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Kinetic Molecular Theory

Pressure

Concepts

What is the origin of the pressure exerted by a gas?

As is evident from the previous exercises, the Kinetic Molecular Theory proposes that a gas is composed of a large number of particles in rapid motion. Each particle occasionally collides with a wall of the container. As a result of that collision, the particle exerts a force on the wall.

What does the Kinetic Molecular Theory predict about the pressure of a gas? The following exercises illustrate several predictions of the Kinetic Molecular Theory.



Effect of Particle Mass

The simulation below depicts a system containing helium atoms (red, FW 4.00) and krypton atoms (blue, FW 83.80). The graph at the left shows the speed of one of the helium atoms (red line) and one of the krypton atoms (blue line) as a function of time. The speeds fluctuate, of course, as a result of the collisions with other particles. Run the simulation, carefully observe the behaviors of the particles, and answer the following questions.

1. Which atoms, helium or krypton, move faster? Explain why this behavior occurs in terms of the postulates of the Kinetic Molecular Theory.

2. Observe collisions between two helium atoms. Do such collisions produce significant changes in the speeds and directions of motion of the helium atoms?

3. Observe collisions between two krypton atoms. Do such collisions produce significant changes in the speeds and directions of motion of the krypton atoms?

4. Observe collisions between a helium atom and a krypton atom. Do such collisions produce a significant change in the speed and direction of motion of the helium atom? Do such collisions produce a significant change in the speed and direction of motion of the krypton atom? Explain why this behavior differs from that for helium-helium and krypton-krypton collisions.

Helium           Krypton


Factors affecting the Pressure

In the simulation below you may select the number of molecules and the molecular mass of the molecules. The manometer at the lower left allows the pressure of the system to be measured.

Systematically vary the molecular mass and number of particles. Remember to reset the simulation after the conditions are changed. Your goals are to identify the effect of particle mass and number of particles on the pressure of the gas and to explain this behavior in terms of the Kinetic Molecular Theory.

Carefully observe the simulation and answer the following questions. Bear in mind that the volume of the system is fixed; thus there is a constant surface area for the container walls.

1. Pressure is associated with collisions of the particles with the container walls. What is the force of each collision? How many collisions occur in a given period of time? (You will need to answer these questions qualitatively.)

2. How does the mass of the particles affect the collision frequency? How does it affect the force of each collision with the container wall?

3. How does the number of particles affect the collision frequency? How does it affect the force of each collision with the container wall?

4. How does the pressure vary with the mass of the particles? Does the identity of a molecular have any influence on the pressure of the gas?

5. How does the pressure vay with the number of particles?

Molecular Mass


Number of Particles










Maxwell Distribution                     Pressure-Volume Relation

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© 2001-2014 David N. Blauch