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Adsorption of class 12 / class 11 explained with presentation

 Adsorption

Objectives:

* Define surface chemistry

* Understand the mechanism of adsorption

*  Classify adsorption

*  Applications of adsorption

 

Surface chemistry

Surface chemistry is referred to as the study of the phenomenon occurring on the surfaces of substances. That is the boundary Separating 2 bulk phases.

It is the study of the chemical phenomena that occur at the

 interface of two surfaces which can be solid-liquid, solid-gas, solid-vacuum, liquid-gas, etc.

 

Who discovered adsorption?

•       Carl Scheele (1742–1786) published the first scientific adsorption measurements in 1777.

 

Who coined the term adsorption?

The word "adsorption" was coined in 1881 by German physicist Heinrich Kayser (1853–1940).

Adsorption

Ø  The term adsorption was first coined in 1881 by a German physicist named Heinrich Kayser.

Ø  It is the phenomenon of attracting and retaining the molecules of a substance on the surface Or a liquid or a solid leading to higher concentration in the surface in comparison to the bulk is called Adsorption

Two components  Adsorption

•       The substance accumulated on the surface of the solid during adsorption is called Adsorbate.

•       The surface on which the adsorption occurs is called Absorbent

 

Absorption:

•          The phenomenon in which a substance is uniformly distributed throughout the bulk is known as Absorption.

•          It is the penetration of a substance through the surface into the bulk of the solid.

QUESTION TASK:
If chalk is dipped into ink then we can say this as adsorption or absorption?

•       Answer is its is adsorption

 

 

Enthalpy of Adsorption

The amount of heat energy liberated when one mole of gas is adsorbed on the unit surface area of the adsorbent is known as enthalpy of adsorption.

Adsorption is always an exothermic

process and enthalpy change ( delta h)

of adsorption is always negative.

 

Positive adsorption:

Ø  When the concentration of the adsorbate is more on the surface of absorbent relative to its concentration in the bulk, it is called Positive adsorption

Negative adsorption

Ø  When the concentration of absorbate is less on the surface of absorbent relative to its concentration of the bulk, it is called Negative adsorption

 

Why adsorption is spontaneous? 

•          Adsorption of exothermic ( Delta H < 0)(H is the enthalpy)
Adsorption is Spontaneous ( Delta G < 0) {G= Gibbs free energy}

•          The rate of adsorption keeps decreasing with time and eventually, reaches equilibrium called adsorption equilibrium.

Factors affecting Adsorption For Gases on solid surfaces

•          Nature of the substance

•          Increasing the surface area

•          To increase this we can make sure that the substance has many pores.

•          By making the surface rough by mechanically 

•          Or chemically rubbing the material. It can also be achieved by subdividing the adsorbent into smaller pieces.

•       Type of absorbent

More easily liquefiable gases are more readily

Absorbed.

•       Effects of Pressure

•       The rate of adsorption increases with 

•       increase in pressure by Adsorption isotherm

graph ( adsorption isotherm is keeping the 

temperature constant and plotting the graph)

•       Effects of Temperature

 

Physical Adsorption or Physisorption 

Characteristics:

v  Sorption:-

 Sometimes the absorbate dissolves into the absorbent. Initially, the absorbate appears on the surface of the absorbent and later passes through the body of the absorbent. Thus adsorption changes into absorption. The simultaneous occurrence of absorption and adsorption is called Sorption.

v  Desorption:-

The process of removing an adsorbed substance from a surface on which it is absorbed is called Desorption

Characteristics

ü  Nature of forces: weak van der Waals forces

ü  Specificity: It is not specific in nature

ü  Reversibility: The process is reversible

ü  Layer: It is a multi-layered process

ü  Enthalpy of adsorption: Low enthalpy of adsorption [ 20 – 40 KJ/mole ]

ü  The energy of activation: Very low

ü  Desorption: Very easy

ü  Factors affecting: Surface area of adsorbent nature of adsorbate, pressure, temperature.

 

 

Chemical Adsorption or Chemisorption

 

q  It is due to strong chemical forces between 

adsorbate and adsorbent. (nature of the chemical bond is

Covalent/Ionic

q  Characteristics:

q  Nature of forces: Strong chemical forces

q  Specificity: Highly specific nature

q  Reversibility: It is irreversible

q  Layer: It is a single-layered process

q  Enthalpy of adsorption: High enthalpy of adsorption 

            [40 – 400 KJ/mole]

q  The the energy of activation: Very high

q  Desorption: Very difficult

q     Factors affecting: Surface area of adsorbent nature of adsorbate Temperature

           Chemisorption---> High activation energy

              Rate of chemisorption decreases with an increase In pressure

 

What is Adsorption Isotherm?

Adsorption isotherms have been of immense importance to researches dealing with environmental protection and adsorption techniques. The two primary methods used for predicting the adsorption capacity of a given material is known as the:

Ø  Freundlich isotherms

Ø   Langmuir isotherms

 

Quick question 

 

The rate of physisorption increases with

•       a) Decrease with temperature

•       b) Increase with temperature

•       c) Decrease with pressure

•       d) Decrease in surface area

Answer Option (a)

 

Applications of adsorption

Gas masks:

These contain activated charcoal or a mixture of adsorbents and are used in breathing in the atmosphere containing poisonous gases.

Removing of coloring matter:

Animal charcoal removes colors of solutions as coloring impurities

 are preferentially adsorbed on charcoal.

Controlling humidity:

Silica and alumina gels are used as adsorbents for removing

Moisture and controlling humidity.

Adsorption indicators:

Dyes like eosin and fluorescein  are used as an indicator in titrating 

Bromide solution with AgNO3.

 

Chromatography separation:

•       Components of mixtures especially of organic compounds can be separated and identified by chromatographic analysis which is based upon adsorption phenomenon:- Gas-Liquid chromatography

•       Froth rotation 

•       Pulverized ore is suspended in water containing some pine oil and froth agent. When air is blown into it, the ore particles are adsorbed on the air-oil  interface and come to the surface in the form of froth.

 

Separation of Gases:

•       Due to the difference in the degree of adsorption of gases by charcoal at different temperatures, a mixture of  inert gases can be separated by  adsorption on coconut charcoal.


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