Intermolecular forces — Chemistry, 14–17
Molecules attract one another without forming new covalent bonds. These attractions help explain boiling points, viscosity and surface tension.
Attractions between molecules
Intermolecular forces are small attractions between separate molecules. They are weaker than the bonds inside a molecule, but many of them together can hold a liquid together or make it difficult for particles to escape into a gas. Tu meet three useful types: temporary dipoles, permanent dipoles and hydrogen bonds.
Why substances boil differently
A simple model of particles was not enough to explain why liquids with similar-sized molecules have different boiling points. Chemists needed to include the attractions between particles: stronger attractions need more energy to overcome. This connects an invisible molecular detail to a measurement you can make in the lab.
Ethanol and ethane
Compare ethanol, C₂H₅OH, with ethane, C₂H₆. Step 1: both have similar-sized molecules, so their temporary attractions are comparable. Step 2: ethanol has an O–H group, allowing hydrogen bonds; ethane does not. Step 3: ethanol boils at about 78 °C, while ethane boils at about −89 °C, because ethanol molecules attract one another much more strongly.
Forces between are not bonds inside
It is reasonable to think that boiling breaks the bonds in a substance, because the liquid visibly changes into a gas. Usually, boiling only separates whole molecules by overcoming attractions between them; the covalent bonds inside each molecule remain. Tu can test this idea by noting that water vapour is still H₂O.
Soap, ink and cooling
Intermolecular forces appear in ordinary materials. Soap changes how water molecules pull on one another, helping water spread across greasy surfaces; ink uses different attractions to stay on paper; evaporation cools your skin because the fastest molecules escape first. Tu are seeing the same particle idea in three technologies.
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