Diffusion and Effusion

Learning Goals

You will learn the difference between effusion and diffusion, and be able to calculate the difference in effusion rates of different gases.


When a person enters a room wearing a lot of a perfume, everyone knows it, beginning with the ones closest to the person and continuing to the person the furthest away. The perfume molecules mix with the other gases in the room and diffuse throughout the room. Gases being emitted from a chemical plant quickly diffuse throughout the region, and we quickly know if a paper mill is in the area - not as much so since the anti-pollution laws began affecting such emission. Diffusion of gases in the atmosphere is going on all around us all the time.

The second phenomenon studied in this section is effusion, which is the movement of gases through a porous plug of some sort between two containers at different pressures. The gases will low spontaneously (without outside help) from the region of high to the region of low pressure. Graham noted that the rate of flow of different gases was inversely proportional to the square root of their molecular weight. This observation leads us to Graham's law of effusion for the ratio of the effusion rates for two different gases. Experimental determinations of the effusion rates of two gases, one of which is an unknown gas, can lead to the determination of the molecular weight of the unknown gas.

Review Questions

  1. Work Exercise 12.14 in your text.
  2. Consider the diffusion of after-shave scent from a source as the molecules traverse throughout the room. The molecules are heavy compared with the other molecules in the air. Why don't the molecules just drop to the floor? In other words, what is the source of their velocity so that they traverse the room? "Talk" about the answers to the above and the molecular collisions as they travel throughout the room on the discussion forum.


Web Author: Dr. Leon L. Combs
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