Graham's Law
Definition and meaning of Graham's Law in chemistry.
Graham's law states that the rate of effusion of a gas is inversely proportional to the square root of its molar mass, at a given temperature and pressure.
In more detail
Formulated by Thomas Graham in 1848, the law follows from kinetic molecular theory: at the same temperature, all gas molecules have the same average kinetic energy, so lighter molecules must move faster on average than heavier ones. Faster-moving molecules escape more readily through a small orifice into a vacuum (effusion), giving the relationship rate1/rate2 = √(M2/M1). The same reasoning approximately describes diffusion, the spreading of one gas through another. Graham's law has practical importance in separating gases of different molar mass, most famously in enriching uranium-235 by gaseous diffusion of uranium hexafluoride (UF6).
Key facts
| Formula | rate1/rate2 = √(M2/M1) |
|---|---|
| Field | Physical Chemistry |
| Discovered by | Thomas Graham (1848) |
| Key application | Gaseous diffusion enrichment of UF6 (uranium-235) |
Comparing hydrogen (M = 2 g/mol) and oxygen (M = 32 g/mol): rate(H2)/rate(O2) = √(32/2) = √16 = 4, so H2 effuses four times faster than O2 under identical conditions.
Frequently asked questions
Does Graham's law apply to diffusion or only effusion?
It is derived rigorously for effusion, the escape of gas molecules through a tiny orifice into a vacuum. It also approximately describes diffusion (mixing of gases), though effusion is the precise, textbook case.
Why do lighter gas molecules effuse faster?
At a given temperature, all gases share the same average kinetic energy (½mv²). Since mass differs, a lighter molecule must have a higher average speed to match that energy, letting it pass through an opening more quickly.