Introduction: Introduction: Properties of GasesProperties of Gases

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 shape and volume Gases take shape and volume of their containerof their container

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

Properties of GasesProperties of Gases

1.1. Gases have massGases have mass

2.2. Gases take shape & volume of Gases take shape & volume of their container their container

3.3. Gases are compressibleGases are compressible

4.4. Gases move through each other Gases move through each other easilyeasily

“ “Diffusion”Diffusion”perfume, skunks!perfume, skunks!

5.5. Gases exert pressureGases exert pressure

Gases take shape of containerGases take shape of container

Why don’t these Why don’t these balloons balloons keep expanding until keep expanding until they pop?they pop?

What is pressure?What is pressure?

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

AreaArea

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?

•pressure exerted on us by pressure exerted on us by weight of gases above our weight of gases above our heads (& all around our body)heads (& all around our body)

•at sea level:at sea level: air pressure = 1 atmosphereair pressure = 1 atmosphere

TorricelliTorricelli

• discovered air pressure discovered air pressure • invented mercury barometer in invented mercury barometer in

16431643

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

Mercury Barometer 1

To report air pressure: take short-cut & report height of Hg column

at 1 atm column is: 29.92 in Hg76 cm Hg760 mm Hg

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?

water barometer has to be 13.6 water barometer has to be 13.6 times taller than Hg barometer times taller than Hg barometer (D(DHgHg = 13.6 g/ml) because … = 13.6 g/ml) because …

DDHH22OO = 1.0 g/ml = 1.0 g/ml

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

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

It’s lessIt’s less

It’s greaterIt’s greater

When you drink through a When you drink through a straw, you reduce the pressure straw, you reduce the pressure in the strawin 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 the

moon? Why or why not?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 serviceU.S. weather service

• 1.013 bars1.013 bars Physics & AstronomyPhysics & Astronomy

CHEMISTRY:CHEMISTRY:

1 atm = 760 torr = 760 mm Hg1 atm = 760 torr = 760 mm Hg = = 101.3 101.3 kPa kPa SI SI Units Units

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 walls molecules with walls of containerof container

Microscopic ViewMicroscopic View

Anything that Anything that ↑↑ # of # of impacts per second impacts per second or force of each or force of each impact impact ↑↑ pressure pressure

Light molecules move Light molecules move faster & hit walls more faster & hit walls more oftenoften

Heavy molecules hit Heavy molecules hit walls with greater walls with greater forceforce

These 2 effects These 2 effects essentially balance essentially balance outout**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)1) concentrationconcentration • # of gas molecules per unit # of gas molecules per unit

volume (moles!!)volume (moles!!)

andand

2) 2) temperaturetemperature

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

• Depends on massDepends on mass• Light molecules are faster than Light molecules are faster than

heavy molecules at same heavy molecules at same temperaturetemperature

• Temperature = measure of Temperature = measure of average KE of particles in systemaverage KE of particles in 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

1080 miles per 1080 miles per hourhour

Pressure – Microscopic ViewPressure – Microscopic View

• Gas molecules hit walls of Gas molecules hit walls of containercontainer

• 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 volume gas molecules per volume

and and • temperaturetemperature

• same amount of gas exerts different same amount of gas exerts different pressure at different temperatures pressure at different temperatures • TiresTires

• BikeBike• CarCar• etcetc

Describing Gas Phase Describing Gas Phase SystemSystem

Need 4 variables to describe gasNeed 4 variables to describe gas

phase system from macroscopic or phase system from macroscopic or

lab viewlab view

1. Pressure (P)1. Pressure (P)

2. Volume (V)2. Volume (V)

3. Temperature (T)3. Temperature (T)

4. Amount of gas (# moles)4. Amount of gas (# moles)

Exit TicketExit Ticket1.1. List 5 common properties of all gases?List 5 common properties of all gases?

2.2. Draw a particle diagram substance in gas Draw a particle diagram substance in gas phasephase

3.3. Use picture from #2 to explain at least 2 Use picture from #2 to explain at least 2 properties of gasesproperties of gases

Bonus:Bonus:

At 0At 0ooC, a He atom is moving at 120 C, a He atom is moving at 120 meters/sec.meters/sec.

How fast is this in miles per hour? Show allHow fast is this in miles per hour? Show all

work! work!