1. When Intermolecular Forces Rule
CHM 101
Sinex
When Intermolecular Forces Rule
PGCC CHM 2000 Sinex

2. Comparison of properties for the states of matter Property Solid
Liquid
Gas
Volume
definite
indefinite
Shape
Relative density
high
~1 g/mL
very low
Compressibility
low-none
very high
Fluidity/mobility no
yes
Particle packing
close
far apart
Diffusion rate
very slow
medium
fast
Motion of particles
vibrations
only
translation rotation
Forces between particles
IMF’s or bonding
IMF’s
none if ideal gas

3. The liquid state of matter
Molecules are closely packed
but
free to move about randomly.
There must be a force of attraction between the molecules.

4. As molecules start to vaporize
Molecules are great distances apart
in gaseous state
but
free to move about randomly.
What about in solids?
Click here for a simulation of the states of matter

5. Force-distance relationship
For the Fattraction proportional to 1/distance
molecules in contact with each other

6. Force-distance relationship1

7. Force-distance relationship2

8. Force-distance relationship3

9. Force-distance relationship4

10. Force-distance relationship5
Less than 20% the strength at 5 molecular distances

11. Inverse functions of distance to various powers
The strength of IMF’s
H-bonding (fixed distance ~200 pm)
Ion-ion (1/r)
Ion-dipole (1/r2)
Dipole-dipole (1/r3)
Ion-induced dipole (1/r4)
Dipole-induced dipole (1/r6)
Induced dipole-induced dipole
(1/r6)
Inverse functions of distance to various powers
Increasing strengths of IMF
How do the other functions behave compared to 1/r?
Click here for an interactive Excel spreadsheet to explore

12. Hydrogen bonding in water
O-H pm
covalent bond
O H 186 pm …

13. Base Pairs in DNA: H-bonding
AT pair                            GC pair
Why not here?

14. What holds the iodine molecules together in solid iodine?
272 pm for I2 covalent bond
427 pm between two molecules in crystalline structure
About half an iodine atom space between molecules

15. weak intermolecular forces between layers
Graphite
weak intermolecular forces between layers
(van der Waals forces)
C-C bond distance: 142 pm
C to C distance between layers – 336 pm
graphene