Diffusion of gases

this is for class 9 students only

2. Diffusion occurs in liquids and gases but hardly at all in
solids.
What is diffusion?
Diffusion is the movement of
particles that allows them to spread
out and mix with other particles.
For example, the smell of aftershave
or perfume diffuses and is detected by
people on the other side of the room.
Use the particle model to explain these facts about diffusion:
Diffusion happens more quickly for gases than for liquids.
Diffusion happens more quickly at warm temperatures
than at cooler temperatures.

3. How do smells spread out?
Where is the smell coming from and how does it spread out?

4. Diffusion animation 1Diffusion animation 1

5. Diffusion animation Diffusion animation
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6. When the gases meet, they react to form a ring of ammonium When the gases meet, they react to form a ring of ammonium
chloride which is closer to one end of the tube than the other.chloride which is closer to one end of the tube than the other.
cotton wool soaked
in ammonia
solution
cotton wool soaked
in concentrated
hydrochloric acid
Which gas particles diffuse faster, ammonia or acid, and why?
By the time the gases meet, the ammonia particles
have travelled further down the tube so they must
have diffused at a faster speed.
Diffusion experimentDiffusion experiment
In this experiment, two gases diffuse towards each other in
a sealed glass tube.
gases
meet
here

7. The distribution of speeds
for nitrogen gas molecules
at three different temperatures
The distribution of speeds
of three different gases
at the same temperature
5.7
u
rms
=
3RT
MÖ
Velocity of a Gas

8. Gas diffusion is the gradual mixing of molecules of one gas
with molecules of another by virtue of their kinetic properties.
5.7
NH
3
17 g/mol
HCl
36 g/mol
NH
4
Cl

9. Gas DiffusionGas Diffusion
relation of mass to rate of diffusionrelation of mass to rate of diffusion
•HCl and NHHCl and NH
33 diffuse diffuse
from opposite ends of from opposite ends of
tube. tube.
•Gases meet to form Gases meet to form
NHNH
44ClCl
•HCl heavier than NHHCl heavier than NH
33
•Therefore, NHTherefore, NH
44Cl forms Cl forms
closer to HCl end of closer to HCl end of
tube.tube.

10. GAS DIFFUSION AND GAS DIFFUSION AND
EFFUSIONEFFUSION
diffusiondiffusion is the is the
gradual mixing gradual mixing
of molecules of of molecules of
different gases.different gases.
effusioneffusion is the is the
movement of movement of
molecules molecules
through a small through a small
hole into an hole into an
empty empty
container.container.

11. Graham’s Law: Diffusion
and Effusion of Gases
•Diffusion the process whereby a gas spreads out through another
gas to occupy the space with uniform partial pressure.
•Effusion the process in which a gas flows through a small hole in a
container.
•Graham’s law of Effusion the rate of effusion of gas molecules
through a hole is inversely proportional to the square root of the
molecular mass of the gas at constant temperature and pressure.
E.g. determine the molecular mass of an unknown compound if it
effused through a small orifice if it effused 3.55 times slower
than CH
4.
E.g. A compound with a molecular mass of 32.0 g/mol effused through
a small opening in 35 s; determine the effusion time for the same
amount of a compound with a molecular mass of 16.0.
MW
k
Rate=

12. GAS DIFFUSION AND GAS DIFFUSION AND
EFFUSIONEFFUSION
Graham’s law governs Graham’s law governs
effusion and diffusion effusion and diffusion
of gas molecules. of gas molecules.
KE=1/2 mvKE=1/2 mv
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Thomas Graham, 1805-1869. Thomas Graham, 1805-1869.
Professor in Glasgow and London.Professor in Glasgow and London.
Rate of effusion is Rate of effusion is
inversely proportional inversely proportional
to its molar mass.to its molar mass.
M of A
M of B
Rate for B
Rate for A

13. GAS DIFFUSION AND GAS DIFFUSION AND
EFFUSIONEFFUSION
Molecules effuse thru holes in a Molecules effuse thru holes in a
rubber balloon, for example, rubber balloon, for example,
at a rate (= moles/time) that isat a rate (= moles/time) that is
proportional to Tproportional to T
inversely proportional to M.inversely proportional to M.
Therefore, He effuses more Therefore, He effuses more
rapidly than Orapidly than O
22 at same T. at same T.
HeHe