INTERMOLECULAR FORCES Chap. 13: Day 2 INTERMOLECULAR FORCES

Type of Bond determines Forces Generally strongest “Interparticle” Ionic Bond Ion-ion force Metalic bond Electronic force Generally weaker “intermolecular forces” Covalent bonds Polar molecules Dipole forces Nonpolar molecules London Disperson Induced dipole forces

Molecules will be polar if Molecular Polarity Molecules will be polar if a) bonds are polar AND the molecule is NOT “symmetric”

Compare CO2 and H2O. Which one is polar? Polar or Nonpolar? Compare CO2 and H2O. Which one is polar?

Polar or Nonpolar? Consider AB3 molecules: BF3, Cl2CO, and NH3.

B—F bonds in BF3 are polar. But molecule is symmetrical and NOT polar Molecular Polarity, BF3 B atom is positive and F atoms are negative. B—F bonds in BF3 are polar. But molecule is symmetrical and NOT polar

Molecular Polarity, HBF2 B atom is positive but H & F atoms are negative. B—F and B—H bonds in HBF2 are polar. But molecule is NOT symmetrical = POLAR.

Methane is symmetrical Is Methane, CH4, Polar? Methane is symmetrical and is NOT polar.

C—F bonds are MUCH more polar than C—H bonds. Substituted Ethylene C—F bonds are MUCH more polar than C—H bonds. Because both C—F bonds are on same side of molecule, molecule is POLAR.

Why do the intermolecular forces vary between nonpolar molecules? What causes the differences in intermolecular forces between molecules? Why do the intermolecular forces vary between nonpolar molecules?

Solid Liquid WHY?

Bigger Molecules interact more

DISPERSION FORCE Nonpolar Molecules

Why do the intermolecular forces vary between polar molecules?

Molecules—such as HCl and H2O— can be POLAR (or have a dipole). Molecular Polarity Molecules—such as HCl and H2O— can be POLAR (or have a dipole). They have a DIPOLE MOMENT. The polar HCl molecule will turn to align with an electric field. Figure 9.15

DIPOLE FORCE Polar Molecules

More polar stronger force

Why different? liquid gas

?

Dipole force with Hydrogen SOME POLAR Molecules Have “special” Dipole force with Hydrogen HYDROGEN FORCE or HYDROGEN BOND

NEED TO BE POLAR & H is attached to N, F, or O HYDOGEN BOND NEED TO BE POLAR & H is attached to N, F, or O

ALL have LONDON DISPERSION force

Melting Point Energy needed over come the intermolecular forces between molecules in the solid phase Temperature at which molecules in a solid gain enough potential energy to change position (become L)

Boiling Point Energy needed over come the intermolecular forces between molecules in the liquid phase Temperature at which molecules in a liquid gain enough potential energy to change position (become gas)

POLAR vs NONPOLAR

Graphing: Intermolecular Forces See handout 4 different Families Boiling point vs period 100 C Boiling point -200 C 4 period 7