1. Why is this needle floating?

2. Intermolecular Forces:
(inter = between) between molecules
and the temperature (kinetic energy) of the molecules.
What determines if a
substance is a solid,
liquid, or gas?

4. Gases: The average kinetic energy of the gas molecules is much larger than the average energy of the attractions between them.
Liquids: the intermolecular attractive forces are strong enough to hold the molecules close together, but without much order.
Solids: the intermolecular attractive forces are strong enough to lock molecules in place (high order).
Are they temperature
dependent?

5. The strengths of intermolecular forces are generally weaker than either ionic or covalent bonds.
16 kJ/mol (to separate molecules) + - + -
431 kJ/mol (to break bond)

6. .. .. : : : .. .. .. : : : .. + S O O - - + S O O - -
Types of intermolecular forces (between neutral molecules):
Dipole-dipole forces: (polar molecules) .. + S .. :
dipole-dipole attraction O O : : .. - - .. + S .. : O O : : .. - -
What effect does this attraction have on the boiling point?

7. dipole-dipole attractions for
Polar molecules have
dipole-dipole attractions for
one another.
+HCl----- +HCl-
dipole-dipole attraction

8. Types of intermolecular forces (between neutral molecules):
Hydrogen bonding: cases of very strong dipole-dipole interaction (bonds involving H-F, H-O, and H-N are most important cases).
+H-F- --- +H-F-
Hydrogen bonding

9. Hydrogen bonding is a weak to moderate attractive force that exists between a hydrogen atom covalently bonded to a very small and highly electronegative atom and a lone pair of electrons on another small, electronegative atom (F, O, or N).

10. Boiling points versus molecular mass
100
-100
Predict a trend for: NH3, PH3, AsH3, and SbH3

11. Now let’s look at HF, HCl, HBr, and HI
SbH3
NH3 HI
AsH3
HBr
HCl
PH3

12. Types of intermolecular forces (between neutral molecules):
London dispersion forces: (instantaneous dipole moment)
( also referred to as van der Waal’s forces)
attraction - + - +
“electrons are shifted to overload one side of an atom or molecule”.

13. In general big molecules are more easily polarized
polarizability: the ease with which an atom or molecule can be distorted to have an instantaneous dipole. “squashiness”
In general big molecules
are more easily polarized
than little ones.
Big and
“squashy”
little

14. Which one(s) of the above are most polarizable?
Hint: look at the relative sizes.

15. Other types of forces holding solids together:
ionic: “charged ions stuck together by their charges”
There are no individual molecules here.

16. Metallic bonding: “sea of electrons”
Copper wire: What keeps the atoms together?
Cu atoms
an outer shell electron
To which nucleus does the electron belong?

17. Metallic Bonding: “sea of e-’s”

18. Covalent Network: (diamonds, quartz) very strong.
1.42 Å
1.54 Å
3.35 Å

21. Pentane isomers: C5H12
neo-pentane
n-pentane
iso-pentane
Hvap=22.8 kJ/mol
Hvap=25.8 kJ/mol
Hvap=24.7 kJ/mol
All three have the same formula C5H12
Why do they have different enthalpies of vaporization?
London and “Tangling”

22. C-C-C-C C
iso-pentane C
C-C-C
neo-pentane
Hvap=24.7 kJ/mol
Hvap=22.8 kJ/mol
n-pentane
Hvap=25.8 kJ/mol
London and “Tangling”

23. Structure effects on boiling points

24. Ion-dipole interactions: such as a salt dissolved in water
cation
polar molecule
anion

26. Energy changes accompanying phase changes

27. Heating curve for 1 gram of water

28. Heating curve for 1 gram of water
Specific Ht. Steam = 1.84 J/g•K
Hvap=2260 J/g
Specific Heat of water = J/g•K
Hfus=334 J/g
Specific Heat of ice = 2.09 J/g•K

29. Vapor pressure

30. VAPOR PRESSURE CURVES
A liquid boils when its vapor pressure =‘s the external pressure.

31. normal boiling point is the temperature at which a
liquid boils under one atm of pressure.
pressure = 1 atm
vapor pressure = 1 atm
liquid
BOILING

32. PHASE DIAGRAMS: (Temperature vs. Pressure)
gas and liquid are
indistinguishable.
critical temperature
and critical pressure
(all 3 phases exists here)

33. H2O
CO2
note slope with pressure
note slope with pressure

34. Crystal Structures:

35. unit cells:
contains 2 atoms
contains 1 atom