Kinetic Molecular Theory of Gases:
Scientists use the kinetic molecular theory (KMT) to describe the
behavior of gases.
The KMT consists of several postulates:
1. Gases consist of small particles (atoms or molecules) that move
randomly with rapid velocities.
2. Gas particles have little attraction for one another. Therefore, attractive
forces between gas molecules can be ignored.
3. The distance between the particles is large compared to their size.
Therefore the volume occupied by gas molecules is small compared to
the volume of the gas.
4. Gas particles move in straight lines and collide with each other and the
container frequently. The force of collisions of the gas particles with the
walls of the container causes pressure.
5. The average kinetic energy of gas molecules is directly proportional to
the absolute temperature (Kelvin).
What Is The Difference Between Diffusion and
Effusion?
when a volume of gas is opened to another volume with less pressure, the
gas can either diffuse or effuse into the container. The main difference
between diffusion and effusion is the barrier between the two volumes.
Effusion : When there is a barrier with one or many small holes that
prevent the gas to expand into the new volume unless a gas molecule
happens to travel through the hole. The term "small" when referring to
the holes are holes with diameters less than the mean free path of the gas
molecules. The mean free path is the average distance traveled by an
individual gas molecule before it collides with another gas molecule.
Diffusion: occurs when the holes in the barrier are larger than the mean
free path of the gas. If there is no barrier at all, you can think of a barrier
with one large hole big enough to cover the boundary between the two
volumes. This would mean the gas will diffuse into the new container.
This diagram shows effusion (left) and diffusion (right). Effusion
occurs through a hole smaller than the mean path of moving particles
while diffusion occurs through a hole large enough allow multiple
particles to pass at once.
Graham's law :
Graham's law, also known as Graham's law of effusion, was formulated
by Scottish physical chemist Thomas Graham in 1848. Graham found
experimentally that the rate of effusion of a gas is inversely proportional
to the square root of the mass of its particles.
1- Graham's Law of Diffusion
The rate at which gases diffuse is inversely proportional to the
square root of their densities.
Since volumes of different gases contain the same number of
particles , the number of moles per liter at a given T and P is
constant. Therefore, the density of a gas is directly proportional to
its molar mass (MM).
2- Graham's Law of Effusion
The rate of effusion of a gas is inversely proportional to the square
root of either the density or the molar mass of the gas.