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.

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

Diffusion animation 1Diffusion animation 1

Diffusion animation 2Diffusion animation 2

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 Diffusion experimentexperimentIn this experiment, two gases diffuse towards each other in

a sealed glass tube.

gasesmeethere

The distribution of speedsfor nitrogen gas molecules

at three different temperatures

The distribution of speedsof three different gases

at the same temperature

5.7

urms = 3RTM

Velocity of a Gas

Gas diffusion is the gradual mixing of molecules of one gas with molecules of another by virtue of their kinetic properties.

5.7

NH3

17 g/molHCl

36 g/mol

NH4Cl

Gas DiffusionGas Diffusionrelation of mass to rate of relation of mass to rate of

diffusiondiffusion

Gas DiffusionGas Diffusionrelation of mass to rate of relation of mass to rate of

diffusiondiffusion

• HCl and NHHCl and NH33 diffuse diffuse from opposite ends from opposite ends of tube. of tube.

• Gases meet to form Gases meet to form NHNH44ClCl

• HCl heavier than HCl heavier than NHNH33

• Therefore, NHTherefore, NH44Cl Cl forms closer to HCl forms closer to HCl end of tube.end of tube.

• HCl and NHHCl and NH33 diffuse diffuse from opposite ends from opposite ends of tube. of tube.

• Gases meet to form Gases meet to form NHNH44ClCl

• HCl heavier than HCl heavier than NHNH33

• Therefore, NHTherefore, NH44Cl Cl forms closer to HCl forms closer to HCl end of tube.end of tube.

GAS DIFFUSION AND GAS DIFFUSION AND EFFUSIONEFFUSION

diffusiondiffusion is the is the gradual mixing of gradual mixing of molecules of molecules of different gases.different gases.

effusioneffusion is the is the movement of movement of molecules through a molecules through a small hole into an small hole into an empty container.empty container.

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 CH4.

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.

MWk

Rate

GAS DIFFUSION AND GAS DIFFUSION AND EFFUSIONEFFUSION

Graham’s law governs Graham’s law governs effusion and diffusion of effusion and diffusion of gas molecules. gas molecules. KE=1/2 KE=1/2 mvmv22

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.

Rate of effusion is Rate of effusion is inversely proportional inversely proportional to its molar mass.to its molar mass.

M of AM of B

Rate for B

Rate for A

GAS DIFFUSION AND GAS DIFFUSION AND EFFUSIONEFFUSION

Molecules effuse thru holes in a Molecules effuse thru holes in a rubber balloon, for example, at a rubber balloon, for example, at a rate (= moles/time) that israte (= moles/time) that is

proportional to Tproportional to T inversely proportional to M.inversely proportional to M.

Therefore, He effuses more rapidly Therefore, He effuses more rapidly than Othan O22 at same T. at same T.

HeHe