1. Condensed Phases and Intermolecular Forces

2. Fundamentals
How do particle diagrams of liquids & solids compare to those of gases?

3. Describe relative positions and motions
of particles in each of 3 phases

4. The Question
Why do some substances exist as gases, some as liquids, and some as solids
at room temp?

5. Part of answer has to do with forces between separate molecules
2 broad categories of forces
need to be aware of

6. Dispersion Dipole-Dipole Covalent Ionic Metallic
Forces
INTERMOLECULAR
INTRAMOLECULAR
Dispersion
Dipole-Dipole
Hydrogen Bonding
Covalent
Ionic
Metallic

7. #1: Intramolecular Forces
Intramolecular forces = attractive forces that hold particles together in bonds (ionic, covalent, or metallic)
Intra means “within”
Intramolecular forces
= bonding forces

8. # 2: Intermolecular Forces-IMF (aka: van der Waals forces)
Inter means “between” or “among”
Intermolecular forces = forces between neighboring molecules
Intermolecular forces are weaker than Intramolecular forces

9. Intermolecular forces determine phase
“Competition” between strength of IMF & KE determines phase

10. Particles want to clump together
If IMF are strong, substance will be solid or liquid at room temp
Particles want to clump together
If IMF are weak, substance will be gas at
room temp
Particles free to spread apart

11. [this substance = a gas at room temperature]
It’s a balancing act!
Kinetic Energy
Intermolecular Forces
[this substance = a gas at room temperature]

12. Intermolecular Forces vs. Kinetic Energy
[this substance = a condensed phase (solid/liquid)]

13. Why Temperature Changes Affect Phase
Since T is measure of average KE, changing T can change phase
Changing T will change average KE

14. Changing the temperature
Intermolecular
Forces
Kinetic
Energy

15. Intermolecular Forces:
≈ 5% to 15% of strength of intramolecular or bonding forces
Account for phase at room temp
Strong IMF  condensed phase
Weak IMF  gas phase

16. 3 types of intermolecular forces (IMF):
Dispersion forces
Dipole-Dipole forces
Hydrogen bonds

17. occur between nonpolar molecules
1. Dispersion Forces:
weakest IMF
occur between nonpolar molecules

18. 2. Dipole-dipole forces:
intermediate IMF
occur between polar molecules

19. 3. Hydrogen bonds:
strongest IMF
occur between molecules that have:
H-F H-O or H-N bonds

20. Dispersion Forces & Nonpolar molecules
instantaneous and momentary
fluctuate
results from motion of electrons
if charge cloud not symmetrical will induce asymmetry in neighbor’s charge cloud!

21. Nonpolar means no poles Can’t tell one end of molecule from other end
Nonpolar molecules
Nonpolar means no poles
Can’t tell one end of molecule from
other end
electrons are evenly distributed
Symmetrical

22. Examples of Nonpolar Molecules
monatomic gas molecules:
He, Ne, Ar, Kr, Xe, Rn
diatomics if both atoms are same:
H2, N2, O2, Cl2, F2, I2, Br2
symmetrical molecules:
CH4, C2H6, C3H8

23. Dispersion Forces and Size
Dispersion forces ↑ with molecule size
larger the electron cloud, the greater the fluctuations in charge can be
Rn > Xe > Kr > Ar > Ne > He
I2 > Br2 > Cl2 > F2
C8H18 > C5H12 > C3H8 > CH4

24. Boiling point of N2 is 77 K (-196˚C)
IMF are very weak dispersion forces

25. Dipole-dipole Forces & Polar Molecules
Molecule shows permanent separation of charge; has poles:
one end partly (-) & one end partly (+)

26. Polar Molecules
Polar means molecule has poles: (+) & (-)
geometry and electron distribution are not symmetrical

27. What do you know about charge?
Opposites Attract!
this time, situation is permanent!
Examples: HI, CH3Cl

28. Hydrogen Bonding H-O N-H
Occurs between molecules with H-F, H-O, or H-N bonds (FON!!!)

29. Hydrogen Bonding Hydrogen bonding is extreme case of
dipole-dipole bonding
F, O, and N are all small and electronegative
strong electrons attraction
H has only 1 electron, so if being pulled away H proton is almost “naked”
H end is always positive &
F, O, or N end is always negative

30. Hydrogen bonding:
strongest IMF
influences physical props a great deal

32. Strength of Hydrogen Bonding
Fluorine most electronegative element, so
H-F bonds are most polar and exhibit
strongest hydrogen bonding
H-F > H-O > H-N

33. IMF vs. Physical Properties
If IMF  then:
Boiling point 
Melting point 
Heat of Fusion 
Heat of Vaporization 
while:
Evaporation Rate 

34. Intermolecular Force vs. Temperature
IMF more important as temperature is lowered
Low temperature – low evaporation rate
High temperature – high evaporation rate

35. Indicate type of IMF for each molecule:
NH3 Ar N2
HCl HF Ne O2
HBr
CH3NH2
Hydrogen bonding
Dispersion forces
Dipole-dipole forces
Dispersion
Dipole-dipole