Diffusion and Effusion of Gases
Diffusion of Gases
Gases have a general tendency to mix with one another even against the law of gravitation. The process of free mixing of gases at the same temperature and pressure is called diffusion.
Example: When a gas jar is full of hydrogen is inverted over aa gas jar full of brown NO2 gas and the two jars are left as such for some time, it is found that both the jars are filled with brown vapours. This is because of diffusion. Heavier gas NO2 diffuses from lower jar to upper jar and lighter hydrogen gas diffuses from upper jar to lower jar against the law of gravitation. As a result of this a brown mixture of hydrogen and NO2 are found to be present in both the jars.
Effusion of Gases
The process by which a gas is escapes out of a container through a small orifice is called effusion.
In case of effusion either temperature or pressure of the system must be different from its surrounding.
Example: A container is filled with gas at high pressure and the gas is moving out through a small orifice (hole) when someone make a orifice at the wall of container. This is due to effusion of gas.
Another example: If a balloon is filled with air at high pressure is picked by means of a needle, a small orifice is formed in the balloon and air from balloon comes out of it. As a result of this, the balloon shrinks. This is due to effusion of air.
Both diffusion and effusion are affected by the molecular weight of the gas particles. For example, a lighter gas will have a faster effusion rate than a heavier gas.
Rate of Diffusion or Effusion
Change in volume or change in pressure of a gas in unit time due to diffusion or effusion is called rate of diffusion or effusion of the gas.
If 'V' volume of gas diffused in time 't' then-
Rate of diffusion = V/t
If pressure of a gas drops from P1 mm to P2 mm in time 't' minutes due to effusion then
Rate of effusion = (P1 − P2)/t
The rate of effusion is inversely proportional to the square root of the molar mass of the gas, which is known as Graham's law.
