Gases

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Gases

• We ALL know about them!

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Gases

• What does one molecule in a container look like?
  This
• What do several molecules look like? This

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What if I had a hole in the container?

• How would a bunch of gas particles behave in a
  container with a hole in it?
• Like this

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Balloons

• If a balloon filled with gas is plunged into a
  tub of liquid Nitrogen,
• What would happen to the Particles of the gas?
  This

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Pressure

• What is Pressure?
• How do we measure it?
• What pressure are we at now?
• What would happen in a syringe if we cap it and
  push the plunger in?
• Do it!
• This
• What would happen to the syringe if we heated it
  up? This

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Pressure

• What does pressure feel like?
• What pressure is pushing on you from the
  Atmosphere?
• What if I removed the pressure on your skin?
• How much is Atmospheric Pressure? (Demo)
• How can pressure push up and down at the same
  time? (think swimming)

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UNITS OF PRESSURE FOR GAS

• mmHg (or Torr)
• Atmospheres (atm)
• Pascals (used in physics
• 1 pascal 1 newton per square meter)
• Equivalences
• 1 atm 760 mmHg
• 1 atm 101,325 Pa 101.325 kPa

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GAS CHARACTERISTICS

• 1. Have NO attractive force (except dispersion).
  Gases in a mixture act totally independent of
  each other
• 2. Pressure is caused by the collision of gas
  particles with the walls of the container.
• 3. The speed of the gas particles is directly
  proportional to the temperature.
• 4. Most of the volume of a gas is empty space.
• 5. Gas molecules are in constant motion, moving
  in straight lines. All collisions are perfectly
  elastic.

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How do we measure gas pressure?
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What have we measured?

• What Variables have we looked at?
• T
• P
• V
• n
• Can we change all of these at once?
• This
• This

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Applications

• Air bags

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Practical Applications of gas laws

• Why do your ears hurt when an airplane lands or
  takes off?

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• What causes the bends in diving?
• The concentration of a gas in a solution is
  directly proportional to its pressure above the
  solution.
• What happens when a diver rapidly ascends?

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• 3. What causes the massive fish kill in Lake
  Sammamish during the hot days of summer?

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How to Gas Laws

• Chapter 13

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The Ideal Gas Law

• A given set of conditions
• The Equation PVnRT
• P Pressure (atm or kPa)
• V Volume (L)
• n Moles of Gas
• T Temperature (K)

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The Gas Constant

• R the universal gas constant
• R 0.08206 L.atm
• K.mol
• R 8.30 L.kPa
• K.mol

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Example 1

• What is the volume of 3.0moles of a gas at 25ºC
  and 2.0atm?
• P V n R
  T
• 2.0atm V 3.0mol 0.08206L.atm 298K
• K.mol
• Numerically? V 3.0 0.08206 298 / 2.0
• V 36.7L

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S.T.P.

• STP Standard Temperature and Pressure
• These are conditions that are universal
• Standard Temperature is 0ºC or 273K
• Standard Pressure is 1atm or 101.325kPa

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Example 2

• What is the volume of 1 mole of CO2 at STP?
• P V n R
  T
• 1atm V 1mol 0.08206 L.atm 273K
• K.mol
• Numerically? V 1 0.08206 273 / 1
• V 22.4L

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Example 3

• What is the volume of 1 mole of O2 at STP?
• P V n R
  T
• 1atm V 1mol 0.08206 L.atm 273K
• K.mol
• Numerically? V 1 0.08206 273 / 1
• V 22.4L THE SAME!!!!!!!

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The combined gas law

• Lets say we have the O2 gas in Example 3 AND we
  change some conditions. Would there be anything
  similar between the two gasses?
• Lets label each condition as either an initial
  or final condition
• Initial Pi Vi ni Ri Ti
• Final Pf Vf nf Rf Tf

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Continued

• Lets divide both sides by Pf Vf

• Now, Substitute for Pf Vf what we know
• Pf Vf nf Rf Tf

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What can we cancel?

• Remember 34.5 1 So Ri 1
• 34.5 Rf

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What else? NOTHING!

• Unless we know that something is remaining
  constant
• For Example ? moles
• Are we changing the number of moles? If the
  answer is NO, then we can cancel it out!

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Leaving

• If we hold the number of moles AND the pressure
  constant then

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Example 4

• If a balloon at a pressure of 150kPa and a temp
  of 25ºC is heated to 50ºC, what will the new
  pressure be if the volume does not change?
• Use the combined gas law (its a change of
  conditions)

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• Cancel out the unchanged variables

• Substitute numbers

• Solve for Pf

• Pf 173kPa

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Example 5

• If a 45mL sample of gas at 26.5ºC is heated to
  55.2ºC, what is the new volume of the gas sample
  (at constant pressure)?
• This is a change of conditions so we will use the
  combined gas law.
• Canceling out any constants we get

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• Substituting in what we know
• Vf 0.050L

• Example 6

• If 46.2g of oxygen gas occupies a volume of 100.L
  at a particular temperature and pressure, what
  volume will 5.00g of oxygen gas occupy under the
  same conditions?

• Canceling out Pressure and Temperature we are
  left with

10.8L