1. Intermolecular bonds

2. Covalent and ionic bonds take place between atoms in a molecule they are Intramolecular bonds
However there are bonds that take place between molecules these are Intermolecular bonds

3. 3 types of Intermolecular bonds
Van der Waals forces
Dipole-Dipole forces
Hydrogen bonding

4. Van Der Waals Forces
Dutch scientist discovered small attractive forces exist between molecules as a result of the shifts in position of the electrons within a molecule
When the electrons are temporarily concentrated at one end of the molecule they induce a slight negative charge here and the other side of the molecule becomes slightly positive as a result

5. A temporary Dipole is set up and this may induce a similar dipole in a nearby molecule
There is then an attraction between the oppositely charged parts of the molecule
FIG 5.39 p63

6. Van der Waals forces are the weak attractive forces between molecules resulting from the formation of temporary dipoles

8. Since electrons are moving at high speed the attractions between any two molecules exist for only an instant
Van der Waals forces are very weak only about 1 thousandth as strong as the covalent bond holding the molecule together
Van der Waals forcesa re the only forces of attraction that exist between non polar molecules

9. Evidence for Van der Waals forces exists in the fact that we can liquefy non polar gases suggesting there must be some attractive force between them
The strenght of Van der Waals forces increases as the molecules get bigger as therea re more electrons in the electron cloud

10. Cl2 is a gas, Br2 is a liquid and I2 is a solid at room temperature due to the increasing strength of the Van der Waals forces as the atomic mass increases
H2 has a boiling point of -252⁰C while O2 of -183⁰C as more heat is required to break the stronger Van der Waals forces between the larger O2 molecules

11. Nobel Gases
Van der Waals forces also exist between atoms of the Nobel gases
The boiling points of these gases increases with the size of the atom as the Van der waals forces between larger atoms is greater
Van der Waals forces also account for the differences in boiling points between molecules such as propane C3H8 and butane C4H10
There are more electrons contributing to the Van der Waals forces in butane so it ahs a higher Boiling point

12. Dipole-Dipole Forces
In compounds whose molecules are polar the dipole-dipole forces are permanent not temporary
Dipole-Dipole forces are forces of attraction between the negative pole of one molecule and the positive pole of another

13. HCl is a polar molecule and has permanent dipole-dipole interactions
Because these forces are between partial electric charges they are weaker than ionic bonds

14. These attractive forces between molecules give rise to higher boiling points than those of similar non polar molecules
Eg.
HCl has a much higher boiling point than H2
NB fig 5.41 p 65

15. Hydrogen Bonding
Experiments have shown that H2O , NH3 and HF all have boiling points much higher than expected
This indicated that these molecules must have very strong intermolecular forces pulling the together ( stronger than Van der Waals forces or dipole-dipole forces)
These intermolecular forces are called hydrogen bonds

16. Hydrogen bonding arises when a hydrogen atom is bonded to a small elecrtronegative atom such as Oxygen, Nitrogen or Flourine
The hydrogen atoms gain quite a strong positive charge as they are bonded to a strongly electronegative atom
This causes strong polarity in the molecules and they line up in chains or groups
A considerable amount of energy is required to break these attractive forces giving such compounds high boiling points

18. Hydrogen bonds are particular types of dipole-dipole attractions between molecules in which hydrogen atoms are bonded to nitrogen, oxygen or fluorine. The hydrogen atom carries a partial positive charge and is attracted to the electronegative atom in another molecule. Thus it acts as a bridge between two electronegative atoms

19. The strength of a hydrogen bond is about 10% that of a covalent bond
Water has a higher boiling point than HF because there are twice as many hydrogen atoms involved in the hydrogen bonding even thought the individual hydrogen bonds in HF are stronger as Fluorine is more electronegative than Oxygen

20. Significance of Hydrogen Bonding
The Du Pont company exploit the added strenght of hydrogen bonding to produce Kevlar an extremely strong synthetic material used in bullet proof vests, sports wear and fences

21. Hydrogen boding in water explains the high surface tension in water whereby a “skin” is almost formed on water enabling insects to walk on it