Kinetic Molecular Theory
Introduced| English |
|---|
| kinetic molecular theory/kɪˈnetɪk məˈlekjʊlə ˈθɪəri/ |
| average kinetic energy/ˈævrɪdʒ kɪˈnetɪk ˈenədʒi/ |
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 ____.
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.
In the kinetic molecular theory, collisions are elastic (no kinetic energy is lost).
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...
Temperature tracks the average kinetic energy of the particles.
Equal temperature means all gases have equal average speed.
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...
Same average KE plus smaller mass means a higher speed.
Raising the temperature shifts the speed distribution toward...
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.