Kinetic Molecular Theory: Study with Video Lessons, Practice Problems & Examples

The kinetic molecular theory explains the behavior of ideal gases, which are theoretical and act independently in a container. It consists of three postulates: 1) Gas particles are negligible in size compared to the container's volume; 2) As temperature increases, the velocity of gas molecules also increases; 3) Collisions between gas particles and container walls are elastic, meaning no attractive or repulsive forces exist. Understanding these principles helps predict real gas behavior, emphasizing concepts like kinetic energy and the ideal gas law.

Kinetic Molecular Theory

Now before we talk about the kinetic molecular theory, let's talk about an ideal gas. We did ideal gas law theory; we looked at different calculations and stoichiometry dealing with ideal gas theory. What we need to realize is that the ideal gas is an imaginary gas and it acts as though it is alone by behaving independently of other gases around it. Ideal gases pretend as though they're the only gas within a container, that they are not influenced by any other gases around them. In reality, all containers have 100, 1,000, maybe even millions of gas molecules moving around, bouncing into each other, affecting one another's trajectories. Some of them, if they hit hard enough and in the right spot, they can actually stick together. Now, if we're talking about ideal gases and they're not real, how can we talk about them? Well, the kinetic molecular theory uses data of real gases to predict how ideal gases would behave if they existed. So, even though ideal gases are not real, we can actually look at real gases around us and predict their behavior if they were around. Alright. So now that we know the usefulness of the kinetic molecular theory, click on the next video, and let's take a look at an example question.

Kinetic Molecular Theory Example 1

Here's the example question: Which conditions of pressure and temperature make for the most ideal gas? Alright. So we just learned that ideal gases are imaginary. But remember, if they did exist, how would they behave? Well, remember we just said that an ideal gas acts as though it is alone inside of a container. So think about the conditions that foster this whole thing of being isolated, being by yourself. So remember pressure, think of it as this piston, and outside pressure could push down on it. And here we have gas molecules, and each one is far enough that they imagine themselves as being alone. So think about it. Do we want pressure to be high or low? If pressure is high, this piston will get pushed down, and if that piston got pushed down, the volume will get smaller. That would cause these gas molecules to come closer together, which they don't want. They want to act as though they are alone. So we don't want high pressure, we want low pressure. So, an option for high pressure is out. Next, temperature. So let's just not worry about pressure now. Let's look at temperature, and here's my ugly flame that I drew. Think about it. When I'm adding heat to a container, what happens to the volume? Remember, when we add heat to a container, it will cause the volume to increase. We know this because of Charles' Law. With a bigger volume, gases can spread out, be by themselves because there's nothing around them. So, we want the temperature to be high. We want low pressure and high temperature. This will foster a larger volume inside the container and allow these gases to behave as though they are by themselves and independent of one another. So just remember, if you had a little bit of trouble with this, just remember, ideal gas laws are imaginary, they want to be alone. Remembering the chemistry gas laws, Boyle's Law, Charles' Law, Avogadro's Law also helps with our understanding of what helps to make a larger volume inside the container so these gases can be by themselves.

Kinetic Molecular Theory

So remember the kinetic molecular theory is a way that we try to understand ideal gas laws if they did exist. Remember, ideal gas laws themselves are completely imaginary. With the kinetic molecular theory we have 3 postulates. Postulate 1 deals with the volume of these ideal gas molecules. Here we're going to say under the first postulate, the size of the particle is significantly smaller and negligible, meaning not important, when compared to the volume of the container. Here we're going to say the volume of a gas particle itself represents less than 0.01% of the total volume in the container. So, a simple gas molecule particle doesn't take up very much space at all, so much in fact that its volume is not important. Here I'm showing the gas molecules in a larger format so we can better see them. But in actuality, they're going to be incredibly incredibly small, each one.

Postulate 2 deals with temperature. We're going to say, here under postulate 2, as the temperature increases, molecules moving at higher velocities will also increase. Here we have the root mean square speed for 3 curves, 3 gases. And here, we're going to say that each of these curves can be found at different temperatures. So let's say that this one here is at 330 degrees Celsius. We'd say that this one here is at 200 degrees Celsius, and let's say here that this one here is at 25 degrees Celsius. Actually, let's make this 100, bigger differences in temperature. Their differences in temperature will result in different overall speeds or velocities for the gas molecules. Here the root mean square speed is just under 1000 here. Here it's just between 600 and 100, and here it's just over 400 meters per second. As the temperature is increasing for each of the curves, we see that their speed or velocity is higher.

Finally, the final and third postulate deals with the forces of gases. Here we say that the collisions between gas particles and the walls of the container are completely elastic if we were to imagine ideal gases. Now, what does it mean to be elastic? Well, that means that these ideal gas particles will behave as though they have no attractive or repulsive forces between the gas around them and the walls of the container. So, a good way to think about this is, if you've ever watched ping pong balls within a container bouncing around, that's kind of an elastic collision. They're bouncing around, hitting the walls, hitting each other, but they're not sticking together. They're not pushing each other away, they're just moving around with their own momentum moving around, moving around, and that's what we talk about with ideal gas laws and ideal gases behaving elastically. They may bump into each other, but there's no repulsive or attractive forces between those gas molecules.

Right? So these are the 3 postulates that we use for us to better understand if ideal gas did exist; these are the behaviors that they would have.

Which of the following statements would correctly explain the non-ideal behavior of a gas based on the Kinetic Molecular Theory (KMT)?

• A. At high temperatures the attractive forces between molecules becomes negligible.
• B. At high pressure the volume of gas molecules become significant.
• C. An increase or decrease in the moles of gas causes the gas constant value to change.

Which of the following statements is/are true for gas molecules according to the Kinetic Molecular Theory?

I.Increasing the amount of gas molecules increases the pressure by increasing the force of the collisions.
II.Decreasing the temperature of a gas decreases the pressure by increasing the force of the collisions.
III.Decreasing the volume of a gas increases pressure by increasing the frequency of the collisions.

• A. I only
• B. II only
• C. III only
• D. I and II
• E. II and III

Which statement is TRUE about kinetic molecular theory?

• A. A single particle does not move in a straight line.
• B. The size of the particle is large compared to the volume.
• C. The collisions of particles with one another is completely elastic.
• D. The average kinetic energy of a particle is not proportional to the temperature.

Based on the kinetic-molecular theory, which of the following is/are true?
I.At a given temperature, all gases have the same average kinetic energy.
II.At a given temperature, different gases have the same average velocities.
III.The average kinetic energy is proportional to the absolute temperature.

• A. I only
• B. II only
• C. I & III only
• D. I, II & III

Here’s what students ask on this topic:

What is the kinetic molecular theory and its postulates?

The kinetic molecular theory explains the behavior of ideal gases, which are theoretical and act independently in a container. It consists of three postulates: 1) Gas particles are negligible in size compared to the container's volume; 2) As temperature increases, the velocity of gas molecules also increases; 3) Collisions between gas particles and container walls are elastic, meaning no attractive or repulsive forces exist. Understanding these principles helps predict real gas behavior, emphasizing concepts like kinetic energy and the ideal gas law.

How does temperature affect the velocity of gas molecules according to the kinetic molecular theory?

According to the kinetic molecular theory, as the temperature of a gas increases, the velocity of its molecules also increases. This is because temperature is a measure of the average kinetic energy of the gas molecules. Higher temperatures provide more energy to the molecules, causing them to move faster. This relationship is quantified by the root mean square speed, which increases with temperature, indicating higher velocities for gas molecules at higher temperatures.

What does it mean for gas particle collisions to be elastic in the context of the kinetic molecular theory?

In the context of the kinetic molecular theory, elastic collisions mean that when gas particles collide with each other or with the walls of the container, there is no net loss of kinetic energy. The particles bounce off each other and the walls without any attractive or repulsive forces acting between them. This behavior is similar to ping pong balls bouncing around in a container, where they do not stick together or push each other away, maintaining their momentum.

Why are gas particles considered negligible in size according to the kinetic molecular theory?

According to the kinetic molecular theory, gas particles are considered negligible in size because their individual volumes are extremely small compared to the total volume of the container. This means that the space occupied by the gas particles themselves is so tiny that it can be ignored when analyzing the behavior of the gas. This assumption simplifies calculations and helps in understanding the properties of gases, such as pressure and temperature, without the need to account for the actual size of the particles.

How does the kinetic molecular theory help in understanding real gas behavior?

The kinetic molecular theory helps in understanding real gas behavior by providing a framework to predict how gases would behave if they were ideal. Although ideal gases are theoretical, the principles of the kinetic molecular theory can be applied to real gases to approximate their behavior. By considering factors like particle size, temperature, and elastic collisions, the theory allows us to make predictions about pressure, volume, and temperature relationships in real gases, aiding in the application of the ideal gas law and other gas-related calculations.