AP Chemistry · Lesson 1

Intermolecular forces and properties

Know Know it

Intermolecular forces act between molecules. When a liquid boils or a solid melts, these forces are overcome; the covalent bonds inside the molecules stay intact.

London dispersion forces act between every pair of molecules. They come from temporary dipoles, and they grow with the number of electrons, because a larger electron cloud is more polarizable. A long, straight molecule also has more surface for contact than a compact one with the same formula.

Dipole-dipole forces add to London forces for polar molecules. Hydrogen bonds are an especially strong dipole attraction, between an H attached to N, O or F and a lone pair on N, O or F of another molecule.

Stronger forces mean a higher boiling point, a higher melting point, a higher heat of vaporization and a lower vapor pressure. Compare forces for molecules of similar size first; a very large nonpolar molecule can have stronger London forces than a small polar one.

Solids come in four kinds. Ionic solids (ions, high melting, conduct only when melted or dissolved), metallic solids (cations in mobile electrons, conduct as solids), covalent network solids (atoms in a bonded network, very hard, very high melting) and molecular solids (molecules held by intermolecular forces, soft and low melting).

Worked example Easy

Which substance can form hydrogen bonds between its own molecules?

  1. CH₄
  2. CH₃F
  3. HCl
  4. CH₃OHAnswer

How to solve it

  1. A hydrogen bond needs an H that is bonded directly to N, O or F.
  2. It also needs a lone pair on N, O or F on the neighbor.
  3. Only CH₃OH has an O–H bond, so only it hydrogen bonds with itself.

Why each choice is right or wrong

  • A. Incorrect. CH₄ has only C–H bonds and no lone pairs on carbon, so its molecules have only London forces.
  • B. Incorrect. CH₃F has a fluorine atom, but its hydrogens are bonded to carbon, not to fluorine, so it cannot hydrogen bond to itself.
  • C. Incorrect. HCl is polar, but chlorine is not N, O or F, so its molecules attract by dipole-dipole forces, not hydrogen bonds.
  • D. Correct. Methanol, CH₃OH, has an H bonded to O and lone pairs on that O, so one molecule's O–H can hydrogen bond to a neighbor's oxygen.

Trap A hydrogen bond is not a bond to hydrogen

The O–H bond inside a water molecule is a covalent bond. A hydrogen bond is the weaker attraction between that H and an oxygen on a different water molecule. Boiling water breaks hydrogen bonds, not O–H bonds.

Explain Explain it

Task verbs: explain, compare

A complete comparison names the strongest force each substance has, says which is stronger and why (more electrons, a permanent dipole, an N–H, O–H or F–H bond), and links the stronger force to the property: more energy is needed to separate the molecules.

When two molecules have nearly the same number of electrons, the polar one or the hydrogen-bonding one usually has the higher boiling point. When the numbers of electrons are very different, London forces often decide.

Worked example Medium

Boiling points of three compounds with similar molar masses

CompoundFormulaMolar mass (g/mol)Boiling point (°C)
PropaneC₃H₈44.1−42
AcetaldehydeCH₃CHO44.120
EthanolC₂H₅OH46.178

Which statement best explains the order of the boiling points?

  1. Ethanol has the most electrons, so its London forces are far stronger
  2. Acetaldehyde has a C=O double bond, which is harder to break than single bonds
  3. Propane is nonpolar, acetaldehyde adds dipole-dipole forces, and ethanol adds hydrogen bondsAnswer
  4. Ethanol has the strongest covalent bonds of the three, so it boils last

How to solve it

  1. The molar masses are about the same, so London forces are about the same.
  2. Acetaldehyde is polar (C=O), so it adds dipole-dipole forces; ethanol has an O–H, so it adds hydrogen bonds.
  3. Stronger total forces need more energy to separate the molecules: propane < acetaldehyde < ethanol.

Why each choice is right or wrong

  • A. Incorrect. The three have nearly the same number of electrons, so their London forces are similar; London forces cannot explain a 120 °C spread.
  • B. Incorrect. Boiling does not break the C=O bond or any other covalent bond; it separates whole molecules.
  • C. Correct. With London forces about equal, the extra forces decide: none for propane, dipole-dipole for polar acetaldehyde, and hydrogen bonds from ethanol's O–H.
  • D. Incorrect. Covalent bonds inside the molecules are not broken when a liquid boils, so their strength does not set the boiling point.

Explain Your turn

At room temperature, chlorine, Cl₂, is a gas and bromine, Br₂, is a liquid. Both are nonpolar. Explain the difference in terms of the forces between the molecules.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Names London dispersion forces as the only intermolecular force in both substances
  • Says Br₂ has more electrons, so its electron cloud is more polarizable and its London forces are stronger
  • Links the stronger forces in Br₂ to more energy needed to separate the molecules, so it stays a liquid at room temperature

Model answer

Both molecules are nonpolar, so the only forces between them are London dispersion forces. A Br₂ molecule has 70 electrons and a Cl₂ molecule has 34, so the electron cloud of Br₂ is larger and more polarizable, and its temporary dipoles are stronger. More energy is needed to pull Br₂ molecules apart, so bromine has the higher boiling point and is still a liquid at room temperature, while chlorine is already a gas.

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