1. Intermolecular Forces
Unit 8
Intermolecular Forces

2. Intermolecular forces are attractive forces between molecules.
(Example: water molecule to water molecule)
Intramolecular forces hold atoms together within in a molecule.
(Example: H to O bond within a water molecule).
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
DHvap
DHfus
DHsub
Generally, intermolecular forces are much weaker than intramolecular forces.
11.2

4. Intermolecular Forces
1. London Forces (a.k.a. Dispersion Forces) Weakest
Dipole-Dipole Interactions
Ion-Dipole Interactions
-(Salt dissolving in solution; Na+ and Cl- )
4. Hydrogen Bonding (STRONGEST)

5. London Dispersion Forces: Weakest
Occur between every compound and arise from the net attractive forces
amount molecules which is produced from induced charge imbalances
Figure 10-8 Olmsted Williams
The larger the molecule the greater
it’s Dispersion Forces are.
Animation:
Figure 10-9 Olmsted Williams

6. The boiling point of long molecules increase with the length of the carbon
chain.
Olmsted Williams Fig Pg 437

7. How molecular shape affects the strength of the dispersion forces
The shapes of the molecules also matter. Long thin molecules can develop bigger temporary dipoles due to electron movement than short fat ones containing the same numbers of electrons.
                                                                                                                                                            
Butane has a higher boiling point because the dispersion forces are greater.

8. Polarizability
the ease with which the electron distribution in the atom or molecule can be distorted.
Polarizability increases with:
greater number of electrons
more diffuse electron cloud
Dispersion forces usually increase with molar mass.
11.2

9. Is the Molecule Polar?
The more Electronegative atom will pull the electron density of the bond
Closer to itself giving it a partial negative charge leaving the other
Atom with a partially positive charge. This is a dipole moment.

10. Molecules with 3 Atoms CO2 HCN SO2
Even though the C-O bond is polar, the bonds cancel each other out because the molecule is linear the dipole moments are equal and in opposite directions.Therefore CO2 is non-polar.
CO2
The dipole moment between H-C points in the direction of C. The dipole moment points between C-N points in the direction of the N. Therefore the dipole vectors are additive and HCN is polar
HCN
SO2
SO2 is a polar molecule because the S-O dipole Moments don’t cancel each other out due to the angle

11. Molecules with 4 Atoms
CCl4 is non-polar
CHCl3 is polar

12. How to Determine if a Molecule
Is Polar
Draw Lewis Structure
If all of the regions of electron density are
bound to the same thing (CCl4; CO2 ) than the
molecule is non-polar
If the regions of electron density are not bound to
the same thing (atom) than the molecule
is polar (HCN; SO2)

13. Which of the following molecules are polar (have a
dipole moment)?H2O, CO2, SO2, and CH4 O H S O
dipole moment
polar molecule
dipole moment
polar molecule C H C O
no dipole moment
nonpolar molecule
no dipole moment
nonpolar molecule
10.2

14. Orientation of Polar Molecules in a Solid
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
Animation:
11.2

15. Dipole Forces occur between molecules containing a dipole moment.
The positive end of the dipole moment on one mole is attracted to the
Negative end of the dipole moment on a nearby molecule.
2-methyl propane
(left) and acetone (right)
Both compounds are about
Equal in size and shape
Figure 10-11
Olmsted Williams

16. Ion-Dipole Interaction
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
The larger the charge the stronger the force
11.2

17. A molecular picture showing the ion-dipole
Olmsted Williams
Fig 10-34
A molecular picture showing the ion-dipole
Interaction that helps a solid ionic crystal dissolve
in water. The arrows indicate ion-dipole interactions.

18. 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. S O
SO2
SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.
11.2

19. The Hydrogen Bond STRONGEST INTERMOLECULAR FORCE
Animation

20. water has the highest heat of fusion
water has the highest melting point
water has the highest boiling point
water has the highest heat of vaporization
water has the lowest molar mass
The melting point, boiling point, heat of fusion and heat of vaporization of water are extremely high and do not fit the trend of properties relative to molar mass within Group VIA.

21. Water exhibits these unusual properties because of hydrogen bonding between water molecules.

22. A hydrogen bond is an intermolecular bond.
A hydrogen bond is formed between polar molecules that contain hydrogen covalently bonded to a small, highly electronegative atom: F, O, N.
F—H
O—H
N—H

23. it will be attracted to another F, O, or N, on another molecule.
A dipole-dipole bond bond will be formed between the two molecules which is called a hydrogen bond.
hydrogen bond
covalent bond
covalent bond

24. Water in the liquid and solid states exists as groups in which the water molecules are linked together by hydrogen bonds.
13.8