Unit 12 Gases

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Unit 12 - Gases

• Chapter 12 in Addison/Wesley Textbook

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Early Studies of Air you know this!

• Low density matter
• Has mass
• Easily compressed
• Fills container completely (flows)
• Diffuses passes through another gas
• Exerts a pressure
• Pressure depends upon temperature

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Kinetic-Molecular Theory of Gases

• Explains previous observations
• Good picture to have of a gas
• Has 7 main points

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Points

• 1. Gases made of tiny particles with mass but not
  their own volume (called point masses)
• 2. Large distance between particles
• 3. Particles in constant, rapid motion
• 4. Particles exert pressure by colliding with
  walls and follow straight-line paths
• (next page)

5
continued

• 5. Collisions are perfectly elastic demo
• 6. Particles do not exert a force on each other
• 7. Increasing temperature increases kinetic
  energy of particles

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Kinetic Energy

• Equation for kinetic energy is
• KE 1/2mv2
• Therefore, because mass remains the same,
  increasing the temperature of a sample of gas
  increases its velocity and therefore its pressure
  on the sides of a container

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Animations of this kind of motion

• http//comp.uark.edu/jgeabana/mol_dyn/KinThI.html
  
• http//www.falstad.com/gas/

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Measuring Gases

• Four variables must be considered
• Amount (mass or moles)
• Volume of container
• Temperature Kelvin!!
• Pressure container and/or atmosphere

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Atmospheric Pressure

• Blow up a balloon inside the pressure is equal
  to atmospheric pressure
• Definition pressure exerted from mass of air
  pressing down on an object.
• How much is that?
• 1 atmosphere 101.3 kPa (sea level)
• You may have heard of 14 lbs/sq. inch
• More or less pressure on a mountain?

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Measuring Pressure

• Barometer measures atmospheric pressure
• Pushes mercury up 760 mm in a glass tube
• Therefore, 1 atm 101.3 kPa
• 760 mm Hg (called Torr)
• Practice conversions

11
Magnitude of Atmospheric Pressure

• Need differences to realize how large this is
• Card demo
• Soda Can activity and Journal Entry
• http//www.delta.edu/slime/cancrush.html

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Manometer Problems

• Measures pressure of any gas (barometer is a kind
  of manometer)
• Called open manometer
• Try two types of problems.

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STP

• Standard Pressure 1 atm
• Standard temperature 0 C 273 K

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Gas Laws

• Mathematical representations of all that we just
  observed
• Exploration of P,T, and V for a sample of gas
• Boyles Law, Charless Law, Gay-Lussacs Law, and
  Combined Gas Law

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Boyles Law

• Relationship of pressure and volume when
  temperature is held constant.
• At constant temperature, the volume of a gas
  varies inversely with pressure
• P k(1/V)
• or PV k
• For 2 situations, P1V1 P2V2
• Try one

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Graph Inverse Relationship

• Volume
• Pressure

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Practical Examples

• The Bends
• Cartesian Diver or Diving Tony
• Expanding marshmallow

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Charles Law

• Came from observing hot air balloons in Paris
• Relationship of V and T when P is constant
  (balloon)
• When the pressure of a sample of gas is held
  constant, its volume varies directly with its
  Kelvin temperature.
• V kT or V/T k

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Problems

• V1/T1 V2/T2
• Try one

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Graph

• What is the shape of the graph for a direct
  relationship?
• V
• T
• Look at V vs. Celsius Temperature graph

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Gay-Lussacs Law

• Relationship of pressure and temperature when
  volume is held constant rigid container
• When the volume of a sample of gas is held
  constant, pressure and Kelvin temperature vary
  directly when temperature increases, pressure
  increases.

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Problems

• P1/T1 P2/T2
• Try one
• Graph straight line

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Combined Gas Law

• For any given amount of gas
• PV/T k
• Therefore, P1V1/T1 P2V2/T2
• Practice and derive other equations

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Daltons Law

• The total pressure of a gas is the sum of the
  partial pressures of each gas in the container.
• Ptot p1 p2 p3 p4.
• Try one

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Collecting Gas Over Water

• Most common use of this law is when a gas is
  collected through water.
• Must use a table and correct for water vapor
  pressure.
• Try one.

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Ideal Gas Law

• This determines all factors of one sample of gas.
• Derivation.
• PV nRT
• R depends upon which pressure units you use.
• Try one

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Virtual Lab

• Try this lab online
• http//jersey.uoregon.edu/vlab/Piston/

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Finding Molar Mass

• n m/M
• PV m (RT)
• _____
• M
• Try one

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Deviations

• Many gases do not show ideal behavior
• Cold temperatures
• High pressure
• Interactions become important

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Grahams Law of Diffusion

• Lighter gases diffuse faster!
• Derivation