van der Waals solids see: http://www.chemguide.co.uk/atoms

CHEM 2060 Lecture 12: vdW and H-bonding L12-1
van der Waals solids
see: http://www.chemguide.co.uk/atoms/bonding/vdw.html
RIGHT: a comparison
between a covalent molecular
bond and a much weaker vdW
bond in He
• Molecular solids in which
only van der Waals
intermolecular bonding
exists generally melt at low
temperatures..
…because thermal energy is
able to overcome vdW
attraction very easily.
• Liquid and solid He exist
only below 4.6 K.
CHEM 2060 Lecture 12: vdW and H-bonding L12-2
van der Waals bonds get stronger as atoms get bigger (more polarizable).
The boiling points of the
noble gases are
helium
-269°C
neon
-246°C
argon
-186°C
krypton
-152°C
xenon
-108°C
radon
-62°C
All of these elements have
monatomic molecules.
CHEM 2060 Lecture 12: vdW and H-bonding L12-3
vdW attractions are
responsible for the
liquid state in
hydrocarbons.
Question:
Do branched
hydrocarbons tend to
have lower or higher
boiling points than
linear hydrocarbons?
Why?
CHEM 2060 Lecture 12: vdW and H-bonding L12-4
Hydrogen Bonding
Polar molecules are held together in molecular solids by dipoles.
i.e. Opposite Ends of Dipole attract each other.
The hydrogen bond is one of the very important types of polar bonds.
H-bonds are relatively weak ~ 20 kJ mol-1
(compared to Covalent & Ionic Bonds ~ 400 kJ mol-1)
Hydrogen bonds require an electronegative H-bond “acceptor”,
typically an O, N or F atom.
Hydrogen bonds also require that the H atom be covalently bonded to an
electronegative atom.
X-H --- X
“donor” “acceptor”
So, C-H does not make for a good H-bond “donor”.
CHEM 2060 Lecture 12: vdW and H-bonding L12-5
EXAMPLES:
ALL GASEOUS DIMERS
CHEM 2060 Lecture 12: vdW and H-bonding L12-6
Features Common To H-Bonded Systems
1
Molecular units retain their integrity.
e.g. X-H bond length is the same regardless of whether it is involved in Hbonding or not.
2
H-bond is linear… so the geometry is: X-H --- X
3
Bond distances to the H atom are asymmetric (one short, one long).
(NOTE: Arguably, FHF- is an example of an H-bond between HF and F-,
but this would be a very strong and completely symmetric H-bond.
Although some texts disagree, I think it is more accurate to call this
covalent bonding.)
4
Angle θ (see HF) is between 100 and 120°.
In solid HF the bonding is zig-zag.
solid held together by H-bonds
CHEM 2060 Lecture 12: vdW and H-bonding L12-7
ICE – solid H2O is the most widely used (and arguably the most important)
example of H-bonding.
• Each H2O molecule is bonded to 4 others
(tetrahedral).
• Although bonds are weak, they are
important.
• Hydrogen bonding in water is responsible
for many of its important properties.
CHEM 2060 Lecture 12: vdW and H-bonding L12-8
mp & bp of water are unexpectedly high due to H bonding.
• Since H-bonding creates an
open network, ice is less
dense than water!
• Only about 1/3 of H-bonds
are broken when ice melts.
• In liquid phase water, there
are still H-bonds.
• Just above mp, as T↑ clusters
of H-bonded water break up.
∴ volume continues to shrink.
• Then as T↑ further, thermal
expansion occurs.
- molecules need more room.
CHEM 2060 Lecture 12: vdW and H-bonding L12-9
So liquid water has minimum
volume (Max. Density) at 4°C.
(WHY Lakes Don’t Freeze Solid)
H-bonds are EXTREMELY
important in biochemistry!
e.g. DNA Sequence and Replication
CHEM 2060 Lecture 12: vdW and H-bonding L12-10
CHEM 2060 Lecture 12: vdW and H-bonding L12-11
One of the biggest sources of difficulty for a chemistry student is the distinction
between chemical bonds and intermolecular forces.
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