|Diffusion is the spontaneous intermingling of one
with another. This occurs with perfumes and aftershaves and also with
fragrance of irate skunks. Diffusion is over when the molecules
the fragrance are evenly spread within a container, be it a room or a
flask. Another way to look at it is to describe the process using
partial pressures. Diffusion is over when the partial pressures
all the gases involved become identical in all parts of the
When a fragrance is more or less concentrated in a corner of a room,
partial pressure is higher there.
|Another necessary term is the gradient. A
is used to describe how the concentration or partial pressures change
one place to another. When sugar is added to coffee it sinks to
bottom. If left alone, (and the coffee stayed hot), the sugar
dissolve and gradially spread on its own accord throughout the
( Of course, we stir it because the time involved here is quite
At the bottom of the cup we have a lot of sugar. At the top of
cup we have zero dissolved sugar. A concentration gradient is set
up from bottom to top. Sugar will gradually move down this
until it is evenly spread out. When a freshly skunked pet comes
the room, it brings with it a high concentration of skunk perfume
Where the pet enters the house there will be a high conentration.
Everywhere else in the house there will be a zero concentration of
molecules. (Unless of course, this is not your pets first time, or your
a skunk rancher!) The point is, one of the great facts
natural processes is that "Nature abhors a vacuum." Another way
say this is that nature abhors gradients. Given the chance,
in nature tend to disappear, some rapidly and some only over eons of
until there is as much an evenness as possible.
|The effusion of a gas is its movement through an extremely
opening into a region of lower pressure. The term diffusion
really only speaks to the direction of gas movement. Effusion speaks
not only the direction but the rate that a change occurs.
|An English scienetist, Thomas Graham (1805-1869), studied
at which various gases effuse, and he found that the more dense the gas
is, the slower it effuses. The exact relationship between
and gas density, d, is called Graham's Law of Effusion.
|(effusion rate)A X (dA)1/2
= (effusion rate)B X (dB)1/2
|Finding the densities of gases at various temperatures is
difficult to do. With a little chemical slight of hand we can get
a formula for a much simpler answer.
|(effusion rate)A X
massA)1/2 = (effusion rate)B X
|Example: Under the same
temperature and pressure, does hydrogen iodide or ammonia effuse
Calculate the relative rates at which they effuse.
| molecular mass of HI =
molecular mass of NH3 = 17.04
( effusion rate of NH3 ) X (17.04)1/2 = ( effusion rate of HI ) X (127.91)1/2
This rearranges to rate for NH3
This rearrange into rate for NH3 =
|As a general rule the more a molecule masses the slower it
To find a mathematical value to this rate use Graham's Law.