Question

During the practical in the lab, when hydrogen sulphide gas haing offensive odour prepared for some test, we can smell the gas even 50 meters away. Explain the phenomenon.

Answer 1

Hint: There is a law related to this phenomenon and is stated by Graham. This occurs because of the movement of gas molecules from an area of high chemical potential to low chemical potential.

Complete answer:
We are able to smell hydrogen sulphide gas even at a distance of 50 meters due to the phenomenon known as ‘diffusion’.
The diffusion is a net movement of atoms or molecules from a highly concentrated region to a low concentration region.
In other words, the movement of atoms or molecules from a high chemical potential region to the low chemical potential region.
The word diffusion is derived from a Latin word – diffundere, to spread out. Which means a substance that spreads out or moves from an area to another.
Diffusion should not be confused with other transport phenomena like advection or convection, where it moves particles from one place to another in a bulk motion.
The movement of molecules in solids is very small, relatively large in liquids and very large in gases. And this movement of molecules has led to a phenomenon called diffusion.
The thermal motion of gas particles at above absolute zero temperature is called molecular diffusion.
The rate of this phenomenon movement is a function of the viscosity of the gas, temperature, and size of the particles.
Lighter gas molecules can diffuse at a faster rate than heavier gas molecules.
$rate\quad of\quad diffusion\quad =\dfrac { amount\quad of\quad gas\quad passing\quad through\quad a\quad unit\quad area }{ unit\quad of\quad time } $
Rate of diffusion of particular gas can be found by using Graham’s law of diffusion, which says that at constant temperature and pressure, the diffusion of a gas is inversely proportional to the square root of molar mass or density.
$rate\quad of\quad diffusion\quad \alpha \dfrac { 1 }{ \sqrt { molar\quad mass/density } } $
We can now look at an example to understand the diffusion of gases.
Take a container, separate it into two partitions.
 Keep two gases P and Q at same pressure in two parts of the container.
The molecules of gas P and Q are in continuous random motion in its respective compartments.
 Now, remove the partition of the container.
The molecules of gas P will begin to stray into gas Q due to the random motion.
In the same way, the molecules of gas Q will begin to stray into gas P due to the random motion.
As time passes, the molecules of both gases continue to stray with each other.
 In a period of time, the whole mass of gas in the container will be a homogeneous mixture of gas P and gas Q and this results because of the phenomenon called diffusion.

Note: Temperature and density are the two main factors affecting diffusion. As temperature increases diffusion also increases. Molecules with high densities diffuse at a lower rate when compared with molecules with low densities.