Kinetic Molecular Theory
Introduced| English | Meaning |
|---|---|
| kinetic molecular theory/kɪˈnetɪk məˈlekjʊlə ˈθɪəri/ | kinetic molecular theory |
| average kinetic energy/ˈævrɪdʒ kɪˈnetɪk ˈenədʒi/ | average kinetic energy |
A storm of invisible collisions
- A gas seems still, but its particles are in a frenzy.
- Billions of them zip about and slam into the walls.
- Those collisions are what we feel as pressure.
- A simple model explains gas behaviour from this chaos.
Gas pressure comes from particles colliding with the container ____.
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Countless collisions with the walls produce the pressure.
The model's rules
- The kinetic molecular theory 分子运动论 pictures gas particles as tiny and far apart, with no attractions.
- They move in constant, random, straight-line motion.
- Their collisions are elastic, losing no kinetic energy.
The Maxwell-Boltzmann distribution of molecular speeds shifts right when heated
In the kinetic molecular theory, collisions are elastic (no kinetic energy is lost).
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Elastic collisions conserve the total kinetic energy.
Temperature is average energy
- Temperature measures the average kinetic energy 平均动能 of the particles.
- Hotter means faster on average.
- At the same temperature, all gases share the same average kinetic energy.
Temperature is a measure of the particles' average...
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Temperature tracks the average kinetic energy of the particles.
Equal temperature means all gases have equal average speed.
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They share average kinetic energy, not speed -- mass differs.
A spread of speeds
- The particles do not all move at one speed; they follow a distribution.
- Lighter particles move faster than heavier ones at the same temperature.
- Raising the temperature shifts the whole spread to higher speeds.
Pressure from collisions
Kinetic theory: gas pressure comes from countless particles colliding with the walls. Shrink the volume and the collisions - and the pressure - increase.
At the same temperature, a lighter gas moves on average...
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Same average KE plus smaller mass means a higher speed.
Raising the temperature shifts the speed distribution toward...
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More thermal energy makes the particles faster on average.
At the same temperature, which moves faster on average, $\text{He}$ or $\text{O}_2$?
- Both have the same average kinetic energy.
- Helium is much lighter, so it must move faster.
Equal temperature means equal average kinetic energy, not equal speed — a lighter gas moves faster to match. Temperature depends on speed and mass together. And this theory is the idealization behind $PV = nRT$; real gases stray from it at high pressure and low temperature.
The kinetic molecular theory models a gas as tiny, non-attracting particles in constant random motion with elastic collisions. Temperature measures their average kinetic energy, so at one temperature all gases share it — and a lighter gas moves faster. This model underlies the ideal gas law.