Lesson 5: 3.5 Kinetic Molecular Theory

What does temperature actually measure at the particulate scale?
If a heavy gas and a light gas are at the same temperature, how do their average kinetic energies and average speeds compare?

Kinetic Molecular Theory
Maxwell-Boltzmann Distribution
Root-Mean-Square Speed
Elastic Collision
Absolute Temperature

Learning Objective 3.5.A; Suggested Skill 4.A (Model Analysis)

KMT states that average kinetic energy is directly proportional to absolute temperature (KEavg = 3/2 RT). Students analyze Maxwell-Boltzmann distribution curves to understand how particle speeds vary within a sample based on molar mass and temperature. They must recognize that lighter gases have a wider, lower distribution skewed toward higher molecular velocities.

Support: Use a digital animation showing light and heavy particles moving simultaneously at the same temperature, pointing out that light particles zip along faster to maintain the same average impact energy.
Extension: Present a graph with three unnamed Maxwell-Boltzmann curves and ask students to assign each curve to a different halogen gas (F2, Cl2, Br2) at 300K, explaining their reasoning.

A rigid container holds an equal molar mixture of Helium gas (4.00g/mol) and Neon gas (20.18g/mol) at a constant temperature of 298 K.

1. Compare the average kinetic energy of the Helium atoms to that of the Neon atoms.

2. Compare the average root-mean-square speed (vrms) of the Helium atoms to that of the Neon atoms. Justify your answer.