1. Diffusion, Effusion, and Graham’s Law of Diffusion

2. What is diffusion?
The movement of particles from regions of higher concentrations to regions of lower concentration.
Eventually, the particles will disperse evenly throughout the space.
Low
High

3. What is effusion?
The passage of a gas under pressure through a tiny opening.

4. Graham’s law of diffusion
Thomas Graham studied effusion in detail and determined that the rate of effusion is indirectly proportional to the square root of the molar mass of the gas, if pressure and temperature are kept constant.
In other words: vA vB =
Molar MassB
Molar MassA 

5.  How does that work? Let’s go back to the Kinetic Molecular Theory
We saw that the movement of particles is proportional to the amount of kinetic energy.
KE= ½ m v 2
So, if the KE is constant, ½ mA vA2 = ½ mB vB2
With a little algebra, we get: vA vB =
Molar MassB
Molar MassA 

6. Example Problems
At the same temperature, which molecule travels faster, O2 or N2? How much faster?
At room temperature, Xe atoms have an average speed of 240 m/s. At the same temperature, what is the speed of H2 molecules?
What is the molar mass of a gas if it diffuses times the speed of argon gas?

7. Answers 1) N2 (lower molar mass) (32/28) = 1.07 times faster
2) x/240 =  (131.3/2)
x/240 = 8.10
x = 1944 m/s
3) 1/0.907 = (x/39.9)
(1/0.907)2 = ((x/39.9))2
1.216 = x/39.9
x = 48.5 g/mol