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KINETIC THEORY OF MATTER
- molecules in matter are always in motion
- speed of molecules is proportional to the temperature
THE STATES OF MATTER
1. Gas
a) ideal gas
- molecules move freely
- molecules have no interactions with each other
- no attraction or repulsion
- like ping-pong balls
b) real gas
- molecules move freely
- molecules have small attractions for each other
- like “sticky” ping-pong balls
- attractions decrease pressure of gas
- molecules have a volume
- inclusion of the molecular volume increases the pressure of gas (more
crowding)
2. Liquid
- molecules move but movement is heavily influenced by other molecules
- molecules must move through a crowd of molecule
- ideal gas can never condense to be a liquid.
3. Solid
- molecules (or ions) vibrate about fixed position in crystal lattice
- motion is not great enough to overcome attractions
- molecules (or ions) stay in a specific arrangement
INTERMOLECULAR FORCES
Overriding Principles
1. Opposite charges attract.
2. The greater the charge, the greater the attraction.
- These principles are a restatement of Coulomb’s Law.
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CHARGE DISTRIBUTIONS OF MATTER
1. Ion
2. Dipole
- A polar molecule has a dipole.
- The value of a dipole depends on
a) polarity of bonds
b) molecular geometry
H
O
H
H C N
3. Induced Dipole
- A molecule can have its electron cloud polarized by neighboring charge.
- Consider F2.
F

F
F
F
- Fluorine molecule has no dipole.
- Consider F2 next to sodium ion, Na+.
Na+
F
-
F
+
- Negative electron cloud adjusts to move closer to positive ion.
- Fluorine molecule now has a dipole!
- Sodium ion induces dipole in fluorine.
- Thus fluorine may have an induced dipole.
Some atoms in molecules have electron clouds that adjust easier than others.
Na+
Na+
F
F
I
I
- Sodium ion induces larger dipole in iodine molecule than in fluorine.
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The ability of an atom or molecule to adjust its electron cloud in response to an
outside charge is called its polarizability.
The less tightly bound to the nucleus the electrons are, the more polarizable they are.
- As a periodic trend, polarizability increases as row increases.
Each electron in an atom, molecule or ion contributes to the overall polarizability.
- Thus, the more electrons a molecule has, the greater the strength of its induced
dipole.
Example: Arrange the following atoms in order of increasing polarizability: Ar, He,
Kr, Ne and Xe.
Answer: He, Ne, Ar, Kr, Xe
Xe – very polarizable He – barely polarizable
TYPES OF INTERMOLECULAR FORCES
1. Ion – Dipole
Example: salt water
O
H
H
O
+
H
O
H
-
O
H
H
H
O
H
H
H
O
H
H
H
H
O
H
H
O
- strongest type of intermolecular force
- as the charge increases, the strength of the force increases
Ca2+ --- H2O forces are stronger than Na+ --- H2O forces
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2. Dipole – Dipole
Example: Formaldehyde
H
-
O
+
C
H
H
H
C
dipole – dipole forces
O
H
H
C
O
Molecules with greater dipole moments have greater attractions to each other.
Intermolecular forces (such dipole-dipole forces) can occur between different
molecules.
+
H
+

C C N
H
H
HH
C
O
3. Hydrogen bonding
- A hydrogen bond is a very strong dipole – dipole bond.
- Molecules with N, O, F covalently bonded to H can form hydrogen bonds.
- N, O, F atoms have high electronegativity
- Electrons in sigma bond are greatly polarized.
O
H

+
O
H
- Hydrogen is “bare naked”.
- A hydrogen bond is formed when polarized H is attracted to nonbonding pair of
electrons.
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Example: H2O
H
O
H
H
H
hydrogen bond
O
- Hydrogen bond is much stronger than dipole – dipole bond.
- Hydrogen bonding gives water many important and distinctive properties.
- Hydrogen bonding holds together DNA helix and proteins.
4. Induction forces (Dipole – Induced Dipole)
Example: HCl in I2.
I
I
I
I
+
-
H
Cl
I
I
I
I
- Dipole can induced by other dipoles.
- Induction forces are weaker than dipole – dipole forces.
5. Dispersion forces (Induced dipole – induced dipole)
- also known as London (dispersion) forces
Charge distribution of molecules fluctuates over time.
- i. e., charge distribution is unequal in nonpolar molecules at times
- i. e., nonpolar molecules become polar for a short time, though average dipole is
zero.
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Spontaneous fluctuation of electron cloud.
Consider a single bromine molecule.
-
+
Br
Br
Br
Br
Br
Br
Br
Br
+
- dipole appears, then disappears, then appears reversed, then disappears, etc…
Fluctuating dipole induces dipole in another molecule so that they fluctuate together.
Consider two bromine molecules
Br
Br
Br
Br
Br
Br
+
-
Br
+
Br
-
Strength of dispersion forces depends on strength of induced dipole.
- Molecules with greater polarizability have greater dispersion forces.
Illustration:
Br
Br
Br
stronger bond
Br
Cl
Cl
Cl
Cl
weaker bond
Bond between bromine molecules is stronger than bond between chlorine molecules.
- degree of contact with other molecules is also important
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Aside: abbreviated carbon chain structure

C
C
C
C
Example: Pentane, C5H12 has stronger dispersion forces than butane, C4H10
pentane
butane
Bonding between butane molecules is weaker than bonding
between pentane molecules.
pentane
ane
2-methylbutane
Bonding between 2-methylbutane molecules is weaker than bonding
between pentane molecules.
Butane
2-methylbutane
Pentane
Boiling Point
-0.5 ºC
27.8 ºC
36.1 ºC
Increasing
Intermolecular
Forces
ane
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Summary of Intermolecular Forces
Molecule A
ion
Molecule B
ionic bonding
ion
ion
- dip
ole
dipole
hydrogen bonding
dipole - dipole
dipole
in du
ctio
n
induced
dipole
dispersion
induced
dipole
Principles of Intermolecular Force Strength
1. Opposite charges attract.
2. The greater the charge, the greater attraction.
3. Permanent dipole has greater attraction than induced dipole.
- True when molecules have same number of electrons.
4. Polar molecules have dipole-dipole, induction forces and dispersion forces (and
possibly hydrogen bonding).
5. Nonpolar molecules have only dispersion forces.
6. Intermolecular forces are additive.
Example: Fluoromethane, CH3F (18 e-) has stronger total forces between molecules
than the force between fluorine gas molecules, F2 (18 e-).
Example: Iodine, I2 (nonpolar, 106 e-) has stronger forces in its molecules than does
hydrogen sulfide (polar, 18 e-). Why?
- Because the induced dipole of iodine is much greater than induced dipole and
permanent dipole of hydrogen sulfide since iodine has so many more electrons.
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INTERMOLECULAR FORCES AND PHASE TRANSITIONS
Solid
condensation
Gas
Liquid
evaporation
Phase changes occur when intermolecular bonds are affected by an increase or
decrease in kinetic energy.
Temperature during a phase transition is always constant.
- All energy input goes into phase transition, not temperature increase.
Heating Curve of Water
250
Temperature (C)
200
150
100
50
63
0
60
0
57
0
54
0
51
0
48
0
45
0
42
0
39
0
36
0
33
0
30
0
27
0
24
0
21
0
18
0
15
0
90
12
0
60
0
30
0
Tim e (sec)
liquid water is
heated
water is boiling
steam is getting
hotter
PROPERTIES OF LIQUIDS
Viscosity
- resistance to flow
- “thickness” of a fluid
- corn syrup has a higher viscosity than water
- viscosity is dependent on dynamic interaction between liquid molecules
- intermolecular forces and geometry of molecule are important.
Surface Tension
- The ability of a liquid to form a film on its surface that can resist puncture.
- Water and Hg are liquids with high surface tension, dish soap has low surface
tension.
- Bugs walk on water because water has high surface tension.
- Meniscus forms in graduated cylinder because of surface tension.
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PHASE DIAGRAMS
- indicates phase of substances at different temperatures and pressures
- indicates under what conditions phase changes occur
Example: CO2
s
critical point
(31.1 C, 73 atm)
l
g
triple point
(-56.4 C, 5.11 atm)
Temperature
- Lines indicate phase boundaries where both phases exist at the same time.
- At the triple point, all three phases exist at the same time.
- Each substance has only one triple point and it is unique to that substance.
- The critical point is where distinction between gas and liquid is lost.
- Substance is called a supercritical fluid.
- Note how the phases diagram illustrates how the boiling, freezing and
sublimation points change with pressure.
- Slope of the phase boundary between liquid and solid is a consequence of the
solid having a higher density than the liquid.
- The boundary in most substances has a positive slope.
Clausius – Clapeyron Equation
The line between the liquid and gas phase is described by the Clausius – Clapeyron
equation.
 P  H vap  1 1 
ln  2  
T T 
P
R
2 
 1
 1
- ln is the natural logarithm (more in 1190)
- With the heat of vaporization, the boiling point of a substance can be predicted.
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Example: H2O
critical point
(374 C, 224 atm)
s
l
g
triple point
(0.0098 C, 0.006 atm)
Temperature
- Note triple point is much different than CO2.
- Note negative slope of solid-liquid boundary
- Indicates solid water has lower density than liquid water.
- Thus ice floats!
Estimating the relative values of boiling points and freezing points
Boiling point of substance is temperature where vapor pressure of liquid equals
atmospheric pressure.
The boiling point and freezing point of substances depends on
1. Intermolecular forces.
- The stronger the forces between molecules, the harder it is to separate them
- Therefore the stronger the forces, the higher the boiling points and melting points
of substances.
2. Kinetic energy of molecules
- The greater the mass of the molecules, the greater the boiling and freezing points.
- The higher the temperature, the faster the molecules move and the more easily
the molecules can break away from each other.
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Increasing boiling points and melting points imply increasing heats of vaporization
and fusion.
Example: Order the following according to increasing boiling point: Br2, Cl2, F2, I2.
Recall: Dispersion forces increase as the total polarizability increases.
Polarizability increases as row of element increases and the number of
electrons increases.
Therefore answer is: F2, Cl2, Br2, I2
Example: Order the following according to decreasing boiling point.
Acetaldehyde
Acetonitrile
Methyl Ether
Methyl Chloride
CH3CHO
CH3CN
CH3OCH3
CH3Cl

2.7 D
3.9 D
1.3 D
1.9 D
 - strength of dipole
D – Debye unit
The strength of the intermolecular force is proportional to the strength of the dipole.
As the value of the dipole increases, the strength of the intermolecular force increases.
Therefore answer is: CH3CN, CH3CHO, CH3Cl, CH3OCH3