Molecular Effusion and Diffusion

Molecular Effusion and Diffusion

Kinetic-molecular theory stated that

The average kinetic energy of molecules is proportional to absolute temperature

where M is the molar mass


Calculate the rms speed, u, of an N2 molecule at room temperature (25C)

T = (25+273)K = 298K

M = 28 g/mol = 0.028 kg/mol

R = 8.314 J/mol K = 8.314 kg m2/s2 mol K

Note: this is equal to 1,150 miles/hour!


The rate of escape of a gas through a tiny pore or pinhole in its container.

The effusion rate, r, has been found to be inversely proportional to the square root of its molar mass:

and a lighter gas will effuse more rapidly than a heavy gas:

Basis of effusion

Diffusion: the spread of one substance through space, or though a second substance (such as the atmosphere)

Diffusion and Mean Free Path

the rates of diffusion are slower than molecular speeds due to molecular collisions

The average distance traveled by a molecule between collisions is the mean free path

1996 Michael Blaber