Intermolecular Forces

1
Intermolecular Forces

• Forces Between Molecules

2
Why are intermolecular forces important?

• They determine the phase of a substance at room
  temperature.

3
3 Types of Intermolecular Forces

• Dispersion or Van der Waals
• Dipole-dipole
• Hydrogen Bonds

4
Strongest Intermolecular Force

• Hydrogen Bonds

5
When do Hydrogen bonds occur?

• Between molecules containing F, O, N bonded to a
  H atom

6
Strongest hydrogen bonds?

• Between molecules containing F-H bonds

7
Weakest hydrogen bonds?

• Between molecules containing N-H bonds

8
What effect does H-bonding have on the properties
of the substance?

• Hydrogen bonding leads to substantial increases
  in the expected boiling point.

9
Weakest Intermolecular Force

• Dispersion or Van der Waals forces

10
When do dispersion forces occur?

• Between nonpolar molecules

11
Nonpolar

• No Poles!
• The molecule is symmetric!
• Cannot tell 1 end from the other.
• (Kickballs footballs)

12
2 Easy categories of Nonpolar Molecules

• Monatomic Gases (Column 18)
• Diatomic Elements

13
What are the diatomic elements?

• H2, N2, O2, F2, Br2, Cl2, I2

14
3rd category of Nonpolar Molecules

• Larger molecules with a very symmetric shape

15
What can you say about dispersion forces as a
function of molecular size?

• The larger the molecules the stronger the
  dispersion forces!

16
When do dipole-dipole forces occur?

• Between polar molecules.

17
Polar

• Has Poles!
• The molecule is NOT symmetric!
• Can tell 1 end from the other.

18
Vapor

• Gas phase of a substance that is normally a
  liquid at room temperature.

19
Vapor Pressure

• The partial pressure of a vapor above its liquid.

20
What does vapor pressure depend on? (Hint see
Table H)

• Vapor pressure depends on the temperature of the
  liquid.

21
What happens to the vapor pressure as the
temperature of the liquid is increased?

• The vapor pressure always increases with
  temperature.

22
Does the vapor pressure depend on the amount of
liquid present?

• No. A thimbleful is as good as an ocean.

23
What are some differences between evaporation
boiling?

• Evaporation occurs at all temperatures. Boiling
  occurs at a definite temperature.
• Evaporation occurs at the surface. Boiling
  occurs throughout the liquid.

24
Heat of fusion

• Amount of energy required to convert 1 gram of a
  pure solid to the liquid phase at its melting
  point.

25
Heat of vaporization

• Amount of energy required to convert 1 gram of a
  pure liquid to the gas phase at its boiling point.

26
What happens to the boiling point as the
intermolecular forces increase?

• The stronger the intermolecular forces, the
  higher the boiling point.

27
What happens to the melting point as the
intermolecular forces increase?

• The stronger the intermolecular forces, the
  higher the melting point.

28
What happens to the heat of fusion as the
intermolecular forces increase?

• The stronger the intermolecular forces, the
  higher the heat of fusion.

29
What happens to the heat of vaporization as the
intermolecular forces increase?

• The stronger the intermolecular forces, the
  higher the heat of vaporization.

30
What happens to the evaporation rate as the
intermolecular forces increase?

• The stronger the intermolecular forces, the lower
  the evaporation rate.

31
What happens to the vapor pressure as the
intermolecular forces increase?

• The stronger the intermolecular forces, the lower
  the vapor pressure.

32
Boiling Point

• Temperature at which the vapor pressure of a
  liquid is equal to the external pressure.

33
Normal Boiling Point

• Temperature at which the vapor pressure of a
  liquid is equal to 1 atm or 101.3 kPa or 760 torr.

34
What happens to the boiling point when the
external pressure is reduced?

• The boiling point is reduced.

35
Sublimation

• Solid to gas

36
Deposition

• Gas to solid

37
Vaporization

• Liquid to Gas

38
Condensation

• Gas to liquid

39
Fusion

• Solid to liquid
• Or
• Melting

40
Freezing

• Liquid to solid
• Crystallization
• Solidification

41
Which phase changes are endothermic?
G L S

• Solid to liquid
• Liquid to Gas
• Solid to Gas

42
Which phase changes are exothermic?

• Gas to Liquid
• Liquid to Solid
• Gas to Solid

G L S
43
Given 3 substances at room temperature 1 solid,
1 liquid, 1 gas, which has the strongest
intermolecular forces?

• Solid gt Liquid gt Gas

44
Describe the mp bp of gases.

• Low!

45
H?F
--------
H?F
Hydrogen Bonding!
46
Ne
--------
Ne
Van der Waals or dispersion forces
47
HCl
--------
HCl
Dipole-dipole forces
48
H
H
-
-
H-N
--------
---------
H
-
--------
H-F
O
-
H
Hydrogen Bonding