CHEM 2060 Lecture 12: vdW and H-bonding L12-1 2060 Lecture 12: vdW and H-bonding L12-1 van der Waals solids see: RIGHT: a comparison between a covalent molecular

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.


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.


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?


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”.


EXAMPLES: ALL GASEOUS DIMERS


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 H-bonding 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


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.


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. DNA Sequence and Replication



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

Documents

CHEM 2060 Lecture 12: vdW and H-bonding L12-1 2060 Lecture 12: vdW and H-bonding L12-1 van der Waals solids see: RIGHT: a comparison between a covalent molecular