Intermolecular Forces

1. Intermolecular Forces Understanding the Reason behind the physical and Chemical Behavior of Covalent Molecules

2. F H F H d- d+ The electronegativity of an atom can create a dipole, or polar molecule. electron rich region electron poor region Polar covalent bond or polar bond : covalent bond with greater electron density around one of the two atoms

3. Generally, intermolecular forces are much weaker than intramolecular forces. Intermolecular forces: attractive forces betweenmolecules. Intramolecular forces: hold atoms together, attractive forces within a molecule.

4. 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

5. F F F F F H H H H H Dipole-Dipole Forces • Attractive forces between polar molecules • Partially + end of one dipole is attracted to the partially -- end of another dipole

6. Hydrogen Bonding The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. http://www.ck12.org/user:krogers/section/Intermolecular-Forces-of-Attraction/

7. Decreasing molar mass Decreasing boiling point Why is the hydrogen bond considered a “special” dipole-dipole interaction?

8. Hydrogen Bonding in Water

9. Induced-Dipole Forces Attractive forces when a polar molecule causes a nonpolar molecule to temporarily have a dipole induced dipole interaction by polar molecule induced dipole interaction (by an ion)

10. London Dispersion Forces • Attractive forces between two nonpolar molecules • In polar or nonpolar atoms, electrons are in continuous motion. • At any instant the electron distribution may be uneven. A momentary uneven charge can create a positive pole at one end of an atom of molecule and a negative pole at the other.

11. O O S What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH4 CH4 is nonpolar: dispersion forces. SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.