The Kinetic-Molecular Theory of Matter

1. The Kinetic-Molecular Theory of Matter

2. Objectives • State the kinetic-molecular theory of matter, and describe how it explains certain properties of matter. • List the five assumptions of the kinetic-molecular theory of gases. Define the terms ideal gas and real gas. • Describe each of the following characteristic properties of gases: expansion, density, fluidity, compressibility, diffusion, and effusion. • Describe the conditions under which a real gas deviates from “ideal” behavior.

3. In this section, you will study the theory as it apples to gas molecules. • In that form, it is called the kinetic molecular theory of gases.

4. Kinetic-Molecular Theory • KMT is based on the idea that particles are always in motion. • Kinetic energy: The energy an object has because of its motion. • KMT can be used to explain the properties of solids, liquids, and gases in terms of the energy of particles and the forces that act between them. • KMT can help you understand the behavior of gas molecules and the physical properties of gases.

5. The KMT is Based on the Following 5 Assumptions: 1. • A gas is composed of particles – either molecules or atoms – that are far apart relative to their size. • These particles usually molecules or atoms, typically occupy a volume about 1000 times greater than the volume occupied by particles in the liquid or solid state.

6. Continued… • Particles are small, hard spheres with negligible volume, and are very far away from each other. • Between particles is empty space. • Gases fill up any container they are in no matter the shape or volume.

7. 2. • Particles in a gas move rapidly independently of each other. • They change direction only when they rebound from collisions with each other or other objects. • Oxygen molecules travel so fast, that odor molecules from a hot pizza in Washington DC should reach Mexico in 90 minutes. They don’t because molecules are constantly being rebound from collisions with each other. • Their aimless path is called a random walk.

8. 3. • All collisions are perfectly elastic. • An elastic collision is one in which there is no net loss of kinetic energy. • Total kinetic energy in a collision remains constant. • KE is transferred between two particles during collisions. • However, the total KE of two particles remains the same as long as temperature is constant.

9. 4. • There are no forces of attraction or repulsion between gas particles: They are too far apart. *You can think of ideal gas molecules as behaving as small billiard balls. When they collide, they do not stick together but immediately bounce apart.

10. 5. • The average kinetic energy of gas particles depends on the temperature of the gas. • All gases at the same temperature have the same average KE. Therefore, lighter particles (Hydrogen, for example), have higher average speeds than heavier particles (oxygen, for example).

11. 5. Continued… • The KE of any moving object including a particle is given by the equation KE = ½ mv2 • In the equation, m is the mass of the particle, and v is the speed. • Because all particles of a specific gas has the same mass, their KE depends only on their speed.