Intermolecular and Interparticle Forces
Introduced| English | Meaning |
|---|---|
| London dispersion forces/ˈlʌndn dɪˈspɜːʃn ˈfɔːsɪz/ | Attractions arising when temporary fluctuations in electron density produce correlated dipoles in neighbouring particles. |
| hydrogen bonding/ˈhaɪdrədʒn ˈbɒndɪŋ/ | An attraction involving a hydrogen atom bonded to an electronegative atom and an electron-rich site on another atom. |
| dipole-dipole forces/ˈdaɪpəʊl ˈdaɪpəʊl ˈfɔːsɪz/ | dipole-dipole forces |
The weak grip between molecules
- Water is a liquid at room temperature, but similar-sized gases are not.
- Something makes molecules stick lightly to their neighbours.
- These attractions are far weaker than the bonds inside a molecule.
- Yet they decide whether a substance is a gas, liquid, or solid.
London dispersion forces
- Every molecule has London dispersion forces 伦敦色散力.
- Fleeting uneven electron clouds create tiny, temporary attractions.
- Bigger molecules, with more electrons, have stronger dispersion.
Hydrogen bonding 氢键: an H on N/O/F is attracted to a lone pair on another molecule
London dispersion: an instantaneous dipole induces a dipole in a neighbour
All molecules experience London dispersion forces.
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Temporary uneven electron clouds occur in every molecule.
Larger molecules with more electrons have weaker dispersion forces.
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False. More electrons make larger temporary dipoles, so dispersion forces get stronger with size; that is why boiling points rise down a group of nonpolar molecules.
Dipole-dipole forces
- Polar molecules also feel dipole-dipole forces 偶极-偶极作用.
- The positive end of one molecule attracts the negative end of another.
- These are stronger than dispersion alone.
Hydrogen bonding
- The strongest of these is hydrogen bonding.
- It happens when hydrogen is bonded to N, O, or F.
- It gives water its unusually high boiling point.
Which intermolecular force?
Identify the strongest force acting between each pair of molecules.
Which is the strongest intermolecular force?
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Hydrogen bonding (H to N, O, or F) is the strongest of the three.
Hydrogen bonding requires hydrogen bonded to...
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Only the small, very electronegative N, O, F give true hydrogen bonds.
Select all forces a polar molecule like $\text{HCl}$ experiences.
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A polar molecule has both dispersion and dipole-dipole forces.
Why does water boil at a higher temperature than $\text{H}_2\text{S}$?
- Water has hydrogen bonds (H on O); $\text{H}_2\text{S}$ does not.
- The stronger attraction takes more energy to break, so water boils higher.
When a molecular liquid boils, what breaks?
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Boiling separates molecules by breaking the weaker forces between them.
These forces act between molecules and are much weaker than the covalent bonds within them — boiling breaks the forces, not the bonds. All molecules have dispersion; polar ones add dipole-dipole; only H bonded to N, O, or F gives hydrogen bonding. Stronger forces mean a higher boiling point and higher viscosity.
Attractions between molecules set the physical state. Every molecule has London dispersion forces (stronger for more electrons); polar molecules add dipole-dipole forces; and H bonded to N, O, or F gives the strongest, hydrogen bonding. Stronger forces raise the boiling point — as with water.