The Kinetic Model of a Gas
The Kinetic Model of a Gas
- The kinetic model treats a gas as a very large number of tiny particles in motion.
- First look at the evidence that the particles really do move. Then state what the model assumes.
Background evidence: Brownian motion
Background only: the current Topic 15 syllabus does not ask you to define Brownian motion. Use this short example to see why random molecular motion is a reasonable model.
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- The visible dots are positions of one smoke particle, not positions of air molecules. Its irregular movement is called .
- Air molecules hit the particle from all directions. The impacts are not perfectly balanced at each instant, so the particle changes direction.
- The changing path is evidence that gas molecules are moving in random directions. It is not a picture of an air molecule's own path.
The assumptions of the kinetic model
- Papers ask you to state two or three of these assumptions, and each one is worth one mark (9702/42/O/N/24 Q4(a)) (9702/42/O/N/25 Q2(b)) (9702/41/M/J/23 Q4(a)).
A gas contains a very large number of molecules.
Even a small volume contains an enormous number.
The molecules are in constant random motion.
There is no preferred direction.
There are no intermolecular forces except during collisions.
Between collisions, each molecule moves freely.
Molecular volume is negligible compared with gas volume.
Most of the container is empty space.
Collisions are perfectly elastic and take negligible time.
Total kinetic energy is conserved in each collision.
Learn them as five short phrases: very many molecules; constant random motion; no forces except in collisions; negligible molecular volume; perfectly elastic collisions. Write them in that plain form and each one scores.
- Where the model breaks. The third and fourth assumptions are the first to fail. At very high pressure, the molecules are close together. Their volume and the forces between them are no longer negligible.
- When those assumptions fail, the ideal-gas relationship is no longer a close model of the real gas.