2.2.2 Intermolecular Forces

What are intermolecular forces?

Forces of attraction between molecules that affect physical properties like melting and boiling points.

What types of intermolecular forces exist?

Permanent dipole–dipole interactions, induced dipole–dipole interactions (London forces), and hydrogen bonding.

Where do induced dipole–dipole interactions occur?

Between all molecular substances and noble gases; not in ionic substances.

What are induced dipole–dipole interactions also called?

London forces or dispersion forces.

How do induced dipole–dipole interactions form?

Electrons in molecules move constantly and randomly, causing temporary dipoles that induce dipoles in neighbouring molecules with opposite charge.

What causes a temporary or instantaneous dipole?

A fluctuation in electron density in a molecule.

What is an induced dipole?

A dipole formed in a neighbouring molecule by attraction to a temporary dipole.

What factor affects the strength of induced dipole–dipole interactions?

The number of electrons in the molecule — more electrons → stronger interactions → higher boiling point.

Why do boiling points increase down group 7 (halogens)?

Because larger halogen molecules have more electrons, creating stronger induced dipole–dipole forces.

Why does I₂ exist as a solid while Cl₂ is a gas?

I₂ has more electrons, producing stronger induced dipole–dipole forces.

Why do boiling points increase down the alkane homologous series?

Larger molecules have more electrons and larger surface areas for stronger induced dipole–dipole interactions.

How does molecular shape affect induced dipole–dipole interactions?

Long, unbranched molecules have a larger surface area for contact, so stronger induced dipole–dipole forces than spherical molecules.

What are permanent dipole–dipole forces?

Forces between polar molecules due to the attraction between permanent dipoles in neighbouring molecules.

How do permanent dipole–dipole forces compare in strength?

They are stronger than induced dipole–dipole interactions, leading to higher boiling points.

What type of molecules show permanent dipole–dipole forces?

Polar molecules with a significant difference in electronegativity and an asymmetric shape.

Give examples of bonds that create permanent dipoles.

C–Cl, C–F, C–Br, H–Cl, and C=O.

What are van der Waals forces?

The collective term for induced dipole–dipole and permanent dipole–dipole interactions.

What is hydrogen bonding?

A strong type of intermolecular force between molecules containing N, O, or F atoms and a hydrogen atom bonded to them (–NH, –OH, or –HF).

How does hydrogen bonding form?

The lone pair on a highly electronegative atom (N, O, F) attracts a hydrogen atom from another molecule, forming a strong dipole-dipole attraction.

What elements can form hydrogen bonds?

Nitrogen, oxygen, and fluorine.

Why is hydrogen bonding stronger than other dipole interactions?

Because of the large difference in electronegativity and the small size of the hydrogen atom allowing close approach.

What are the anomalous properties of water caused by hydrogen bonding?

Ice is less dense than water, and water has unusually high melting and boiling points.

Why is ice less dense than water?

Hydrogen bonding holds water molecules in an open tetrahedral lattice with gaps, making ice less dense.

Why does water have high melting and boiling points?

Strong hydrogen bonds require a lot of energy to break.

What is a simple molecular lattice?

A solid structure made of covalently bonded molecules held together by intermolecular forces.

Give examples of simple molecular lattices.

I₂ (iodine), ice, CO₂ (dry ice).

Describe the structure of iodine (I₂).

Iodine molecules (I₂) are held together by weak induced dipole–dipole interactions in a simple molecular lattice.

Describe the structure of ice.

Each water molecule forms four hydrogen bonds in a tetrahedral arrangement, creating an open lattice.

How do intermolecular forces affect melting and boiling points in covalent substances?

Stronger intermolecular forces lead to higher melting and boiling points.

How does molecular size affect melting and boiling points?

Larger molecules have more electrons and stronger London forces, increasing melting and boiling points.

What is the relationship between structure, bonding, and physical properties?

Simple molecular substances have low melting and boiling points because only weak intermolecular forces must be overcome when they change state.