Kinetic-Molecular Theory Chapter 14

Models Scientists use models because they are simpler than real life. Kinetic-molecular theory can be extended to all phases of matter. We begin with gases.

Kinetic-Molecular Theory 1) Gases consist of spherical molecules in constant, random, straight-line motion. Random, straight-line? They move in straight lines until they hit something and then they can bounce off at any angle.

Kinetic-Molecular Theory 2) Gas particles undergo “elastic” collisions with each other and with the walls of their container. “Elastic” means the total kinetic energy is conserved. K.E. does not get converted to heat or sound or any other kind of energy. The kinetic energy can be transferred between particles.

Kinetic-Molecular Theory 3) Gas molecules are separated by such large distances that the volume of the molecule itself is negligible. The distance between the molecules is more important than the actual size of the molecule. “Point Masses”

Kinetic-Molecular Theory 4) Gas molecules do not attract or repel each other. There are no interactions between gas molecules.

Best Assumptions Gas molecules are spherical and they move in constant, random, straight-line motion. Collisions between gas molecules are “elastic.”

Worst Assumptions Gas molecules are point masses. There are no attractions between molecules. Gases liquefy and occupy space as liquids. This wouldn’t happen if these two assumptions were true.

Real vs. ideal gas Ideal gas obeys the assumptions of the Kinetic theory all the time. It’s a model. It doesn’t exist. Real gases follow the kinetic theory most of the time, but not at high pressure and low temperature. Kinetic theory breaks down when molecules are close together.

Review of Kinetic Theory What features of the Kinetic-Molecular model are close to real gases? What features of the Kinetic-Molecular model break down?

Real gases behaving well? When the molecules are far apart from one another & interactions aren’t important. Low Pressure High Temperature

Real gases behave poorly? When the assumptions of the kinetic theory break down. When the molecules are close together and the interactions are important. High pressure Low temperature.