B.Sc. Second year Undergraduate degree course (CBCS Pattern)
Semester Third
Physical Chemistry (CHE-312)
Chapter – Surface Chemistry
Characteristics of adsorption
Adsorption is a surface phenomenon in which
molecules of a substance (the adsorbate) accumulate on the surface of another
substance (the adsorbent). The adsorbate is always present in higher
concentrations on the surface of the adsorbent than in the bulk.
Here are some of the characteristics of adsorption:
a.
Adsorption
is a spontaneous process. This means that it occurs without any outside
intervention and that the free energy of the system decreases as a result of
the adsorption.
b.
Adsorption
is exothermic. This means that heat is released when adsorption occurs.
c.
Adsorption
is reversible. The adsorbate can be desorbed from the adsorbent by
increasing the temperature or decreasing the pressure.
d.
Adsorption
depends on the surface area of the adsorbent. The greater the surface area
of the adsorbent, the more adsorbate can be adsorbed.
e.
Adsorption
depends on the temperature and pressure of the adsorbate. Adsorption
increases with decreasing temperature and increasing pressure.
Types of adsorption
The adsorption of a gas into a solid
surface is mainly of two types:
(a) Physical Adsorption
This is due to the gas molecules being
held to the solid surface by van der Waal’s attractive forces. It is also
referred to as van der Waal’s Adsorption. For example, adsorption of hydrogen
or oxygen on charcoal is Physical Adsorption.
(b) Chemical Adsorption or
Chemisorption
In this kind of adsorption, the gas
molecules or atoms are held to the solid surface by chemical bonds. These bonds
may be covalent or ionic in nature. For example, hydrogen is chemisorbed on
nickel. Hydrogen molecules is first adsorbed by van der Waal’s forces and then
dissociates. The hydrogen atoms are thus chemisorbed on nickel.
Adsorption isotherms
The relationship between the
equilibrium pressure of a gas and its amount adsorbed on the solid adsorbent at
any constant temperature is called an Adsorption isotherm. It may be given in
the form of an equation or graphical curve.
Langmuir
adsorption isotherm
Langmuir
(1916) derived a simple adsorption isotherm based on theoretical
considerations. It was named after him.
Assumptions
of Langmuir Isotherm
Langmuir
proposed his theory by making following assumptions.
(i)
Surface is energetically uniform. Fixed number of vacant or adsorption sites
are available on the surface of the solid.
(ii)
All the vacant sites are of equal size and shape on the surface of adsorbent.
Each
site can hold maximum of one gaseous molecule and a constant amount of heat energy
is released.
(iii)
Heat of adsorption is constant throughout the surface and it ranges from 0 to
1.
(iv)
Dynamic equilibrium exists between adsorbed gaseous molecules and the free
gaseous molecules.
(v)
Adsorption is monolayer or unilayer.
Derivation
Langmuir Equation depicts the relationship
between the extent of adsorption and pressure. Langmuir proposed that dynamic
equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules.
Using the equilibrium equation, equilibrium constant can be calculated.
B(s) -is unoccupied metal surface and
AB- is adsorbed gaseous molecule
According
to Kinetic theory,
Rate
of forward reaction = Ka [A] [B]
Rate
of backward reaction = Kd [AB]
At
equilibrium, Rate of forward reaction is equal to Rate of backward reaction
Ka [A] [B] = Kd [AB]
A new
parameter ‘’ θ is introduced.
Let θ be the number of sites of the surface which are covered with gaseous molecule and (1–è) be the fraction of surface unoccupied by gaseous molecule. Rate of forward direction
depends upon two factors, number of sites available on the surface of adsorbent, (1 – θ ) and pressure, P.
Divide numerator and denominator on RHS by Kd , we get
Substituting in the above equation we get
This is known as Langmuir Adsorption Equation.
Limitations of Langmuir Adsorption Equation
(i) The adsorbed gas has to behave ideally in the vapour phase. Langmuir
equation is valid under low pressure only.
(ii) Langmuir Equation assumes that adsorption is monolayer. But,
monolayer formation is possible only under low pressure condition. Under high
pressure condition the assumption breaks down as gas molecules attract more and
more molecules towards each other.
(iii) Another assumption is the surface of solid is homogeneous but in
real solid surfaces is heterogeneous.
(iv) Langmuir equation assumed that molecules do not interact with each
other. This is impossible as weak force of attraction exists even between
molecules of same type.
(v) In adsorption liquefaction of gases taking place, which results in decrease in randomness but the value is not zero.
Surface Chemistry: Introduction,some basic terms, adsorption materials, factors affecting adsorption
Surface Chemistry:characteristics of adsorption, types of adsorption, classification of adsorption isotherms, Langmuir adsorption isotherm
Surface Chemistry:Freundlich's adsorption isotherm, BET theory , application of adsorption
Surface Chemistry: Multiple Choice questions
