Chapter 12
Chapter 5
Intermolecular forces
Physical States of Matter
Chapter 12
Section 12.3
Physical States of Matter
¾Three phases of matter
– solid
Definite shape and volume
solid
– liquid
Definite volume, shape of
container
– gas
Shape and volume of
container
liquid
gas
Certain forces keep the molecules together in solids and liquids
1
Chapter 5
Chapter 12
Physical States of Matter
Chapter 12
Physical States of Matter
¾ Physical state determined by the relative strength of
cohesive and disruptive forces
¾ Cohesive forces
¾Intramolecular forces
Intermolecular Forces
¾ Ion-Ion forces (ionic compounds)
¾ Ion-Dipole Forces
¾ Solvation/Hydration
S l ti /H d ti
¾Intermolecular forces
¾Disruptive forces
2
Chapter 12
Chapter 12
Section 12.3
Chapter 12
Intermolecular Forces
Intermolecular Forces
Dipole moment and boiling point
¾ Dipole-dipole forces
¾ For compounds of similar mass, the boiling point
increases with increasing dipole moment
¾ Exists between polar molecules
¾ This is a fairly weak force except when one atom
is hydrogen
δ+
C
δ−
O
δ+
C
δ−
O
δ+
C
δ−
O
3
Chapter 12
Chapter 12
Intermolecular Forces
¾
Chapter 12
Intermolecular Forces
Hydrogen bond
¾
Molecules capable of hydrogen bonding
¾
Effect of hydrogen bonding
Hydrogen bonding and boiling point
• A particularly strong type of dipole-dipole interaction
• It exist between an electronegative atom and a hydrogen atom
bonded to another electronegative atom.
δ+
δ+
¾
It is an especially strong
attractive force due to the
small size and mass of
hydrogen and the high
electronegativity of oxygen
H
δ−
H
O
H
δ+
H
δ−
O
δ+
H
δ+
δ+
δ+
O
δ−
O
δ+
H
δ−
O
δ+
H
δ−
O
δ+
H
δ+
H
δ+
H
H
δ−
H
H
δ+
H
δ+
δ−
O
δ+
δ−
O
δ+
H
H
4
Chapter 12
Chapter 12
Intermolecular Forces
Chapter 12
Intermolecular Forces
¾
¾ Which of the following molecules can form
hydrogen bonding?
Intermolecular Forces
Dipole-induced dipole force
¾ Exists between a non-polar compound and a polar
compound
¾ Polarization: the process of inducing a dipole.
¾ Polarizability: the ease with which the electron cloud of
an atom or a molecule can be distorted.
CH3CH2OH, CH3CHO, and CH3COOH
¾ Larger the molar mass, the greater the polarizability of
the molecules
Symmetrical distribution
of electron clouds
¾
Polarizability of atoms
Symmetry is lost
Rank the following molecules in the order of increasing
polarizability
Cl2, Br2, and I2
5
Chapter 12
Chapter 12
Intermolecular Forces
¾
Induced dipole-induced dipole (London
dispersion forces)
¾ Exists between non-polar molecules
¾ Very weak attractive force resulting from
momentary (fleeting) distortions in electron
distributions
Chapter 12
Intermolecular Forces
¾
London Dispersion forces
¾
Why do LDF get stronger with increasing size?
Intermolecular Forces
¾
London Dispersion forces
¾ F2 , Cl2 , Br2 , I2
¾ Which halogen should have the largest
dispersion forces and why?
¾ What is the impact?
Symmetrical distribution
of electron cloud
At a given instant of time,
there is an unsymmetrical distribution
of electron clouds
6
Chapter 12
Section 12.3
Chapter 12
Chapter 13
Intermolecular Forces
Summary for intermolecular forces
¾
Relative ranking intermolecular forces….which is the
strongest force, which is the weakest force?
3.1 Impact of IMF
¾
The relative strengths of IMF help explain….
¾
Physical states of matter
¾
B ili ttemperature
Boiling
t
¾
Solubility (miscibility)
7
Chapter 12
Chapter 12
Evaporation and Condensation
Chapter 12
Evaporation and Vaporization
¾ Evaporation or vaporization
¾ Molecules escape the surface of a liquid
¾ Vapor pressure
¾ Condensation
¾ Molecules strike and re-enter the surface of a liquid
¾ What does equilibrium mean?
Vapor Pressure
δ+
vaporization
¾ Vapor pressure depends
on strength
t
th off cohesive
h i
forces
δ+
H
O
H
O
δ+
O
δ+
H
H
δ−
δ+
H
δ−
O
δ+
H
H
O
H
δ−
O
δ+
H
δ+
condensation
H
H
δ−
H
O
H
H
O
H
H
8
Chapter 12
Chapter 12
Vapor Pressure
¾ Hydrogen bonds versus LDF
Polarity
Vapor pressure
(torr)
P t
Pentane
72
Nonpolar
414 5
414.5
Hexane
86
Nonpolar
113.9
Heptane
100
Nonpolar
37.2
Ethanol
46
Polar (hydrogen bonds)
43.9
1-Propanol
60
Polar (hydrogen bonds)
17.3
1-Butanol
74
Polar (hydrogen bonds)
7.1
Water
18
Vapor Pressure
¾ Effect of Temperature
Molecular
weight (μ)
Liquid
Chapter 12
Vapor Pressure
Polar (hydrogen bonds)
17.5
¾ Why does T affect
vapor pressure?
Temperature (oC)
Vapor pressure of
water (torr)
0
4.6
20
17.5
40
55.3
60
149.2
80
355.5
100
760.0
As T increases
9
Chapter 12
Chapter 12
Chapter 12
Boiling and Boiling Point
¾ What happens to vapor pressure as temperature
rises?
Boiling Point
¾
Boiling Point
Compare propanol, butane and acetone (58 - 60
amu). What IMF are possible?
¾
Another comparison
¾ Standard
St d d or normall b
boiling
ili point
i t
propanol
butane
acetone
propane
octane
C17-ane
10
Chapter 12
Chapter 13
Chapter 13
Boiling Point
¾ Why does water have a high boiling point?
Solubility
Miscibility
¾ Solubility
¾ A nice sounding but technically WRONG answer:
¾ Miscibility
“Water has high boiling point because the bonds in water
require a lot of energy to break. Therefore, a high
temperature is needed to break the bonds and produce
the gas.”
¾ Compounds with IMF of similar strength tend to be
soluble
¾ “like (IMF) dissolves like (IMF)”
11
Chapter 13
Solubility
*Expressed in mol alcohol/100g solvent at 20° C.
Can you explain the data shown above?
12