Vocabulary: intermolecular force dipole-dipole force hydrogen bond London dispersion forces boiling point melting point heat capacity surface tension capillary action momentary dipole induced dipole “The Creation of Adam,” Sistine Chapel, Michelangelo, Italian, 1512

 Weak forces of attraction between molecules of molecular compounds.  ALL intermolecular forces are due to dipoles in molecules. The difference is whether those dipoles are permanent or not.  Intermolecular forces are what allow molecules to come together and form liquids and solids.

 Dipole-dipole force  Dipole-dipole force: Attraction between two polar molecules.  Hydrogen bonding  Hydrogen bonding: Attraction between two molecules containing one of the very polar bonds of O-H, N-H, and F-H.  London dispersion force  London dispersion force: Attraction between two non-polar molecules due to momentary and induced dipoles.

 Intermolecular forces (between molecules) are always weaker than bonds (inside molecules).

 Attraction between two polar molecules

 What happens to molecules when you heat them up?  What happens to molecules when you cool them down?  How will this affect the attraction of the molecules due to the intermolecular force?  dapplet.html dapplet.html

 Conclusions: 1. The lower the temperature, the easier it is for the molecules to attract one another. 2. The stronger the dipole-dipole force, the higher the temperature can be for the molecules to still attract one another.

 Attraction between two molecules containing one of the very polar bonds of O-H, N-H, and F-H. It is the strongest of the intermolecular forces.  It is NOT a bond like covalent and ionic bonding. It is much, much weaker and is BETWEEN molecules, not inside molecules.

Boiling point seems to increase with mass of molecule.

But the molecule with the smallest mass has the highest boiling point.

The flow of water through xylem against gravity. The attraction of base pairs in DNA. The coiling of wool fibers (which can be permanently destroyed by heat.)

 Most of water’s unique properties are due to its extensive hydrogen bonding. How many hydrogen bonds can each water molecule make?

Extensive hydrogen bonding in ice

 Weird properties of water due to hydrogen bonding:

1. Solid form is less dense than liquid form.

 Weird properties of water due to hydrogen bonding: 2. Relatively high melting and boiling points.

 Weird properties of water due to hydrogen bonding: 3. Relatively high heat capacity (amount of heat required to raise temperature by 1 o C.)

 Weird properties of water due to hydrogen bonding: 4. High surface tension ( An increased attraction of molecules at the surface of a liquid resulting from forces of attraction on fewer sides of the molecules.)

 Weird properties of water due to hydrogen bonding: 5. Strong capillary action (The movement of a liquid along the surface of a solid caused by the attraction of molecules of the liquid to the molecules of the solid.

 Attraction between two non-polar molecules due to momentary dipoles and induced dipoles. Momentary dipole

 What do fish breathe????

 When a non-polar molecule has a momentary dipole, it can induce a dipole in a neighboring molecule. Then the two temporarily polar molecules can attract one another with a dispersion force. ningwj/modules/CI_induced_dipo les_neighboring_I2_molecules_1 3_5b.html viningwj/modules/CI_dipoleind uced_dipole_forces_13_5a.html

 Non-polar molecules with higher molecular weights (and therefore more electrons) have stronger dispersion forces than lighter molecules because the heavier molecules are more polarizable. vdw.htm

 Dipole-dipole forces and London dispersion forces are collectively called Van der Waal’s forces.

 London dispersion forces  Dipole-dipole forces  Hydrogen bonds  The strongest intermolecular forces are only 5% as strong as the weakest covalent bond! stronger

 The following are some of the many properties caused by the strength of intermolecular forces:  Boiling point/condensation point  Melting point/freezing point  Solubility  Viscosity  Surface tension  Capillary action

 What type of intermolecular force do you think will cause a substance to have a high melting point?  What type of intermolecular force do you think will cause a substance to have a low boiling point?  What type of intermolecular force do you think will cause a substance to have high surface tension? hydrogen bonds London dispersion forces hydrogen bonds