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 the shape of their 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?
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What is air pressure?

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

• 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)?

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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? Why
  or why not?

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Units of pressure 1 atm

• 14.7 lb/in2 U.S. pressure gauges
• 29.9 in Hg U.S. weather
• 101.3 kPa SI Units (Regents)
• (kPa kilopascal)
• 1.013 Bars Physics Astronomy
• 760 Torr or 760 mm Hg CHEMISTRY

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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 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
• Bonus At 0oC, a He atom is moving at 1200
  meters/sec. How fast is this in miles per hour?
  Show all work!