Intermolecular and Interparticle Forces
Introduced| English |
|---|
| London dispersion forces/ˈlʌndn dɪˈspɜːʃn ˈfɔːsɪz/ |
| dipole-dipole forces/ˈdaɪpəʊl ˈdaɪpəʊl ˈfɔːsɪz/ |
| hydrogen bonding/ˈhaɪdrədʒn ˈbɒndɪŋ/ |
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.
All molecules experience London dispersion forces.
Temporary uneven electron clouds occur in every molecule.
Larger molecules with more electrons have stronger dispersion forces.
More electrons make bigger temporary dipoles, so stronger dispersion.
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 氢键.
- 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?
Hydrogen bonding (H to N, O, or F) is the strongest of the three.
Hydrogen bonding requires hydrogen bonded to...
Only the small, very electronegative N, O, F give true hydrogen bonds.
Select all forces a polar molecule like $\text{HCl}$ experiences.
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?
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.