF$ ) possessing stronger individual hydrogen bonds than water ( $H_2O$ ), liquid water has a significantly higher boiling point ( $100^\circ\text{C}$ ) than liquid hydrogen fluoride ( $19.5^\circ\text{C}$ ). What is the primary structural reason for this higher boiling point in water?

Each water molecule can form an average of four hydrogen bonds in an extensive three-dimensional network, whereas each hydrogen fluoride molecule forms an average of only two hydrogen bonds.Answer
B
The intramolecular covalent bond between hydrogen and oxygen in water is significantly stronger than the intramolecular covalent bond in hydrogen fluoride.
C
Oxygen has a higher electronegativity than fluorine, resulting in a larger permanent dipole moment in water molecules.
D
Hydrogen fluoride molecules in the liquid state are predominantly attracted by weak London dispersion forces rather than hydrogen bonds.

Answer

Each water molecule can form an average of four hydrogen bonds in an extensive three-dimensional network, whereas each hydrogen fluoride molecule forms an average of only two hydrogen bonds.

The correct answer correctly identifies that the number of hydrogen bonds per molecule determines the bulk physical property. Water possesses 2 hydrogen atoms and 2 lone pairs on oxygen, creating an optimal ratio that allows 4 hydrogen bonds per molecule in a 3D network. Hydrogen fluoride has 3 lone pairs but only 1 hydrogen atom, creating a hydrogen deficit that limits the system to an average of 2 hydrogen bonds per molecule.

Step-by-Step Solution

Analyze the structural capacity for hydrogen bonding in hydrogen fluoride (

HF

HF

molecule has

1

hydrogen atom and

3

lone pairs on the fluorine atom. Because hydrogen atoms are the limiting factor, each molecule can only participate in an average of

2

hydrogen bonds (

1

donated,

1

accepted).

Hydrogen bonding requires both a hydrogen atom bonded to a highly electronegative atom and an available unshared electron pair.

Analyze the structural capacity for hydrogen bonding in water (

H_2O

H_2O

molecule has

2

hydrogen atoms and

2

lone pairs on the central oxygen atom. This 1:1 stoichiometry of hydrogens to lone pairs allows each water molecule to form an average of

4

hydrogen bonds in a 3D network.

The equal number of hydrogen atoms and lone pairs maximizes the overall density of the hydrogen-bonding network.

Compare the total intermolecular energy required to separate the molecules during boiling.

Even though a single

F-H\cdots F

bond is stronger than a single

O-H\cdots O

bond, twice as many hydrogen bonds must be broken per mole of water, requiring significantly more thermal energy to vaporize liquid water.

Boiling point depends on the total energy required to overcome all intermolecular attractions in the liquid bulk.

Key Concept

Hydrogen bonding capacity and network density

Estimated Time:1m 0s

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