Molecular+Effusion+and+Diffusion

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[[image:effusion.JPG]]
Effusion is the process of individual gas molecules flowing through an opening one-by-one with no molecular collisions. An example of this is the gas escaping from a balloon.

A requirement for effusion to take place is that the diameter of the area where gas is escaping must be significantly smaller than the mean free path. As Scottish physical chemist Thomas Graham discovered, the rate at which molecules effuse is dependent of molecular weight. To be precise, the rate of effusion is inversely proportional of the square root of its molar mass (equation below).

Graham's Law also states that gases of lower molecular weights effuse more quickly than gases of greater molecular weight. For example, helium balloons will "go flat" sooner than a balloon filled with oxygen, because helium has a lesser molar mass. This law is more accurate when being applied to effusion, but is also relevant to diffusion. Effusion can be practically applied to the separation of isotopes, which in turn led to the development of the atomic bomb.

[[image:diffusion.JPG]]
Diffusion is faster for lighter molecules than it is for heavier ones. At room temperature, nitrogen moves at a speed of 515 m/s, or 1,150 mph. However, if perfume is sprayed at one end of the room it will take a few minutes to get to the other side and fill the room.This is because the molecules are constantly going in different directions and changing speeds. The average distance traveled by a molecule between collisions is the **mean free path**. When pressure goes up then the mean free path is short, and when the pressure is low the mean free path is long. This is like driving on the highway. When there are many cars (high pressure) you must stop a lot and maybe change lanes. When there are not many cars (low pressure) you do not need to change lanes as much or stop a lot, making a longer mean free path because other cars are not in your path.

Pictures
[|http://www.balloonbrites.com] [|http://en.wikipedia.org/wiki/Graham%27s_Law_of_Effusion] http://www.immediart.com/catalog/product_info.php?cPath=61_62&products_id=270 http://epswww.unm.edu/coursinf/eps462/graphics/diffusion.gif

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