What Is The Difference Between Diffusion and Effusion?

Diffusion vs Effusion: Gas Transport Mechanisms

This diagram shows effusion (left) and diffusion (right).
This diagram shows effusion (left) and diffusion (right). Effusion occurs through a hole smaller than the mean path of moving particles while diffusion occurs through a hole large enough allow multiple particles to pass at once. Astrang13, Creative Common License

When a volume of gas is opened to another volume with less pressure, the gas can either diffuse or effuse into the container. The main difference between diffusion and effusion is the barrier between the two volumes.

Effusion occurs when there is a barrier with one or many small holes that prevent the gas to expand into the new volume unless a gas molecule happens to travel through the hole. The term "small" when referring to the holes are holes with diameters less than the mean free path of the gas molecules.

The mean free path is the average distance traveled by an individual gas molecule before it collides with another gas molecule.

Diffusion occurs when the holes in the barrier are larger than the mean free path of the gas. If there is no barrier at all, you can think of a barrier with one large hole big enough to cover the boundary between the two volumes. This would mean the gas will diffuse into the new container.

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 get to their destination. Essentially, negative pressure causes quick movement. 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.

Examples of Diffusion