IB Chemistry Intermolecular Forces: FAQ

Intermolecular forces are the attractive forces between molecules — London dispersion, dipole-dipole and hydrogen bonding — covered in Structure 2.2 of the current IB Chemistry guide, first examined 2025. They appear on Paper 1 as multiple-choice or short-answer items, and on Paper 2 in longer questions linking forces to physical properties like boiling point or solubility.

The syllabus recognises three: London (dispersion) forces, present in every molecule regardless of polarity; dipole-dipole forces, between polar molecules with a permanent dipole; and hydrogen bonding, a stronger dipole interaction that requires hydrogen bonded directly to nitrogen, oxygen or fluorine. Examiners expect the strongest force present named, not a generic list of all three.

Covalent bonds are intramolecular — strong shared-electron bonds holding atoms together within a molecule, typically 150-900 kJ/mol. Intermolecular forces act between separate molecules and are far weaker, usually under 50 kJ/mol. That gap explains why water boils at 100°C, while breaking its O-H covalent bonds needs vastly more energy than boiling ever supplies.

Generally yes, for molecules of similar size — hydrogen bonding typically contributes 10-40 kJ/mol, compared with just a few kJ/mol from dispersion forces in small molecules. But size matters: dispersion forces in a large molecule like iodine can outweigh the hydrogen bonding in a small one like hydrogen fluoride.

The commonest mistake I mark is writing "van der Waals forces" as a catch-all when the mark scheme wants the specific type named — dispersion, dipole-dipole or hydrogen bonding. Examiners also penalise answers that ignore the strongest force present; if a molecule can hydrogen bond, citing only dispersion forces won't earn full marks.

The three core forces are identical at SL and HL — nothing new is introduced. The difference is application depth: HL exam questions expect intermolecular forces linked to a wider range of properties (viscosity, vapour pressure, enthalpy of vaporisation) and combined with Reactivity 1 energetics, which SL Paper 2 questions rarely demand.

There's no fixed percentage in the guide, but intermolecular forces underpin so much else — solubility, phase changes, chromatography, even protein structure in longer questions — that some form of this reasoning turns up in at least one Paper 2 structured question in most exam sessions I've marked.

Structure your answer in three steps: name the strongest intermolecular force present in each molecule, state the physical property being compared, then link force strength to that property with a causal word like "therefore." Answers that just say "intermolecular forces are stronger" without naming the type rarely earn full marks on Paper 2.

Boiling and melting require enough energy to overcome the intermolecular forces holding molecules together — the stronger the force, the higher the boiling or melting point, all else being equal. Hydrogen-bonded ethanol (boiling point 78°C) boils far higher than dispersion-only propane (boiling point -42°C), despite the two having similar molar mass.

Run a quick three-question check on any molecule: does it have hydrogen bonded directly to nitrogen, oxygen or fluorine (hydrogen bonding)? Is it polar with an uneven electron distribution (dipole-dipole)? Every molecule also has dispersion forces — so your job is identifying the strongest one present, not listing all three by default.

Intermolecular forces sit within Structure 2.2, "Models of bonding and structure," in the current IB Chemistry guide examined from 2025. The topic follows directly from covalent bonding and molecular shape, and it resurfaces later when explaining solubility, states of matter and several Reactivity 2 rate-of-reaction discussions.

Yes. Paper 1 typically tests recognition and naming through multiple-choice items, while Paper 2 asks you to apply intermolecular-forces reasoning to explain data — a boiling point trend, a solubility difference, an anomalous property — usually worth 2-4 marks within a longer structured question rather than as a standalone item.

The underlying three forces are the same for both levels — your child won't meet a "different" force at HL. What changes is application depth: HL questions expect intermolecular forces linked to more properties and combined with quantitative energy concepts, which is why HL Paper 2 questions on this topic tend to carry more marks per point.

Ask your child to explain, out loud, why one substance boils at a higher temperature than another. If they can name the strongest intermolecular force and link it to the property without prompting, they've understood it. Topical worksheets and past-paper questions on revisionprep.com are a useful way to test that, even without a chemistry background yourself.