Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 7.2 Hydrogen bonding-Study Notes - New Syllabus

7.2 Hydrogen Bonding in Specific Molecules

Hydrogen bonding arises in molecules such as \( \mathrm{H_2O} \), \( \mathrm{NH_3} \), and \( \mathrm{HF} \) due to the presence of highly polar bonds and lone pairs on electronegative atoms.

General Requirement Recap

• H bonded to \( \mathrm{N} \), \( \mathrm{O} \), or \( \mathrm{F} \)
• Lone pair on neighbouring molecule

(i) Water (\( \mathrm{H_2O} \))

Structure and Bonding

• O–H bonds are highly polar.
• Oxygen has 2 lone pairs.

Hydrogen Bonding Pattern

• Each molecule can form up to 4 hydrogen bonds:
  • 2 via hydrogen atoms
  • 2 via lone pairs on oxygen

Result

• Extensive 3D hydrogen-bonded network.
• Very high boiling point for small molecule.

(ii) Ammonia (\( \mathrm{NH_3} \))

Structure and Bonding

• N–H bonds are polar.
• Nitrogen has 1 lone pair.

Hydrogen Bonding Pattern

• Each molecule can form fewer hydrogen bonds than water:
  • 3 H atoms available
  • Only 1 lone pair available

Result

• Less extensive hydrogen bonding network than water.
• Lower boiling point than water.

(iii) Hydrogen Fluoride (\( \mathrm{HF} \))

Structure and Bonding

• H–F bond is very polar.
• Fluorine has 3 lone pairs.

Hydrogen Bonding Pattern

• Forms chains of molecules via hydrogen bonding.
• Each molecule typically forms 2 hydrogen bonds.

Result

• Strong hydrogen bonding due to very high electronegativity of F.
• Higher boiling point than expected for its size.

Comparison Summary

• \( \mathrm{H_2O} \): extensive 3D network → strongest overall effect
• \( \mathrm{NH_3} \): fewer H-bonds → weaker network
• \( \mathrm{HF} \): strong but forms chains, not 3D network

Key Insight

• Strength of hydrogen bonding depends on:
  • Electronegativity (F > O > N)
  • Number of hydrogen bonds formed
  • Structure (network vs chain)

The nature and extent of hydrogen bonding depend on both electronegativity and molecular structure.