Gases

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Gases

• Unit 4
• Chapter 10
• AP Chemistry

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Characteristics of Gases

• Unlike liquids and solids, they
• Expand to fill their containers.
• Are highly compressible.
• Have extremely low densities.
• Volumes are proportional so can be used as mole
  values when comparing amounts.

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Pressure

• Pressure is the amount of force applied to an
  area.

• Atmospheric pressure is the weight of air per
  unit of area.

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Units of Pressure

• Gas pressure is caused by collisions of molecules
  with the side of the container and other
  molecules.
• Pascals
• 1 Pa 1 N/m2
• Bar
• 1 bar 105 Pa 100 kPa

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Units of Pressure

• mm Hg or torr
• These units are literally the difference in the
  heights measured in mm (h) of two connected
  columns of mercury.

• Atmosphere
• 1.00 atm 760 torr

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Standard Pressure

• Normal atmospheric pressure at sea level.

• It is equal to
• 1.00 atm
• 760 torr (760 mm Hg)
• 101.325 kPa

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Barometer

• Used to measure atmospheric pressure
• More air pressure results in higher level of Hg
  supported (warm, dry days)
• Less air pressure results in lower level of Hg
  supported (stormy, unstable air)

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For a barometer, shape doesnt matter, only
height.
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Manometer

• Used to measure the difference in pressure
  between atmospheric pressure and that of a gas in
  a vessel.

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Boyles Law

• The volume of a fixed quantity of gas at
  constant temperature is inversely proportional to
  the pressure.

P1V1 P2V2
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As P and V areinversely proportional

• A plot of V versus P results in a curve.

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Charless Law

• The volume of a fixed amount of gas at constant
  pressure is directly proportional to its absolute
  temperature.
• T Kelvin (absolute scale)
• K Celsius 273

V1 V2 T1 T2
A plot of V versus T will be a straight line.
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Temperature Pressure

• Boyles Law
• P1V1 P2V2
• Charles Law
• V1 V2
• T1 T2

• Combined Gas Law
• P1V1 P2V2
• T1 T2

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Avogadros Law

• The volume of a gas at constant temperature and
  pressure is directly proportional to the number
  of moles of the gas.

V1 V2 n1 n2
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Ideal-Gas Equation

• So far weve seen that

• V ? 1/P (Boyles law)
• V ? T (Charless law)
• V ? n (Avogadros law)

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Ideal-Gas Equation

• The constant of proportionality is known as R,
  the gas constant.

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Ideal-Gas Equation
PV nRT
Temp in Kelvin
Pressure in atmospheres or kPa
Universal gas constant 0.08206 Latm
Kmole
Volume in Liters
moles
8.31 LkPa Kmole
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