Properties of Gases Chapter 14

What do you know about gases? Particle DiagramsParticle Diagrams Ar(g), H 2 (g), CO 2 (g)Ar(g), H 2 (g), CO 2 (g) Gases take the shape and volume of their containerGases take the shape and volume of their container Gases flowGases flow Gases have low densitiesGases have low densities

Properties of Gases 1.Gases have mass. 2.Gases take the shape and volume of their container (Mickey Mouse balloon). 3.Gases are compressible. 4.Gases move through each other easily (perfume, skunks!). “Diffusion” 5.Gases exert pressure. Gas mixing

Gases take the shape of their container. Why don’t the balloons on the right expand until they pop?

What is pressure? Pressure = force per unit areaPressure = force per unit area P = ForceP = Force Area Force = mass X acceleration Would you rather have your foot stepped on by someone wearing sneakers or stilleto heels?

What is air pressure? The pressure exerted on us by the weight of the gases above our heads.The pressure exerted on us by the weight of the gases above our heads. At sea level, air pressure = 1 atmosphere.At sea level, air pressure = 1 atmosphere. Tiger GraphicTiger GraphicTiger GraphicTiger Graphic

Torricelli Discovered air pressure and invented the mercury barometer in 1643.Discovered air pressure and invented the mercury barometer in st person to propose correct explanation for wind.1 st person to propose correct explanation for wind.

V2, CCA: Mercury Barometer 1 & Mercury Barometer 2Mercury Barometer 1 Mercury Barometer 2

To report air pressure, we take a short-cut & report the height of the Hg column. At 1 atm, the column is in or 76 cm or 760 mm.

1.How does the downward pressure of the Hg in the column compare to the pressure of the atmosphere? 2.A water barometer has to be 13.6 times taller than a Hg barometer (D Hg = 13.6 g/ml) because … Large Barometers

How does the air pressure at the top of Mt. Whitney (14,494 ft) compare to the air pressure at John Jay? What about Death Valley (86 m below sea level)?

When you drink through a straw, you reduce the pressure in the straw. Why does the liquid in the cup go up the straw?Why does the liquid in the cup go up the straw? Could you drink a soda this way on the moon? Why or why not?Could you drink a soda this way on the moon? Why or why not?

Units of pressure = 1 atm 14.7 lb/in 2 U.S. pressure gauges14.7 lb/in 2 U.S. pressure gauges 29.9 In. HgU.S. weather*29.9 In. HgU.S. weather* kPa SI Units (Regents)101.3 kPa SI Units (Regents) (kPa = kilopascal) BarsPhysics & Astronomy1.013 BarsPhysics & Astronomy 760 Torr or 760 mm Hg CHEMISTRY760 Torr or 760 mm Hg CHEMISTRY

What causes the pressure of a gas in a closed container? Impacts of gas molecules with the walls of the container. Anything that increases the number of impacts per second or the force of each impact increases the pressure. Microscopic View

Light molecules move faster and hit the walls more often. Heavy molecules hit the walls with greater force. These 2 effects exactly balance out. **Gas pressure doesn’t depend on the identity of the gas.**

Pressure Depends on 1) the concentration or # of gas molecules per unit volume and 2) the temperature.

How fast do the molecules in the air move? Depends on the mass.Depends on the mass. Light molecules are faster than heavy molecules at the same temperature.Light molecules are faster than heavy molecules at the same temperature. Temperature = measure of the ave. translational K.E. of the particles of a system.Temperature = measure of the ave. translational K.E. of the particles of a system.

Molecular Speeds at 298 K H X 10 5 cm/secH X 10 5 cm/sec He1.36 X 10 5 cm/secHe1.36 X 10 5 cm/sec O X 10 4 cm/secO X 10 4 cm/sec Ar4.31 X 10 4 cm/secAr4.31 X 10 4 cm/sec Xe2.38 X 10 4 cm/secXe2.38 X 10 4 cm/sec cm X 1 in X 1 ft X 1 mile X 3600 sec = sec 2.54 cm 12 in 5280 ft 1 hour 1080 miles per hour

Average Speeds Boltzmann AppletBoltzmann AppletBoltzmann AppletBoltzmann Applet Maxwell-Boltzmann Velocity DistributionMaxwell-Boltzmann Velocity DistributionMaxwell-Boltzmann Velocity DistributionMaxwell-Boltzmann Velocity Distribution

Molecular Speed vs. Temperature

Pressure – Microscopic View Gas molecules hit the walls of their container.Gas molecules hit the walls of their container. Pressure depends onPressure depends on – Number of impacts per unit time – Force of each impact

Pressure – Macroscopic View Pressure depends on how many gas molecules per unit volume and on the temperature.Pressure depends on how many gas molecules per unit volume and on the temperature. The same amount of gas exerts different pressure at different temperatures (tires).The same amount of gas exerts different pressure at different temperatures (tires).

Describing a Gas Phase System Need 4 variables to completely describe a gas-phase system from the macroscopic or lab view. 1.Pressure 2.Volume 3.Temperature 4.Amount of gas in moles.

Exit Ticket What are 5 common properties of all gases?What are 5 common properties of all gases? Draw a particle diagram of the gas phase.Draw a particle diagram of the gas phase. Use this picture to explain at least 2 of the properties of gases.Use this picture to explain at least 2 of the properties of gases. Bonus: At 0 o C, a He atom is moving at 1200 meters/sec. How fast is this in miles per hour? Show all work! (Use the factor-label method.)Bonus: At 0 o C, a He atom is moving at 1200 meters/sec. How fast is this in miles per hour? Show all work! (Use the factor-label method.)