Effusion
Definition and meaning of Effusion in chemistry.
Effusion is the process by which gas molecules escape through a tiny opening into a vacuum or a region of lower pressure, passing through one at a time without colliding with each other. It occurs when the diameter of the opening is smaller than the mean free path of the gas molecules.
In more detail
The rate of effusion depends only on the average speed of the gas molecules, which in turn depends on molar mass: lighter molecules move faster on average and effuse more quickly. This relationship is quantified by Graham's law of effusion, which states that the rate of effusion is inversely proportional to the square root of the molar mass. Effusion rates can be used experimentally to compare or determine unknown molar masses, and the effect underlies techniques such as gaseous diffusion/effusion cascades once used to enrich uranium isotopes via uranium hexafluoride (UF6).
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing law | Graham's law of effusion |
| Rate relationship | rate1/rate2 = √(M2/M1) |
| Key application | Isotope separation (e.g., UF6 enrichment) |
A helium-filled balloon deflates noticeably faster than an air-filled one because the small, low-mass helium atoms effuse through the microscopic pores of the rubber more rapidly than the heavier N2 and O2 molecules in air.
Frequently asked questions
How is effusion different from diffusion?
Effusion is the escape of gas molecules one at a time through a small hole into a vacuum without intermolecular collisions, whereas diffusion is the gradual mixing of gases as molecules collide and spread through a mixture or another gas.
What does Graham's law say about effusion rates?
Graham's law states that the effusion rate of a gas is inversely proportional to the square root of its molar mass, so lighter gases effuse faster than heavier ones under the same conditions.