Introduction to the Introduction to the 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 shape & volume of Gases take shape & volume of

their containertheir container•Gases flowGases flow•Gases have low densitiesGases have low densities

Properties of GasesProperties of Gases1.1. Gases have massGases have mass2.2. Gases take shape & volume of Gases take shape & volume of

container container 3.3. Gases are compressibleGases are compressible4.4. Gases easily move through each Gases easily move through each

other (perfume, skunks!) other (perfume, skunks!) “ “Diffusion”Diffusion”

5.5. Gases exert pressureGases exert pressure

Gases take the shape of their Gases take the shape of their containercontainer Why don’t the balloons Why don’t the balloons on on the right keep the right keep expanding expanding until they pop?until they pop?

What is pressure?What is pressure?•Pressure = force per unit areaPressure = force per unit area•P = P = ForceForce

AreaAreaForce = 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 atmospherethis is standard pressurethis is standard pressure

TorricelliTorricelli• Discovered air pressure & invented Discovered air pressure & invented

mercury barometer in mercury barometer in 16431643

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

Mercury Barometer 1

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

At 1 atm: column height is 29.92 in Hg or 76 cm Hg or 760 mm Hg

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

Can we use H2O in a barometer?

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 …

DDwaterwater = 1.0 g/ml = 1.0 g/ml

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

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

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

straw?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 walls molecules with walls of containerof container

Anything that Anything that increases # of increases # of impacts per second impacts per second or force of each or force of each impact increases impact increases pressurepressureMicroscopic ViewMicroscopic View

Light molecules move Light molecules move faster and hit walls faster and hit walls more oftenmore often

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) concentration concentration or # gas or # gas

molecules per unit volumemolecules per unit volume

andand

2) 2) temperaturetemperature

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

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

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 hour1080 miles per 1080 miles per hourhour

Speeds of various gasses according to mass

Molecular Speed vs. Temperature

Pressure – Microscopic ViewPressure – Microscopic View• Gas molecules hit walls of their Gas molecules hit walls of their

containercontainer

• Pressure depends on:Pressure depends on:– # impacts per unit time# impacts per unit time– Force each impactForce each impact

Pressure – Macroscopic ViewPressure – Macroscopic View• Pressure depends on:Pressure depends on:

• how manyhow many gas molecules per unit gas molecules per unit volume & volume &

• temperaturetemperature

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

Describing Gas Phase Describing Gas Phase SystemSystem

Need 4 variables to completely Need 4 variables to completely describe gas-phase system describe gas-phase system from macroscopic or lab viewfrom macroscopic or lab view

1. Pressure1. Pressure2. Volume2. Volume3. Temperature3. Temperature4. Amount gas (moles)4. Amount gas (moles)

Exit TicketExit Ticket• Name 5 common properties of all gasesName 5 common properties of all gases• Draw particle diagram of oxygen in gas Draw particle diagram of oxygen in gas

phasephase• Use your particle diagram to explain at Use your particle diagram to explain at

least 2 properties of gasesleast 2 properties of gases

• Bonus: At 0Bonus: At 0ooC, a He atom is moving at C, a He atom is moving at 1200 meters/sec. How fast is this in 1200 meters/sec. How fast is this in miles per hour? Show all work! (Use miles per hour? Show all work! (Use the factor-label method)the factor-label method)