Introduction to the Properties of Gases

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Introduction to the Properties of Gases

• Chapter 14

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What do you know about gases?

• particle diagrams
• Ar(g), H2(g), CO2(g)
• gases take shape volume of their container
• gases flow
• gases have low densities

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Properties of Gases

• gases have mass
• gases take shape volume of container
• gases are compressible
• gases easily move through each other (perfume,
  skunks!)
• diffusion
• gases exert pressure

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gases take shape of container

Why dont the balloons on
the right keep expanding

until they pop?
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What is pressure?

• pressure force per unit area
• P force

area
force mass x acceleration
Would you rather have your foot stepped on by
someone wearing sneakers or stilleto heels? Why?
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What is air pressure?

• pressure exerted on us by weight of gases above
  our heads ( all around our body)
• 0
• at sea level
• air pressure 1 atmosphere
• this is standard pressure

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• http//www.youtube.com/watch?vJsoE4F2Pb20

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Torricelli

• discovered air pressure
  invented mercury
  barometer in 1643
• 1st person to propose correct
  explanation for wind

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(No Transcript)
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reporting air pressure - take short-cut
report height of Hg column at 1 atm column
height is 29.92 in Hg (? 30) or 76 cm Hg or 760
mm Hg
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How does downward
pressure of Hg in column
compare to pressure
of atmosphere?
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Can we use H2O in a barometer?

• sure, but consider the densities of the 2
  liquids
• DHg 13.6 g/ml and Dwater 1.0 g/ml
• therefore
• a barometer made with water has to be
• 13.6 times taller than a Hg barometer!

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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)?
its LOWER than standard atmospheric pressure

its HIGHER than standard atmospheric pressure
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When you drink through straw, you reduce pressure
in straw so

• Why does liquid in cup go up the straw?
• Could you drink a soda
  this way on the moon?
  Explain why or why not?

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Units of pressure

• 14.70 lb/in2 U.S. pressure gauges
• 29.9 in Hg U.S. weather
• 101.325 kPa SI Units (Regents)
• (kPa kilopascal)
• 1.013 Bars Physics Astronomy
• CHEMISTRY also use
• 760 Torr, 760 mm Hg, 1 atm

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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
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light molecules move faster and hit walls more
often heavy molecules hit walls with greater
force these 2 effects essentially balance out
gas pressure doesnt depend on the identity of
the gas
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Pressure Depends on

• concentration
• ( gas molecules per unit volume)
• and
• 2) temperature

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How fast do the molecules in the air move?

• depends on mass
• light molecules faster than heavy molecules at
  same temperature
• temperature measure of average KE of particles
  in system

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Molecular Speeds at 298 K

• H2 1.93 X 105 cm/sec
• He 1.36 X 105 cm/sec
• O2 4.82 X 104 cm/sec
• Ar 4.31 X 104 cm/sec
• Xe 2.38 X 104 cm/sec

48200 cm x 1 in x 1 ft x 1 mile x
3600 sec
sec
2.54 cm
5280 ft
1 hour
1080 miles per hour
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speeds of various gasses according to mass
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Molecular Speed vs. Temperature
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Pressure Microscopic View

• gas molecules hit walls of their container
• pressure depends on
• impacts per unit time
• Force each impact

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Pressure Macroscopic View

• pressure depends on
• how many ( moles) gas molecules per unit
  volume
• temperature
• same amount of gas exerts different pressure at
  different temperatures (tires)

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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)

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Exit Ticket

• name 5 common properties of all gases
• draw particle diagram of oxygen in gas phase
• use your particle diagram to explain at least 2
  properties of gases (state the prop indicate
  how your diagram illustrates it)
• Bonus
• At 0oC, a He atom is moving at 1200 m/s. How
  fast is this in miles per hour? Show all work!