READING FOR TUESDAY: Chapter 18 sections 1 – 3 HOMEWORK – DUE THURSDAY 12/3/15
HW-BW 12.2 CH 12 #’s 12, 13, 17-25 (all), 28-31 (all) HOMEWORK – DUE TUESDAY 12/8/15
HW-BW 13 CH 13 #’s 9, 11, 12, 65, 67, 70, 71, 73, 75, 77, 84, 92, 93, 99-103 (all), 107, 126
Lab Wednesday/Thursday – EXP 15 continued Monday/Tuesday – EXP 16
The Molecular DanceMolecules in the liquid are constantly in motion
vibrational, and limited rotational and translationalThe average kinetic energy is proportional to the
temperatureHowever, some molecules have more kinetic energy than
the average, and others have less
If these high energy molecules are at the surface, they may have enough energy to overcome the attractive forces therefore – the larger the surface area,
the faster the rate of evaporation This will allow them to escape the
liquid and become a vapor
Vapor Pressure
Some molecules of the vapor will lose energy through molecular collisions and will get captured back into the liquid when they collide with it
Also some may stick and gather together to form droplets of liquidparticularly on surrounding surfaces
We call this process condensation
Vapor Pressure
Evaporation vs. Condensation Vaporization and condensation are opposite processes In an open container, the vapor molecules generally spread out
faster than they can condense So rate of vaporization is greater than the rate of condensation, and there
is a net loss of liquid In a closed container, the vapor is not allowed to spread out
indefinitely So in a closed container the rates of vaporization and condensation will
be equal at some point
Dynamic Equilibrium In a closed container, once the rates of vaporization and
condensation are equal, the total amount of vapor and liquid will not change
Evaporation and condensation are still occurring, but because they are opposite processes, there is no net gain or loss of either vapor or liquid
When two opposite processes reach the same rate so that there is no gain or loss of material, we call it a dynamic equilibrium this does not mean there are equal amounts of vapor and liquid – it
means that they are changing by equal amounts
Dynamic Equilibrium
Vapor–Liquid Dynamic Equilibrium If the volume of the chamber is increased, it will decrease the
pressure of the vapor inside the chamber fewer vapor molecules in a given volume, causing the rate of
condensation to slow
For a period of time, the rate of vaporization will be faster than the rate of condensation, and the amount of vapor will increase
Eventually enough vapor accumulates so that the rate of the condensation increases to the point where it is once again as fast as evaporation equilibrium is reestablished the vapor pressure will be the same
as it was before
The weaker the attractive forces between molecules, the less energy they will need to vaporize weaker attractive forces means that more energy will need to be
removed from the vapor molecules before they can condense Results in more molecules in the vapor phase, and a liquid
that evaporates faster – the weaker the attractive forces, the faster the rate of evaporation
Liquids that evaporate easily are said to be volatile e.g., gasoline, fingernail polish remover
Liquids that do not evaporate easily are called nonvolatile e.g., motor oil
The higher the vapor pressure, the more volatile the liquid
Vapor Pressure
Distribution of Thermal Energy Only a small fraction of the molecules in a liquid have
enough energy to escape As the temperature increases, the fraction of the molecules with
“escape energy” increases The higher the temperature, the faster the rate of
evaporation
Heat of Vaporization The amount of heat energy required to vaporize one mole of
the liquid is called the heat of vaporization, DHvap sometimes called the enthalpy of vaporization
Always endothermic, therefore DHvap is + Somewhat temperature dependent
· DHcondensation = −DHvaporization
Boiling PointWhen the temperature of a liquid reaches a point
where its vapor pressure is the same as the external pressure, vapor bubbles can form anywhere in the liquid
This phenomenon is what is called boiling and the temperature at which the vapor pressure = external pressure is the boiling point
The normal boiling point is the temperature at which the vapor pressure of the liquid = 1 atm
The lower the external pressure, the lower the boiling point of the liquid
Boiling Point
Heating Curve of Water
Energy put in
Temperature
Heating Curve of Water
Solid Liquid
Heating Curve of Water
Energy put in
Temperature
melting
melting point
Heating Curve of Water
Liquid
Heating Curve of Water
Liquid
Heating Curve of Water
Energy put in
Temperature
melting
melting point
Heating Curve of Water
LiquidGas
Heating Curve of Water
Energy put in
Temperature
melting
boiling
boiling point
melting point
Heating Curve of Water
Energy put inm
elting
boilin
g
S L G
heat of vaporization:2260 J
1 gheat of fusion:334 J
1 gspecific heat water:
4.184 J
1 g 1 C
specific heat steam:
1.998 J
1 g 1 C
specific heat ice:2.11 J
1 g 1 C
Heating Curve of Water
Clausius–Clapeyron Equation The Clausius-Clapeyron equation can be used with just two
measurements of vapor pressure and temperature Can also be used to predict the vapor pressure if you know the heat
of vaporization and the normal boiling point remember: the vapor pressure at the normal boiling point is 760 torr
Melting = FusionAs a solid is heated, its temperature rises and the
molecules vibrate more vigorouslyOnce the temperature reaches the melting point,
the molecules have sufficient energy to overcome some of the attractions that hold them in position and the solid melts (or fuses)
The opposite of melting is freezing
Heat of Fusion The amount of heat energy required to melt one mole of the
solid is called the Heat of Fusion, DHfus sometimes called the enthalpy of fusion
Always endothermic, therefore DHfus is + Somewhat temperature dependent
· Generally much less than DHvap
· DHsublimation = DHfusion + DHvaporization
Phase DiagramsPhase diagrams describe the different states and
state changes that occur at various temperature/pressure conditions
Regions represent statesLines represent state changes
liquid/gas line is vapor pressure curveboth states exist simultaneouslycritical point is the furthest point on the vapor pressure
curveTriple point is the temperature/pressure condition
where all three states exist simultaneously
Phase DiagramsP
ress
ure
Temperature
vaporization
condensation
criticalpoint
triplepoint
Solid Liquid
Gas
1 atm
normalmelting pt.
normalboiling pt.
Fusion Curve
Vapor PressureCurve
SublimationCurve
melting
freezing
sublimation
deposition
Phase Diagram of Water
Temperature
Pre
ssur
e
criticalpoint
374.1 °C217.7 atm
triplepoint
Ice Water
Steam
1 atm
normalboiling pt.
100 °C
normalmelting pt.
0 °C
0.01 °C0.006 atm
Phase Diagram of CO2P
ress
ure
Temperature
criticalpoint
31.0 °C72.9 atm
triplepoint
Solid Liquid
Gas1 atm
-56.7 °C5.1 atm
normalsublimation pt.
-78.5 °C
• 20.0 °C, 72.9 atm liquid
• −56.7 °C, 5.1 atm solid, liquid, gas
• 10.0 °C, 1.0 atm gas
• −78.5 °C, 1.0 atm solid, gas
• 50.0 °C, 80.0 atm scf
Consider the phase diagram of CO2 shown. What phase(s) is/are present at each of the following conditions?
• 20.0 °C, 72.9 atm
• −56.7 °C, 5.1 atm
• 10.0 °C, 1.0 atm
• −78.5 °C, 1.0 atm
• 50.0 °C, 80.0 atm
Phase Diagram