What Is the Difference Between Diffusion and Effusion?

When a volume of gas is released from one smaller area to another larger area with less pressure, the gas either diffuses or effuses into the container. The primary difference between diffusion and effusion is the barrier, which filters the gas as it moves between the two volumes.

The Barrier Is Key

Effusion occurs when a barrier with one or many small holes prevents gas from expanding into the new volume unless a gas molecule happens to travel through the hole. The term "small" refers to holes with diameters less than the mean free path of the gas molecules. Mean free path is the average distance traveled by an individual gas molecule before it collides with another gas molecule.

Diffusion occurs when holes in a barrier are larger than the mean free path of a gas. If no barrier exists, consider a "barrier" with one large hole big enough to cover the boundary between the two volumes.

Handy reminder: small holes = effusion, big holes = diffusion

Which Is Faster?

Effusion typically transports particles more quickly because they don't have to move around other particles to reach their destination. Essentially, negative pressure causes quick movement. 

Lacking the same level of negative pressure, the rate at which diffusion occurs is limited by the size and kinetic energy of the other particles in the solution, in addition to the concentration gradient.