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UNIT 5
SURFACE CHEMISTRY
SYLLABUS:
Adosrption – physisorption and chemisorption; factors affecting adsorption of gases on solids; catasysis:
homogeneous and heterogeneous; activity and selectivity: enzyme catalysis; colloidal state: distinction
between true solutions, colloids and suspensions; lyophilic, lyophobic, multimolecular and macromolecular
colloids; properties of colloids; Tyndal effect, Brownian movement, electrophoresis, coagulation; emulsion –
type of emulsions.
1 Mark Question –Answer, (VSA) Type Questions
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What is adsorption?
The accumulation of molecular species at the surface rather than in the bulk of a solid or lquid is
termedadsorption.
Why are powdered substances more effective adsorbent than their crystalline forms?
Powdered substances have greater surface area as compared to their crystalline forms. Greater the surface
area, greater is the adsorption.
What is collodion?
The usual colloidion is a 4% solution of nitrocellulose in a mixture of alcohol and ether.
What is meant by critical mecellisation concentration (CMC) and Kraft temperature (Tk)?
The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk)
and above a particular concentration called critical micelle concentration (CMC)
For soaps, the CMC is 10–4 to 10–3mol L–1.
What is Helmholtz electrical double layer?
The combination of the two layers of opposite charges around the colloidal particle is
called
Helmholtz electrical double layer. (Example: AgI/I-K+ sol, AgI/Ag+I- sol etc.) According modern views,
the first layer of ions is firmly held and is termed fixed layer while the second layer is mobile which is
termed diffused layer.
What isElectrokinetic potential or zeta potential?
Since separation of charge is a seat of potential, the charges of opposite signs on the fixed and diffused
parts of the double layeraround the colloidal particle results in a difference in potential between these
layers. This potential difference between the fixed layer and the diffused layer of opposite charges is
called the electrokinetic potential or zeta potential.
What do you mean by the term electrophoresis?
The movement of colloidal particles under an appliedelectric potential is called electrophoresis.
What do you mean by the term Electro-osmosis?
When electrophoresis, i.e., movement of particles isprevented by some suitable means, it is observed
thatthe dispersion medium begins to move in an electricfield. This phenomenon is termed electroosmosis.
What is sorption?
Sorption is the process in which adsorption and absorption take place simultaneously, e.g., dyeing of
cotton fibers by azo dye?
What is occlusion?
The adsorption of gases on the surface of metals is called occlusion.
2 Marks Question – answer, (SA) Type Questions
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Comment on the statement on the statement that” Colloid is not a substance but state of a substance.”
The given statement is true. This is because the same substance may exists as a colloid under certain
conditions and as a crystalloid under certain other conditions. For example, NaCl in water behaves as a
crystalloid while in benzene, it 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 1 nm to 1000 nm, it is in the colloid state.
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Write any four points of differences between phyisorption and chemisorption.
Physical adsorption or physorption
Chemical adsorption of chemisorption
1. It arises because of van der Waals’ forces.
1. It is caused by chemical bondformation.
2. It is not specific in nature.
2. It is highly specific in nature.
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3. It is reversible in nature.
3. It is irreversible.
4. It depends on the nature of gas. More easily 4. It also depends on the natureof gas. Gases which
liquefiable gases are adsorbed readily.
can reactwith the adsorbent showchemisorption.
5. Enthalpy of adsorption is low (20-40 kJ mol– 5. Enthalpy of adsorption is high(80-240 kJ mol–1)
1
)in this case.
in this case.
6. Low temperature is favourable for adsorption. 6. High temperature is favourable for adsorption. It
It decreases with increase of temperature.
increases with the increase of temperature.
7. No appreciable activation energy is needed.
7. High activation energy is sometimes needed.
8. It results into multimolecular layers on 8. It results into unimolecular layer.
adsorbent surface under high pressure.
What is the difference between a colloidal sol, gel and emulsion?
In a colloidal sol, the dispersed phase is a solid and the dispersion medium is a liquid. In a gel, it is
opposite. In emulsion, both the dispersed phase and dispersion medium are liquids.
Explain the term: Peptization.
Peptization may be defined as the process of converting a precipitate into colloidal sol by shaking it with
dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose
is called peptizing agent. This method is applied, generally, to convert a freshly prepared precipitate into a
colloidal sol.
Cause of peptization:
During peptization, the precipitate adsorbs one of the ions of the electrolyte on its surface. This causes the
development of positive or negative charge on precipitates, which ultimately break up into smaller
particles of the size of a colloid.
Example: Fe(OH)3 precipitate + FeCl3(Electrolyte) = Redish brown coloured colloidal sol
Fe3+ions are preferentially adsorbed on the surface of molecules of Fe(OH) 3 forming Fe(OH)3. Fe3+ which
being identically charged repels each other.
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Explain the term: Coagulation.
Coagulation or precipitation of lyophobic sols: (Irreversible) The process of aggregation of colloidal
particles of lyophobic sol a in to an insoluble precipitate by the addition of some suitable electrolyte is
called coagulation.
Flocculation :( Reversible) At lower concentration of electrolytes the aggregation of colloidal particles is
called flocculation that can be reversed by shaking while at high concentration of electrolyte coagulation
takes place and the same can not be reversed simple by shaking.
Coagulation/ Flocculation value o f electrolyte:The coagulation of a colloidal sol by an electrolyte does
not take place until the added electrolyte has certain minimum concentration in the sol.
The minimum amount of electrolyte (in millimoles) that must be added to
one liter of colloidal sol so as to cause its complete coagulation is called coagulation value of
flocculation value.
(The minimum concentration of an electrolyte in millimoles per litrerequired to cause precipitation of a
sol in two hours is called coagulating value.The smaller the quantity needed, the higher will be the
coagulating power of an ion.)
Discuss the ways by which coagulation of the lyophobic sols can be carried out.
The coagulation of the lyophobic sols can becarried out in the following ways:
(i) By electrophoresis: The colloidal particles move towards oppositelycharged electrodes, get discharged
and precipitated.
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(ii) By mixing two oppositely charged sols: Oppositely charged sols whenmixed in almost equal
proportions, neutralise their charges and getpartially or completely precipitated. Mixing of hydrated ferric
oxide(+ve sol) and arsenioussulphide (–ve sol) bring them in the precipitatedforms. This type of
coagulation is called mutual coagulation.
(iii) By boiling: When a sol is boiled, the adsorbed layer is disturbeddue to increased collisions with the
molecules of dispersionmedium. This reduces the charge on the particles and ultimatelylead to settling
down in the form of a precipitate.
(iv) By persistent dialysis: On prolonged dialysis, traces of theelectrolyte present in the sol are removed
almost completely andthe colloids become unstable and ultimately coagulate.
(v) By addition of electrolytes: When excess of an electrolyte is added,the colloidal particles are
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precipitated. The reason is that colloidsinteract with ions carrying charge opposite to that present
onthemselves. This causes neutralisation leading to their coagulation.The ion responsible for
neutralisation of charge on the particles iscalled the coagulating ion. A negative ion causes the
precipitationof positively charged sol and vice versa.
State Hardy – Schulze Rule.
Hardy – Schulze Rule:
(i) The ions carrying charge opposite to that of sol particles are effective in causing the coagulation of the
sols.
(ii) Coagulation power of an electrolyte is directly proportional to the fourth power of the valency of the
ions causing coagulation.
In the coagulation of a negative sol (e.g. As2S3), the flocculating power is in the order:
Al3+> Mg2+/Ba2+> Na+
Similarly, in the coagulation of a positive sol (e.g. Fe(OH)3), the flocculating power is in the order:
[Fe(CN)6]4–> PO43-> SO42-> ClWrite two differences between multimolecular colloids and macromolecular colloids.
Multimolecular
Macromolecular
On dissolution, a large number of atoms Macromolecules in suitable solvents form solutions in
or smaller molecules of a substance which the size of the macromolecules may be in the
aggregate together to form species having colloidal range. Such systems are called macromolecular
size
in
the
colloidal
range colloids. These colloids are quite stable and resemble true
(diameter<1nm). The species thus formed solutions in many respects.
are called multimolecular colloids.
Examples of naturally occurring macromolecules are
Forexample, a gold sol, a Sulphur sol.
starch, cellulose, proteins and enzymes; and those of manGenerally lyophilic.
made macromolecules are polythene, nylon, polystyrene,
synthetic rubber, etc.
Generally lyophobic.
Explain the term associated colloids (Micelles).
There are some substances which at low concentrations behave as normal strong electrolytes, but at higher
concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles
thus formed are called micelles. These are also known as associated colloids.
Surface active agents such as soaps and synthetic detergents belong to this class.
These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules
or more
Explain Shape- Selective Catalysis by Zeolites.
Shape- Selective Catalysis by Zeolites:
The catalytic reaction that depends upon the pore structure of thecatalyst and the size of the
reactant and product molecules is calledshape-selective catalysis. Zeolites are good shape-selective
catalystsbecause of their honeycomb-like structures. They are microporous aluminosilicates with three
dimensional network of silicates in which some silicon atoms are replaced by aluminium atoms giving
Al–O–Si framework. The reactions taking place in zeolites depend upon the size and shape of reactant and
product molecules as well as upon the pores and cavities of the zeolites. They are found in nature as well
as synthesised for catalytic selectivity.
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Zeolites are being very widely used as catalysts in petrochemicalindustries for cracking of
hydrocarbons and isomerisation. An importantzeolite catalyst used in the petroleum industry is ZSM-5. It
convertsalcohols directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons.
3 Marks Question – answer, (SA) Type Questions
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Explain what is observed when
(i)
An electrolyte, NaCl is added to hydrated ferric oxide sol.
(ii)
Electric current is passed through a colloidal sol.
(iii)
When a beam of light is passed through a colloidal sol.
(i)
Coagulation (ii) Electrophoresis resulting in to coagulation. (iii) Tyndal effect.
Tyndall effect: If a colloidal solution placed in dark is observed in the direction of light, it appears
reasonably clear or translucent by the transmitted light but they show a mild to strong opalescence, when
viewed at right angles to the passage of light, i.e., the path of the beam is illuminated by a bluish light.
This effect was first observed by Faraday and later studied in detail by Tyndall and is termed as Tyndall
effect.
What is Tyndall effect? What is the cause of Tyndall effect? What are the essential conditions for the
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observation of this effect?
Tyndall effect: As above.
Cause of Tyndall effect: The Tyndall effect is due to the fact that colloidal particles scatter light in all
directions in space. This scattering of light illuminates the path of beam in the colloidal dispersion.
Tyndall effect can be observed during
the projection of picture in the cinema hall
due to scattering of light by dust and smoke
particles present there.
Tyndall effect is observed only
when the following two conditions are
satisfied.
(i) The diameter of the dispersed particles is
not much smaller than the wavelength of the
light used; and
(ii) The refractive indices of the dispersed
phase and the dispersion medium differ
greatly in magnitude.
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Tyndall effect is used to distinguish between a colloidal and true solution.
Discuss the classification colloids on the basis of Nature of interaction between dispersed phase and
dispersion medium.
ClassificationBased on Natureof Interactionbetween DispersedPhase andDispersionMedium:
Depending upon the nature of interaction between the dispersed phaseand the dispersion medium,
colloidal sols are divided into two categories,namely, lyophilic (solvent attracting) and lyophobic (solvent
repelling).
If water is the dispersion medium, the terms used are hydrophilic andhydrophobic.
Lyophilic sols
Lyophobic sols
The word ‘lyophilic’ means liquid-loving.
The word ‘lyophobic’ means liquid-hating.
Particles of the dispersed phase have greater
Particles of the dispersed phase have no affinity
affinity for the dispersion medium.
for the dispersion medium.
Colloidal sols directly formed by mixing Substances like metals, their sulphides, etc., when
substances like gum, gelatine, starch, rubber,
simply mixed with the dispersion medium do not
form the colloidal sol. Their colloidal sols can be
etc., with a suitable liquid (the dispersion prepared only by special methods. Such sols are
medium) are called lyophilic sols.
called lyophobic sols.
These sols are quite stable and cannot be easily These sols are readily precipitated (or coagulated)
coagulated.
on the addition of small amounts of electrolytes,
by heating or by shaking and hence, are not
stable.
An important characteristic of these sols is that if Once precipitated, they do not give back the
the dispersion medium is separated from the colloidal sol by simple addition of the dispersion
dispersed
medium. Hence, these sols are also called
phase (say by evaporation), the sol can be irreversible sols.
reconstituted by simply remixing with the
dispersion medium. That is why these sols are
also called reversible sols.
Self stablised because of strong attractive forces Lyophobic sols need stabilizing agents for their
between the suspended particles and suspension preservation because there are no attractive forces
medium.
between the suspended particles and suspension
medium.
They do not show Tyndal effect.
They show Tyndal effect.
Examples: Glue, Gelatin, Starch, Protein, Rubber Examples: Sols of metals like silver and gold,
Hydroxides like Al(OH)3 , Fe(OH)3, Metal
sulphide like As2S3
Describe Chemical method and Electrical disintegration or Bredig’s Arc method for the preparation of
colloids.
Preparation of Colloids:
A few important methods for the preparation of colloids are as follows:
(a) Chemical methods: Colloidal solutions can be prepared by chemical reactions leading to formation
of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then
aggregate leading to formation of sols.
(b) Electrical disintegration or Bredig’s Arc method:
This process involves dispersion as well as
condensation. Colloidal sols of metals such
as gold, silver, platinum, etc., can be
prepared by this method. In this method,
electric arc is struck between electrodes of
the metal immersed in the dispersion
medium.
The
intense
heat
produced
vapourises the metal, which then condenses
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to form particles of colloidal size.
Bredig’s Arc method
Explain the following colloids around us.
(i)
Blue colour of the sky
(ii)
Formation of delta
(iii)
Electrical precipitation of smoke
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(i) Blue colour of the sky: Dust particles along with water suspended in air scatter blue light which
reaches our eyes and the sky looks blue to us.
(ii) Formation of delta: River water is a colloidal solution of clay. Sea water contains a number 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.
(iii) Electrical precipitation of smoke:
Smoke is a colloidal solution ofsolid
particles
such
as
carbon,
arsenic
compounds, dust, etc., inair. The smoke,
before it comes out from the chimney, is led
througha chamber containing plates having
a charge opposite to that carriedby smoke
particles. The particles on coming in contact
with theseplates lose their charge and get
precipitated. The particles thussettle down
on the floor of the chamber. The precipitator
is calledCottrell precipitator.
5 Marks Question – answer, (LA) Type Questions
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Discuss the effect of pressure and temperature on the adsorption of gases on solids?
ADSORPTION ISOTHERMS: The variation in the amount of gas adsorbed (Adsorbate) by the
adsorbent withpressureat constant temperature can be expressed by means of acurve termed as adsorption
isotherm.
The extent of adsorption of a gas on a solid is generally expressed as (
x
). Where m is the mass
m
of the adsorbent and x is the no. of moles of adsorbate.
x
) and pressure of gas at constant temperature is called adsorption
m
isotherm.
Extent of adsorption increases with pressure and becomes
maximum corresponding to pressure (Ps) called saturation or
equilibrium pressure. Since adsorption is a reversible process (in
case of physisorption) the desorption also takes place
x
simultaneously. At this pressure Ps, the amount of gas adsorbed
( )
m
becomes equal to amount of gas desorsped. So that extent of
Ps
adsorption becomes constant even though pressure is increased.
P
This state is alsoincalled
and Ps
is called between
saturationthe
Freundlich adsorption isotherm:Freundlich,
1909,saturation
gave an state
empirical
relationship
pressure
x
quantity of gas adsorbed by unit mass of solid adsorbent i.e. (extent of adsorption ( )) and pressure (P)
m
A relation or graph between (
at a particular temperature.
The following observation can be made easily from the adsorption isotherm:
(i) At low pressure, (
(
x
x
x
x
)  P or ( ) = K P (ii) At high pressure, ( )  P0 or ( ) = K P0
m
m
m
m
x
)=K
m
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(
x
)
m
(
x
)
m
PP
(iii) In the intermediate range of pressure (
x
) will depend upon the power of pressure (P) which lies
m
between 0 to 1 i.e. fractional power of pressure.
1
(
1
x
x
)  P n or ( ) = k. P n (n>1) Where n can take any whole number values which depend upon nature
m
m
of adsorbate and adsorbent. The above relationship is called Freundlich adsorption isotherm.
Taking logarithm of above equation:
log
x
1
= log k + log P
n
m
Freundlich isotherm explains
the behaviour adsorption in an
approximate manner. The factor 1/n can
have values between 0 and 1 probable
range 0.1 to 0.5). Thus, equation holds
well over a limited range of pressure.
. Effect of temperature: (Adsorption isobar): A plot between amounts of gas adsorbed Vs temperature
at constant pressure is called adsorption isobar.[(
x
) Vs P]
m
Since adsorption is exothermic, increase in temperature should decrease the amount of gas
adsorbed. Therefore decreases with increasing temperature. This is always true for physical adsorption.
(
x
)
m
T
T
(Physical adsorption)
(
x
)
m
(Chemical adsorption)
x
In chemisorption ( ) initially increases and then decreases and then decreases. This is because
m
chemisorption requires some activation energy just as ordinary chemical reaction.