1. Chapter 8.3-8.4 – Intermolecular Forces
CHM1111 Section 04
Instructor: Dr. Jules Carlson
Class Time: M/W/F 1:30-2:20
Wednesday, November 16th

2. Boiling Point/Melting Point Example
Arrange the following sets of compounds from lowest to highest boiling point or melting point.
I2, F2, Cl2
C3H7, C12H26, C8H18
CH3OH, CH3CH2CH2OH, CH3CH2COCH3
NH3, NH4+, NH2NH2
H2O, CH3OH, NaCl

3. Liquid Properties from Intermolecular Forces: Surface Tension
Surface tension is the resistance of a liquid to increase its surface area (measured in J m-2).
Molecules in the interior of the liquid are surrounded by other molecules and affected by intermolecular forces in all directions.
However, molecules at the surface are not subjected to intermolecular forces at the air-liquid interface, so a net force draws the liquid in.
Polar compounds have higher surface tensions than non-polar compounds.

4. Liquid Properties from Intermolecular Forces: Surface Tension
Surface tension is proportional to the magnitude of liquid intermolecular forces.

5. Liquid Properties from Intermolecular Forces: Capillary Action
Upward movement of water inside a capillary against the force of gravity.
Molecules in contact with their container experience two sets of intermolecular forces:
Cohesive forces attract molecules in the liquid to each other.
Adhesive forces attract molecules in the liquid to the molecules of the container walls.
Produces a meniscus.

6. Liquid Properties from Intermolecular Forces: Viscosity
Viscosity – a liquid’s resistance to flow.
Viscosity depends upon molecular shapes and sizes. Small molecules like water and acetone have lower viscosity while sugars (i.e. honey) and oils have higher viscosity.
Viscosity decreases with increasing temperature.

7. Vapour Pressure
Remember that the most probable kinetic energy is directly proportional to temperature.
In order for molecules to escape to the gas phase, the molecule must overcome the attractive forces in the liquid.
Br2 only has dispersion forces so has a lower escape energy than H2O.
As temperature increases, vapour pressure increases.

8. Vapour Pressure
If a container is open, molecules can slowly escape to the gas phase until no liquid remains.
With a closed container, as molecules escape to the gas phase, vapour pressure increases, and more gas molecules strike the liquid and are recaptured.
There is an equilibrium that reached where escape and recapture are at equal rates.

9. Vapour Pressure Problem
For the following pairs of liquids, which has the lower vapour pressure at room temperature and why?
Water (H2O) or Methanol (MeOH)
1-pentanol (C5H12OH) or 1-hexanol (C6H13OH)
Methyl chloride (CH3Cl) or chloroform (CHCl3)

10. Forces in Solids
In solids, molecules are locked in position producing higher rigidity – cannot move freely but bonds can vibrate and occasionally rotate.
The forces holding solids together can vary widely in strength relating to the types of intermolecular forces, like in liquids.

11. Types of Solids Four types of solids:
Molecular solids – aggregates of molecules held together by dispersion forces, dipolar forces, and/or hydrogen bonding.
Network Solids – molecules in array of covalent bonds
Metallic Solids – metals bonding from electrons in highly delocalized valence electrons.
Ionic Solids – have anions and cations strongly attracted to each other by electrical forces.

12. Molecular Solids
Held together by weakest forces, including dispersion forces (naphthalene), dipolar interactions, or hydrogen bonding (benzoic acid)
Glucose shows extensive hydrogen bonding.
Naphthalene
MP = 80⁰ C
Benzoic Acid
MP = 122⁰ C
Glucose
MP = 155⁰ C

13. Molecular Solids
Benzoic acid arranges in pairs with H-bonds between O and H.
Pairs are held together by weaker dispersion forces.

14. Network Solids
Held together by much stronger covalent bonds ( kJ mol-1) so many have much higher melting points.
Si-Si covalent bonds have an energy of 225 kJ mol-1.
Si is sp3 hybridized.
Melting point of Si is 1683 K vs 317 K for P4, a molecular solid.
Some network solids are very durable – gemstones.

15. Network Solids Bonding can have a significant effect on properties.
Diamond is hybridized sp3 and graphite is hybridized sp2.
Diamond is extremely strong while graphite is a brittle lubricant. Corborundum (C and Si) is also very strong.
Graphite also has delocalized π bonding above and below the plane of the bonds.

16. Metallic Solids
Metallic solids have highly delocalized valence orbitals. Produces a “sea” of mobile valence electrons.
Metals in group 1 are soft, have low melting points, and have poor electrical conductivity.
Metals are ductile and malleable.
Metals in the middle of the d-block are very strong and have the highest melting points. (Tungsten’s MP = 3407 ⁰C). Have high numbers of valence electrons that can occupy bonding orbitals in the metal lattice.
Metals in group 11 (Copper, Silver, Gold) are have high electrical conductivity.