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.
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Analogy
A room full of bouncing balls: warm the room and the balls bounce faster and hit the walls harder.
Example
u = √(3RT/M)M = 0.0280 kg/molu = √(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.