Unit 3 · Properties of Substances and Mixtures

Intermolecular vs Intramolecular Forces

What each one holds together, how much stronger bonds really are, and which one a phase change or a reaction has to overcome.

The two words differ by one prefix, and mixing them up is one of the most common ways to lose a point on an AP explanation. Intramolecular forces are the bonds inside a molecule: the covalent bonds from Unit 2. Intermolecular forces are the attractions between molecules: dispersion, dipole-dipole, hydrogen bonding, and ion-dipole, covered one by one in AP Chemistry intermolecular forces. This page puts the two side by side, with real energy values, so you can say which one a process overcomes and why.

Intermolecular vs Intramolecular Forces Compared in One Table

The two are connected. The polar bonds and shape of a molecule, both intramolecular features, decide whether it has a permanent dipole, and that dipole decides which intermolecular forces it can have. That chain is worked through in molecular polarity, and the dashed-line convention is practiced in particulate diagrams.

How Much Stronger Intramolecular Bonds Are Than Intermolecular Forces

A fair comparison puts both on the same basis: the energy to boil one mole of a substance, which separates the molecules, against the energy to break every covalent bond in one mole of its molecules, using average bond enthalpies.

Even water, with the strongest intermolecular forces in the table, needs about 23 times more energy to break its bonds than to boil. A single hydrogen bond between water molecules is roughly 20 kJ/mol against about 463 kJ/mol for one O–H bond, the comparison made in AP Chemistry hydrogen bonding. The vaporization values are what the flat boiling plateau on a heating curve measures. Values are rounded; bond enthalpies are averages and vary slightly between tables.

What Boiling Water Overcomes Compared With Electrolysis

Boiling, H₂O(l) → H₂O(g), takes 40.7 kJ per mole at 100°C. The gas is still made of H₂O molecules: only hydrogen bonds and other intermolecular forces were overcome, so it is a physical change. Electrolysis, 2 H₂O(l) → 2 H₂(g) + O₂(g), has ΔH = +572 kJ, or 286 kJ per mole of water, about seven times as much. It breaks O–H bonds and forms H–H and O=O bonds, so new substances appear and it is a chemical change. Which process breaks bonds is the test used in physical and chemical changes, and the reaction itself is covered in electrolysis.

Why a Stronger Covalent Bond Does Not Mean a Higher Boiling Point

Nitrogen has one of the strongest bonds in chemistry, the N≡N triple bond at about 945 kJ/mol, yet N₂ boils at −196°C. Boiling never touches that bond; it only has to overcome the weak dispersion forces between nonpolar N₂ molecules. Compare Cl₂ and HCl: the H–Cl bond (431 kJ/mol) is much stronger than the Cl–Cl bond (242 kJ/mol), but Cl₂ boils at about −34°C and HCl at about −85°C. Cl₂ has 34 electrons to HCl’s 18, so its larger, more polarizable electron cloud gives stronger London dispersion forces, enough to outweigh HCl’s dipole.

The rule: for a molecular substance, boiling point, melting point, and vapor pressure depend on the intermolecular forces only. Bond enthalpy belongs in reaction energy questions, such as the bond-enthalpy estimate in endothermic vs exothermic and the bond-breaking wavelength found with the wavelength calculator.

Interparticle Forces in Ionic, Metallic, and Network Covalent Solids

The intramolecular and intermolecular labels only work cleanly for molecular substances. NaCl, copper, and quartz (SiO₂) have no separate molecules, so the CED speaks of interparticle forces: ionic bonds between ions, metallic bonding between cations and delocalized electrons, and a continuous network of covalent bonds. Melting these solids has to disrupt the bonds themselves, which is why NaCl melts at 801°C, copper at 1085°C, and quartz near 1700°C, while molecular CO₂ sublimes at −78°C. How to recognize each kind of solid is in types of solids, and ranking ionic melting points with Coulomb’s law is in ionic vs covalent bonds.

How to Word an Intermolecular Force Explanation on an AP FRQ

A full-credit explanation names the particles, names the force, and says which force is stronger and why. Three habits help:

• Say “between molecules.” “Ethanol has a higher boiling point because hydrogen bonds between ethanol molecules are stronger than the dipole-dipole forces between dimethyl ether molecules.”
• Never say a bond breaks during a phase change. Write “more energy is needed to overcome the intermolecular forces,” not “more energy is needed to break the bonds.”
• Name the bond type for network, ionic, and metallic solids. “Melting SiO₂ requires breaking covalent bonds throughout the network, while melting CO₂ only overcomes dispersion forces between molecules.”

These explanations show up across Unit 3; the Unit 3 review lists every topic, and the Unit 3 practice test checks them in multiple-choice form.

Five Intermolecular vs Intramolecular Practice Questions With Answers

1. When liquid bromine, Br₂, evaporates, what is overcome?
   Show answer

   London dispersion forces between Br₂ molecules. The Br–Br covalent bond inside each molecule stays intact, so the vapor is still Br₂.
2. Which takes more energy: melting one mole of ice (6.01 kJ) or breaking one mole of O–H bonds?
   Show answer

   Breaking the O–H bonds, about 463 kJ/mol, roughly 77 times more. Melting only loosens the hydrogen-bonded arrangement of the molecules.
3. A student writes that HF boils higher than HCl because the H–F bond (about 565 kJ/mol) is stronger than the H–Cl bond. Correct the reasoning.
   Show answer

   Boiling does not break H–F bonds. HF boils higher because its molecules form hydrogen bonds with each other (H bonded to F), which are stronger than the dipole-dipole and dispersion forces between HCl molecules.
4. Which of these is an intramolecular force: a hydrogen bond between two water molecules, the attraction between Na⁺ and water in salt water, a C=O bond in CO₂, or dispersion forces between I₂ molecules?
   Show answer

   The C=O bond in CO₂. It joins atoms within one molecule; the other three are attractions between separate particles.
5. Why does SiO₂ melt near 1700°C while CO₂ sublimes at −78°C?
   Show answer

   SiO₂ is a covalent network solid, so melting must break Si–O covalent bonds throughout the network. CO₂ is a molecular solid, so subliming only overcomes weak dispersion forces between CO₂ molecules.

Common Intermolecular vs Intramolecular Mistakes

• Saying boiling or melting breaks covalent bonds in a molecular substance. It overcomes intermolecular forces; the molecules stay whole.
• Calling a hydrogen bond a chemical bond. It is an intermolecular attraction, about 20 times weaker than the O–H bond.
• Using bond strength to rank boiling points. N₂ has a triple bond and boils at −196°C.
• Mixing up the prefixes. Inter- means between, as in interstate highways; intra- means within, as in an intramural league.
• Treating ionic and network solids like molecular ones. Their melting does break bonds, which is why their melting points are so high.

Intermolecular forces sit in Unit 3, the most heavily weighted unit at 18–22% of the multiple-choice section. To see how a strong Unit 3 would move your overall result, try the AP Chem Score Calculator.

Related Resources

• AP Chem Score Calculator
• AP Chemistry Intermolecular Forces
• AP Chemistry Covalent Bonds
• AP Chemistry Hydrogen Bonding
• AP Chemistry London Dispersion Forces
• AP Chemistry Types of Solids
• AP Chemistry Physical and Chemical Changes
• Unit 2 Review: Compound Structure and Properties
• Unit 3 Review: Properties of Substances and Mixtures
• AP Chemistry Unit 3 Practice Test

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