Vapor Pressure
Emily Humpton, Ben Klein, Julia Kuzel
Vapor Pressure- the constant pressure
exerted by a vapor (gaseous state of a
typically solid or liquid substance) in
equilibrium with its liquid or solid
phase
Dynamic Equilibrium- when two opposing processes occur
at the same rate (ex. Condensation and evaporation).
The vapor pressure of a liquid is the pressure exerted by its
vapor when the liquid and vapor states are in dynamic
equilibrium
When this occurs the vapor is said to be saturated
Volatility, Vapor Pressure and Temperature
Dynamic equilibrium does not occur in an open container,
therefore open containers do not have vapor pressure
High vapor pressure evaporates more quickly and is volatile
(gasoline)
Low vapor pressure evaporates more slowly and is not
volatile (motor oil)
As temperature increases, vapor pressure increases
Vapor pressure and boiling point
Liquids boil when the external and vapor pressures are
equal
Temperature of boiling point increases with increasing
pressure
Normal Boiling Point-boiling point at 1 atm
http://hyperphysics.phy-astr.gsu.edu/Hbase/Kinetic/vappre.html
The varying boiling points depending on
pressure are used in real life for pressure
cookers and central cooling systems
http://media3.washingtonpost.com/wpdyn/content/graphic/2007/02/20/GR2007022000797.gif
Clausius-Clapeyron Equation
Questions
11.37 Explain how each of the following affects the vapor pressure of a
liquid:
A) Volume of liquid No Affect
B) Surface area No Affect
C) Intermolecular attractive forces When forces increase vp decreases
D) Temperature Vp increases with temp. increase
11.38 A liquid that has an equilibrium vapor pressure of 130mm Hg at
298 K is placed into a 1-L vessel. What is pressure difference shown on
manometer, and what is composition of the gas in the vessel if:
A) 200 mL of liquid is added and frozen at the bottom, then the gas is
evacuated. Liquid melts to 298 K Pressure is 130 mm Hg and gas is all
evaporated liquid
B) 200 mL of liquid is added at atmospheric pressure, and after a few
minutes the vessels is closed off 760mm Hg gas is from part A
C) A few mL of the liquid is introduced into the vessel at 298 K while it
has 1atm pressure of air in it, no air escapes. After some time a few
drops liquid remain Still 890mm Hg some gas
added from liquid
Questions
11.39 Place the following substances in order of increasing volatility:
CH4, CBr4, CH2Cl2, CH3Cl, CHBr3, CH2Br2
CBr4 < CHBr3 < CH2Br2 < CH2Cl2 < CH3Cl < CH4
11.40 PCl3 and AsCl3 are similar substances, with similar geometries
and bonding modes.
a) Which of these two substances would you expect to be the
volatile at room temperature?
more
b) Which substance would you expect to have the higher boiling point?
c) In which substance would the kinetic energies of the molecules be
greater than 40º C, a temperature well below either
substance’s
boiling point?
d) In which substance would you expect the intermolecular forces to be
larger?
Questions
11.41
a) Two pans of boiling water are on different burners of a stove. One
pan of water is boiling vigorously, while the other is boiling gently. What
can be said about the temperature of the water in the two pans?
The temperature of the water in the two pans is the same.
b) A large container of water and a small one are at the same
temperature. What can be said about the relative vapor pressures of the
water in the two containers? At the same temperature, the vapor pressures
of water in the two containers is the same
11.42 Explain the following observations:
a) Water evaporates more quickly on a hot, dry day than on a hot, humid
On a dry day, the external pressure is lower,
day.
so the water can more easily evaporate
b) It takes longer to cook hard-boiled eggs at high altitudes than low
altitudes.
At higher altitudes, there is less pressure, and a low
vapor pressure allows water to boil at a lower pressure.
Questions
11.43
a) Use the vapor-pressure curve in Figure 11.22 to estimate the boiling
point of diethyl ether at 400 torr. Approximately 17º C
b) Use the vapor-pressure table in Appendix B to determine the boiling
point of water when the external pressure is 25 torr.
Approximately 28º C
11.44
a) Suppose the pressure inside a pressure cooker reaches 1.2 atm. By
using the vapor-pressure table in Appendix B, estimate the temperature
at which water will boil in this cooker. 105º C
b) Use the vapor-pressure curve in Figure 11.22 to estimate the
external pressure under which ethyl alcohol will boil at 70º C.
500 torr
Questions
11.45 Mt. Denali in Alaska is the tallest peak in the United States (20,320 ft).
a) If the barometric pressure at the top of the mountain is 340 torr, at what
temperature will water boil there? Refer to Appendix B. 75º C
b) If the temperature at the top is 12º C, would a container of diethyl ether
experience a pressure greater than the local atmospheric pressure?
The vapor pressure is approximately 325 torr, which
is less than the atmospheric pressure of 340 torr
11.46 Reno, Nevada, is about 4500 ft above sea level.
a) If the barometric pressure is 680 mm Hg in Reno, at what temperature will
water boil? Refer to Appendix B. 97º C
b) What can you say about the average kinetic energies of the water molecules
at the boiling point in Reno as compared with those at the boiling point in
Chicago, where the barometric pressure is 752 mm Hg? If you believe the
average kinetic energies are different, explain how it is that boiling of water can
occur at two different average kinetic energies of the water molecules.
Works Cited
Brown, Theodore E., H. Eugene LeMay, and Bruce Edward Bursten.
Chemistry : The Central Science. Upper Saddle River: Prentice Hall,
2002.
Nave, Rod. "Vapor Pressure." Hyperphysics. Aug. 2000. Georgia State University.
3 Dec. 2008 <http://hyperphysics.phyastr.gsu.edu/hbase/kinetic/vappre.html>.
"Vapor Pressure." Chem Purdue. 2004. Purdue University. 3 Dec. 2008
<http://www.chem.purdue.edu/gchelp/liquids/vpress.html>.