5.4 Intermolecular Forces

This topic explains intermolecular forces — the attractive forces that exist between molecules (not within them).

Definition

An intermolecular force is the attractive force that exists between the molecules.

Simple explanation: Intermolecular forces are the forces that hold different molecules together — like the glue between separate molecules.

Main Idea

Intermolecular forces are weaker than chemical bonds (ionic, covalent, metallic). They determine many physical properties of substances, such as:

• Melting point
• Boiling point
• Solubility
• Surface tension
• Viscosity

The book discusses two main types of intermolecular forces:

• Dipole–dipole forces
• Hydrogen bonding

Simple explanation: Intermolecular forces are the "weak attractions" between molecules that decide whether a substance is a solid, liquid, or gas at room temperature.

Core Concept

Types of Intermolecular Forces

Detailed Explanation

Dipole–Dipole Forces

Dipole–dipole interactions occur between polar molecules.

You know that paints and dyes are used to protect solid surfaces from atmospheric effects. They also give visual appeal. Resins are used to coat materials that give toughness, flexibility, adhesion, and chemical resistance.

• Dams, bridges, floors, trains, buses, cars, etc. are painted with resins.
• Synthetic resins are used where water resistance is required.
• Chemically, resins are either adhesive or they form bond linkages with the material being bonded together.

Notice that the slightly negative end of a polar molecule is weakly attracted to the slightly positive end of another molecule. Such attracting forces are called dipole–dipole interactions.

Simple explanation: In polar molecules, one end is slightly positive (δ+) and the other is slightly negative (δ−). The δ+ end of one molecule attracts the δ− end of another molecule.

Example: HCl molecules attract each other: - The H (δ+) of one HCl is attracted to the Cl (δ−) of another HCl.

Hydrogen Bonding

Molecules in which hydrogen is covalently bonded to a very electronegative atom such as oxygen, nitrogen, or fluorine are also weakly bonded to a lone pair of electrons of another electronegative atom.

• In the same molecule, or
• In a nearby molecule

This intermolecular interaction is called hydrogen bonding.

Why does hydrogen bonding happen? Oxygen, nitrogen, or fluorine makes hydrogen very electron-deficient. Thus, this interaction of such a highly electron-deficient hydrogen and a lone pair on a nearby electronegative atom compensates for the deficiency.

Definition (from the book): The interaction of a highly electron-deficient hydrogen and a lone pair on a nearby highly electronegative atom such as N, O, or F is called a hydrogen bond. This phenomenon is called hydrogen bonding.

Simple explanation: When H is attached to N, O, or F, it becomes very positive (δ+). This H is attracted to a lone pair on another N, O, or F atom. This attraction is a hydrogen bond.

Importance of Intermolecular Forces

The book states:

> "These intermolecular forces are extremely important in determining properties of water, biological molecules, such as proteins, DNA, etc. and synthetic materials such as glue, paints, resins, etc. The adhesive action of paints and dyes is developed due to hydrogen bonding. Synthetic resins bind two surfaces together by hydrogen bonding or dipole–dipole interactions."

• Properties of water
• Structure of proteins and DNA
• Adhesion of paints and glues
• Strength of resins and synthetic materials

Intermediate Level

Dipole–Dipole Forces — In Detail

• Molecules must be polar (have a permanent dipole).
• The δ+ end of one molecule attracts the δ− end of another.

Strength: Moderate — stronger than dispersion forces, weaker than hydrogen bonds.

Example: HCl molecules - H (δ+) of one HCl is attracted to Cl (δ−) of another HCl.

Diagram from the book (Figure 5.1): Shows polar molecules arranged so that opposite ends attract each other.

Hydrogen Bonding — In Detail

• H must be covalently bonded to N, O, or F.
• There must be a lone pair on another N, O, or F atom nearby.

• N, O, and F are highly electronegative.
• They pull electron density away from H.
• H becomes very electron-deficient (δ+).
• This δ+ H is strongly attracted to a lone pair on another N, O, or F.

Strength: Strongest of the intermolecular forces (but still weaker than covalent bonds).

• Water (H₂O): Each water molecule can form up to 4 hydrogen bonds.
• Ammonia (NH₃): Forms hydrogen bonds.
• Hydrogen fluoride (HF): Forms hydrogen bonds.

• Water's high boiling point — due to hydrogen bonding.
• DNA structure — held together by hydrogen bonds between base pairs.
• Protein structure — hydrogen bonds help maintain shape.
• Adhesion of paints and glues — due to hydrogen bonding.

Comparison of Intermolecular Forces

Important Points

• - Intermolecular forces: attractive forces between molecules.
• - Dipole–dipole forces: between polar molecules; δ+ end of one attracts δ− end of another.
• - Hydrogen bonding: interaction between a highly electron-deficient hydrogen (bonded to N, O, or F) and a lone pair on a nearby N, O, or F atom.
• - Hydrogen bonds are the strongest of the intermolecular forces.
• - Intermolecular forces are weaker than chemical bonds.
• - They determine melting point, boiling point, solubility, and other physical properties.
• - Importance: water properties, biological molecules (proteins, DNA), synthetic materials (glue, paints, resins).
• - Examples:
• - Dipole–dipole: HCl
• - Hydrogen bonding: H₂O, NH₃, HF

Remember

• - Intermolecular forces = between molecules.
• - Intramolecular forces = within molecules (chemical bonds).
• - Dipole–dipole = between polar molecules.
• - Hydrogen bonding = H bonded to N, O, or F + lone pair on nearby N, O, or F.
• - Hydrogen bonds are the strongest intermolecular forces.
• - Examples: HCl (dipole–dipole); H₂O, NH₃, HF (hydrogen bonding).
• - Importance: water, DNA, proteins, paints, glues, resins.

Common Mistakes

• 1. Confusing intermolecular and intramolecular forces. Intermolecular = between molecules; intramolecular = within molecules (chemical bonds).
• 2. Thinking hydrogen bonding only occurs in water. It also occurs in NH₃, HF, and biological molecules.
• 3. Thinking hydrogen bonds are chemical bonds. They are intermolecular forces, much weaker than covalent bonds.
• 4. Forgetting that H must be bonded to N, O, or F. Hydrogen bonding does not occur with H–C bonds.
• 5. Thinking dipole–dipole forces occur in non-polar molecules. They occur only in polar molecules.
• 6. Thinking all intermolecular forces are equally strong. Hydrogen bonds > dipole–dipole > dispersion forces.

Quick Revision

• Intermolecular force: attractive force that exists between molecules.
• Dipole–dipole interactions: occur between polar molecules; the slightly negative end of one polar molecule is weakly attracted to the slightly positive end of another.
• Hydrogen bonding: interaction of a highly electron-deficient hydrogen (bonded to N, O, or F) and a lone pair on a nearby highly electronegative atom (N, O, or F).
• Hydrogen bonds are the strongest of the intermolecular forces.
• Importance: determining properties of water, biological molecules (proteins, DNA), and synthetic materials (glue, paints, resins).
• Adhesion of paints and dyes is due to hydrogen bonding.
• Synthetic resins bind two surfaces by hydrogen bonding or dipole–dipole interactions.

60-Second Revision

• Intermolecular forces = between molecules.
• Dipole–dipole = polar molecules.
• Hydrogen bonding = H bonded to N, O, or F.
• Hydrogen bonds = strongest of the weak forces.
• Examples: HCl (dipole–dipole); H₂O, NH₃, HF (hydrogen bonding).
• Warning: hydrogen bonds are not chemical bonds — they are intermolecular forces.