Adsorption: Meaning, Types, Properties, Adsorption Isotherm

Adsorption: Dip a chalk piece in coloured solution for one minute. Cut the chalk piece and observe? Yes, you will watch wet chalk with the colour present only on the outer surface. In this activity, water is absorbed, and the colour is adsorbed. In this article, let us understand everything about adsorption, its types, properties, etc.

What is Adsorption?

When charcoal is dipped in the coloured solution, the solution decolorizes due to the adsorption of coloured particles by the charcoal.  

The process in which molecular, atomic, decolorizing ionic species of one substance get accumulated at the surface of another is called adsorption. 

Adsorption is also defined as the phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or a solid, resulting in a higher concentration of the molecules on the surface.

Components of Adsorption

1. Adsorbate: The substance which gets adsorbed on any surface is called adsorbate. For example, if gas gets adsorbed on a solid surface, then the gas is termed as the adsorbate.
2. Adsorbent: The substance on the surface of which adsorption takes place is called adsorbent. Adsorbents may be a solid or a liquid. Metal powders, powdered charcoal, silica powder, etc., are the commonly used adsorbents. Activated charcoal, silica, alumina, etc., are very effective adsorbents due to their large surface area.
3. Desorption: The removal of the adsorbed substance from the surface is called desorption. Heating and lowering of pressure favour desorption.
4. Sorption: The process in which adsorption and absorption take place simultaneously is called sorption.
5. Occlusion: The adsorption of gases on the surface of the metal is called occlusion.

How Adsorption is Exothermic Process?

During adsorption, the surface energy of the adsorbent decreases. This decrease in energy appears in the form of heat of adsorption. Hence, adsorption is an exothermic process, i.e., \({\rm{\Delta }}{{\rm{H}}_{{\rm{adsorption }}}}\) is always negative. During adsorption, entropy (randomness of the system)  also decreases, i.e., \({\rm{\Delta S}}\) is negative. For the spontaneity of the adsorption, energy should be negative. 

\({\rm{\Delta G = \Delta H – T\Delta S}}\)

As \({\rm{\Delta S}}\) is negative, \({\rm{\Delta G}}\) can be negative only if \({\rm{\Delta H}}\) is negative and \(\Delta {\rm{H}} > {\rm{T}}\Delta {\rm{S}}\) in magnitude.

What are the Factors Affecting Adsorption?

1. Nature and surface area of adsorbent: Different adsorbents have different absorbing power. The greater the surface area greater is the adsorption. For example, porous and finely powdered charcoal absorbs more as compared to the hard non-porous charcoal.

The surface area per gram of the adsorbents is called the specific surface area of the adsorbent.
The greater the ease of liquefaction of gas, the greater is the adsorption. For example, adsorption on charcoal is in the order:

\({{\rm{H}}_{\rm{2}}}{\rm{ < }}{{\rm{N}}_{\rm{2}}}{\rm{ < CO < C}}{{\rm{H}}_{\rm{4}}}{\rm{ < C}}{{\rm{O}}_{\rm{2}}}{\rm{ < HCl < N}}{{\rm{H}}_{\rm{3}}}{\rm{ < S}}{{\rm{O}}_{\rm{2}}}\).

2. Temperature: Adsorption is an exothermic process. Therefore, as temperature increases the adsorption decreases at a constant pressure. The relationship between the extent of adsorption and the temperature at any constant pressure is called adsorption isobar.

3. Pressure: Adsorption increases with the increase of pressure at a constant temperature. The effect is greater at a lower temperature.

4. Activation of solid adsorbent: This is done by increasing the adsorbing surface area by subdividing solid adsorbents are by removing the gases already adsorbed.

Types of Adsorption

Depending on the nature of the force existing between the adsorbate and adsorbents molecule, the absorption is classified into two types, i.e., physisorption and chemisorption.

1. Physical adsorption or physisorption: If the adsorbate is held on the surface by weak forces such as van der Waals’ force then the absorption is called van der Waals adsorption or physical adsorption or physisorption.
Example 1: Adsorption of gases on animal charcoal.
Example 2: Adsorption of water vapor on silica gel.

2. Chemical adsorption or chemisorption: If the adsorbate is held on the surface by the strong chemical bond, the adsorption is called chemical adsorption or chemisorption. Example: Rusting of iron.

Difference between Physisorption and Chemisorption

What is Positive and Negative Adsorption?

Positive absorption is defined as the adsorption in which the concentration of adsorbate is more on the surface of the absorbent than its concentration in bulk.
Example: Shaking concentrated solution of \({\rm{KCl}}\) with blood charcoal shows positive adsorption.

Negative absorption is defined as the adsorption in which the concentration of adsorbate is less on the surface of the absorbent than its concentration in bulk.
Example: Shaking dilute solution of \({\rm{KCl}}\) with blood charcoal shows positive adsorption.

Adsorption Equation

A graph between the amount of the gas adsorbed per gram of the adsorbent(x/m) and the equilibrium pressure of the adsorbate at constant temperature is called the adsorption isotherm.

1. Freundlich Adsorption Isotherm: 

The relation between the amount absorbed \(\left( {\frac{{\rm{x}}}{{\rm{m}}}} \right)\) equilibrium pressure \(\left( {\rm{P}} \right)\) is given by the equation, \({\rm{ k}}{{\rm{P}}^{\frac{{\rm{1}}}{{\rm{n}}}}}{\rm{,}}\) where \({\rm{n}}\) is a positive integer and \({\rm{n}}\) and \({\rm{k}}\) are constant depending upon the nature of the adsorbate and adsorbent at a particular temperature. The factor \(\frac{{\rm{1}}}{{\rm{n}}}\) has values between \(0\) and \(1.\) This equation is applicable only for physisorption.

This relationship is put forward by Freundlich and hence is known as Freundlich adsorption isotherm.

2. Langmuir Adsorption Isotherm

The Langmuir Adsorption Isotherm equation is \(\frac{{\rm{x}}}{{\rm{m}}}{\rm{ = }}\frac{{{\rm{aP}}}}{{{\rm{1 + bP}}}}{\rm{,}}\) where \({\rm{a}}\) and \({\rm{b}}\) are constants. The constants \({\rm{a}}\) and \({\rm{b}}\) depend upon the nature of the gas adsorbate, the nature of the solid adsorbent, and the temperature. \(\frac{{\rm{x}}}{{\rm{m}}}\) is the amount absorbed and \({\rm{P}}\) is the pressure. This equation is applicable for physisorption as well as chemisorption. 

This relationship is put forward by Langmuir and hence is known as Langmuir adsorption isotherm.

Applications of Adsorption

Few applications of Adsorption are:

1. In preserving vacuum
2. In gas masks
3. In decolorizing sugar
4. In chromatography
5. In heterogeneous catalysis
6. In adsorption indicators
7. In softening hard water
8. In the separation of inert gases
9. Adsorption helps to moisture from the air around certain electronic gadgets

Study Adsorption Isotherms Here

Summary

In this article, you are happy about learning adsorption, types, properties, uses, etc. It will help differentiate adsorption from absorption.

Adsorption Frequently Asked Questions

Q.1. What is an adsorption isotherm?
Ans: A graph between the amount of the gas adsorbed per gram of the adsorbent \(\left( {\frac{{\rm{x}}}{{\rm{m}}}} \right)\) and the equilibrium pressure of the adsorbate at constant temperature is called the adsorption isotherm.

Q.2. What does adsorption mean?
Ans: The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or a solid, resulting in a higher concentration of the molecules on the surface, is called adsorption.

Q.3. Where is adsorption used in daily life?
Ans: Adsorption is used in our daily life for different purposes like decolorizing sugar, gas masks, softening hard water, etc.

Q.4. What is physical adsorption? Give an example.
Ans: If the adsorbate is held on the surface by weak forces such as van der Waals’ force then the absorption is called van der Waals adsorption or physical adsorption or physisorption.
Example: Adsorption of gases on animal charcoal.

Q.5. Why is adsorption exothermic?
Ans: During adsorption, the surface energy of the adsorbent decreases. This decrease in energy appears in the form of heat of adsorption. Hence, adsorption is an exothermic process.