What is the Kinetic Molecular Theory?

The kinetic molecular theory states that molecules are in constant motion. Since molecules are in constant motion, the thing that determines whether something is a solid, liquid, or gas depends on how much motion those molecules are in.

Kinetic Energy and Intermolecular Forces

There are a couple of things you should know about substances. First of all, kinetic energy drives molecules apart. The higher the kinetic energy, the farther apart the molecules are going to be.

Now, on the other hand, intermolecular forces hold molecules together. They’re doing the opposite thing of kinetic energy. They’re opposing kinetic energy, because the closer intermolecular- or the larger the intermolecular forces are, the closer the molecules are going to be together.

The closer the molecules are, the intermolecular forces are therefore stronger. Now, I want to look at three different phases of substances and look at the balance between intermolecular forces and kinetic energy, because that’s what determines whether a substance is a solid, liquid, or gas.

In a solid, the molecules are close together. Since the molecules are close together, the intermolecular forces are going to be very strong. Intermolecular forces are going to be much larger than the kinetic energy.

Since the molecules are close together, that’s why it’s a solid. Now, many times the kinetic energy is low depending on the temperature. That’s what makes intermolecular force much greater than KE, because KE is generally low and intermolecular forces are high, creating a big difference between the two.

Now, in this case, the molecules really only vibrate because they can’t move around that much. Now, in a liquid molecules have absorbed energy, so they’re going to move faster. Intermolecular forces are generally equal to, or slightly bigger than, KE.

That right there means bigger than or equal to. If they are bigger than, it’s just going to be by a little bit. Molecules break free from each other, but the forces still hold them close by. The molecules can just slide past each other.

Now, we’re going to move on gases. I hope you’re seeing the trend here. The more something is moving, the more it’s going to be to the gas phase. Solids are when things aren’t moving very much, and gas is when the molecules are moving a lot.

That kind of makes sense in your head. Solid, like you think about, for example, water. When it’s frozen, it makes sense that everything’s frozen in place. When it’s a gas, it’s free to move everywhere.

You can guess that, if originally intermolecular forces were bigger, now they’re equal, this time kinetic energy is generally going to be bigger than the intermolecular forces. In this case, molecules have absorbed even more energy.

Now they’re moving faster. The intermolecular forces become much less than KE. KE is much larger than the intermolecular forces. Now molecules are breaking free, because the intermolecular forces cannot hold them close. Now they’re moving everywhere, and that allows the molecules to be in the form of a gas.