Read Section 6.5 before viewing the slide show.

Unit 23 Gases Five Premises of Kinetic Molecular Theory (6.5)

The Kinetic-Molecular Theory of Gases Our understanding of gases is based the kinetic-molecular theory of gases. The theory is based on five postulates and explains well the pressure-volume- temperature behavior observed for gases. The five postulates are: Particles of a gas (could be molecules or individual atoms) are in constant rapid motion and move in straight lines. Particles of a gas are tiny compared with the distances between them. Particles of a gas are very far apart and thus the intermolecular forces are not important – there is little attraction or repulsion between the particles of a gas. Particles of a gas collide with one another and the energy is conserved in every collision. Temperature is a measure of the average kinetic energy (energy of motion) of the gas particles. On the ensuing slide you will use an animation to visually look at implications of these postulates.

Gases at the Particle Level Let’s get an appreciation at the particle level. Go to the site: and click on either the Download button or the Run Now button. An image similar to that to the right will appear. Use your mouse cursor to raise the pump handle as high as it will go and then lower it. (Press the Alt and Tab keys simultaneously to toggle between the simulation and the activities on the next couple of slides.) Consider how much of the kinetic- molecular theory you see demonstrated in the simulation by considering the questions on the next slides. Remember, however, the simulation is built on the kinetic-molecular theory – it doesn’t “prove” anything, it just illustrates it.

The premises of the kinetic-molecular theory are below. Consider each one while working with the simulation. Things to look for are indicated below each bullet. Particles of a gas (could be molecules or individual atoms) are in constant rapid motion and move in straight lines. Watch the trajectory of a particle as it goes through the container. Does it deviate from a straight line? Particles of a gas are tiny compared with the distances between them. If you added up the volume of all of the particles in the container the result would be much less than the volume of the container. This isn’t so true for the simulation but in real life it is. Particles of a gas are very far apart and thus the intermolecular forces are not important – there is little attraction or repulsion between the particles of a gas. Do the particles tend to attract or repel each other or do they continue to move in a straight line between collisions? Gases at the Particle Level

Particles of a gas collide with one another and the energy is conserved in every collision. This is a little hard to measure. Think of the energy as being related to the motion of the particles. Put not too many particles in the container and follow the motion of one of them. It will appear to speed up or slow down after a collision. Notice the particle it collides with – what happens to its speed? Temperature is a measure of the average kinetic energy (energy of motion) of the gas particles. The best way to see this is to increase the temperature of the container and notice what happens to the speed of the particles overall. The speed is related to the kinetic energy. Gases at the Particle Level

Believe it or not, that is the end of Unit 23.