Kinetic Molecular Theory

Kinetic molecular theory (KMT) is a description of how the particles that make up a gas behave. Understanding KMT will allow you to explain how and why the various properties of gases are related.

There are a number of different ways to state KMT, your instructor may present it differently, but underneath it will be comprised of the same ideas being stated here.

• Gases are made of particles that move in straight lines until they hit something. 
• When particles hit something, they collide elastically. 
• The particles themselves have no volume. 
• The temperature of a gas is directly related to the kinetic energy of the particles.

Hopefully, you recognize that there are flaws in these statements. At the very least, you should recognize that the third violates the definition of matter. The first two statements are incorrect because intermolecular attractions would alter the path of particles that pass close to each other, and the attractions would make the collisions a little “sticky” resulting in a loss of kinetic energy.
What that means is that the statements above are approximations. The correct, complete statements would look like this.

• Gases are made of particles that are constantly moving. 
• This motion is commonly so fast that the relative attractions have very little effect on the paths followed by the particles (see South Street Effect). This means that under most conditions the pathways followed by gas particles are indistinguishable from straight lines. 
• Collisions between particles and between particles and the container are very quick and due to the simple structure of most gases, very little loss of kinetic energy occurs under normal conditions. 
• The volume occupied by the particles under normal conditions is so small compared to the volume of the container that the particle volume is negligible.

The good news for us is that under normal conditions (pressures similar to those we experience normally and temperatures at which humans can survive) these statements are so close to true that we can pretend that we believe them.