1. Intermolecular Forces
Section 6.5

2. Introduction We will consider ionic and covalent bonds between atoms
If there are no attractive forces between molecules, then all substances would be gases
There must be some force that attracts molecules to other molecules that does not create a real bond
These are known collectively as intermolecular forces

3. Types of Intermolecular Forces
In order of increasing strength:
van der Waals' forces (AKA London forces or dispersion forces)
Dipole-dipole forces
Hydrogen bonding

4. van der Waals' Forces Exist between all species
Is the only intermolecular force between non- polar species
Result of temporary (instantaneous) dipoles when one side of the molecule becomes partially negative because of the random motion of electrons

5. Continued
This results in a weak dipole moment that then attracts other molecules
Strength of this force increases with molar mass as more electrons are available for temporary dipoles

7. Effect of van der Waals' Forces on Boiling Point
Higher boiling point means more energy is needed to break the intermolecular forces
As the molar mass increases, the boiling point increases
As the surface area increases, the boiling point increases (again, more electrons available)
More elongated the molecule, the stronger the van der Waals' forces, so the higher the boiling point

8. Continued
Sometimes the van der Waals' force can be quite strong as in the case of some polymers that have high mass and are very long molecules

9. Dipole-dipole forces
Dipole moment: a measure of the polarity of a molecule
Arrows are used to represent the polarity of the bond (heading toward the partially negative part)
The permanent dipoles formed cause electrostatic attraction between molecules that have the permanent dipoles
Stronger than van der Waals' forces in molecules of similar size

10. Dipole-dipole Forces

11. Continued

12. Effect of Dipole-dipole Forces on Boiling Point
Compared to molecules of similar mass with only van der Waals' forces, much higher boiling point
Polar molecules have van der Waals' forces and dipole-dipole forces
Stronger intermolecular force: higher boiling point

13. Hydrogen Bonding Not a “true” bond
Occurs when hydrogen is bonded to highly electronegative, small atoms, like N, O, or F
Creates a very high dipole moment as the more electronegative atom attracts the electrons, leaving the hydrogen very partially positive
Can be thought of as partway between a dipole-dipole force and a dative covalent bond

14. Continued
For maximum strength, the 2 atoms and the hydrogen should be in a straight line
When X and Y are N, O, or F:
Xδ-: hydrogen bond Hδ+ - Yδ-
Usually much stronger than other intermolecular forces

15. Hydrogen Bonding

16. H-bonding

17. Effect of H-bonding on Boiling Point
Consider HF and HCl:
HF has van der Waals' forces, dipole-dipole forces, and H-bonding
HCl has van der Waals' forces and dipole- dipole forces
Boiling point of HF is higher than HCl
More energy is needed to break the intermolecular forces

18. Consider H2O and H2S H-bonding present in H2O, but not in H2S
Boiling point of water is higher than H2S
H-bonding allows water to form tetrahedral shapes when it is a liquid
H-bonding allows water to form hexagons when it freezes, so ice is less dense than liquid water
H-bonding in water also forms temporary hexagon arrays on the surface of water, giving it a high surface tension

19. Basilisk Lizard

20. Consider NH3 and PH3
NH3 can hydrogen bond with itself and with the water when the ammonia is aqueous
PH3 can only hydrogen bond with water, not with itself
Ammonia has the higher boiling point

21. Biological Importance
The weak bond between the nitrogenous bases in nucleic acids is a H-bond
Occurs between thymine and adenine as well as between cytosine and guanine
Easily broken by enzymes