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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