Types of Intermolecular Forces

Intermolecular forces (IMFs) are attractions that exist between molecules (not within them). They are much weaker than intramolecular bonds (ionic, covalent) but play a crucial role in determining a substance's physical properties. There are three main types, ranked from weakest to strongest.

Relative Strength of Intermolecular Forces

Dispersion Forces (London Forces)

Weakest. Present in ALL molecules. Caused by temporary dipoles from electron movement.

Dipole-Dipole Forces

Moderate. Between polar molecules. Permanent partial charges attract.

Hydrogen Bonding

Strongest IMF. When H is bonded to N, O or F and attracted to a lone pair on another N, O or F.

Bar length indicates relative strength ↑

Important: Dispersion forces increase with molecular size (more electrons = larger temporary dipoles). This is why hexane (C₆H₁₄) has a higher boiling point than methane (CH₄) despite both being non-polar.

Polarity and Molecular Shape

A molecule is polar if it has an uneven distribution of electron density, creating a permanent dipole. Polarity depends on both bond polarity (difference in electronegativity) and molecular geometry.

Polar Molecule: H₂O

O

H

Bent shape. Bond dipoles do not cancel. Net dipole exists. Can form hydrogen bonds.

Non-polar Molecule: CO₂

O

C

O

Linear shape. Bond dipoles are equal and opposite, so they cancel. No net dipole. Only dispersion forces.

Rule of thumb: Symmetrical molecules (e.g. CO₂, CH₄, CCl₄) tend to be non-polar because bond dipoles cancel. Asymmetrical molecules (e.g. H₂O, NH₃, HCl) tend to be polar.

Effect on Physical Properties

Stronger intermolecular forces require more energy to overcome, which directly affects melting point, boiling point, viscosity and solubility.

Boiling Point

Stronger IMFs = higher boiling point. More energy needed to separate molecules from liquid to gas.

Viscosity

Stronger IMFs = higher viscosity. Molecules resist flowing because they are held together more tightly.

Solubility

"Like dissolves like." Polar solutes dissolve in polar solvents; non-polar solutes dissolve in non-polar solvents.

Why Water is Special

Water has an unusually high boiling point (100 °C) for its small molecular mass because of its extensive hydrogen bonding network. Each water molecule can form up to four hydrogen bonds with neighbours. This also explains water's high surface tension, high specific heat capacity, and its expansion on freezing (ice is less dense than liquid water).

Key Vocabulary

Dispersion Force

A weak, temporary attraction caused by instantaneous dipoles from random electron movement. Present in all molecules.

Hydrogen Bond

A strong intermolecular force between a hydrogen atom bonded to N, O or F and a lone pair on another N, O or F atom.

Electronegativity

The ability of an atom to attract shared electrons in a covalent bond. Differences in electronegativity create bond polarity.

Dipole

A separation of positive and negative charge. A permanent dipole exists in polar molecules; a temporary dipole causes dispersion forces.

Worked Examples

1

Explain why ethanol (C₂H₅OH) has a higher boiling point than dimethyl ether (CH₃OCH₃), despite having the same molecular formula.

Step 1: Both molecules have the formula C₂H₆O, so they have similar dispersion forces.

Step 2: Ethanol has an O-H group, which can form hydrogen bonds between molecules.

Step 3: Dimethyl ether has no O-H bond (only C-H and C-O-C), so it cannot form hydrogen bonds. Ethanol's stronger IMFs give it a higher boiling point (78 °C vs -24 °C).

2

Why does oil not dissolve in water?

Step 1: Water is polar with strong hydrogen bonds between molecules.

Step 2: Oil consists of long-chain hydrocarbons that are non-polar, with only weak dispersion forces.

Step 3: "Like dissolves like" -- non-polar oil molecules cannot form favourable interactions with polar water molecules, so they separate into layers.

3

Rank these substances by boiling point (lowest to highest): CH₄, H₂O, HCl.

Step 1: CH₄ is non-polar -- only dispersion forces (weakest IMFs). Boiling point = -161 °C.

Step 2: HCl is polar -- dipole-dipole forces plus dispersion forces. Boiling point = -85 °C.

Step 3: H₂O has hydrogen bonding (strongest IMF here). Boiling point = 100 °C. Order: CH₄ < HCl < H₂O.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Which type of intermolecular force is present in ALL molecules?

Dispersion forces Dipole-dipole forces Hydrogen bonding Ionic bonds

Question 2

Hydrogen bonding occurs when hydrogen is bonded to which of the following?

Carbon, silicon or germanium Nitrogen, oxygen or fluorine Any non-metal element Sodium, potassium or calcium

Question 3

CO₂ has polar bonds but is a non-polar molecule. Why?

Carbon is not electronegative It is a gas at room temperature The linear shape means bond dipoles cancel out Oxygen atoms repel each other

Question 4

Which substance would you expect to have the highest boiling point?

CH₄ (methane) N₂ (nitrogen gas) Ne (neon) HF (hydrogen fluoride)

Question 5

"Like dissolves like" means:

Polar solutes dissolve in polar solvents; non-polar in non-polar All substances dissolve in water Larger molecules always dissolve better Ionic compounds dissolve in oil

Intermolecular Forces