CHEM 2060 Lecture 12: vdW and H-bonding 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.
L12-1

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 neon argon
-269°C
-246°C
-186°C krypton xenon
-152°C
-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
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”.
L12-4

EXAMPLES:
ALL GASEOUS DIMERS
CHEM 2060 Lecture 12: vdW and H-bonding L12-5

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 H
2
O is the most widely used (and arguably the most important) example of H-bonding.
• Each H
2
O 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.

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.
CHEM 2060 Lecture 12: vdW and H-bonding L12-9

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. http://mysite.verizon.net/kdrews47/interactions/interactions.html