1. Intermolecular forces
Questions
Why do some solids dissolve in water but others do not?
Why are some substances gases at room temperature, but others are liquid or solid?
The answers have to do with …
Intermolecular forces

2. Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
“Measure” of intermolecular force - boiling point
- melting point
- Surface tension
- evaporation rate
Generally, intermolecular forces are much weaker than intramolecular forces.

3. Types Of Intermolecular Forces:
There are three main types of IMFs
London Dispersion Forces (IMF)
Dipole-Dipole Forces (DD)
Hydrogen Bonding Forces (HB)

4. London Dispersion Forces
The temporary separations of charge that lead to the London force attractions are what attract one nonpolar molecule to its neighbors.
London forces increase with the size of the molecules.
Fritz London

5. London Dispersion Forces
Non-polar molecules do not have dipoles like polar molecules. How, then, can non-polar compounds form solids or liquids?
London forces are attractive forces caused due to small temporary dipoles that exist in non-polar molecules
Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical
The resulting tiny dipoles cause attractions between molecules

6. London forces Instantaneous dipole: Induced dipole:
Eventually electrons shift so that tiny dipoles form
A dipole forms in one atom or molecule, inducing a dipole in the other

7. London Dispersion Forces

8. Orientation of Polar Molecules in a Solid
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid

9. Hydrogen Bond
The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond in one molecule and an electronegative O, N, or F atom.

10. Hydrogen Bonding
Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen
Hydrogen bonding between ammonia and water

11. Ice is less dense than water
Water is a Unique Substance
Maximum Density
40C
Density of Water
Ice is less dense than water

12. The hexagonal structure of ice

13. The H-bonding abilitiy of the water molecule

14. Forces & Properties
Bigger size = Stronger force = higher melting and boiling point because more energy is needed to separate the molecules
Ex. H2 Vs. Cl2
Heavier the molecule = stronger force
Ex. F2 Vs. Cl2

15. Molecular shape and boiling point
Longer molecule = stronger force
C5H12 (straight structure) Vs C5H8 (branched structure)

16. Strong intermolecular forces
Properties of Liquids
Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.
(polar molecules, liquid metals).
Strong intermolecular forces =
High surface tension
(liquid beads up)

17. Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion

18. stronger cohesive forces
Figure 12.20
Shape of water or mercury meniscus in glass
stronger cohesive forces
adhesive forces

19. Like Dissolves Like
This is why oil and water will not mix! Oil is nonpolar, and water is polar.
The two will repel each other, and so you can not dissolve one in the other
Polar substances dissolves Polar substances
Non-Polar Dissolves Non-polar sbubstances

20. Bond Polarity “Like Dissolves Like” Polar dissolves Polar
Nonpolar dissolves Nonpolar