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Intermolecular forces and boiling points – London, dipole–dipole, hydrogen bonding

ChemistryChemical BondingAges 15–16

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Compare real (approximate, 1 atm) boiling points across six series: alkanes C₁–C₈, the three isomers of C₅H₁₂, HCl – HBr – HI, substances with M ≈ 44–46 g/mol, alcohol–alkane pairs of similar mass, and H₂O vs H₂S. Heat or cool to watch molecules in the particle box separate as they pass the boiling point while the bar chart shows which substances have boiled, with London, dipole–dipole and hydrogen-bond attractions drawn between molecules and an explanation of which force dominates.

Lesson: Intermolecular forces and boiling points

What it shows

The boiling point of a molecular substance depends on how strongly its molecules attract one another. This simulation pairs a particle model with real boiling-point data chosen to isolate one factor at a time: chain length and number of electrons in the alkanes, molecular shape in the pentane isomers, polarity versus electron count in HCl, HBr and HI, dipole–dipole forces versus hydrogen bonding at equal molar mass, and water versus hydrogen sulfide. Data are rounded handbook values; the particle picture is illustrative and not to scale.

How to use

Choose a series and a substance, or tap a bar in the chart. Drag Temperature, or press Heat or Cool, and watch the molecules leave the liquid when the boiling point is passed while that bar changes to gas. Tick Show attractions between molecules to see London (grey), dipole–dipole (orange) and hydrogen bonds (blue), then read the explanation under the chart.

Parameters you can change

  • Series Straight-chain alkanes C₁–C₈, The three isomers of C₅H₁₂, HCl, HBr, HI, M ≈ 44–46 g/mol: propane, dimethyl ether, ethanal, ethanol, Alcohols and alkanes of similar M, H₂O and H₂S
  • Starting temperature -200–150 °C
  • Heating and cooling rate 2–40 °C/s
  • Show attractions between molecules

Questions to explore

  1. Why does HI boil at a higher temperature than HCl even though HCl is more polar?
  2. Ethanol and dimethyl ether are both C₂H₆O. Why do their boiling points differ by more than 100 °C?
  3. Which C₅H₁₂ isomers are liquids at 20 °C, and how does molecular shape explain this?

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