1. Diffusion

2. Atomic Collision  Molecules in a gas make collisions with each other as well as the wall.  There is an average time and average distance between collisions.

3. Mean Free Path  A particle of radius r must avoid other particles so it traces a cylinder of radius 2r.  The length of the cylinder is the speed times the time.  There are N/V particles in the volume, but they move reducing the path.  The combination gives the mean free path.

4. Random Walk  A molecule will move in random directions after each collision.  If it starts in one place it will eventually move away.  The motion is called a random walk.

5. Concentrated Region  Molecules in a concentrated region will disperse into the rest of the medium.  The difference in concentrations is the concentration gradient. C1C1 xx C2C2

6. Diffusion Constant  Different materials pass through an area at different rates. Depends on concentrateDepends on concentrate Depends on mediumDepends on medium C1C1 xx C2C2 A D O 2 in air: D = 1.8 x 10 -5 m 2 /s O 2 in water: D = 1.0 x 10 -9 m 2 /s DNA in water: D = 1.3 x 10 -12 m 2 /s

7. Fick’s Law  The rate of diffusion depends on the concentration gradient, area, and diffusion constant.  J measures mol/m 3.  There is a diffusion time t for diffusion in one direction over a distance  x.