Difference between Adsorption and Absorption - Practice Questions & MCQ

What is adsorption?

There are several examples, which reveal that the surface of a solid has the tendency to attract and retain the molecules of the phase with which it comes into contact. These molecules remain only at the surface and do not go deeper into the bulk. The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. 

The term adsorption was first used by Kayser which refers strictly to the existence of a higher concentration of any particular component (adsorbate) at the surface of liquid or solid phase (adsorbent).

Now, what are these terms Adsorbate and Adsorbent?

The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.

Till now, you must have learnt about adsorption. But do you know there is another phenomenon which takes place throughout the bulk of the solid and this phenomenon is known as absorption.

Let's learn about the distinction between the two phenomena by using examples.

When a chalk stick is dipped in ink, the surface retains the colour of the ink due to adsorption of coloured molecules while the solvent of the ink goes deeper into the stick due to absorption. On breaking the chalk stick, it is found to be white from inside, because only water enters the bulk of chalk not ink.

A distinction can be made between absorption and adsorption by taking an example of water vapour. Water vapours are absorbed by anhydrous calcium chloride but adsorbed by silica gel. In other words, in adsorption the concentration of the adsorbate increases only at the surface of the adsorbent, while in absorption the concentration is uniform throughout the bulk of the solid.

What is the mechanism of adsorption?

Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. Inside the adsorbent, all the forces acting between the particles are mutually balanced but on the surface, the particles are not surrounded by atoms or molecules of their kind on all sides, and hence they possess unbalanced or residual attractive forces. These forces of the adsorbent are responsible for attracting the adsorbate particles on its surface. The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure.

Concept of residual forces.

In the interior of a liquid or solid, molecules experiences attractions from all sides, whereas molecules on the surface have other neighboring molecules below and on the sides. The surface thus possesses residual unbalanced forces and molecules on the surface possess higher energy. The adsorption on the surface takes place due to these residual unbalanced forces at the surface of adsorbent.

Now as you have learned about the mechanism of adsorption, you must be curious as to what decides the extent of adsorption. So there are some factors listed below that govern the extent of adsorption.

1. Nature of gas or Adsorbent - gases with higher critical temperature or easily liquefiable due to stronger Vander Waal's force of attraction are more adsorbed. Example, NH₃, Cl₂, HCl
2. Effect of Pressure - At constant temperature, adsorption increases with an increase of pressure.
3. Effect of Temperature - Adsorption decreases with an increase in temperature because adsorption is an exothermic process. 
4. Surface area of Adsorbent - The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure. The larger is the surface area of the adsorbent more is adsorption. The surface area per gm of adsorbent is called the specific surface area of adsorbent. That is why a porous or finely divided form of adsorbents adsorb larger quantities of adsorbate.
5. Activation of Solid Adsorbent - When already adsorbed gases are removed or the solid is divided further, it becomes activated adsorbent and rate of adsorption increases.
6. Heat of Adsorption - Another important factor featuring adsorption is the heat of adsorption. During adsorption, there is always a decrease in residual forces of the surface, i.e., there is a decrease in surface energy which appears as heat. Adsorption, therefore, is invariably an exothermic process. In other words,  of adsorption is always negative.

This concept will tell you about the thermodynamic science behind the process of adsorption and when equilibrium is achieved. Let's look at the below text.

Adsorption is accompanied by decrease in enthalpy as well as decrease in entropy of the system. For a process to be spontaneous, the thermodynamic requirement is that, at constant temperature and pressure, G must be negative, i.e., there is a decrease in Gibbs energy. On the basis of equation, G = H – TS, G can be negative if H has sufficiently high negative value as – TS is positive. Thus, in an adsorption process, which is spontaneous, a combination of these two factors makes G negative. As the adsorption proceeds, H becomes less and less negative ultimately H becomes equal to TS and G becomes zero. At this state, equilibrium is attained.