GRAHAM’S LAW OF DIFFUSION
Diffusion is a process of
intermixing of gases. This is a natural tendency of any gas to occupy the whole
volume available to it. So even without pressure difference gases mix with each
other. If the pressure difference is increased the diffusion process becomes
faster.(GRAHAM’S LAW OF DIFFUSION).
Effusion is flow of a gas through
small openings due to pressure difference that is it is a case of fast
diffusion.
(a) Diffusion is mixing of gas molecules by
random motion under conditions where molecular collisions occur. Diffusion is
the process by which molecules move and travel from one place to another
without requiring bulk motion. Diffusion results in molecules moving or mixing
by only using kinetic energy.
(b)
Effusion is the escape of a gas through a pinhole without molecular collisions.
Effusion is the process by which molecules travel through a pinhole from a
place of high concentration to low concentration. The process describes the
ability of gas to travel through a small hole without collisions between
molecules. This occurs when the diameter of the hole is considerably smaller
than the mean free path of the molecules. The mean free path is the average
distance traveled by a moving particle between successive collisions.
According
to Graham’s Law of diffusion the rate of diffusion depends on pressure and molecular weight
of a gas. Rate of Diffusion is directly proportional to Partial Pressure of the
gas and inversely proportional to the square root of molar mass of the gas.
That is, greater the pressure faster is the movement of molecules and heavier
the molecules, slower are their movement.
Rate
of diffusion α
P (partial pressure)
Also,
Rate of diffusion α
1/ √M
Therefore
we can write:
Rate,
r α P/√M
Rate
of diffusion may be defined in various ways. It may be number of moles
transferred per unit time, Volume transferred per unit time, distance travelled
per unit time and even pressure drop per unit time. While comparing the rates
of two gases, we should take similar definitions of rate, it may be n/t, V/t or
d/t.
Graham’s Law of diffusion can be applied to diffusion and effusion
both.
Worked example 1:
What
is the ratio of the rate of effusion of neon gas to that of helium gas at the
same temperature and pressure?
Worked example 2:
The
pressure in a vessel that contained pure oxygen dropped from 2000 torr to 1500
torr in 55 min as the oxygen leaked through a small hole into a vacuum. When
the same vessel was filled with another gas, the pressure dropped from 2000
torr to 1500 torr in 85 min. What is the molecular weight of the second gas?
