When
Intermolecular Forces
Rule
in Chemistry and Biology
PGCC CHM 101 Sinex
Comparison of properties for the states of matter
Property
Solid
Liquid
Gas
Volume
Shape
Relative density
definite
definite
high
definite
indefinite
~1 g/mL
indefinite
indefinite
very low
Compressibility
low-none
low-none
very high
no
close
yes
close
yes
far apart
Fluidity/mobility
Particle packing
Diffusion rate
Motion of
particles
Forces between
particles
very slow
vibrations
only
IMF’s or
bonding
medium
fast
translation translation
rotation
rotation
IMF’s
only
none if
ideal gas
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.
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
Force-distance relationship
For the Fattraction proportional to 1/distance
molecules in contact with each other
Relative attractive force
1
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
8
Relative molecular distance
9
10
Force-distance relationship
Relative attractive force
1
1
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
8
Relative molecular distance
9
10
Force-distance relationship
Relative attractive force
1
2
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
8
Relative molecular distance
9
10
Force-distance relationship
Relative attractive force
1
3
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
8
Relative molecular distance
9
10
Force-distance relationship
Relative attractive force
1
4
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
8
Relative molecular distance
9
10
Force-distance relationship
Relative attractive force
1
5
0.8
0.6
Less than 20% the strength
at 5 molecular distances
0.4
0.2
0
0
1
2
3
4
5
6
7
8
Relative molecular distance
9
10
Increasing strengths of IMF
The strength of IMF’s
•
•
•
•
•
•
•
H-bonding (fixed distance ~200 pm)
Ion-ion (1/r)
Ion-dipole (1/r2)
Inverse
3
Dipole-dipole (1/r )
functions
of
Ion-induced dipole (1/r4)
distance
6
Dipole-induced dipole (1/r )
to various
powers
Induced dipole-induced dipole
(1/r6)
How do the other functions behave compared to 1/r?
Click here for an interactive Excel spreadsheet to explore
Hydrogen bonding in water
O-H 96 pm
covalent bond
… H 186 pm
O
Click here for Chime activity
Base Pairs in DNA: H-bonding
AT pair
GC pair
Why not
here?
Click here for Chime structure of DNA
Click here for Chime activity
Lipid Bilayer: Induced Dipole – Induced
Dipole Interaction for Hydrocarbon Chains
phospholipid
Click here
for Chime
activity
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
Why do low molar mass substances that
are non-polar molecules, such as H2, N2
O2, or CO2 have very low boiling points?
Since they are non-polar, the IMF will be
induced dipole- induced dipole, which is
the weakest IMF.
To get the molecules to interact with one
another, the temperature needs to be
decreased considerably (lower kinetic
energy).
For an ideal gas we assume there are no
interactions between the atoms or
molecules. Why is this a good
assumption?
Atoms or molecules in the gaseous state
at room conditions are too far apart for
IMF’s to operate. IMF’s are short range
attractive forces.
Attractive forces become important at low
temperatures or high pressures. WHY?
Some review questions
1. If a gas is compressed to high
pressures, it will liquefy. Why?
Click here for Chime structures to
address the next two questions.
2. What holds a protein in the alphahelix form?
3. How about a beta-sheet protein?
LESSON LEARNED
All of biology
is held together
by chemistry!
Go tell a biologist!!!!