The Nature of Gases

Objectives:

1.    Describe the nature of gases and recognize that gases have mass, can be compressed, exert pressure, occupy the space available to them, and diffuse.

2.    State the six postulates of the Kinetic-molecular theory and explain how they account for the physical properties of gases.

Key Terms:

diffusion    elastic     Kinetic-molecular theory

Notes: (13-1)

Nature of Gases:

We are constantly surrounded and buried in gas molecules.  They allow us to move freely through them while constantly filling in behind us.  We breath them in and out and use them to make energy within our bodies.  They do have mass and can create widespread damage if moving at a high enough velocity.  Some familiar molecules that we have talked about are He, N2, O2, CO2, and CH4.  As you can see, gas molecules exist as monatomic, diatomic and polyatomic molecules.  Below are some basic properties of gas molecules.

 Gases have mass Gases are compressible Gases will fill the space available to them Gases diffuse through each other very rapidly Gases exert pressure The pressure of a gas depends on its temperature
(chemed.chem.purdue.edu/.../bp/ ch22/graphics/22_4fig.gif)

Kinetic Molecular Theory:

The Kinetic-molecular Theory attempts to explain the behavior of gas molecules based on the nature of gas.  The theory is grounded on three basic assumptions.

 All gases have mass Gas molecules are separated from each other by extremely large distances - explains how gases can be compressed and the low density of gases Gases particles are constantly moving at a very rapid rate - explains how gases immediately fill a container.

Looking at the three assumptions you can see why a ball filled with air feels hard.  As you increase the amount of air molecules within the ball the ball becomes harder.  This increase in tension is due to the increase in both the mass of the air and the number of collisions on the wall of the ball.  There are two more assumptions that make the theory complete.

 The collisions are considered to be elastic (without gain or loss in energy) The kinetic energy (1/2 mv2) of the gas molecules increases with added temperature - this explains the expansion of gases when heated