CIE AS/A Level Chemistry 3.6 Intermolecular forces, electronegativity and bond properties Study Notes- 2025-2027 Syllabus

Van der Waals’ Forces — Definition and Use

Van der Waals’ forces are all intermolecular forces that occur between molecules, apart from those involving bond formation (i.e., not covalent, ionic, or metallic bonds). They include London dispersion forces, permanent dipole–dipole forces, and hydrogen bonding.

Definition of Van der Waals’ Forces

Van der Waals’ forces are the intermolecular forces between molecular entities other than those due to bond formation.

The term van der Waals’ forces is used as a generic term to describe all intermolecular forces.

Types of Intermolecular Forces Included Under Van der Waals’ Forces

Types of Van der Waals’ Forces

Van der Waals’ forces include all intermolecular forces that occur between molecules. These can be classified into three main types: instantaneous dipole–induced dipole forces (London dispersion forces), permanent dipole–permanent dipole forces, and hydrogen bonding.

1. Instantaneous Dipole–Induced Dipole Forces (id–id)

Also called: London dispersion forces

• Present in all molecules (polar and non-polar).
• Caused by random movement of electrons → creates a temporary dipole.
• This temporary dipole induces a dipole in a neighbouring molecule → weak attraction.
• Strength increases with:  
  • number of electrons   
  • molar mass   
  • surface area / contact area

Greater electrons → stronger London forces → higher boiling point.

2. Permanent Dipole–Permanent Dipole Forces (pd–pd)

• Occurs between molecules with permanent dipoles.
• Bonds are polar because of electronegativity differences.
• The positive end of one polar molecule attracts the negative end of another.
• Stronger than London forces, but weaker than hydrogen bonding.

Example: \( \mathrm{HCl} \), \( \mathrm{CH_3Cl} \), \( \mathrm{C=O} \)-containing molecules.

3. Hydrogen Bonding (a special type of pd–pd force)

• Strongest type of van der Waals’ force.
• Occurs when H is bonded to highly electronegative atoms: \( \mathrm{N, O} \).
• Requires: 
  •  a \( \mathrm{N–H} \) or \( \mathrm{O–H} \) bond  
  •  a lone pair on an electronegative atom
• Examples: \( \mathrm{H_2O} \), \( \mathrm{NH_3} \), alcohols, amines.

Hydrogen bond: \( \mathrm{\delta^+ H \cdots \delta^- O} \) or \( \mathrm{\delta^+ H \cdots \delta^- N} \)

Comparison Table

Hydrogen Bonding as a Special Case of Permanent Dipole–Permanent Dipole Forces

Hydrogen bonding is the strongest type of intermolecular attraction. It is a special type of permanent dipole–permanent dipole (pd–pd) force that occurs only when hydrogen is bonded to a highly electronegative atom: nitrogen or oxygen.

What is Hydrogen Bonding?

• Occurs when H is covalently bonded to a highly electronegative atom:   
  •  \( \mathrm{N–H} \)   
  • \( \mathrm{O–H} \)
• The electronegative atom pulls electron density away from hydrogen → H becomes strongly partially positive \( (\delta^+) \).
• The electronegative atom has a lone pair that carries partial negative charge \( (\delta^-) \).
• A strong attraction forms between:   
  • \( \delta^+ \) hydrogen on one molecule   
  • lone pair on \( \mathrm{N} \) or \( \mathrm{O} \) of another molecule

Hydrogen bond: \( \mathrm{\delta^+H\ \cdots\ \delta^-O} \) or \( \mathrm{\delta^+H\ \cdots\ \delta^-N} \)

Why It Is a Special Case of pd–pd Forces

• Hydrogen bonding is a very strong permanent dipole–permanent dipole force.
• It is stronger than other pd–pd forces because:   
  • \( \mathrm{N} \) and \( \mathrm{O} \) are extremely electronegative   
  • The small size of H allows very close approach of molecules   
  • Large dipole created when electrons are pulled strongly from H
• Thus, hydrogen bonding is simply an extreme, strong form of pd–pd interaction.

Where Hydrogen Bonding Occurs

Occurs only when hydrogen is bonded to:

• \( \mathrm{N} \) → e.g., \( \mathrm{NH_3} \), amines
• \( \mathrm{O} \) → e.g., \( \mathrm{H_2O} \), alcohols, carboxylic acids

Comparison Table