Introduction to the 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 shape & volume of their containergases take shape & volume of their container gases flowgases flow gases have low densitiesgases have low densities

Properties of Gases 1.gases have mass 2.gases take shape & volume of container 3.gases are compressible 4.gases easily move through each other (perfume, skunks!)  “Diffusion” 5.gases exert pressure

gases take shape of container Why don’t the balloons on Why don’t the balloons on the right keep expanding the right keep expanding until they pop? 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? pressure exerted on us by weight of gases above our heads (& all around our body)pressure exerted on us by weight of gases above our heads (& all around our body)0 at sea level:at sea level: air pressure = 1 atmosphere air pressure = 1 atmosphere this is standard pressure

2Pb20

Torricelli discovered air pressure & invented mercury barometer in 1643discovered air pressure & invented mercury barometer in st person to propose correct explanation for wind1 st person to propose correct explanation for wind

reporting air pressure - take short-cut & report height of Hg column At 1 atm: column height is in Hg or 76 cm Hg or 760 mm Hg

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

Can we use H 2 O in a barometer? D Hg = 13.6 g/ml and D water = 1.0 g/ml therefore … a barometer made with water has to be 13.6 times taller than a Hg barometer () 13.6 times taller than a Hg barometer ()

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

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

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

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

light molecules move faster and hit walls more often heavy molecules hit walls with greater force these 2 effects essentially balance out **gas pressure doesn’t depend on the identity of the gas**

Pressure Depends on 1)concentration (# gas molecules per unit volume) (# gas molecules per unit volume) and and 2) temperature

How fast do the molecules in the air move? depends on massdepends on mass light molecules faster than heavy molecules at same temperaturelight molecules faster than heavy molecules at same temperature temperature = measure of average KE of particles in systemtemperature = measure of average KE of particles in 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/sec *O 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

speeds of various gasses according to mass

Molecular Speed vs. Temperature

Pressure – Microscopic View gas molecules hit walls of their containergas molecules hit walls of their container pressure depends on:pressure depends on: – # impacts per unit time – Force each impact

Pressure – Macroscopic View pressure depends on:pressure depends on: how many (# moles) gas molecules per unit volume & how many (# moles) gas molecules per unit volume & temperaturetemperature same amount of gas exerts different pressure at different temperatures (tires)same amount of gas exerts different pressure at different temperatures (tires)

Describing Gas Phase System need 4 variables to completely describe gas-phase system from macroscopic (lab) view 1. pressure 2. volume 3. temperature 4. amount gas (# moles)

Exit Ticket name 5 common properties of all gasesname 5 common properties of all gases draw particle diagram of oxygen in gas phasedraw particle diagram of oxygen in gas phase use your particle diagram to explain at least 2 properties of gasesuse your particle diagram to explain at least 2 properties of gasesBonus: At 0 o C, a He atom is moving at 1200 m/s. How fast is this in miles per hour? Show all work!