Vapor Pressure

Evaporation H 2 O(g) molecules (water vapor) H 2 O(l) molecules

Vapor Pressure more “sticky” less likely to vaporize In general: LOW v.p. not very “sticky” more likely to vaporize 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.

TEMPERATURE ( o C) 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. = 78 o C b.p. = 100 o C atmospheric pressure is kPa

Vapor Pressure o C78.4 o C100 o C chloroform ethyl alcohol water Pressure (KPa) Temperature ( o C) 101.3

Water Molecules in Liquid and Steam

Microscopic view of a liquid near its surface The high energy molecules escape the surface.

Behavior of a liquid in a closed container

Water rapidly boiling on a stove

Copyright © 2006 Pearson Benjamin Cummings. All rights reserved.

Formation of a bubble is opposed by the pressure of the atmosphere Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 452

Boiling Point and Pressure

The Gas Laws The density of a gas decreases as its temperature increases.

The Earth’s Atmosphere 78% Nitrogen 20% Oxygen 1% Argon 0.03% CO 2 Trace Elements ‘The Precious Envelope’ Helium Nitrogen Oxygen Water vapor Argon Neon Carbon dioxide Air

Kinetic Molecular Theory

Postulates of the Kinetic Molecular Theory of Gases 1.Gases consist of tiny particles (atoms or molecules) 2.These particles are so small, compared with the distances between them, that the volume (size) of the individual particles can be assumed to be negligible (zero). 3. The particles are in constant random motion, colliding with the walls of the container. These collisions with the walls cause the pressure exerted by the gas. 4. The particles are assumed not to attract or to repel each other. 5. The average kinetic energy of the gas particles is directly proportional to the Kelvin temperature of the gas

Collisions of Gas Particles

Kinetic Molecular Theory (KMT) 1.…are so small that they are assumed to have zero volume 2.…are in constant, straight-line motion 3.…experience elastic collisions in which no energy is lost 4.…have no attractive or repulsive forces toward each other 5.…have an average kinetic energy (KE) that is proportional to the absolute temp. of gas (i.e., Kelvin temp.) AS TEMP., KE  explains why gases behave as they do  deals w /“ideal” gas particles…

Elastic vs. Inelastic Collisions 8 3

Real Gases Particles in a REAL gas… –have their own volume –attract each other Gas behavior is most ideal… –at low pressures –at high temperatures –in nonpolar atoms/molecules Courtesy Christy Johannesson

Characteristics of Gases Gases expand to fill any container. –random motion, no attraction Gases are fluids (like liquids). –no attraction Gases have very low densities. –no volume = lots of empty space Courtesy Christy Johannesson

Characteristics of Gases Gases can be compressed. –no volume = lots of empty space Gases undergo diffusion & effusion. –random motion Courtesy Christy Johannesson

high pressureslow temps Theory “works,” except at high pressures and low temps. **Two gases w /same # of particles and at same temp. and pressure have the same kinetic energy. KE is related to mass and velocity (KE = ½ m v 2 ) To keep same KE, as m, v must OR as m, v must

Particle-Velocity Distribution (various gases, same T and P) # of particles Velocity of particles (m/s) H2H2 N2N2 CO 2 (SLOW)(FAST) More massive gas particles are slower than less massive gas particles (on average).

Particle-Velocity Distribution (same gas, same P, various T) # of particles Velocity of particles (m/s) O 10 o C (SLOW)(FAST) O 50 o C O 100 o C

Hot vs. Cold Tea Kinetic energy Many molecules have an intermediate kinetic energy Few molecules have a very high kinetic energy Low temperature (iced tea) High temperature (hot tea) Percent of molecules

Pressure Is caused by the collisions of molecules with the walls of a container is equal to force/unit area SI units = Newton/meter 2 = 1 Pascal (Pa) 1 standard atmosphere = 101,325 Pa 1 standard atmosphere = 1 atm = 760 mm Hg = 760 torr

Pressure F A = P A P = F Pressure occurs when a force is dispersed over a given surface area. If F acts over a large area… But if F acts over a small area… P = FAFA

Pressure Which shoes create the most pressure? Courtesy Christy Johannesson Bed of Nails demonstration

Pressure KEY UNITS AT SEA LEVEL kPa (kilopascal) 1 atm 760 mm Hg 760 torr 14.7 psi Courtesy Christy Johannesson Sea level

= 14.7 lb/in 2 (1.44 x 10 6 in 2 ) A = [100 ft ] 2 1 ft 100 ft. Find force of air pressure acting on a baseball field tarp… At sea level, air pressure is standard pressure: 1 atm, kPa, 760 mm Hg, 14.7 lb/in 2 12 in = 1.44 x 10 6 in 2 F = 2 x 10 7 lb lb. 1 ton = 10,000 tons F = P A= 2 x 10 7 lb.

Force = Pressure / Area Air pressure = 14.7 psi (pounds per square inch) Area = length x width x height Force holding down ruler = (14.7 psi) (250 in sq) Force = 10 tons (20,000 pounds) To remove the tarp at Wrigley field, workers must first shake the tarp to get air below the tarp to create lift to balance the air pressure above the tarp.

Key: Gases exert pressure in all directions. Atmospheric pressure changes with altitude: as altitude, pressure Demo: graduated cylinder holding column of water upside down with piece of paper

Pressure and Balloons A B = pressure exerted ON balloon A = pressure exerted BY balloon B When balloon is being filled: P A > P B When balloon is filled and tied: P A = P B When balloon deflates: P A < P B

The first device for measuring atmospheric pressure was developed by Evangelista Torricelli during the 17 th century. The device was called a “barometer” Baro = weight Meter = measure Evangelista Torricelli Measuring Pressure

How to Measure Pressure Aneroid Barometer Courtesy Christy Johannesson Barometer –measures atmospheric pressure vacuum P atm P Hg

barometer barometer: device to measure air pressure air pressure vacuum mercury (Hg)

Barometer Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 401 Empty space (a vacuum) Hg Weight of the mercury in the column Weight of the atmosphere (atmospheric pressure)

Barometer Mercury filled 760 mm = 1 atm Water filled mm = 1 atm The barometer measures air pressure Water column (34.0 ft. high or 10.4 m) Atmospheric pressure Mercury column (30.0 in. high or 76 cm )

Barometers Mount Everest Sea level On top of Mount Everest Sea level

Barometer Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 451 (a) (b)(c)

STP Standard Temperature & Pressure 0°C 273 K 1 atm kPa - OR - STP Courtesy Christy Johannesson

Temperature ºF ºC K K = ºC Always use absolute temperature (Kelvin) when working with gases. Courtesy Christy Johannesson

Convert 25 o C to Kelvin. K = o C How many mm Hg is kPa? How many kPa is 1.37 atm? 25 o C K = X kPa = 1.37 atm kPa 1 atm = kPa X mm Hg = kPa 760 mm Hg kPa = 1737 mm Hg

Manometer –measures contained gas pressure U-tube ManometerBourdon-tube gauge Courtesy Christy Johannesson

PaPa CONFINED GAS AIR PRESSURE Hg HEIGHT DIFFERENCE manometer: manometer: measures the pressure of a confined gas higher pressure

CONFINED GAS AIR PRESSURE Hg HEIGHT DIFFERENCE manometer: manometer: measures the pressure of a confined gas

101.3 kPa X atm 96.5 kPa 233 mm Hg Atmospheric pressure is 96.5 kPa; mercury height difference is 233 mm. Find confined gas pressure, in atm. SMALL + HEIGHT = BIG atm atm = X atm X = 1.26 atm 96.5 kPa 1 atm mm Hg 760 mm Hg 1 atm = X atm 96.5 kPa mm Hg = X atm