States of Matter

Chapter 13

GASESSection 13.1

Kinetic-Molecular Theory

• Objects in motion have energy called kinetic energy.

• The kinetic-molecular theory describes the behavior of gases in terms of particles in motion.

• The model makes several assumptions about the size, motion, and energy of gas particles.

Particle Size

• Gases consist of small particles that are separated from one another by empty space.

• The volume of the particles is small compared with the volume of the empty space.

• Because gas particles are far apart, there are no significant attractive or repulsive forces among them.

Particle Motion• Gas particles are in constant,

random motion.• Particles move in a straight line

until they collide with other particles or with the walls of their container.

• Collisions between gas particles are elastic.

• An elastic collision is one in which no kinetic energy is lost.

• Kinetic energy may be transferred between colliding particles, but the total kinetic energy of the two particles does not change.

Particle Energy• Two factors determine the kinetic energy of a

particle– Mass– Velocity

• The kinetic energy of a particle can be represented by the equation

• Velocity reflects both speed and direction of motion.

• In a sample of a single gas, all particles have the same mass but do not have the same velocity.

• Therefore, all particles do not have the same kinetic energy.

• Kinetic energy and temperature are related.• Temperature is a measure of the average

kinetic energy of the particles in a sample of matter.

• At a given temperature, all gases have the same average kinetic energy.

Explaining the Behavior of Gases

• Kinetic-molecular theory can help explain the behavior of gases.

• For example, the constant motion of gas particles allows a gas to expand until it fills its container.

• What property of gases makes it possible for an air-filled flotation device to work?

• Low Density– Density is mass per volume– A great deal of space exists between gas particles

• Compression and Expansion– Gases expand until they fill their container

• Diffusion and Effusion– Diffusion is the term used to describe the movement

of one material through another from high to low concentration.

– During effusion, a gas escapes through a tiny opening.– Graham’s law of effusion states that the rate of

effusion for a gas is inversely