The Nature of Gases Kinetic Theory and a Model for Gases

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.

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.

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.

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

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

Second Assumption  Experimental measurements of gas molecules show that they move quite rapidly, even at room temperature.  O 2 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.

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

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

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

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.

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.

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.

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.