INTERMOLECULAR FORCES

Chap. 13: Day 2

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

Melting Melting PointPointEnergy needed over Energy needed over come the come the

intermolecular forces intermolecular forces between molecules in between molecules in

the solid phasethe solid phase

Temperature at which Temperature at which molecules in a solid gain molecules in a solid gain

enough potential energy to enough potential energy to change position (become L)change position (become L)

Boiling Boiling PointPointEnergy needed over Energy needed over come the come the

intermolecular forces intermolecular forces between molecules in between molecules in

the liquid phasethe liquid phase

Temperature at Temperature at which molecules in which molecules in a liquid gain enough a liquid gain enough potential energy to potential energy to

change position change position (become gas)(become gas)

POLAR vs POLAR vs NONPOLARNONPOLAR

8Molecular Molecular PolarityPolarity

Molecular Molecular PolarityPolarity

Molecules will be polar ifMolecules will be polar if

a)a)bonds are polarbonds are polar

ANDANDb)b)the molecule is NOT the molecule is NOT

“ “symmetric”symmetric”

9

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

10

Polar or Nonpolar?Polar or Nonpolar?

• Consider AB3 molecules: BF3, Cl2CO, and NH3.

11Molecular Polarity, Molecular Polarity, BFBF33

Molecular Polarity, Molecular Polarity, BFBF33

F

F FB

F

F FB

B atom is B atom is positive and positive and F atoms are F atoms are negative.negative.

B—F bonds in BFB—F bonds in BF33 are polar. are polar. B—F bonds in BFB—F bonds in BF33 are polar. are polar.

But molecule is symmetrical But molecule is symmetrical and and NOTNOT polar polar

But molecule is symmetrical But molecule is symmetrical and and NOTNOT polar polar

12Molecular Polarity, Molecular Polarity, HBFHBF22

Molecular Polarity, Molecular Polarity, HBFHBF22

B atom is B atom is positive but positive but H & F atoms H & F atoms are negative.are negative.

H

F FB

H

F FB

B—F and B—H bonds in HBFB—F and B—H bonds in HBF22

are polar. But molecule is NOT are polar. But molecule is NOT symmetrical and is polar.symmetrical and is polar.

B—F and B—H bonds in HBFB—F and B—H bonds in HBF22

are polar. But molecule is NOT are polar. But molecule is NOT symmetrical and is polar.symmetrical and is polar.

13

Is Methane, CHIs Methane, CH44, Polar?, Polar?Is Methane, CHIs Methane, CH44, Polar?, Polar?

Methane is symmetrical Methane is symmetrical

and is NOT polar.and is NOT polar.

14Substituted Substituted EthyleneEthylene

Substituted Substituted EthyleneEthylene

• C—F bonds are MUCH more polar than C—F bonds are MUCH more polar than C—H bonds. C—H bonds.

• Because both C—F bonds are on same Because both C—F bonds are on same side of molecule, molecule is side of molecule, molecule is POLARPOLAR..

NONPOLAR MoleculesNONPOLAR Molecules

What causes the differences What causes the differences in intermolecular forces in intermolecular forces

between molecules?between molecules?

Why do the intermolecular Why do the intermolecular forces vary between nonpolar forces vary between nonpolar

molecules?molecules?

Liquid Solid

WHY?

Bigger Molecules interact more

DISPERSION FORCE

Nonpolar MoleculesNonpolar Molecules

POLAR MoleculesPOLAR Molecules

Why do the Why do the intermolecular intermolecular

forces vary forces vary between polar between polar

molecules?molecules?

20Molecular Molecular PolarityPolarity

Molecules—such as HCl and HMolecules—such as HCl and H22O— can be O— can be POLARPOLAR (or (or

dipolar). dipolar).

They have a They have a DIPOLE MOMENTDIPOLE MOMENT. The polar HCl . The polar HCl molecule will turn to align with an electric field.molecule will turn to align with an electric field.

Figure 9.15Figure 9.15

DIPOLE FORCE

Polar MoleculesPolar Molecules

More polar stronger force

Why different?Why different?

liquidliquid gasgas

?

SOMESOME POLAR POLAR MoleculesMolecules

Have “special”Have “special”

Dipole forceDipole force with with HydrogenHydrogen

HYDROGEN HYDROGEN FORCE FORCE oror

HYDROGEN BONDHYDROGEN BOND

HYDOGEN BOND

NEED TO BE POLAR

H is attached to N, F, or O

ALL molecules have LONDON DISPERSION force

Graphing: Intermolecular Forces

See handout 4 different Families

Boiling point vs period

Boi

ling

poin

t

period

100 k

400 k

2 6