Maharashtra Board Class 11 Chemistry Chapter 11 Adsorption and Colloids PDF Download

Adsorption And Colloids

11.1 Introduction

When a metal spoon is dipped in milk and taken out, you will notice that a film of milk particles cover the spoon surface. If a cold water bottle is taken out from the refrigerator and kept on a table for a while, water vapour is seen to condense on the outer surface of the bottle, forming droplets or a film. Here the milk particles or the water molecules from the air get adsorbed on the surface, namely, the spoon and the bottle.

Many surfaces of objects around us are exposed to the atmosphere. Water molecules as well as other gas molecules such as \(\text{N}_2\), \(\text{O}_2\), from the air form an invisible multi molecular film on these objects. This is known as the phenomenon of adsorption.

11.2.1 Adsorption

Consider a surface of liquid or solid. The molecular forces at the surface of a liquid are unbalanced or in unsaturation state. In solids, the ions or molecules at the surface of a crystal do not have their forces satisfied by the close contact with other particles.

Because of the unsaturation solid and liquid surfaces tend to attract gases or dissolved substances with which they come in close contact. Thus the substance accumulates on the surface of solid or liquid.

Teacher's Note

When you dip a spoon in milk, a thin film sticks to it. This is adsorption. It is like how dust sticks to your school furniture.

Exam Trick

Remember: Adsorption = Attraction to surface. Absorption = Soaking inside. When you dip chalk in ink, the color stays on top (adsorption) but water soaks inside (absorption).

Points to Remember

Adsorption happens only on the surface of a solid or liquid.
It happens because molecules on the surface have unbalanced forces.
Gases or dissolved materials stick to the surface.
It is different from absorption which goes inside the material.

11.2.2 Why Does Adsorption Occur?

The adsorption phenomenon is caused by dispersion forces (also known as London dispersion forces), or van der Waals forces. These are short range and additive. The adsorption force is the sum of all interactions between all the atoms. The pulling interactions cause the surface of a liquid to tighten like an elastic film. A measure of the elastic force at the surface of a liquid is called surface tension.

The surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. There is a tendency to have minimum surface tension that is, decrease of free energy, which leads to adsorption.

Terms Involved In Adsorption

Adsorbent: The material or substance present in the bulk, on the surface of which adsorption takes place is called adsorbent.

Adsorbate: The substance getting absorbed on the the adsorbent is called as adsorbate.

Teacher's Note

The spoon is the adsorbent. The milk particles are the adsorbate. Think of adsorbent as the surface and adsorbate as what sticks to it.

Exam Trick

Adsorbent = the solid surface. Adsorbate = the gas or liquid that sticks. Remember: Adsorbent is like a magnet, and adsorbate is what gets attracted to it.

Points to Remember

Adsorbent is the material on which adsorption happens.
Adsorbate is the substance that gets adsorbed.
Both work together in the adsorption process.
The adsorbent is usually a solid material.

11.2.3 Examples Of Adsorption

You know that when cotton is dipped in water, cotton becomes wet with water which is due to absorption.

Some more examples of adsorption:

Adsorption of gases like hydrogen, oxygen, by finely divided metals, namely, platinum, palladium, copper, nickel, etc.

Adsorption of gases like nitrogen, carbon dioxide, by activated charcoal.

Removal of colouring matter like an organic dye, for example methylene blue. When charcoal is added to methylene blue solution and shaken, it becomes colourless after some time, as dye molecules accumulate on the surface of charcoal.

Teacher's Note

Adsorption is on the surface only, like when dust settles on your desk. Absorption is like water soaking into a sponge.

Exam Trick

Adsorption = A (surface). Absorption = AB (inside). This small change in spelling helps you remember the difference.

Points to Remember

Adsorption happens only on the surface.
Absorption spreads through the whole material.
Adsorption depends on temperature and pressure.
Absorption does not depend on temperature and pressure.
Adsorption releases heat.

11.2.4 Desorption

The process of removal of an adsorbed substance from a surface on which it was adsorbed is called desorption.

11.2.5 Sorption

When a chalk is dipped in ink, ink molecules are adsorbed at the surface of chalk and the surface becomes coloured, while the solvent of the ink goes deeper into the chalk due to absorption. When, both adsorption and absorption occur simultaneously it is known as sorption.

Teacher's Note

Sorption means both adsorption and absorption happen at the same time. When chalk absorbs ink, both processes work together.

Exam Trick

Sorption = adsorption + absorption happening together. Remember: when you dip chalk in ink, color sticks to the surface and liquid goes inside both at the same time.

Points to Remember

Sorption is when both adsorption and absorption happen together.
In sorption, the surface gets colored and the liquid goes inside.
Chalk and ink is a perfect example of sorption.
Sorption is common in many daily life activities.

11.3 Types Of Adsorption

There are mainly two types of adsorption phenomenon depending on nature of forces involved.

11.3.1 Physical Adsorption Or Physisorption

When the adsorbent such as gas molecules are accumulated at the surface of a solid on account of weak van der Waals forces, the adsorption is termed as physical adsorption or physisorption.

The van der Waals forces, are similar to forces causing condensation of gas into liquid. Thus, heat is released in physisorption. The heat released during physisorption is of the same order of magnitude as heat of condensation. Due to weak nature of van der Waals forces, physisorption is also weak in nature. The adsorbed gas forms several layers of molecules at high pressures. The extent of adsorption is large at low temperatures. The equilibrium is attained rapidly. The physisorption is readily reversed by lowering of pressure of gas or by raising temperature.

11.3.2 Chemical Adsorption Or Chemisorption

Chemisorption was first investigated in 1916 by American Chemist, Irving Langmuir (1881-1957). When the gas molecules accumulate at the surface of a solid or adsorbate by means of chemical bonds, be it covalent or ionic, the adsorption is called chemical adsorption (or chemisorption). Chemisorption is specific in nature. Chemisorption involving the gas-solid as the adsorbate and adsorbent, is usually exothermic. It means that heat is released during this process. (Exception: the adsorption of hydrogen on glass is endothermic: heat is absorbed during the process. This is due to dissociation of hydrogen.) The heat evolved in chemisorption per mole of adsorbate is nearly the same order of magnitude as that accompanying chemical bonding. Chemisorption involves a large energy of activation and referred as activated adsorption. Chemisorption increases with increase in temperature in the beginning, as more number of molecules can have activation energy. But after certain temperature chemisorption decreases with increase in temperature, as the chemical bonds break.

Sometimes at low temperature, physisorption occurs which passes into chemisorption as the temperature is raised. Besides, chemisorption is dependent on surface area of the adsorbent.

Teacher's Note

Physisorption is weak and temporary, like dust on a fan. Chemisorption is strong and permanent, like paint on a wall that bonds to the surface.

Exam Trick

Physisorption = weak forces (van der Waals). Chemisorption = strong forces (chemical bonds). Think: Physical is weaker, Chemical is stronger.

Points to Remember

Physisorption uses weak van der Waals forces.
Chemisorption uses strong chemical bonds.
Physisorption is reversible, chemisorption is irreversible.
Physisorption happens at low temperature.
Chemisorption happens at high temperature.

This is a preview of the first 3 pages. To get the complete book, click below.