How would you compare diffusion with effusion?

1 Answer
Jul 13, 2017

Here's my interpretation.


Diffusion is when a substance disperses evenly throughout a medium; i.e. a perfume dispersing its scent through a room:

Effusion is when a substance "escapes" through a (tiny) opening; i.e. helium in a balloon escaping through the balloon's rubber network:

We can infer that the rates of diffusion and effusion of a gas depend on the speed of the particles.

In the effusion image, we notice that #"He"# effuses faster than #"C"_2"H"_4"O"_2#....but why?

Graham's law of effusion (or diffusion) explains this:

#(r_1)/(r_2) = sqrt((M_2)/(M_1))#


  • #(r_1)/(r_2)# is the ratio of the rates of effusion of a gas 1 and a gas 2

  • #M_1# and #M_2# are the molar masses of gases 1 and 2

This equation tells us that

The rate of effusion (and diffusion) of a gas is inversely proportional to the square root of its molar mass.

Therefore, the lower the mass of a gas, the higher its rate of effusion and the higher its speed.

This makes intuitive sense, as a gas with less mass should move faster than a gas with more mass.

In terms of rates and speeds, effusion and diffusion are quite similar.

The difference between the two is that diffusion is the spreading out of a substance within a dispersing medium, and effusion is when a substance escapes through a tiny pinhole, or other hole.

Another way to differentiate between diffusion and effusion is to imagine effusion as motion of particles through a hole one-dimensionally (i.e. #+-x#), and diffusion allows for all three dimensions (i.e. #+-x, +-y, +-z#).

#uarr# Credit Truong-Son N. for this addition #uarr#