1. Graham’s Law of Diffusion

2. Graham’s Law of Diffusion
HCl NH3
NH4Cl(s)
100 cm
100 cm
Choice 1: Both gases move at the same speed and meet
in the middle.

3. Diffusion NH4Cl(s) HCl NH3 81.1 cm 118.9 cm
Choice 2: Lighter gas moves faster; meet closer to
heavier gas.

4. Graham’s Law Consider two gases at same temp. Gas 1: KE1 = ½ m1 v12
Since temp. is same, then… KE1 = KE2
½ m1 v12 = ½ m2 v22
m1 v12 = m2 v22
Divide both sides by m1 v22…
Take square root of both sides to get Graham’s Law:

5. Graham’s Law Diffusion Effusion
Spreading of gas molecules throughout a container until evenly distributed.
Effusion
Passing of gas molecules through a tiny opening in a container

6. KE = ½mv2 Graham’s Law Speed of diffusion/effusion
Kinetic energy is determined by the temperature of the gas.
At the same temp & KE, heavier molecules move more slowly.
Larger m  smaller v
KE = ½mv2

7. Graham’s Law Graham’s Law
Rate of diffusion of a gas is inversely related to the square root of its molar mass.
The equation shows the ratio of Gas A’s speed to Gas B’s speed.

8. Br
79.904 35 Kr
83.80 36
Graham’s Law
Determine the relative rate of diffusion for krypton and bromine.
The first gas is “Gas A” and the second gas is “Gas B”.
Relative rate mean find the ratio “vA/vB”.
Kr diffuses times faster than Br2.

9. Put the gas with the unknown speed as “Gas A”. 8 H 1
Graham’s Law
A molecule of oxygen gas has an average speed of m/s at a given temp and pressure. What is the average speed of hydrogen molecules at the same conditions?
Put the gas with the unknown speed as “Gas A”.

10. 1 O 8
Graham’s Law H
2.0
An unknown gas diffuses 4.0 times faster than O Find its molar mass.
The first gas is “Gas A” and the second gas is “Gas B”.
The ratio “vA/vB” is 4.0.
Square both sides to get rid of the square root sign.

11. Graham's Law
        Graham's Law
        Graham's Law

12. Diffusion

13. Gas Diffusion and Effusion
Graham's law
governs effusion and
diffusion of gas molecules.
Rate of effusion is inversely
proportional to its molar mass.
Thomas Graham
( )

14. To use Graham’s Law, both gases must be at same temperature.
diffusion: particle movement from
high to low concentration
NET MOVEMENT
effusion: diffusion of gas particles
through an opening
For gases, rates of diffusion & effusion obey Graham’s law:
more massive = slow; less massive = fast

15. Effusion Particles in regions of high concentration
spread out into regions of low concentration,
filling the space available to them.

17. Weather and Diffusion LOW Air Pressure HIGH Air Pressure
Map showing tornado risk in the U.S.
Highest
High

18. Calculation of Diffusion Rate
V1 = y
M1 = 17 amu
V2 = x
M2 = 36.5 amu
NH3
HCl
Substitute values into equation
V1 moves 1.465x for each 1x move of V2
NH3 (y= 1.465x) HCl x
1.465 x + 1x = x
Thus in the example at the beginning of the ppt:
200 cm / = 81.1 cm for x
In the 200 cm tube, HCl covers 81.1 cm and NH3 covers ( ) 118.9cm
of the distance within the tube