AP Chemistry Intermolecular Forces

The four types, how to decide which a substance has, and how to rank them, in four worked examples.

Almost every Unit 3 question about boiling point, melting point, vapor pressure, or solubility is the same question underneath: which intermolecular forces does each substance have, and which are stronger? The Unit 3 review lists the four forces. This page is the practice for deciding which ones apply to a given molecule, and for ranking substances when the answer is not obvious. The detailed treatment of dispersion forces is on its own page, London dispersion forces.

The Four Intermolecular Forces and How to Identify Them

A good routine is to write down the forces each substance has, then compare the relevant factor for each. For molecules of similar size, a permanent dipole or hydrogen bonding usually decides the ranking. For molecules of very different size, dispersion forces can outweigh a dipole, so never assume dispersion is always weakest. Whether a molecule is polar depends on its shape; see molecular polarity.

Worked Example 1: Listing Every Force a Substance Has

List all intermolecular forces present in pure CH₄, HCl, CH₃OH, and NH₃.

CH₄: nonpolar (tetrahedral and symmetric), so dispersion only.
HCl: polar, with H bonded to Cl (not N, O, or F), so dispersion + dipole-dipole, no hydrogen bonding.
CH₃OH: polar and has an O–H bond, so dispersion + dipole-dipole + hydrogen bonding.
NH₃: polar (trigonal pyramidal) with N–H bonds and a lone pair on nitrogen, so dispersion + dipole-dipole + hydrogen bonding.

Every substance gets dispersion forces; the question is which stronger forces are added on top.

Worked Example 2: Deciding Which Molecules Can Hydrogen Bond With Themselves

Which of HF, CH₃OCH₃, CH₃CH₂OH, CH₃F, and CH₃NH₂ form hydrogen bonds with other molecules of the same substance?

The test is two-part: an H bonded to N, O, or F in one molecule, and a lone pair on N, O, or F in its neighbor.
HF: yes. CH₃CH₂OH: yes (O–H). CH₃NH₂: yes (N–H).
CH₃OCH₃: no. The oxygen has lone pairs, but every H is bonded to carbon, so there is no H to donate.
CH₃F: no. Fluorine has lone pairs, but the hydrogens are on carbon.

This is why ethanol and dimethyl ether, which share the formula C₂H₆O, behave so differently: only ethanol has an O–H bond, the comparison worked through in vapor pressure.

Worked Example 3: Explaining the Boiling Points of the Hydrogen Halides

The boiling points of HF, HCl, HBr, and HI are about 20, −85, −66, and −35°C. Explain why HF is the outlier and why the rest rise in the order HCl < HBr < HI.

HCl, HBr, HI: all three are polar and none can hydrogen bond, so each has dipole-dipole plus dispersion forces. Down the group the molecules have more electrons and a larger, more polarizable electron cloud, so dispersion forces grow and the boiling point rises: HCl < HBr < HI.
HF: although it has the fewest electrons, it forms hydrogen bonds, which are much stronger than the other attractions in this set. That lifts its boiling point above HCl and far above what its size would predict.

The chart shows the same pattern for groups 15 and 16: NH₃ and H₂O sit far above the trend of their heavier relatives, while the group 14 hydrides, which cannot hydrogen bond, climb smoothly with size.

Worked Example 4: Explaining Solubility With Ion-Dipole Forces

Explain why NaCl dissolves readily in water but not in hexane (C₆H₁₄).

In water: water is polar, so its partial negative oxygen atoms attract Na⁺ and its partial positive hydrogen atoms attract Cl⁻. These ion-dipole attractions are strong enough to pull the ions out of the crystal and surround them, releasing energy that offsets the energy needed to separate the ions.
In hexane: hexane is nonpolar and has only dispersion forces. It cannot attract the ions strongly, so the ionic attractions in the crystal win and the salt stays solid.

The pattern “like dissolves like” follows from this comparison of forces: polar and ionic substances dissolve in polar solvents, and nonpolar substances dissolve in nonpolar solvents. How the resulting solution is described is covered in molarity.

Five Intermolecular Force Practice Questions With Answers

1. What is the strongest intermolecular force in pure propane, CH₃CH₂CH₃?
   Show answer

   London dispersion forces. Propane is nonpolar, so dispersion is the only intermolecular force it has.
2. Why does H₂O boil at 100°C while H₂S boils near −60°C?
   Show answer

   Water molecules form hydrogen bonds, and H₂S molecules do not (H is bonded to S). The stronger attractions in water require far more energy to overcome.
3. Which has the higher vapor pressure at the same temperature, CH₃OCH₃ or CH₃CH₂OH?
   Show answer

   CH₃OCH₃. It cannot hydrogen bond, so its molecules are held less tightly and more of them escape into the vapor.
4. Rank HCl, HBr, and HI from lowest to highest boiling point and give the reason.
   Show answer

   HCl < HBr < HI. All are polar without hydrogen bonding, and dispersion forces increase with the larger electron clouds down the group.
5. What force holds Na⁺ ions in solution in water?
   Show answer

   Ion-dipole attraction between the Na⁺ ion and the partial negative oxygen ends of the water molecules.

Common Intermolecular Force Mistakes

• Saying a covalent bond breaks when a substance boils. Boiling overcomes intermolecular forces; the covalent bonds stay intact.
• Calling any H-containing molecule a hydrogen-bonder. The H must be bonded to N, O, or F.
• Forgetting that every substance has dispersion forces. Polar molecules have them too.
• Assuming dispersion forces are always the weakest. In large molecules they can outweigh a dipole.
• Ranking by molar mass alone. Compare all the forces present first, then use size to break ties.

Intermolecular forces also explain the temperature plateaus on a heating curve, and the contrast with the strong attractions inside ionic and covalent compounds. To see how your Unit 3 practice affects an overall estimate, the AP Chem Score Calculator converts section scores to a 1–5 range.

Related Resources

• AP Chem Score Calculator
• Unit 3 Review: Properties of Substances and Mixtures
• AP Chemistry London Dispersion Forces
• AP Chemistry Molecular Polarity
• AP Chemistry Vapor Pressure
• AP Chemistry Heating Curve
• AP Chemistry Ionic vs Covalent Bonds
• AP Chemistry Study Guide

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