Kinetic Molecular Theory: Introduction

1
Kinetic Molecular TheoryIntroduction Phases

• Assumptions of KMT
• Phases Phase Changes According to KMT

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Assumptions of KMT (1 of 3)

• All molecules and atoms are in constant motion
• Three types of motion are possible
• Vibration back and fourth motion where the
  distance between atoms changes
• occurs within molecules and between molecules
• Rotational molecule or atom rotates around an
  axis
• Translational straight line motion that results
  in the molecule or atom changing location

3
Assumptions of KMT (2 of 3)

• The amount of heat an object has (its
  temperature) is a measure of how fast the
  molecules are moving
• The faster the molecules, the hotter the
  substance.
• The slower the molecules, the colder the
  substance.
• All heating and cooling must be a result of
  speeding up or slowing down molecules

4
Assumptions of KMT (3 of 3)

• Molecules in a substance are held together by
  relatively weak Intermolecular Forces
• Intermolecular Forces (IMF) A group of
  attractive forces between molecules. The
  strength of the IMF is affected by
• The type of molecules involved
• The distance between the molecules
• The farther apart the molecules the weaker the
  forces

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Phases According to KMT (1 of 8)

• Solid Phase
• Molecules in a solid vibrate back and fourth but
  are held in place by Intermolecular Forces
• Molecules form a very regular, well organized
  crystal structure
• Molecules cannot rotate or translate

Intermolecular Forces
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Phases According to KMT (2 of 8)

• Melting - Solid to a liquid
• As a substance is heated, the molecules move
  faster, and making it harder for the IMF to hold
  the molecules together
• Once the molecules are vibrating so fast that the
  IMF cannot hold them together, the molecules
  begin to move around
• The solid has melted into a liquid

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Phases According to KMT (3 of 8)

• Liquid Phase
• Molecules are moving fast enough that the IMF
  cannot hold them in place
• IMF still hold the molecules very close together
• All three types of motion are available
• Vibrational, Rotational, Translational

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Phases According to KMT (4 of 8)

• Evaporation Liquid to a gas
• As molecules in a liquid move around they collide
  with one another
• During these collisions, speed (heat) is
  transferred from one molecule to another
• Not all molecules in a liquid are traveling at
  the same speed (Check out this animation)
• If a molecule gains enough speed and is near the
  surface of the liquid, this really fast molecule
  can break free of the IMF and fly off as a vapor.
• Boiling is simply really fast evaporation

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Phases According to KMT (5 of 8)

• Gaseous Phase
• Molecules are moving so fast that the IMF cannot
  hold them together
• Molecules are widely separated
• The IMF are not important except when two
  molecules collide, at which time, the IMF
  attempts to hold the molecules together.
• All three types of motion are available to gases
• Check out this animation to see the molecular
  effects of heating a solid

10
Phases According to KMT (6 of 8)

• Condensation Gas to a liquid
• When gasses cool, the molecules slow down
• When two slow moving gas molecules collide, the
  IMF are able to hold the molecules together
• If more slow molecules collide and stick, a
  small droplet of liquid will form
• Since Vapors are liquids or solids at room
  temperature, they will condense even with small
  changes in temperature
• Ex Water vapor condensing on a cold drink

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Phases According to KMT (7 of 8)

• Freezing Liquid to a solid
• As a liquid cools, the molecules slow down
• The IMF are able to not only hold the molecules
  together, but also to hold them in place
• Now only vibrational motion is possible
• The liquid has frozen into a solid

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Phases According to KMT (8 of 8)

• Sublimation Solid to a gas
• The amount of molecular speed required to move
  melt is the same as the amount of speed required
  to evaporate
• Molecules skip the liquid phase
• Most liquids, including water, can be made to
  sublimate at the right pressure and temperatures
• Sublimation of water is used in the freeze dying
  process