1. 4.7 Intermolecular Forces

2. intermolecular force: the force of attraction and repulsion between molecules

3. Like all chemical bonding, intermolecular forces are explained as mutual electrostatic attractions.
These forces are much weaker than covalent bonds.
The forces have an effect on a chemicals melting point, boiling point, capillary action, surface tension, volatility and solubility.

4. Easy Experimental Evidence for Intermolecular Forces
it takes much less energy to boil water (breaking intermolecular bonds) than it does to decompose water (breaking covalent bonds)

5. van der Waals and Hydrogen Bonding
Collectively some intermolecular forces are classified under the heading of van der Waals forces.
dipole-dipole
London dispersion forces
Hydrogen bonding was added later to explain the anomalies.

6. Dipole-dipole attractions
Occurs in POLAR MOLECULES
attractions between the negative end of one polar molecule and the positive end of another
it occurs as a result of the simultaneous attraction of a dipole by its surrounding dipoles
the strength of a dipole depends on the polarity of the molecule

7. polar molecules can move and rotate to maximize
attractions and minimize repulsions

8. Evidence of hydrogen bonding

9. Hydrogen bonding
exists when hydrogen atoms are bonded to highly electronegative atoms like N, O, F
the hydrogen is simultaneously attracted to a pair of e- on the N, O, or F atom of an adjacent molecule
it’s like covalent bonding but instead of electrons being shared it is a proton
It is unique to these few atoms because of the strong electronegativity of N,O,F and hydrogen’s small size

10. Hydrogen bonding in Ammonia and Water
-Red line indicated the formation of a hydrogen bond

11. London Dispersion Forces
the simultaneous attraction of the electrons of one molecule by the positive nuclei in the surrounding molecules
it occurs in all molecules but is the dominant force in non-polar molecules
the strength is directly related to the number of electrons in a molecule

12. The electrons from one molecule are attracted to the nucleus of surrounding molecules.
Nucleus of atom
Electrons of an atom

13. Predicting Boiling Points with Dipole-Dipole & London Forces
isoelectric molecules (2 different molecules that have the same e- configuration) have approximately the same strength of London Forces
Example: bromine and iodine monochloride have 70 electrons per molecule they would experience the same level of London forces

14. the more polar the molecule, the stronger the dipole-dipole force
Example:
- bromine does not contain a polar bond because the electrons are shared equally; therefore it does not contain dipole-dipole forces
- Iodine monochloride does have a slight polar bond 0.5; therefore it does contain dipole-dipole forces

15. The greater the number of e- per molecule, the stronger the London force and therefore, the higher the boiling point
Example: Which has the highest melting point: methane (CH4), ethane (C2H6), propane (C3H8), or butane (C4H10)
butane

16. You cannot perform a comparison of London forces to dipole-dipole forces because they are not the same thing!

17. Surface Tension
Surface-tension is the resistance of liquid to increase its surface areas
It is caused by the attraction between the liquid's interior molecules to each other. If surface area is to increase molecules from the interior must migrate toward the surface which requires energy and is resisted by the liquid

18. Capillary Action at Work!
Capillary action is the spontaneous rising of a liquid in a narrow tube.
It is a result of cohesion and adhesion.
The xylem of plants uses capillary action to transport water up to the leaves in tress. I

19. Viscosity The measure of a liquid’s resistance to flow
If a liquid experiences a high level of intermolecular forces it will more slowly than one with a lower level of intermolecular forces.

20. Homework
Pg 247 #1-8