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CHAPTER
5
SURFACE
CHEMISTRY
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1.Adsorption: -The accumulation of molecules species at
the surface rather in the bulk of a solid or liquid is termed adsorption.
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2.Desorption:-Removal of adsorbate from the surface of adsorbent is
known as Desorption.
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3.Sorption:-When adsorption
and absorption both takes place simultaneously.
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4. Type of adsorption:
- Onthe basis of interaction between
adsorption andabsorption, adsorbate are of two types:
(i)Physical adsorption/physisorption: -When weak vander Waal’s interaction
involve between adsorbate and adsorbent.
(ii) Chemical adsorption/chemisorption:-When chemical bonds form between adsorbate
and adsorbent.
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5.Application of adsorption:-
(i) Production of high vacuum: The remaining traces of air can be
adsorbed by charcoal from a vessel evacuated by a vacuum pump to give a very
high vacuum.
(ii)
Gas masks: Gas mask (a device which consists of activated charcoal or
mixture of adsorbents) is usually used for breathing in coal mines to adsorb
poisonous gases.
(iii) Control of humidity: Silica and aluminium gels are used as
adsorbents for removing moisture and controlling humidity.
(iv) Removal of colouring matter from solutions: Animal charcoal removes colours of
solutions by adsorbing coloured impurities.
(v) Heterogeneous catalysis: Adsorption of reactants on the solid
surface of the catalysts increases the rate of reaction. Manufacture of
ammonia using iron as a catalyst, manufacture of H2SO4 by
contact process and use of finely divided nickel in the hydrogenation of oils
are excellent examples of heterogeneous catalysis.
(vi) Froth floatation process: A low grade sulphide ore is
concentrated by separating it from silica and other earthy matter by this
method using pine oil and frothing agent
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6.Adsorption isotherm:-The variation in the amount of gas
adsorbed by the adsorbent with pressure at constant temperature can be
expressed by means of a curve termed as adsorption isotherm.
Freundlich adsorption isotherm:-It is a graph which shows relationship
between the quantity of gas adsorbed by unit mass of solid adsorbent and
pressure at a particular temperature.
x/m = k p1/n
This
graph shows that physical adsorption decreases with increase in temperature.
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(i)Surface area: - Adsorption increases with increases of
surface area of adsorbent.
(ii)Nature of adsorbate:- Easily liquefiable gases are readily
adsorbed.
(iii)Temperature:-Low temperature is favorable for physical
adsorption and Hightemperature for chemisorption.
(iv)Pressure: -Pressure
increases, adsorption increases.
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8.CATALYSIS:- Substances which alter the rate of chemical reaction and
themselvesremain chemically and quantitatively unchanged after the reaction
are known as catalyst and the phenomenon is known as catalysis.
Homogenous catalyst–when reactants and catalyst are in same
phase.
Heterogeneous catalyst–the catalytic process in which the
reactants and catalyst are in different phase.
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9.
Adsorption theory of Heterogeneous catalysis–It explains the mechanism ofheterogeneous
catalyst.
The mechanism involves 5 steps:-
a.
Diffusion
of reactants to the surface of catalyst.
b.
Adsorption
of reactant molecules on the surface of catalyst.
c.
Chemical
reaction on the catalyst surface through formation of an intermediate.
d.
Desorption
of reaction product from the catalyst surface.
e.
Diffusion
of reaction product away from the catalyst surface.
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10. Activity-The activity of a catalyst is the ability
of a catalyst to accelerate chemical reactions..Catalytic activity increases
from group 5 to group 11 elements of the periodic table.
Selectivity–The selectivity of a catalyst is its
ability to direct a reaction to yield a particular product. For example, starting with H2
and CO, and using different catalysts, we get different products.
9. SHAPE SELECTIVE CATALYSIS
The
catalytic reaction that depends upon the pore size of the catalyst and the
size of reactant and product molecules is called shape selective catalysis.
e.g. Zeolites are good shape selective catalysts
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10.
CATALYST PROMOTERS AND CATALYST PIOSONS: Promoters are Substances that enhance the activity of a catalyst while
poisons decrease the activity of a catalyst. For example, in Haber’s process
for manufacture of ammonia, molybdenum acts as a promoter for iron which is
used as a catalyst.
An example of catalyst
poison is Pd-BaSO4 (in Rosenmund reduction) where BaSO4
decreases the activity of Palladium catalyst.
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11. ENZYME CATALYSIS
Enzymes are protein molecules of high molecular
mass which catalyse the biochemical reaction.
e.g. Inversion of cane sugar
by invertase enzyme.
Characteristic of enzyme catalysis –
a.
Enzymes
are specific to substrate.
b.
Enzymes
are highly active under optimum temperature.
c.
Enzymes
are specific to pH. e.g. Pepsin act in
acidic medium
d.
Enzymes
are inhabited by the presence of certain substance.
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12. Colloid-A colloid is a
heterogeneous system in which one substance isdispersed (dispersed phase) in
another substance called dispersion medium.
TYPES OF COLLOIDS
(1) On the basis of nature
of interaction between dispersed phase and dispersion medium.
(a)
Lyophobic colloid-Solvent hating colloid, these colloids
can not be prepared bysimply mixing of dispersed phase into dispersion
medium. e.g. metallic sols.
(b)
Lyophobic colloid-solvent loving these colloids can be
prepared by simply mixingof dispersion phase into dispersion medium. e.g.
Starch sol.
(2) On
the basis of types of particles of the dispersed phase
(a)
Multimolecular colloid-on
dissolution, a large number of atoms or smaller molecules of a substance
aggregate together to form species having size in colloidal range. The
species thus formed are called Multimolecular colloids. e.g. Sulphur sol.
(b)
Macromolecular colloids -
macromolecules in suitable solvent form solution in which size of the
particles are in range of colloidal range. e.g. starch sol.
(c)
Associated colloids (micelles)-
Some substances in low concentration behaves as normal strong electrolyte but
at higher concentration exhibit colloidal behavior due to formation of
aggregates. The aggregated particles are called micelles and also known as
associated colloids. E.g. soap solution
(3) On
the basis of dispersion medium
(a) Hydrosols:
Colloids
in water are called hydrosols or aqua sols
(b) Alcosols:
Colloids in alcohol are called alcosols.
(c) Benzosols:
Colloids in benzene are called benzosols
(d) Aerosols:
Colloids in air are called aerosols
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13. Kraft temperature- Temperature
above which formation
of micelles takes place.
Critical
micelle concentration (cmc) - concentration above
which micelleformation takes place is known as cmc.
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14. PREPERATION OF COLLOIDS
(a) Chemical
methods- By double decomposition, oxidation reaction or
hydrolysis
OXIDATION
: Sulphur sol e.g. SO2 +2H2S Ã 3S (SOL) +2H2O
HYDROLYSIS:
Ferric hydroxide sol e.g. FeCl3 +3H2O Ã Fe(OH)3
(b) Bredig’s
arc method – For preparation of metallic sol. It
involves dispersion as well as condensation.
(c) Peptization-
Process
of converting a precipitate into colloidal sol. By shaking it
with
dispersion medium in the presence of a small amount of electrolyte
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15. PURIFICATION
OF COLLIODAL SOLUTION :-
(a)
Dialysis-it is a process of removing a dissolved
substance from a colloidal solution by membrane.
(b)Electro
dialysis-when dialysis is carried out with an
electric field applied around the membrane.
(c)
Ultra filtration- Use of special filters which are
permeable to all ionic substances except colloidal particles.
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16. PROPERTIES
OF COLLOIDAL SOLUTION:-
(1) They
show colligative properties
(2) Brownian
motion-zig-zag motion of colloidal particles is called
Brownian motion.
(3) Tyndall
effect-scattering of light by colloidal particles by which
path of beam becomes clearly visible. This effect is known as Tyndall effect.
(4)
Electrophoresis
- Movement of Colloidal particles towards opposite electrode in presence of
external electric field.
(5) Coagulation –The process of setting
of colloidal particles is called coagulation or precipitation of the sol.
Coagulation of lyophobic sols can be done by (a) addition of small amount of
electrolyte (b) mixing two opposite charged sols (c) by boiling, by electrophoresis
or by persistant dyalisis.
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17. Hardy
Schulze Law –Coagulating power of a coagulating ion
is directly proportional to the charge on the ion.
Eg:
Na +< Ca ++< Al 3+ for negatively
charged sol. Like As2S3 sol
Cl
-< CO32- < PO43-
< [Fe (CN)6]4–for positive sol. Like Fe(OH)3 sol
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18. Emulsion
–Liquid
–liquid colloidal system is known as Emulsion.
There are two types of Emulsion.
O/W type - Oil dispersed in water. Eg:
milk, vanishing cream.
W/O type –Water dispersed in oil. Eg:
Butter & Cream.
Emulsifying Agent –The substance
which stabilizes emulsions are called emulsifying agents or emulsifiers..
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19. Electrokinetic potential or zeta potential: Potential difference between the fixed layer and
the diffused layer of opposite charges is called Electrokinetic potential or zeta potential.
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20. Protective colloids: When a lyophilic sol is added to a lyophobic sol, the lyophilic
particles form a layer around the lyophobic particles and thus protect the
latter from coagulation by electrolytes. Lyophilic sols used for this purpose
are called protective colloids
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Conceptual question: What
are the physical states of dispersed phase and dispersion medium of froth?
Ans
- Dispersed phase is gas, dispersion medium is liquid.
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Conceptual question: What is the cause of Brownian movement among
colloidal particles?
Ans- Due to collision between colloidal
particles of same charge.
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Conceptual question: Why is it necessary to remove CO when
ammonia is obtained by Haber’s process?
Ans: CO acts as a catalyst poison for Haber’s
process ammonia synthesis therefore it is necessary to remove when NH3
obtained by Haber’s
process.
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Conceptual question: How is adsorption of a gas related to its
critical temperature?
Ans-
Higher the critical temperature of the gas greater is the ease of
liquefaction. i.e. greater Vander Waals forces of attraction and hence large
adsorption will occur.
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Conceptual question: What is difference between Sol. & Gel?
Ans –Both
are colloidal solutions. Sol has liquid as dispersion medium. While Gel has
solid as dispersion medium.
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Conceptual question: Comment on the statement that “colloid is
not a substance but a state of substance”?
Ans
–The
given statement is true. This is because the statement may exist asa colloid
under certain conditions and as a crystalloid under certain other
conditions.e.g: NaCl in water behaves as a crystalloid while in benzene,
behaves as a colloid (called associated colloid). It is the size of the
particles which matters i.e. the state in which the substance exists. If the
size of the particles lies in the range 1nm to 1000nm it is in the colloid
state.
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VALUE
BASED QUESTIONS
Q.1 Smoke is colloidal solution of solid particles
such as Carbon, arsenic com pounds dust, etc. in air. Precipitation of smoke
particles coming form the chimney of factories is carried out by
Cottrel Precipitator and Carbon free passes out through the chimney.
(a) Name the principle used in the Cottrel
Precipitator.
(b) How smoke precepitator causes precipitation and
settling of smoke particles.
(c) Name the value learnt by the use of this Cottrel
Precipitator
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S.
No
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CBSE
BOARD EXAM QUESTIONS WITH ANSWER/ VALUE POINTS
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1
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What causes Brownian movement in a
colloidal solution?
This is due to the unequal bombardment
of colloidal particles by the molecules of dispersionmedium.
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2
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Which has a higher enthalpy of
adsorption, physisorption or chemisorption?
Chemisorption
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3
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Give
an example of ‘shape-selective catalyst’.
Zeolites
( ZSM-5)
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4
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Mention
two ways by which lyophilic colloids can be coagulated.
This
can be done (i) by adding an electrolyte. (ii) by adding a suitable solvent.
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5
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Define the term ‘Tyndall effect’.
The
scattering of light by colloidal particles is known as Tyndall effect.
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6
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What
is an emulsion?
Emulsion is a colloidal solution in
which both the dispersed phase and dispersion medium are liquids e.g. milk,
cod liver oil, etc.
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7
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Why is the adsorption phenomenon always
exothermic?
As the adsorption progresses, the
residual forces at the surface decreases resulting in the decrease of
surface energy which appears as heat.
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8
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How are the following colloids different
from each other in respect of their dispersion medium and dispersed phase?
Give one example of each.
(i) Aerosol (ii)
Emulsion (iii) Hydrosol
(i)
Aerosol
: Sols in which the dispersion medium is gas and dispersed phase is solid or
liquid. For example: Smoke, dust
(ii) Emulsion A colloidal system in which
dispersion medium and dispersed phase both are liquids. For example:Milk, cod
liver oil.
(iii) Hydrosol
Sols in which the dispersed phase is solid
and dispersion medium is water. For example: gum sol.
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9
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Write the dispersed phase and dispersion
medium of the following colloidal systems: (i) Smoke (ii) Milk
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10
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Write
three special features of chemisorption which are not found in physisorption.
(i) Chemisorption is caused by
chemical bond formation where as physisorption arises due to Van der Waals’
forces.
(ii) Chemisorption is highly
specific in nature whereas physisorption is not specific.
(iii) Chemisorption results into
unimolecular layer whereas physisorption results into multimolecular layers.
(iv) Chemisorption is irreversible
in nature whereas physisorption is reversible in nature.
(v) Chemisorption first increases
with increase in temperature, then decreases while physisorption decreases
with increase in temperature.
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11
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Distinguish between Multimolecular
colloids, Macromolecular colloids and Associated colloids
Multimolecular colloids: In
this type of colloids, colloidal particles are aggregates of atoms or
molecules each having size less than 1nm, e.g., sulphur sol, gold sol.
Macromolecular colloids: In
this type of colloids, colloidal particles are themselves large molecules of
colloidal dimensions, e.g., starch, proteins, polyethene, etc.
Associated colloids: There
are certain substances which at low concentrations behave as normal
electrolyte, but at higher concentrations exhibit colloidal behaviour due to
the formation of aggregates. Such colloids are known as associated colloids,
e.g., soaps and detergents.
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12
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(a) What happens when a freshly
precipitated Fe(OH)3 is shaken with water containing a small
quantity of FeCl3?
(b) Why is a finely divided
substance more effective as an adsorbent?
(a) It is converted into
colloidal state.
Fe(OH)3 + FeCl3 [Fe(OH)3Fe]3+
+ 3Cl–
(b) Powdered substances have
greater surface area as compared to their crystalline forms. Greaterthe
surface area, greater is the adsorption.
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13
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Explain
what is observed when
(i)
an electrolyte, KCl, is added to a hydrated ferric oxide sol.
(ii)
an electric current is passed through a colloidal solution.
(iii) a beam of strong light
is passed through a colloidal solution.
(i) The positively charged
colloidal particles of Fe(OH)3 get coagulated by the oppositely
chargedCl- ions provided by KCl.
(ii) On passing direct current,
colloidal particles move towards the oppositely charged electrodewhere they lose
their charge and get coagulated.
(iii) Scattering of light by the
colloidal particles takes place and the path of light becomes visible(Tyndall
effect).
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14
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What
are lyophilic and lyophobic sols? Give one example of each type. Which one of
these twotypes of sols is easily coagulated and why?
Lyophilic sols: Lyophilic
sols are those sols in which the particles of dispersed phase have
greataffinity for the dispersion medium, e.g., sols of gum, gelatine,
starch, etc.
Lyophobic sols: In
this type of sols the particles of dispersed phase have little or no affinity
forthe dispersion medium, e.g., gold sol, Fe (OH) 3 sol, As2O3 sol.,
etc.Lyophobic sols easily coagulate on the addition of small amount of
electrolyte because these are not stable. The stability of Lyophobic sols is
only due to the presence of charge on the colloidalparticles, on the other
hand stability of lyophilic sol. is due to charge as well as solvation
ofcolloidal particles.
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15
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Define the following: (i)
Peptization (ii) Reversible sols (iii) Electrophoresis
Peptization: The
process of converting a precipitate into colloidal sol by shaking it with
dispersion medium in the presence of a small amount of suitable electrolyte
is called peptization. During peptization, the precipitate absorbs one of the
ions of the electrolyte on its surface. This causes development of positive
or negative charge on precipitates, which ultimately break up into particles
of colloidal dimension.
Reversible sols: Those
colloids which can be separated back into dispersed phase and dispersion
medium.
Electrophoresis
: The movement of colloidal particles under an applied electric potential is
called electrophoresis.
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16
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Explain how the phenomenon of adsorption
finds application in each of the following processes:
(i) Production of vacuum
(ii) Heterogeneous catalysis
(iii) Froth floatation process
OR
Define each of the following terms:
(i) Micelles
(ii) Peptization
(iii) Desorption
(i) Production of Vacuum: Adsorption
can be successfully applied to create conditions of high vacuum. For this a
bulb of charcoal cooled in liquid air, is connected to vessel which has
already been exhausted as far as possible by vacuum pump. The remaining
traces of air inspite of low pressure are adsorbed by the charcoal almost
completely.
(ii) Heterogeneous Catalysis: There
are many gaseous reactions of industrial importance involving solid catalyst.
Manufacture of ammonia using iron as a catalyst, manufacture of H2SO4 by
contact process using V2O5 catalyst and use of finely divided nickel in the
hydrogenation of vegetable oils are the excellent examples. The gaseous
reactants are adsorbed on the surface of the solid catalyst. As a result, the
concentration of the reactants increases on the surface of the catalyst and
hence the rate of reaction increases.
(iii) Froth Floatation Process: In
froth floatation process the powdered ore is mixed with water. It is then
mixed with pine oil (a frother). The oil particles are adsorbed on the
surface of ore particles. Now, a stream of air is blown through the mixture
from below when froth is formed at the water surface. The ore particles stick
to the bubbles of the air rises to surface along with
the foam while the gangue particles
which are wetted by water settle at the bottom. The foam is separated out and
is collected and in the course, the ore particles also settle down.
OR
(i) Micelles: There
are some substances which at low concentration behave as normal strong
electrolytes but at higher concentration exhibit colloidal behaviour due to
formation of aggregated particles. The aggregated particles thus formed are
called micelles. The formation of micelles takes place only above a
particular temperature called Kraft temperature and above a particular
concentration called critical micelle concentration (CMC). Surface
active agents such as soap and synthetic detergents belong to this class.
(ii) Peptization: The
process of converting a precipitate into colloidal sol by shaking it with
dispersion medium in the presence of a small amount of suitable electrolyte
is called peptization. During peptization, the precipitate absorbs one of the
ions of the electrolyte on its surface. This causes development of positive
or negative charge on precipitates, which ultimately break up into particles
of colloidal dimension.
(iii) Desorption: The
process of removing an adsorbed substance from a surface on which it is
adsorbed is called desorption.
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17
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What is the difference between oil/water
(O/W) type and water/oil (W/O) type emulsions? Give an example of each type.
(i) Oil in water (o/w) type
emulsions: In such emulsions oil is the dispersed
phase and water is the dispersion medium, e.g., milk, vanishing cream.
(ii) Water in oil (w/o) type
emulsion: In such emulsions, water is the
dispersed phase and oil is the dispersion medium, e.g., butter, cod
liver oil.
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18
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What is special about the following
terms: (i) Kraft Temperature (ii) Sorption
(i) Kraft temperature is the
minimum temperature above which the formation of micelles takes place.
(ii) Sorption is the process in
which adsorption and absorption take place simultaneously, e.g., dyeing
of cotton fibres by azo dyes.
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19
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(a) Write the expression for the
Freundlich adsorption isotherm for the adsorption of gases on solids, in the
form of an equation.
(b) What are the dispersed phase
and dispersion medium of butter?
(c) A delta is formed at the
meeting place of sea and river water. Why?
(a) Freundlich adsorption
isotherm equation for adsorption of gases on solids:
where x is the mass of the gas
adsorbed on mass m of the adsorbent at pressure p, k and
n are
constants which depends on the nature of
the gas and adsorbent at a particular temperature.
(b) In butter, water acts as
disperse phase and oil acts as dispersion medium.
(c) River water is a colloidal
solution of clay. Sea water contains a variety of electrolytes. When river
water meets the sea water, the electrolytes present in sea water coagulate
the colloidal solution of clay resulting in its deposition with the formation
of delta.
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