Effusion
Definition and meaning of Effusion in chemistry.
Effusion is the process by which gas molecules escape one at a time through a tiny opening into a region of lower pressure or a vacuum. It happens only when the opening is smaller than the average distance a molecule travels between collisions, called the mean free path. This keeps molecules from bumping into each other as they pass through.
In more detail
The rate of effusion depends on how fast the gas molecules move on average, and that speed depends on molar mass. Lighter molecules move faster than heavier ones at the same temperature, so they effuse more quickly. This relationship is described by Graham's law of effusion.
The law states that effusion rate is inversely proportional to the square root of molar mass. Chemists use effusion rates to compare gases or find an unknown molar mass experimentally. The same principle powers gaseous diffusion cascades, once used to separate uranium isotopes.
In that process, uranium hexafluoride gas (UF6) containing the lighter uranium-235 isotope effuses slightly faster than UF6 containing uranium-238. Repeating this separation through thousands of stages gradually enriches the lighter isotope. This behavior follows directly from kinetic molecular theory.
All gases at the same temperature share the same average kinetic energy, regardless of molar mass. To match that energy, lighter molecules must move faster than heavier ones. Effusion also explains everyday observations.
A punctured tire loses air gradually instead of all at once, because air molecules escape through the tiny hole one at a time.
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing law | Graham's law of effusion |
| Rate relationship | rate1/rate2 = √(M2/M1) |
| Requirement | Opening smaller than the mean free path |
| Key application | Isotope separation (e.g., UF6 enrichment) |
A helium-filled balloon deflates faster than an air-filled one. Helium atoms are much lighter than the nitrogen and oxygen molecules that make up air. Because of their lower molar mass, helium atoms move faster and effuse more quickly through the tiny pores in the rubber.
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 colliding with each other. Diffusion is the gradual mixing of gases as molecules collide and spread through another gas or mixture.
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. Lighter gases effuse faster than heavier ones under the same conditions.
Why do lighter gases effuse faster than heavier ones?
At a given temperature, all gas molecules have the same average kinetic energy. Lighter molecules must move faster to have that same energy, so they reach the opening more often and effuse faster.