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
1
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
2
Kinetic Molecular Description of
Liquids and Solids
gas:
Kinetic energy >> intermolecular forces
Liquid: Kinetic energy ≅ intermolecular forces
Solid:
Kinetic energy << intermolecular forces
Kinetic Energy ∝ T
solid
Mary J. Bojan
Heating: T ↑, KE ↑
→
liquid
Chem 110
→
gas
3
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”.
Mary J. Bojan
Chem 110
4
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
5
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
6
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
7
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
8
CALORIMETRY
Experimental measure of heat flow
q = C m ΔT
q = heat flow
C = specific heat
(heat capacity per gram)
m = mass
ΔT= Tfinal – Tinitial
For H2O:
C = 4.184 J/g °C
Molar heat capacity
= 75.2 J/mole °C
q = C m ΔT
= amount of heat given off (−) or absorbed (+) as temperature changes
Mary J. Bojan
Chem 110
9
CALORIMETRY
ΔH = Heat change
= 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
10
HEATING THE SAMPLE
As heat is added, two types of changes take place:
1. Within single phase (in red) changes are continuous
q = n CpΔT
Since T ↑: kinetic energy ↑
Energy ↑, molecular motion ↑
separation between molecules ↑
molecular attractions ↓, and order ↓.
Mary J. Bojan
Chem 110
11
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
12
EXAMPLE PROBLEM
2 moles of ice at −25 °C are heated to 125 °C.
How much energy is needed?
Cp(ice)
= 37.6 J/mol·K
Cp(water) = 75.3 J/mol·K
Cp(steam) = 33.1 J/mol·K
Mary J. Bojan
ΔHfusion = 6.02 kJ/mol
ΔHvapor = 40.67 kJ/mol
Chem 110
13
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
14
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
15
As T ↑, what happens to vapor pressure?
1  increases
2  decreases
3  stays the same
WHY?
Mary J. Bojan
Chem 110
16
Using Kinetic Molecular Theory
As I.M. forces ↑, what happens to vapor pressure?
Mary J. Bojan
Chem 110
17
Vapor pressure and boiling point
boiling point: T 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
18
Phase Diagrams
Plot of pressure vs. temperature of the 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
19
Phase diagrams
Mary J. Bojan
Chem 110
20
Phase diagrams of H2O and CO2
Mary J. Bojan
Chem 110
21