SURFACE CHEMISTRY
The
branch of chemistry that deals with the study of surface phenomena like adsorption,
catalysis etc. is called surface chemistry
ADSORPTION
It
is a surface phenomenon which involves attracting and retaining the molecules
of a substance on the surface of some other substance.E.g. Adsorption of
ammonia gas on the surface of activated charcoal.
ADSORBENT:
- The substance on the surface of which adsorption takes place is called
adsorbent E.g. activated charcoal, silica gel etc.
ADSORBATE: - The substance whose
particles get adsorbed on the surface of adsorbent is called adsorbate.E.g. NH3,
CO2 etc.
DESORPTION:
- The process of removal of particles of adsorbate from the surface of
adsorbent is known as desorption .It is reverse of adsorption.
ABSORPTION:
- It is a bulk phenomenon in which particles of one substance go deep into the
body of some other substance.
SORPTION:
- The process in which absorption and adsorption occur together is known as
sorption.
TYPES
OF ADSORPTION
(1) PHYSICAL
ADSORTION OR PHYSISORPTION
When
particles of adsorbate are held on the surface of an adsorbent by weak Van der Waal’s
forces or physical forces then it is called physical adsorption.
(2)
CHEMICAL ADSORPTION OR CHEMISORPTION
When
particles of adsorbate are held on the surface of an adsorbent by chemical bonds
then it is called chemical adsorption.
DIFFERENCE
BETWEEN PHYSICAL AND CHEMICAL ADSORPTION
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PHYSISORPTION
1) It involves weak van
der waal’s attractive forces.
2)
It takes place at low temperature.
3)
It is reversible in nature.
4)
It is non-specific.
5)
It is multi-layered
6)
No need of high activation energy.
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CHEMISORPTION
1) It involves strong
chemical bonds
2)
It takes place at high temperature.
3)
It is irreversible in nature.
4)
It is specific in nature.
5)
It is mono-layered
6)
High activation energy is required.
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FACTORS
AFFECTING ADSORPTION OF GASES ON SOLID SURFACE
1) NATURE OF GAS: - Easily liquefiable
gases like ammonia and carbon dioxide are adsorbed to a greater extent than the
gases which are hard to liquefy like N2,O2 etc.
REASON: - Easily liquefiable
gases have greater magnitude of attractive forces.
2) NATURE OF SOLID:-
The solids with porous nature have larger surface area and hence act as good adsorbents.E.g. Charcoal, silica gel,
alumina gel and clay.
3) SURFACE AREA OF THE
ADSORBENT
:- Larger the surface area of the adsorbent greater will be the rate of
adsorption.E.g. adsorption in solid adsorbents is
enhanced when finely divided solid particles are used.
4) ACTIVATION OF
ADSORBENT: - It means increasing
the adsorbing power of the adsorbent. It is done by heating or grinding the adsorbent.
Heating is done to increase the number of active sites and grinding is done to
increase the surface area of adsorbent.
5) EFFECT OF TEMPERATURE: - Effect of
temperature can be studied from adsorption isobar.
ADSORPTION
ISOBAR
The
graph between x/m and temperature at constant pressure is called adsorption
isobar. There are two types of adsorption isobars.
(i) ADSORPTION
ISOBAR FOR PHYSICAL ADSORPTION; -
It
is clear from the isobar that for physical adsorption the rate of adsorption
decreases with the increase in temperature.
REASON:
- With the increase in temperature the already adsorbed gas molecules absorb
heat energy and their kinetic energy is enhanced as result of which molecules
start escaping from the surface of adsorbent hence the rate of adsorption
decreases.
(ii) ASORPTION
ISOBAR FOR CHEMICAL ADSORPTION
In
this case value of x/m first increases and then decreases with further rise in
temperature.
REASON:
- In the beginning rise in temperature helps to provide activation energy to
the gaseous molecules so that chemical bond can be formed. Hence the rate of
adsorption increases in the beginning. But with the further increase in
temperature energy of the bonded molecules increases which results in the
breaking of chemical bonds hence the rate of adsorption decreases.
6) EFFECT OF PRESSURE: - In general the rate
of adsorption increases with the increase in pressure. The effect of pressure
on the rate of adsorption can be studied in detail from adsorption isotherm.
ADSORPTION ISOTHERM
The graph between x/m
and pressure at constant temperature is called adsorption isotherm
ADSORPTION ISOTHERM
In
the beginning the rate of adsorption increases with the increase in pressure
then it becomes constant at a particular pressure called saturated pressure.
TYPES OF ADSORPTION ISOTHERM:
- They
are of two types.
(i)
FREUNDLICH ADSORPTION ISOTHERM: - The variation of rate of adsorption with
pressure at constant temperature can be studied by Freundlich adsorption
isotherm as follow:
AT
LOW PRESSURE: - At low pressure graph is nearly straight which shows that:
x/m α P
AT
HIGHER PRESSURE: - The graph becomes almost constant which reveals that x/m
becomes independent of the pressure.
x/m α P0
AT
INTERMEDIATE RANGE OF PRESSURE: - x/m
depends upon P raised to the power between 1 and 0.
x/m α P1/n
x/m = k × P1/n………………(1)
To
check the validity of this equation (1) take log on both sides.
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