Properties of GasesProperties of Gases

Chapter 14Chapter 14

What do you know about What do you know about gases?gases?

•Particle DiagramsParticle Diagrams

•Ar(g), HAr(g), H22(g), CO(g), CO22(g)(g)

•Gases take the shape and Gases take the shape and volume of their containervolume of their container

•Gases flowGases flow•Gases have low densitiesGases have low densities

Properties of GasesProperties of Gases

1.1. Gases have mass.Gases have mass.

2.2. Gases take the shape and volume of Gases take the shape and volume of their container (Mickey Mouse their container (Mickey Mouse balloon).balloon).

3.3. Gases are compressible.Gases are compressible.

4.4. Gases move through each other Gases move through each other easily (perfume, skunks!). easily (perfume, skunks!). “Diffusion”“Diffusion”

5.5. Gases exert pressure.Gases exert pressure.Gas mixing

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

What is pressure?What is pressure?

•Pressure = force per unit areaPressure = force per unit area

•P = P = ForceForceAreaArea

Force = mass X accelerationForce = mass X acceleration

Would you rather have your foot stepped on Would you rather have your foot stepped on by someone wearing sneakers or stilleto by someone wearing sneakers or stilleto heels?heels?

What is air pressure?What is air pressure?

•The pressure exerted on us The pressure exerted on us by the weight of the gases by the weight of the gases above our heads.above our heads.

•At sea level, air pressure = 1 At sea level, air pressure = 1 atmosphere.atmosphere.

•Tiger GraphicTiger Graphic

TorricelliTorricelli

• Discovered air pressure and Discovered air pressure and invented the mercury barometer invented the mercury barometer in in 16431643..

• 11stst person to propose correct person to propose correct explanation for wind.explanation for wind.

V2, CCA: Mercury 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 29.92 in or 76 cm or 760 mm.

1.1. How does the downward How does the downward pressure of the Hg in the pressure of the Hg in the column compare to the column compare to the pressure of the atmosphere?pressure of the atmosphere?

2.2. A water barometer has to be A water barometer has to be 13.6 times taller than a Hg 13.6 times taller than a Hg barometer (Dbarometer (DHgHg = 13.6 g/ml) = 13.6 g/ml) because …because …Large Barometers

How does the air pressure at How does the air pressure at the top of Mt. Whitney the top of Mt. Whitney (14,494 ft) compare to the (14,494 ft) compare to the air pressure at John Jay?air pressure at John Jay?

What about Death Valley (86 What about Death Valley (86 m below sea level)?m below sea level)?

When you drink through a When you drink through a straw, you reduce the pressure straw, you reduce the pressure in the straw.in the straw.• Why does the liquid in the cup go up Why does the liquid in the cup go up

the straw?the straw?

• Could you drink a soda this way on Could you drink a soda this way on the moon? Why or why not?the moon? Why or why not?

Units of pressure = 1 atmUnits of pressure = 1 atm

• 14.7 lb/in14.7 lb/in22 U.S. pressure gaugesU.S. pressure gauges• 29.9 In. Hg29.9 In. Hg U.S. weather*U.S. weather*• 101.3 kPa 101.3 kPa SI Units (Regents)SI Units (Regents)

(kPa = kilopascal)(kPa = kilopascal)

• 1.013 Bars1.013 Bars Physics & AstronomyPhysics & Astronomy• 760 Torr or 760 mm Hg 760 Torr or 760 mm Hg CHEMISTRYCHEMISTRY

What causes the pressure of a What causes the pressure of a gas in a closed container?gas in a closed container?

Impacts of gas Impacts of gas molecules with the molecules with the walls of the walls of the container.container.

Anything that Anything that increases the increases the number of impacts number of impacts per second or the per second or the force of each impact force of each impact increases the increases the pressure.pressure.

Microscopic ViewMicroscopic View

Light molecules move Light molecules move faster and hit the faster and hit the walls more often.walls more often.

Heavy molecules hit Heavy molecules hit the walls with greater the walls with greater force.force.

These 2 effects These 2 effects exactly balance out.exactly balance out.

**Gas pressure doesn’t depend on **Gas pressure doesn’t depend on the identity of the gas.**the identity of the gas.**

Pressure Depends on Pressure Depends on

1) the concentration or # of gas 1) the concentration or # of gas molecules per unit volumemolecules per unit volume

andand

2) the temperature.2) the temperature.

How fast do the molecules in How fast do the molecules in the air move?the air move?

• Depends on the mass.Depends on the mass.• Light molecules are faster than Light molecules are faster than

heavy molecules at the same heavy molecules at the same temperature.temperature.

• Temperature = measure of the Temperature = measure of the ave. translational K.E. of the ave. translational K.E. of the particles of a system.particles of a system.

Molecular Speeds at 298 KMolecular Speeds at 298 K

• HH22 1.93 X 101.93 X 1055 cm/sec cm/sec

• HeHe 1.36 X 101.36 X 1055 cm/sec cm/sec

• OO22 4.82 X 104.82 X 1044 cm/sec cm/sec

• ArAr 4.31 X 104.31 X 1044 cm/sec cm/sec• XeXe 2.38 X 102.38 X 1044 cm/sec cm/sec

48200 48200 cmcm X X 1 in1 in X X 1 ft1 ft X X 1 mile1 mile X X 3600 3600 secsec = = sese

cc2.54 2.54 cmcm

12 in12 in 5280 ft5280 ft 1 hour1 hour

1080 miles per 1080 miles per hourhour

Average SpeedsAverage Speeds

• Boltzmann Applet

• Maxwell-Boltzmann Velocity Distribution

Molecular Speed vs. Temperature

Pressure – Microscopic ViewPressure – Microscopic View

• Gas molecules hit the walls of Gas molecules hit the walls of their container.their container.

• Pressure depends onPressure depends on– Number of impacts per unit Number of impacts per unit

timetime– Force of each impactForce of each impact

Pressure – Macroscopic ViewPressure – Macroscopic View

• Pressure depends on Pressure depends on how manyhow many gas molecules per unit volume gas molecules per unit volume and on the and on the temperaturetemperature..

• The same amount of gas exerts The same amount of gas exerts different pressure at different different pressure at different temperatures (tires).temperatures (tires).

Describing a Gas Phase SystemDescribing a Gas Phase System

Need 4 variables to completely Need 4 variables to completely describe a gas-phase system describe a gas-phase system from the macroscopic or lab view.from the macroscopic or lab view.

1.1. PressurePressure

2.2. VolumeVolume

3.3. TemperatureTemperature

4.4. Amount of gas in moles.Amount of gas in moles.

Exit TicketExit Ticket• What are 5 common properties of all What are 5 common properties of all

gases?gases?• Draw a particle diagram of the gas Draw a particle diagram of the gas

phase.phase.• Use this picture to explain at least 2 Use this picture to explain at least 2

of the properties of gases.of the properties of gases.• Bonus: At 0Bonus: At 0ooC, a He atom is moving C, a He atom is moving

at 1200 meters/sec. How fast is this at 1200 meters/sec. How fast is this in miles per hour? Show all work! in miles per hour? Show all work! (Use the factor-label method.)(Use the factor-label method.)