Kinetic-Molecular Theory

1
Kinetic-Molecular Theory

• Chapter 13
• The States of Matter

2
The Nature of Gases

• Essential Questions
• What are the three assumptions of the kinetic
  theory of gases, and how does this theory explain
  gas pressure?
• What is the relationship between Kelvin
  temperature and average kinetic energy of
  particles of a gas?

3
Lets Dispel Some Myths

• Why does everything go flying out of an airplane
  under circumstances of explosive decompression?
• Why does nature hate a vacuum?
• How does a straw work?
• Why does the liquid in a straw stay up in the
  straw when the top is covered with your finger?

4
Some Illustrations

• Water in cup with 4x6 card underneath
• Water in a straw
• Picking up a plate of glass with water
  underneath
• Pushion cups

5
Kinetic Theory and a Model For Gases

• The energy an object has because of its motion is
  called kinetic energy.
• Kinetic Theory all matter consists of tiny
  particles that are in constant random motion.

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Kinetic Theory and Gases

• The particles in a gas are considered to be
  small, hard spheres with an insignificant volume.
• The motion of the particles is rapid, constant,
  and random.
• All collisions between particles are perfectly
  elastic (no loss of energy).

7
Gas Pressure

• Gas pressure is the result of simultaneous
  collisions of billions upon billions of rapidly
  moving particles in a gas with an object.
• An empty space with no particles (and no
  pressure) is called a vacuum.

8
Atmospheric Pressure is

• the force exerted by air per unit area.
• the result of simultaneous collisions of
  billions of rapidly moving particles.
• If there are no particles present, the absence of
  collisions and therefore absence of pressure is
  called a vacuum.

9
Atmospheric Pressure

• The result of collisions of atoms and molecules
  of the air with objects.
• A barometer is used to measure atmospheric
  pressure.

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Which Brings Us To The Barometer

• Invented by Evangelista Torrticelli
• Can you tell me how a barometer works?

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MercuryBarometer
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Atmospheric Pressure

• Atmospheric pressure decreases as you climb a
  mountain because the density of the Earths
  atmosphere decreases with an increase in
  elevation.

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MercuryBarometer

• What is the decrease in pressure
• from sea level to the top of Mt. Everest?

14
Units of Pressure

• SI unit is the pascal (Pa).
• Standard Pressure is the pressure required to
  support a column of mercury 760 mm tall.

15
Units of Pressure

• Pressure is force per unit area (such as pounds
  per square foot).
• Units of pressure we will use include
  kilopascals, millimeters of mercury, torr, and
  atmospheres.

16
Standard Pressure

• Standard pressure was initially considered
  atmospheric pressure at sea level on an average
  day (very vague).
• Today, standard pressure is more exactly defined.

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Standard Pressure Equals

• 101.3 kilopascals (kPa)
• 760 millimeters of mercury (mmHg)
• 760 torr
• 1.0 atmosphere (atm)
• 14.7 pounds per square inch (lbs./in2)

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Recall from previous lessons

• Temperature is
• Temperature is a measure of...
• Temperature is a measure of average kinetic
  energy of the particles that make up a substance.

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Average Kinetic Energy

• At any given temperature the particles of all
  substances, regardless of physical state, have
  the same average kinetic energy.
• Kelvin temperature is a measure of that average
  kinetic energy.

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Absolute Zero

• Absolute zero (O K or -273.15 C) is the
  temperature at which all molecular motion
  theoretically ceases.
• The Kelvin temperature is directly proportional
  to the average kinetic energy.

22
The Nature of Liquids

• Essential Question
• What factors determine the boiling temperature,
  vapor pressure and evaporation rate of a liquid?

23
A Model for Liquids

• Substances that can flow are called fluids.
• Gases and liquids are both fluids.
• They both conform to the shape of their
  containers.

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Gases vs Liquids

• According to Kinetic Theory, there are
  (effectively) no attractions between the
  particles in a gas.
• Particles in a liquid are attracted to each
  other.
• This is what causes a gas to condense into a
  liquid.

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Properties of Liquids

• The interplay between the disruptive collisions
  of particles and their attractions for one
  another determine the physical properties of a
  liquid.
• How does this interplay determine rate of
  evaporation?

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Evaporation

• Conversion of a liquid to a gas is called
  vaporization.
• If the liquid is not boiling this process is
  called evaporation.
• During evaporation only particles of a minimum
  kinetic energy can escape.

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Evaporation

• Why does a liquid evaporate more quickly if it is
  heated?
• Why is evaporation a cooling process?

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Vapor Pressure

• Vapor pressure is a measure of the force exerted
  by a gas above a liquid in a sealed container.
• In a sealed container, a dynamic equilibrium is
  established.
• What does this imply?

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Vapor Pressure

• What properties of a liquid will determine its
  vapor pressure?
• At a given temperature, what is vapor pressure
  dependent upon?
• How is a manometer used to measure vapor pressure?

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Boiling Point

• As a young child observing water boiling, I
  wondered, How does all that air get down there?
• What was wrong with my question?

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Boiling Point

• When a liquid is hot enough to allow particles
  throughout the liquid to vaporize, it begins to
  boil.
• This occurs when the vapor pressure is exactly
  equal to the external pressure.
• What can be done to get a liquid to boil?

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Boiling Point and Pressure Changes
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Vapor Pressure and Boiling Point
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Normal Boiling Point
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Temperature of a Boiling Liquid

• never rises above its boiling point.
• If heat is supplied at a faster rate, the liquid
  only boils more rapidly.
• Why is this the case?

36
The Nature of Solids

• Essential Question
• How does the way atoms are organized explain the
  properties of solids, and what determines the
  shape of a crystal?

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A Model For Solids

• Properties of solids reflect the orderly
  arrangement, and fixed location, of the
  particles.
• The melting point is the temperature at which a
  solid turns to a liquid.
• What causes melting?

38
Crystal Structure and Units Cells

• In a crystal the particles are arranged in an
  orderly, repeating 3-dimensional pattern.
• The shape of the crystal is an identifying
  characteristic of the solid.
• The smallest group of particles with this shape
  is called a unit cell.

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Allotropes

• Allotropes are two or more different molecular
  forms of the same substance in the same physical
  state.
• Carbon exists as diamonds, graphite, or
  buckminster fullerene.

40
Amorphous Solids

• Amorphous solids lack an orderly internal
  structure.
• Rubber, plastic, asphalt and glass are amorphous
  solids.
• Glass is also called a supercooled liquid

41
Changes of State

• Essential Question
• What is sublimation, and how are equilibrium
  conditions represented on a phase diagram?

42
Sublimation

• Sublimation occurs when a solid turns directly to
  a gas without melting.
• This happens when a solids vapor pressure is
  greater than atmospheric pressure.
• Carbon dioxide (dry ice) will sublime.
• Solid room air fresheners also sublime.

43
Phase Diagrams

• Show the temperature and pressure conditions at
  which two phases exist in equilibrium.
• The triple point show were all three phases exist
  in equilibrium.

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Phase Diagram