3.3 Kinetic Molecular Theory

Core assumptions of KMT

Kinetic Molecular Theory (KMT) models an ideal gas as tiny particles in constant, random motion with negligible volume compared to their container. Collisions between particles and with container walls are perfectly elastic (no energy lost). There are assumed to be no attractive or repulsive forces between particles, and average kinetic energy depends only on temperature (Kelvin).

Temperature, speed, and pressure

As temperature increases, average kinetic energy and particle speed increase, causing more frequent and forceful collisions with container walls, raising pressure (if volume is fixed). At the same temperature, lighter molecules move faster than heavier ones since KE = ½mv² is equal for all gases at a given T, so mass and velocity are inversely related.

Limits of the ideal model

Real gases deviate from KMT predictions at high pressure (particles are forced close together, so volume isn't negligible) and low temperature (particles move slowly enough for intermolecular forces to matter). Gases with stronger intermolecular attractions or larger molecular size deviate more from ideal behavior under these conditions.

Key terms

Kinetic Molecular Theory (KMT)

A model describing ideal gas behavior: particles in constant random motion, negligible volume, elastic collisions, no intermolecular forces, KE depends only on temperature.

Elastic collision

A collision in which no kinetic energy is lost; total kinetic energy is conserved.

Relationship between temperature and kinetic energy

Average kinetic energy of gas particles is directly proportional to temperature in Kelvin.

Why do lighter gases move faster at the same T?

Because KE = ½mv² is equal for all gases at a given temperature, smaller mass means greater average speed.

When do real gases deviate from KMT?

At high pressure (particle volume matters) and low temperature (intermolecular forces become significant).

Try an AP-style question

At the same temperature, how do the average kinetic energies of He and Ar compare?

1. He's is greater
2. They are equal
3. Ar's is greater
4. It depends on pressure