The word “kinetic” comes from the Greek Word Greek kīnētikós which means moving (http://www.merriam-webster.com). Thus, Kinetic theory deals with the motion of the particles of matter. Kinetic molecular theory (also known as particle theory) states that all matter is made up particles and these particles are always in motion.
Kinetic molecular theory is useful in describing the properties of solids, liquids and gases at the molecular level. We will describe these by their general motion and amounts of kinetic energy as follows.
The particles of a solid will possess only a small amount of kinetic energy. The particles of a solid are always in motion, however, the motion will be so minuscule that we can say that they are simply vibrating in position. Thus, the particles of a solid will have a very ordered structure. Solids have a definite shape, a definite volume and are not easily compressed.
The particles of a liquid possess a greater amount of kinetic energy than the particles of a solid. Thus, the particles of liquid will not be in an ordered arrangement and will take on the shape of the container that they are placed in. These particles will possess enough energy to flow throughout a container and past one another. They do not have enough energy to escape the attraction they have for each other and as a result will remain loosely connected. Liquids have an indefinite shape, a definite volume and are not easily compressed.
The particles of a gas possess a very large amount of kinetic energy and move very rapidly and randomly. The particles of a gas are moving so fast that they have no attraction for one another. As a result of this kinetic energy, the particles of a gas will have large empty spaces between them. Just as with liquids, gases will take on the shape of the container that they are placed in. Gases have an indefinite shape, an indefinite volume and are easily compressed (due to the large empty spaces between particles)
We will spend most of our time exploring kinetic theory as it relates to gases. To understand the theory as it relates to ideal gases we must make the following assumptions:
- The particles of a gas are considered to be extremely small, hard spheres.
- The particles that make up a gas are so tiny that the total volume of the individual particles is negligible.
- The motion of the particles of a gas is very rapid and random. The particles move very rapidly, however, they are constantly colliding with one another and changing directions. Thus, we generally refer to the average motion of the particles rather than the motion of the individual particles.
- The particles of a gas collide in perfectly elastic collisions. A perfectly elastic collision is one in which 100% of the kinetic energy is transferred between the particles involved in a collision. This means that no kinetic energy will be lost to other forms of energy like heat or light.