Lesson 7: Intermolecular Forces

Covalent bonds hold molecules together; intermolecular forces (IMFs) hold molecules to each other, and they govern boiling points, solubility, viscosity and the behaviour of everything from DNA to detergents. There are three types to command (Structure 2.2). London (dispersion) forces act between all molecules: at any instant an electron cloud is uneven, creating a temporary dipole that induces dipoles in neighbours. They strengthen with the number of electrons and with molecular size and surface contact — which is why boiling points rise down the halogens and along the alkanes. Dipole–dipole attractions add on top for polar molecules. Hydrogen bonding, the strongest IMF, is a special attraction — not a true bond — between a hydrogen attached to N, O or F (where it is left very δ+) and a lone pair on N, O or F of a neighbouring molecule. Only H–N, H–O and H–F qualify; C–H never hydrogen-bonds, however often students claim otherwise.

Reading boiling points: the checklist at work

Boiling separates molecules from each other, so boiling point is a direct IMF meter. The comparison routine has three steps: (1) list the IMF types present in each substance; (2) compare sizes (electrons/M_r) for the dispersion contribution; (3) conclude. Watch it on the hydrogen halides: HF boils at 293 K, HCl at 188 K, HBr at 206 K, HI at 238 K. From HCl to HI, rising size strengthens dispersion forces and boiling points climb. HF breaks the trend completely — hydrogen bonding between HF molecules outguns its small size. The same logic explains why ethanol (M_r 46, H-bonding) boils at 78 °C while its isomer methoxymethane (M_r 46, dipole–dipole only) boils at −24 °C: identical mass, identical atoms, 100 degrees of difference written entirely by one O–H bond. Water is the extreme case: H₂O (M_r 18) would boil near −90 °C by extrapolation from H₂S, H₂Se and H₂Te, but hydrogen bonding lifts it to 100 °C — and arranges solid water into an open, H-bonded lattice less dense than the liquid, so ice floats and lakes freeze from the top, insulating the life beneath.

Solubility and chromatography: IMFs doing analytical work

"Like dissolves like" is IMF bookkeeping: a solute dissolves when solute–solvent attractions rival the attractions being broken. Ionic and hydrogen-bonding substances dissolve in water (hydration shells form); nonpolar substances dissolve in nonpolar solvents; ethanol, with an –OH head and a hydrocarbon tail, bridges both worlds — which is why it is the lab's universal solvent and why it appears in hand sanitisers and tinctures. Chromatography converts IMF differences into separations: in paper or thin-layer chromatography, each component partitions between the stationary phase and the moving solvent according to its IMF profile, travelling with a characteristic retention factor R_f = distance moved by spot ÷ distance moved by solvent front.

Worked example 1: ranking boiling points with reasons

Rank CH₄, CH₃Cl and CH₃OH by boiling point and justify the order. CH₄ (M_r 16): London forces only, fewest electrons — lowest, 112 K. CH₃Cl (M_r 50.5): polar molecule, dipole–dipole plus stronger dispersion from more electrons (Cl especially) — middle, 249 K. CH₃OH (M_r 32): hydrogen bonding through its O–H group, the strongest IMF here, beats its smaller size — highest, 338 K. The lesson: IMF type usually outranks size; size decides only between substances with the same IMF types. An answer that names the forces in each substance, compares them, and then ranks earns full marks; a bare order earns none.

Worked example 2: chromatography as quantitative practical work

A food dye mixture is run on paper with water as solvent. The solvent front travels 8.00 cm; three spots travel 1.60, 3.20 and 6.40 cm. R_f values: 1.60 ÷ 8.00 = 0.200; 3.20 ÷ 8.00 = 0.400; 6.40 ÷ 8.00 = 0.800. The component at R_f 0.200 interacts most strongly with the paper (cellulose — covered in –OH groups, a hydrogen-bonding surface) and least happily with the water phase... in this system the lowest R_f is the most polar dye, held back by hydrogen bonding to the stationary phase; the 0.800 component travels nearly with the front, weakly attracted to the paper. Technique points that convert this from a demo into data: draw the baseline in pencil (ink would run and contaminate the chromatogram), spot small and concentrated (a wide, dilute spot tails), keep the solvent level below the baseline (or the spots dissolve into the reservoir), cover the tank (evaporation changes the front), and mark the front the moment the paper comes out. Uncertainty: spot and front positions read to ±0.05 cm, so the small spot's R_f carries ±(0.05 ÷ 1.60 + 0.05 ÷ 8.00) ≈ ±3.7% — quote R_f = 0.200 ± 0.007. Identifying an unknown dye needs a co-run standard on the same sheet: R_f values are system-specific, and comparing across sheets is the classic school-lab error.

Nature of Science — and the real world

Hydrogen bonding was proposed (1920, Latimer and Rodebush) to explain anomalies like water's boiling point before any direct structural evidence existed — a hypothesis vindicated decades later by X-ray and neutron diffraction. It is now known to hold together the DNA double helix (G–C pairs with three H-bonds, A–T with two — a difference that matters to melting temperatures in PCR), the α-helices and β-sheets of proteins, and the setting of nylon and Kevlar, whose exceptional strength is hydrogen bonding between chains. Gecko feet grip by dispersion forces alone, multiplied over millions of tiny hairs — an IMF turned into technology.

Common mistakes

• Calling a hydrogen bond a covalent bond, or drawing it within one molecule when the question is about boiling point (intramolecular forces do not affect boiling).
• Attributing hydrogen bonding to any molecule containing H and O (e.g. CH₄, or ethers with no O–H): the H must be bonded to N, O or F.
• Comparing boiling points by molar mass alone, ignoring the IMF types present.
• In chromatography, comparing R_f values measured in different solvents or on different days as if they were universal constants.

IB Exam Experience

Question 1. Which substance has the highest boiling point?

• (A) CH₄
• (B) C₂H₆
• (C) CH₃OH
• (D) CH₃F

Question 2. Which compound can form hydrogen bonds with water molecules but does not hydrogen-bond with itself as a pure liquid?

• (A) CH₃OH
• (B) CH₃OCH₃
• (C) CH₃NH₂
• (D) HF

Question 3. In paper chromatography a dye travels 4.40 cm while the solvent front travels 8.00 cm. What is its R_f value?

• (A) 0.450
• (B) 0.550
• (C) 1.82
• (D) 0.0550

Answer key. 1. (C) — methanol hydrogen-bonds; the others rely on dispersion (and weak dipole) forces only. 2. (B) — methoxymethane has an oxygen lone pair that accepts hydrogen bonds from water, but no O–H/N–H/F–H bond of its own to donate. 3. (B) — R_f = 4.40 ÷ 8.00 = 0.550.

Structured question (Paper 2 style). (a) Explain why ice is less dense than liquid water. [2] (b) Ethanol and methoxymethane are structural isomers (C₂H₆O). Their boiling points are 78 °C and −24 °C respectively. Explain the difference. [3] (c) State the strongest type of intermolecular force present in liquid hydrogen chloride. [1] Marking notes. (a) In ice each water molecule is hydrogen-bonded tetrahedrally into an open lattice with large empty spaces [1]; on melting some hydrogen bonds break and the structure partially collapses, so the same mass occupies less volume [1]. (b) Ethanol has an O–H bond so hydrogen bonding acts between its molecules [1]; methoxymethane has no O–H (or N–H/F–H) bond, so only dipole–dipole and dispersion forces act [1]; hydrogen bonding is stronger, so more thermal energy is needed to separate ethanol molecules [1]. (c) Dipole–dipole attractions (accept "dipole–dipole forces"; HCl has no H bonded to N/O/F, so no hydrogen bonding despite the polar bond).

IA idea: Investigate how R_f values of a series of dyes (or amino acids) vary with the composition of a water–ethanol solvent, and interpret the trend in terms of competing intermolecular forces.

Intermolecular forces at work: separation science and sticky feet

Gas chromatography is chromatography with the logic exposed: a gaseous mobile phase carries vaporised sample through a column coated with a liquid stationary phase. Each component partitions between the phases according to its intermolecular forces with the coating — hydrogen-bonding analytes linger on a polar coating, nonpolar ones on a hydrocarbon coating — and emerges after its characteristic retention time, to be measured by a detector and identified against standards run under identical conditions. It is R_f thinking with a clock, and it analyses everything from blood alcohol to doping samples to the flavour compounds in coffee. The design variables mirror the paper version: column temperature (the analogue of solvent choice), the stationary phase's polarity, and the carrier flow rate — and the identification caveat is identical: retention times are system-specific, so unknowns are confirmed by co-injection with an authentic standard, or by coupling the column to a mass spectrometer (GC-MS) that fingerprints each peak as it exits.

Two biological set-pieces complete the picture. DNA holds its double helix together with hydrogen bonds between base pairs — three between guanine and cytosine, two between adenine and thymine — which is why GC-rich DNA melts at higher temperatures (a measurable, examinable quantity in PCR work) and why the bonds are individually weak but collectively immense: a zip, not a weld, so the strands can part for replication. Glycerol (propane-1,2,3-triol) pours like syrup because each molecule hydrogen-bonds through three –OH groups into a near-network — viscosity as an IMF census — while ethanol, with one –OH, flows freely, and dimethyl ether, with none it can donate, is a gas at room temperature. Given any new molecule, the IMF checklist from this lesson — dispersion by size, dipole by polarity, hydrogen bonding by the N/O/F rule — predicts its boiling behaviour, its solubility and its chromatographic personality before a single measurement is made. That predictive reach, from three bullet points of theory, is why this topic punches so far above its weight in the IB's marking.

Surface tension, viscosity and detergents: IMFs you can watch

Surface tension is the skin on a liquid: molecules at the surface have neighbours only below and beside them, so they are pulled inwards, minimising surface area — water's high surface tension (strong hydrogen bonding) lets insects walk on it and makes droplets bead. Viscosity is resistance to flow: glycerol's three –OH groups per molecule build a hydrogen-bonded web that must be torn and re-formed for layers to slide, so it pours slowly, while hexane's dispersion-only liquid runs freely. Warming thins both — thermal energy breaks IMFs faster than they reform — which is why cold honey crawls and warm honey pours. Detergents are engineered IMF diplomacy: a long hydrocarbon tail (dispersion forces only — grease-soluble) bonded to an ionic head (water-soluble). In washing water the molecules surround grease globules tail-in, forming micelles whose ionic skins repel each other and stay dispersed, lifting the grease away. Hard water sabotages the design — Ca²⁺ and Mg²⁺ precipitate soap anions as scum — which is why synthetic detergents use sulfonate heads that keep calcium salts soluble. Every element of this paragraph is the lesson's checklist at work: name the forces, compare their strengths, predict the behaviour — on a system you can hold under a tap.