3.5 Kinetic Molecular Theory

Kinetic Molecular Theory of Gases (KMT)

A theory that explains the properties of gases by the assumption that they are made up of individual particles in continuous motion.

Kinetic = Motion

Molecular = Particles

Elastic Collisions

Collisions between gas molecules where there is no net loss of kinetic energy.

Kelvin Temperature

A temperature scale where 0 Kelvin represents absolute zero; the average kinetic energy of gas molecules is directly proportional to this temperature.

Root-Mean-Square (rms) Speed

The speed of a molecule that has the same kinetic energy as the average kinetic energy of a gas sample.

Ideal Gas

A hypothetical gas that perfectly follows all assumptions of the kinetic molecular theory.

Fraction of molecules

The proportion of molecules in a gas sample that move at specific speeds, which can vary with temperature.

Condensation

The process by which a gas transitions to a liquid state due to a decrease in temperature or an increase in pressure.

Freezing

The process by which a liquid transitions to a solid state due to a decrease in temperature or an increase in pressure.

Intermolecular Forces

Forces that mediate interaction between molecules, which become significant in liquids and solids.

MAIN ASSUMPTIONS OF THE KMT

1. The combined volume of all the gas molecules is negligible relative to the total volume in which the gas is contained. Gases are made up of mostly empty space.

Gases are made up of mostly empty space

Easily Compressed

2. Gas molecules are in constant, rapid, straight-line motion

This motion is interrupted by collisions with other molecules or the walls of the container (elastic collisions)

The pressure of a gas is caused by the collisions of the molecules with the walls of the container. The magnitude of the pressure is determined by how often and how forcefully the molecules strike the walls

3. The pressure of an enclosed gas remains the same as long as the temperature and volume are not changed

The pressure of an enclosed gas remains the same as long as the temperature and volume are not changed

4. Molecules of a gas have no attraction or repulsion for one another

5. Energy can be transferred between molecules during collisions, but, as long as temperature remains constant, the average kinetic energy of the molecules does not change with time.

Molecules of gases have elastic collisions

6. The average kinetic energy of the molecules are directly proportional to the Kelvin temperature of the gas

DISTRIBUTIONS OF MOLECULAR SPEED

• Although the molecules in a sample of gas have an average kinetic energy and an average speed, the individual molecules are moving at different speeds

• Each molecule collides with other molecules

• Momentum is conserved in each collision, but one of the colliding molecules might be deflected off at high speed while the other is nearly stopped

• The result, at any instant, the molecules in the sample have a wide range of speeds