Remember Chapter 11 part 1
Introduction to Kinetic Molecular Theory
and
Intermolecular forces
Mary J. Bojan
Chem 110
1
To understand properties, we want to connect what we
see to what is happening on a molecular level.
Start with states of matter: gas à liquidà solid.
Kinetic Molecular Theory
is used to explain what is
happening on a molecular
level.
Key postulate of this
theory is that molecules
have kinetic energy: they
are in constant motion in
all states.
Mary J. Bojan
Chem 110
2
States of Matter
The state a substance is in at a particular temperature
and pressure depends on two antagonistic entities:
–  The kinetic energy of the particles
–  The strength of the attractions between the
particles
Mary J. Bojan
Chem 110
3
If we know the relative strengths of these two
interactions we could predict the state of a substance.
gas:
Kinetic energy >> intermolecular forces
Liquid: Kinetic energy ≅ intermolecular forces
Solid:
Kinetic energy << intermolecular forces
solid
→
Kinetic Energy ∝ T
Mary J. Bojan
liquid
→
gas
Heating: T ↑, KE ↑
Chem 110
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IM Forces: Structure Affects Function
Functional Group
Boiling point
Structure
Hydrocarbon
MW = 72amu
36°C
CH3CH2CH2CH2⎯CH3
Aldehyde
MW = 72amu
75°C
O
CH3
CH2
CH2 C
H
O
Ketone
MW = 72amu
79°C
amine
MW = 73amu
78°C
CH3CH2CH2CH2⎯NH2
ether
MW = 74amu
34°C
CH3CH2⎯O⎯CH2CH3
Alcohol
MW = 74amu
117°C
CH3CH2CH2CH2⎯OH
carboxylic acid
MW = 74amu
141°C
Mary J. Bojan
CH3
CH2 C
CH3
O
CH3
CH2 C
OH
Chem 110
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Chapter 11 part 2
Properties of Liquids
Viscosity
Surface Tension
Capillary Action
Phase Changes
(energy of phase changes)
Dynamic Equilibrium
Vapor pressure
Phase diagram
Mary J. Bojan
Chem 110
6
Properties of Liquids
Intermolecular forces play an important role in the
properties of liquids
Cohesive Forces: forces within liquid
Adhesive forces: forces between the
liquid and a surface.
There is a “competition”.
H 2O
Mary J. Bojan
Chem 110
Hg
7
Properties of Liquids
Viscosity: resistance to flow
↑cohesive forces, ↑ viscosity
For pure compound: as T ↑, viscosity ↓
Motor oil:
SAE 10 has lower viscosity than SAE 40.
Problem:
Need low viscosity at low T and high viscosity at high T
Multi-grade motor oils: e.g. 10w30
Mary J. Bojan
Chem 110
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Properties of Liquids
Surface Tension: energy needed to
increase surface area
↑ cohesive IM forces, ↑ surface tension
Surface Tension
Surface molecules have fewer interactions.
Energy is minimized by minimizing the
surface area.
Intermolecular interactions are favorable
(heat is required to break them)
The more interactions, the better.
Mary J. Bojan
Chem 110
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Properties of Liquids
Capillary Action: result of adhesion
and surface tension
Mechanism for
ground water motion
fluid movement in plants, animals
wicking (sponges, candles, paper towels,
chromatography)
capillary rise
The height depends on weight of
water that can be supported by
surface tension
Mary J. Bojan
Chem 110
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Energy of phase changes
Endothermic
It requires energy to disrupt intermolecular forces.
vaporization
sublimation
melting (fusion)
condensation
deposition
freezing
Exothermic
Energy is released when intermolecular interactions are formed
Mary J. Bojan
Chem 110
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We use CALORIMETRY to measure the energy
changes involved in chemical processes.
Calorimetry: Experimental measure of heat flow
q = Cs m ΔT
q = heat flow
Cs = specific heat
(heat capacity per gram)
m = mass
ΔT= Tfinal – Tinitial
For H2O:
Cs = 4.184 J/g °C
Molar heat capacity
Cm= 75.2 J/mole °C
q = Cs m ΔT (= Cm n ΔT)
= amount of heat given off (−) or absorbed (+) as temperature changes
Mary J. Bojan
Chem 110
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CALORIMETRY
ΔH = Heat change (at constant P)
= amount of heat given off (−) or absorbed (+) when a change occurs
Examples:
ΔHfusion = amount of heat needed to freeze a mole of substance
ΔHvap = amount of heat needed to vaporize a mole of substance
Mary J. Bojan
Chem 110
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HEATING THE SAMPLE
As heat is added, two types of changes take place:
1. Within single phase (in red) changes are continuous
q = n CmΔT
Since T ↑: kinetic energy ↑
Energy ↑, molecular motion ↑
separation between molecules ↑
molecular attractions ↓, and order ↓.
Mary J. Bojan
Chem 110
14
HEATING THE SAMPLE
As heat is added, two types of changes take place:
2. Between phases, (phase transition in blue) the changes are abrupt,
from one physical state to another
q = n ΔHx
(x = melting, vaporization)
Since T is constant, kinetic energy is constant
but energy ↑, molecular separation↑,
molecular attractions ↓, order ↓.
Mary J. Bojan
Chem 110
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EXAMPLE PROBLEM
2 moles of ice at −25 °C are heated to 125 °C.
How much energy is needed?
Cm(ice) = 37.6 J/mol·K
Cm(water) = 75.3 J/mol·K
Cm(steam) = 33.1 J/mol·K
Mary J. Bojan
ΔHfusion = 6.02 kJ/mol
ΔHvapor = 40.67 kJ/mol
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Break problem into steps
1.  ice −25oC " 0oC
2. ice 0oC " water 0oC
(phase transition)
3.  water 0oC " 100oC
4.  water 100oC " steam 100oC
(phase transition)
5.  steam 100oC " 125oC
Mary J. Bojan
Chem 110
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Vapor Pressure
vapor pressure (v.p.): the pressure exerted by a vapor in equilibrium
with its liquid or solid phase.
Dynamic Equilibrium
forward rate = backward rate
evaporation = condensation
No net change, but
change is occurring
on molecular level.
Mary J. Bojan
Chem 110
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As T ↑, what happens to vapor pressure?
1  increases
2  decreases
3  stays the same
WHY?
Mary J. Bojan
Chem 110
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Kinetic Molecular Theory can be used to
explain the trends in vapor pressure
As I.M. forces ↑, what happens to vapor pressure?
Mary J. Bojan
Chem 110
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There is a relationship between the vapor pressure
of a substance and its boiling point
boiling point: temperature at which v.p. = Pext
As Pext ↑, what happens to boiling point?
1.  increases
2.  decreases
3. stays the same
normal boiling point:
boiling point of a liquid when Pext = 1 atm
T at which the v.p. of a liquid is 1 atm.
Mary J. Bojan
Chem 110
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Phase Diagram: Plot of pressure vs.
temperature of a system showing the
boundaries between the phases.
Find these on a phase diagram
•  normal melting point
•  pressure dependence of melting point
•  normal boiling point
•  pressure dependence of boiling point
•  critical point
•  triple point
•  supercritical fluid
•  coexistence curves
What happens when the pressure and/or temperature of the
system is changed?
Mary J. Bojan
Chem 110
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General Phase diagram
Mary J. Bojan
Chem 110
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Phase diagrams of H2O and CO2
Mary J. Bojan
Chem 110
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