Evaporation

H2O(g)molecules

(water vapor)

H2O(l)molecules

Evaporation

H2O(g)molecules

(water vapor)

H2O(l)molecules

Evaporation

H2O(g)molecules

(water vapor)

H2O(l)molecules

How Vapor Pressure is Measured

1 atm = 760 mm Hg

760 mm + 120 mm = 880 mm Hg

Animation by Raymond ChangAll rights reserved

Manometer Atmospheric Pressure

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 401

Manometer A

BIG = small + height

________ = small + __________

760 mm Hg

h = 120 mm

760 mm 120 mm

Small = 640 mm Hg

?

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 401

Manometer B

BIG = small + height

BIG = ________ + _________ 760 mm 120 mm

BIG = 880 mm Hg

760 mm Hg

h = 120 mm

?

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 401

The Manometer and Vapor Pressure

Barometer & Manometer

atmospheric pressure = 101.3 kPa

atmospheric pressure = 100.4 kPa

atmospheric pressure = 101.7 kPa

confinedgas

confinedgas

confinedgas

600 mm

200 mm325 mm

150 mm 100 mm

500 mm

750 mm

(a) (b) (c) (d)

Pressure and TemperatureSTST

PP (Standard Temperature and Pressure)

standard temperature standard pressure1 atm101.3 kPa760 mm Hg

273 K0oC

Equations / Conversion Factors:

K = oC + 273oC = K – 273

1 atm = 101.3 kPa = 760 mm Hg

Convert 25oC to Kelvin.

K = oC + 273

How many mm Hg is 231.5 kPa?

How many kPa is 1.37 atm?

25oC + 273 298 K=

X kPa = 1.37 atm101.3 kPa

1 atm= 138.8 kPa

X mm Hg = 231.5 kPa760 mm Hg

101.3 kPa= 1737 mm Hg

Pa

CONFINEDGAS

AIRPRESSURE

Hg HEIGHTDIFFERENCE

manometermanometer:: measures the

pressure of a confined gas

higherpressure

101.3 kPa

Atmospheric pressure is 96.5 kPa;mercury height difference is 233 mm.

Find confined gas pressure, in atm.

SMALL + HEIGHT = BIGSMALL + HEIGHT = BIG

0.953 atm + 0.307 atm = X atm

X = 1.26 atm

96.5 kPa1 atm

+ 233 mm Hg760 mm Hg

1 atm= X atm

96.5 kPa + 233 mm Hg = X atm

233 mm Hg

96.5 kPa

X atm

BIG

small

1.26 atm

Vapor Pressure

more“sticky”

less likely tovaporize

In general:LOW v.p.

not very“sticky”

more likely tovaporize

In general:HIGH v.p.

measure of the tendency for liquid particles to enter gas phase at a given temp.

a measure of “stickiness” of liquid particles to each other

NOT all liquids have same v.p. at same temp.

0 20 40 60 80 1000

20

40

60

80

100

TEMPERATURE (oC)

PRESSURE (kPa)

CHLOROFORM

ETHANOL

WATER

Volatile substances evaporate easily (have high v.p.’s).

BOILING when vapor pressure = confining pressure (usually from atmosphere)

b.p. = 78oC

b.p. = 100oC

atmospheric pressure is 101.3 kPa

Vapor Pressure

93.3

80.0

66.6

53.3

40.0

26.7

13.3

0 10 20 30 40 50 60 70 80 90 100

61.3oC 78.4oC 100oC

chlo

rofo

rm

ethy

l alc

ohol

water

Pre

ssur

e (K

Pa)

Temperature (oC)

101.3

ETHANOL

WATER

AIR PRESSURE(~100 kPa)

VAPORPRESSURE

(~5 kPa)

VAPORPRESSURE

(~10 kPa)

BOILING when vapor pressure = confining pressure (usually from atmosphere)

At sea level and 20oC…

ETHANOL

WATER

NETPRESSURE

(~95 kPa)

NETPRESSURE

(~90 kPa)

Water Molecules in Liquid and

Steam

Microscopic view of a liquid near its surface

The high energymolecules escapethe surface.

Behavior of a liquid in a closed container

Water rapidly boiling on a stove

Copyright © 2006 Pearson Benjamin Cummings. All rights reserved.

Pressure Cooker

120oC

Formation of a bubble is opposed by the pressure of the atmosphere

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 452

Vapor Pressure

93.3

80.0

66.6

53.3

40.0

26.7

13.3

0 10 20 30 40 50 60 70 80 90 100

61.3oC 78.4oC 100oC

chlo

rofo

rm

ethy

l alc

ohol

water

Pre

ssur

e (K

Pa)

Temperature (oC)

101.3

Boiling Point and Pressure

Heating / Cooling Curve of Water

140

120

100

80

60

40

20

0

-20

Tem

pera

ture

(oC

)

Heat added at a constant rate

liquid water

water and steam

steam

ice andwater

ice

Heatin

g

Cooling

Gas Collected Over Water

Measuring the Vapor Pressure of a Liquid

Davis, Metcalfe, Williams, Castka, Modern Chemistry, 1999, page 376

Gas Mixtures and Dalton’s

Law

Gases Dissolved in Liquids