# Kinetic molecular theory

Kinetic molecular theory models a gas as tiny particles in constant random motion with no attraction between them. Temperature measures average kinetic energy. At the same temperature, lighter molecules move faster.

Chemistry (university) · Part 03: Intermolecular forces, gases and solutions · Card 6 of 9 · Last updated 2026-10-19

## Analogy

A room full of bouncing balls: warm the room and the balls bounce faster and hit the walls harder.

## Example

**Speed of N₂ at 298 K**

u = √(3RT/M)
M = 0.0280 kg/mol
u = √(3 × 8.314 × 298 / 0.0280)
u ≈ 515 m/s

## In practice

The theory explains why gases spread to fill a container and why helium leaks from a balloon faster than air.

## Practice

### Flashcards

- **What does temperature measure in kinetic theory?** The average kinetic energy of the particles.
- **Why does lighter gas effuse faster?** Rate is inversely proportional to the square root of molar mass (Graham's law).

## Questions people ask

### Do all gas molecules at one temperature move at the same speed?

No. At a given temperature, molecules have a wide spread of speeds, described by the Maxwell-Boltzmann distribution. Only the average kinetic energy is the same for all gases. A lighter gas has a higher average speed than a heavier one, but individual molecules vary.

### What is Graham's law of effusion?

Graham's law says the rate at which a gas leaks through a tiny hole is inversely proportional to the square root of its molar mass. Light gases effuse faster. Hydrogen, with a molar mass of 2, effuses about four times faster than oxygen, with a molar mass of 32.

## Related concepts

- [Ideal gas law](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-4)
- [Dalton's law of partial pressures](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-5)
- [Real gases and van der Waals equation](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-7)

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