1. Intermolecular Forces
Ch. 16: Liquids and Solids
Intermolecular Forces

2. Intra- vs. Inter- intramolecular forces intermolecular forces
inside molecules
hold atoms together into molecule
intermolecular forces
between molecules
get weaker as phase changes from S – L – G
When a substance changes state, molecule stays together but intermolecular forces weaken

3. Dipole-Dipole Attraction
when molecules with dipole moments line up to minimize repulsion and maximize attraction
very weak compared to covalent and ionic bonds

4. Hydrogen Bonding happens between H and N, O, or F
very strong type of dipole-dipole attraction
because bond is so polar
because atoms are so small

5. London Dispersion Forces
in every molecular compound
only important for nonpolar molecules and noble gas atoms
weak, short-lived
caused by formation
of temporary
dipole moments

6. Practice which has highest boiling pt?
HF, HCl, or HBr?
Identify the most important intermolecular forces :
BaSO4
H2S Xe
C2H6 P4
H2O
CsI
ionic
dipole-dipole
H-bonding
London Dispersion

7. Which has stronger IMF’s?
CO2 or OCS
CO2: nonpolar so only LD
OCS: polar so dipole-dipole
PF3 or PF5
PF3: polar so dipole-dipole
PF5: nonpolar so only LD
SF2 or SF6
SF2: polar so dipole-dipole
SF6: nonpolar so only LD
SO3 or SO2
SO3: nonpolar so LD only
SO2: polar so dipol-dipole

8. Liquids and Solids

9. Properties of Liquids low compressibility no rigidity
high density compared to gases
beads up as droplets
molecules on surface are subject to attractions from side and below
surface tension
resistance to an increase in surface area
greater tension means stronger IMF’s

10. Properties of Liquids capillary action viscosity
spontaneous rising of a liquid up
a tube
from cohesive forces (IMF’s) and
adhesive forces (between liquid
and container)
viscosity
measure of a liquid’s resistance
to flow
strong IMF’s  highly viscous
large, complex molecules  highly viscous

11. Practice Quiz