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