1. Intermolecular Forces and
Liquids and Solids
Chapter 12

2. Tokay gecko.

3. Tokay Gecko inverted on glass panel.
Geckos can cling to surfaces such as walls and ceilings because of intermolecular interactions called van der Waals forces.
Gecko feet have tiny hairs that split many times to make many very fine tips, giving the hairs a very large total surface area.
The fine hairs are able to form many contacts with any surface, and the surface-to-hair contact is adhesive due to van der Waals forces.
A gecko can support its entire body weight from a wall with only one toe.
One gecko foot can support the weight of an entire human.
Tokay Gecko inverted on glass panel.
Scanning electron microscope image of a gecko’s toe.

4. Van der Waals attractions work because at close range, atoms and molecules notice each other's dipole moments.
The dipole moments are due to the fact that at any given instant, the electron cloud is not evenly distributed over a molecule (or atom).
Even for a nonpolar molecule or a single atom, the electron distribution will not be symmetrical at a given instant (although it will be centered on average).
This instantaneous dipole moment causes atoms (or molecules) in the vicinity to arrange their instantaneous dipoles so as to lower their energy, which causes attraction.

5. The fundamental difference between states of matter is the distance between particles (atoms or molecules).

6. Ice cubes- solid Water- liquid Water vapor- gas 12.1
For a given substance, possible phases include solid, liquid, and gas.
Ice cubes- solid
Water- liquid
Water vapor- gas
condensed states
gaseous state
Principle difference between the condensed states and the gaseous state is the distance between molecules.
gases > liquids > solids
12.1

7. 12.1

8. States of Matter
Because in the solid and liquid states particles are closer together, we refer to them as condensed phases.

9. The States of Matter
The state a substance is in at a particular temperature and pressure depends on two antagonistic entities:
The kinetic energy of the particles
The strength of the attractions between the particles

10. Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule
(chemical bonds).
12.2

11. Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Intermolecular vs Intramolecular
41 kJ to vaporize 1 mole of water (inter)
930 kJ to break all O-H bonds in 1 mole of water (intra)
“Measure” of intermolecular force
boiling point
melting point
DHvap
DHfus
DHsub
The attractions between molecules are not nearly as strong as the intramolecular attractions (bonds) that hold compounds together.
12.2

12. Intermolecular Forces
Dipole-Dipole Forces
Ion-Dipole Forces
Dispersion Forces
The Hydrogen Bond
12.2

13. Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
Polar molecules orient to maximize attractive interactions.
12.2

14. Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
12.2

15. Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-dipole interactions allow ionic substances to dissolve in polar solvents.
12.2

16. Ion-Dipole Forces Charge and size of ion
Magnitude of dipole moment and size of molecule
Strongest interaction – high charge, small ionic radius (i.e., high charge density)
12.2

17. Will a cation or an anion (of equal charge) make the stronger ion-dipole interaction with water? Explain.
Cation, because it is smaller. For two ions with equal but opposite charge, the cation will exhibit the greater charge density because it is smaller.
12.2

18. Intermolecular Forces
Dispersion Forces (London forces)
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules that are otherwise nonpolar
These forces are present in all molecules, whether they are polar or nonpolar.
ion-induced dipole interaction
dipole-induced dipole interaction
instantaneous dipole-induced dipole interaction
12.2

19. Intermolecular Forces
Dispersion Forces (London forces)
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
Spherical charge distribution in a helium atom.
ion-induced dipole interaction
dipole-induced dipole interaction
12.2

20. Intermolecular Forces
Dispersion Forces (London forces)
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
Instantaneous dipole-induced dipole
He atom 2 produces an instantaneous dipole that induces a dipole in neighbouring He atom 1.
12.2

21. Intermolecular Forces
Dispersion Forces (London forces)
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
Instantaneous-induced dipole
The strength of dispersion forces tends to increase with increased molecular weight.
Larger atoms have larger electron clouds, which are easier to polarize.
The tendency of an electron cloud to distort in this way is called polarizability.
12.2