Kinetic Molecular Theory - Online Tutor, Practice Problems & Exam Prep

The Kinetic Molecular Theory provides a framework for understanding the behavior of gases as if ideal gases truly existed. Ideal gases are hypothetical, assuming no interaction with other gases and negligible volume compared to their containers. The theory comprises three key postulates:

First, the volume of gas particles is so small it's considered insignificant. Second, as temperature increases, the velocity of gas molecules also increases, with root mean square speed illustrating this relationship at various temperatures. Third, gas particle collisions with container walls are elastic, meaning there are no attractive or repulsive forces affecting them, akin to ping pong balls bouncing in a container. This model helps predict real gas behaviors by applying these principles, despite the non-existence of ideal gases.

Kinetic Molecular Theory

Now before we talk about the kinetic molecular theory, let's talk about an ideal gas. Now we did ideal gas law theory. We looked at different calculations. We looked at stoichiometry deal dealing with ideal gas theory. But 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.

Now, 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 hundreds, thousands, 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, what it does is it 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.

All right, 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 the example question says which conditions of pressure and temperature make for the most ideal gas. Alright, so we just learned that ideal gas is our 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 push 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 A 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 key to a container, what happens to the volume? Remember, when we add heat to container, it will cause the volume to increase. We know this because of Charles Law. With bigger volume, gases can spread out be by themselves because there's nothing around them. So we want the temperature to be high, so we want low pressure, 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 an ideal gas law is imaginary. It wants to be alone. Remembering the chemistry, gas laws of Boyle's law, Charles law, Avogadro's law also helps with our understanding on 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, we say the molecules moving at higher velocities will also increase. Here we have the root mean square speed for three curves, 3 gases. And here we're going to say that each of these curves can be found at a different temperature.

So let's say that this one here is at 330°C. We'd say that this one here is at 200°C and let's say here that this one here is at 25°C. And actually let's make this 100 bigger differences in temperature. Their differences in temperature will result in different overall speed of velocity. For the gas molecules he had, the root mean square speed is just under 1000 here. Here it's just between 400 and 600, 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 3rd postulate deals with the forces of gases. Here we say that the collision between gas particles in 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 gases 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 under their own momentum, moving around, moving around.

And that's what we talk about 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 three 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.

• 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.

• 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?

The kinetic molecular theory is a scientific theory that explains the behavior of gases, relating their macroscopic properties to the motion of their molecules. According to this theory, a gas is composed of a large number of small particles, usually molecules or atoms, that are in constant, random motion. These particles are considered to be perfectly elastic, meaning they do not lose energy when they collide with each other or with the walls of their container.

The main postulates of the kinetic molecular theory include:

1. Gas particles are in continuous, random motion.
2. The volume of the individual particles is negligible compared to the volume of the container.
3. There are no attractive or repulsive forces between the particles; their collisions with each other and with the walls of the container are perfectly elastic.
4. The average kinetic energy of the gas particles is directly proportional to the temperature of the gas in kelvins.

This theory helps to explain various gas laws, such as Boyle's law, Charles's law, and Avogadro's law, and it provides a basis for understanding temperature, pressure, and volume relationships in gases.

What are the basic postulates of kinetic molecular theory?

The Kinetic Molecular Theory (KMT) is a model that helps us understand the behavior of gases at the molecular level. Here are its basic postulates:

1. Gas particles are in constant, random motion. They move in straight lines until they collide with other particles or the walls of their container.
2. The particles are considered to be point masses; that is, they have mass but occupy negligible space. The volume of the particles themselves is much less than the volume of the container they are in.
3. The collisions between gas particles and between particles and the container walls are perfectly elastic. This means that there is no net loss of kinetic energy from the collisions.
4. There are no forces of attraction or repulsion between the particles. They do not influence each other unless they are in direct contact during a collision.
5. The average kinetic energy of the gas particles is directly proportional to the temperature of the gas in kelvins. Higher temperatures correspond to higher average kinetic energies.

These postulates help explain gas laws and the behavior of gases under various conditions, such as changes in pressure, volume, and temperature.

What are the three main points of the kinetic molecular theory of gases?

The kinetic molecular theory of gases is a model that helps explain the behavior of gases and is based on several key postulates:

1. Gas Particles are in Constant, Random Motion: The theory posits that gas particles are small and far apart relative to their size, and they move in constant, random motion. This motion is straight-line until the particles collide with each other or with the walls of their container.
2. Elastic Collisions: When gas particles collide, either with each other or with the walls of a container, the collisions are perfectly elastic. This means that there is no net loss of kinetic energy from the collisions. The total energy of the system remains constant, although energy can be transferred between particles during collisions.
3. No Forces of Attraction or Repulsion: The theory assumes that there are no forces of attraction or repulsion between the particles in a gas. This implies that the gas particles do not influence each other unless they are in direct contact during a collision.

Together, these points explain why gases can fill any volume, why they have low densities, and how changes in temperature and pressure affect their behavior.