The Nature of Gases

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The Nature of Gases

• Kinetic Theory and a Model for Gases

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Objectives

• Upon completion of this presentation, you will be
  able to
• state the Kinetic Theory of Gases.
• state the assumptions of the Kinetic Theory of
  Gases.
• apply the assumptions of the Kinetic Theory to
  ideal and real gases.

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Introduction

• Kinetic means motion.
• Kinetic energy, KE, is the energy of motion.
• The kinetic theory is a model of matter where ...
• ... all matter is composed of tiny particles ...
• ... in constant motion.
• The kinetic theory applies to all states of
  matter.
• We will apply it first to gases and then extend
  the theory to liquids and solids.

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Introduction

• The kinetic theory describes the behavior of
  matter in its various states.
• There are a series of assumptions we make about
  matter in order to use the theory.
• These assumptions help us to understand how to
  apply the theory to ...
• ... gases ...
• ... liquids ...
• ... and solids.

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First Assumption

• The particles in a gas are considered to be very
  small, very hard spheres with an insignificant
  volume.
• This means that the individual gas particles ...
• are atoms or small molecules
• are very far apart in relation to the size of the
  particles
• have very little attraction or repulsion towards
  one another
• move independently of each other

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Second Assumption

• The motion of the particles in a gas is rapid,
  constant, and random.
• This means that the individual gas particles ...
• spread out to fill any volume or shape of
  container
• travel in straight lines until they encounter
  another particle or another object
• change direction only after a collision

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Second Assumption

• Experimental measurements of gas molecules show
  that they move quite rapidly, even at room
  temperature.
• O2 molecules have an average speed of 1,700 km/hr
  (1,060 mph).
• However, they only travel about 70 nm until they
  encounter another particle.
• This is about 500 times their diameter
• Each gas molecule travels in a very crooked path
  called a random walk.

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Third Assumption

• All collisions between gas particles are
  perfectly elastic.
• This means that the individual gas particles ...
• transfer kinetic energy during a collision
• collide without a loss of kinetic energy
• have a total kinetic energy that remains constant

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Applications

• We can use these assumptions to understand the
  behavior of real gases.
• Compressibility
• Gases are compressible.
• This can be explained by the first assumption
• the small size of gas particles
• the large distance between gas particles

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Applications

• We can use these assumptions to understand the
  behavior of real gases.
• Expansion
• Gases expand to fill all available space of a
  container.
• This can be explained by the second assumption
• gas particles move rapidly, constantly, and
  randomly
• this movement will allow the gas particles to
  move to the limits of the container

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Applications

• We can use these assumptions to understand the
  behavior of real gases.
• Density
• Gases are by far the least dense of the states of
  matter.
• This can be explained by the first assumption
• the combination of the small size of gas
  particles and the large distance between gas
  particles leads to a very low density.

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Note

• The textbook lists only 3 assumptions of the
  kinetic theory of matter.
• There are, in fact, many more assumptions in a
  complete treatment of the kinetic theory.
• All of these assumptions help us to better
  understand the nature of matter in all of its
  states.

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Note

• Some examples of additional assumptions include
• The number of molecules is so large that
  statistical treatment can be applied.
• The average kinetic energy of the gas particles
  depends only on the temperature of the system.
• The time during collision of molecule with the
  container's wall is negligible as comparable to
  the time between successive collisions.
• The equations of motion of the molecules are
  time-reversible.

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Summary

• First Assumption
• The particles in a gas are considered to be very
  small, very hard spheres with an insignificant
  volume.
• Second Assumption
• The motion of the particles in a gas is rapid,
  constant, and random.
• Third Assumption
• All collisions between gas particles are
  perfectly elastic.