The Behavior of Gases

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The Behavior of Gases
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Properties of Gases (Review)

• No definite shape
• No definite volume
• compressible

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Kinetic Molecular Theory
moving
molecules
well supported ideas
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Basic Kinetic Theory of Gases

• Composed of particles like atoms (ex He) or
  molecules like (O2 and CO2)
• There are no attractive/repulsive forces.
• Lots of empty space!!

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Basic Kinetic Theory of Gases

• 2. Particles move in random, constant,
  straight-line motion.
• Move independently of each other.

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Basic Kinetic Theory of Gases

• 3. All collisions are elastic meaning that KE is
  transferred without loss of energy.
• No change in kinetic energy.
• Gases tend to diffuse towards areas of lower
  concentration.

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Gas Pressure

• Pressure- force exerted on container walls by
  particles in a gas
• Units used- kPa, atm, Torr, mmHg
• STP (Standard Temperature and Pressure) Table A
• 273 K or 0C and
• 101.3 kPa 1 atm 760 Torr (mmHg)

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Factors Affecting Pressure
Amount of Gas (number of moles) Increasing amount will increase P (and vice versa) Ex bicycle tires, car tires
Temperature Increasing temp. will increase P (and vice versa) Ex Tires deflate in winter
Volume Decreasing volume will increase P, increasing volume decreases P Ex press down on a balloon and it pops
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• Pressure and volume have an inverse relationship,
  if temperature remains constant.
• If volume is increased, pressure is decreased by
  the same factor.

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Mathematically, the product of PV is constant or
PV k (where k is some constant).
Boyle Law P1 V1 P2 V2 P3 V3
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Summary
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• Volume and temperature have a direct
  relationship, if pressure is held constant.
• If temperature (K) is increased, volume is
  increased by the same factor.

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Mathematically, the relationship of volume
divided by Kelvin temperature is constant or V/T
k.
Charles Law V1 /T1 V2 /T2 V3 /T3
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Summary
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• Pressure and temperature have a direct
  relationship, if volume remains constant.
• If temperature (K) is increased, pressure will be
  increased by the same factor.

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Mathematically, the relationship of volume
divided by Kelvin temperature is constant or P/T
k.
Gay-Lussacs Law P1 /T1 P2 /T2 P3 /T3
Pressure
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Combined Gas Law Equation

• P1 V1 P2 V2
• T1 T2

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Combined Gas Law Equation

• Steps
• Determine which variable (if any) is kept
  constant.
• Cancel those terms and remove them from the
  equation (Ex If the question says that
  temperature remains constant the new equation
  becomes P1V1 P2V2).
• Plug in values that are given.
• Solve for the unknown.
• Be sure to always use temperature in Kelvins.

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Ideal Gases vs. Real Gases

• Ideal gases behave as predicted by Kinetic
  Molecular Theory.
• Examples H2 and He
• Gases are most ideal at high temperature and low
  pressure (also have low mass and low polarity).

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• Real gases deviate from ideal behavior.
• Why?
• At low temps, gas particles become attracted to
  each other (KMT says they are not).
• Under high pressure, gases occupy a specific
  volume (KMT says they dont).

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Avogadros Law

• Avogadros number 6.02 x 1023
• Simply refers to the quantity of particles found
  in a mole.
• At STP, 6.02 x 1023 particles of a gas occupies
  22.4 L.
• At STP, 3.01 x 1023 particles of a gas occupies
  11.2 L.

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• Avogadro also hypothesized that equal volumes of
  different gases at the same temperature and
  pressure contain equal number of particles (or
  equal moles).

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Vapor Pressure

• In a sealed container, vapor pressure can be
  measured above a liquid.
• Evaporation occurs when some particles from the
  surface of a liquid escape causing pressure to
  build up above the liquid (not to be confused
  with boiling).

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Factors that Increase the Rate of Evaporation

• Heating a liquid (not to boiling point)
• Increasing surface area
• Create air currents
• (blow across the surface)

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Liquid-Vapor Equilibrium

• Some of the gas particles condense and then we
  find both evaporating and condensing occurs at
  the same rate.
• Rate of Evaporation Rate of Condensation

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Related to Boiling

• Boiling occurs when the vapor pressure becomes
  equal to the external pressure.
• At normal atmospheric pressure, we call this
  normal boiling point.

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Boiling and Attractive (Intermolecular Forces)

• Boiling occurs when heat energy overcomes
  attractive forces between molecules.
• The stronger the intermolecular forces, the
  higher the boiling point.
• The weaker the intermolecular forces, the lower
  the boiling point.

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Table H
Notice, increasing temperature increases vapor
pressure. Line drawn at 101.3 kPa corresponds to
normal boiling point.