Surface Chemistry
 Adsorption: It is the phenomenon of attracting & retaining the
molecules of a substance on the surface of a liquid or a solid
resulting into a higher concentration of molecules on the
surface.
 The substance adsorbed is called adsorbate & the substance on
which it is absorbed is called adsorbent.
 The reverse of adsorption is removal of adsorbed substance
from the surface is called desorption.
 The adsorption of gases on the surface of metals is called
occlusion.
 Cause of adsorption: Adsorption arises at the surface of solids
as a result of the presence of unbalanced forces at the surface.
 Characteristics of adsorption:
1. It is specific & selective in nature.
2. It is accompanied by decrease in the free energy of the
system.
Since it is a spontaneous process, ▲G is –ve.
 Absorption: It is a phenomenon in which a substance is
uniformly distributed throughout the substance.
 Adsorption id a surface phenomenon whereas absorption is a
bulk phenomenon.
 Factors on which adsorption of gases on solid depends:1. Nature of adsorbent.
2. Surface area of adsorbent: - Creates the surface area
greater in the adsorption.
3. Nature of gas.
4. Temperature: - Adsorption is an exothermic. Therefore
increase of temperature decreases the adsorption & vice
versa.
5. Pressure: - Adsorption increases with pressure at const.
temp.
6. Activation of solid adsorbent: - Activation means
increasing the adsorbing power of solid adsorbent.
This can be done by removing the gases already adsorbed by
heating or by subdividing the adsorbent.
 Enthalpy of adsorption: - ( heat adsorption)
It is the amount of heat evolved when one mole of an adsorbate
is adsorbed on the surface of adsorbent.
 Types of adsorption :1. Physical adsorption
2. Chemical adsorption
 Physical adsorption : - Particles of adsorbate are held to
surface of adsorbent by physical forces such as wander waals
forces.
 Chemical adsorption:- It is a process in which the molecules
of adsorbent by chemical forces.
 Distinction: PHYSICAL
CHEMICAL
1. Molecules are attracted to
surfaces by Vander Waals
forces.
2. Heat of adsorption is low
(20-40kj/mol).
3. Process is reversible.
 Molecules are held by
chemical bonds.
4. Decreases with temperature.
 It first increases then
 Heat of adsorption is high
(40-400kj/mol).
 Process is irreversible.
decreases with temperature.
5. Non-specific in nature.
 Specific in nature.
6. It forms multi molecular layer.
 It forms unimolecular.
 Adsorption isotherm: - It is the plot of the mass of gas
adsorbed per gram of adsorbent(x/m) versus equilibrium
pressure at const. temp.
 Freundlieh adsorption isotherm
 Adsorption of gases on solids: For low pressure,
x/m α P
 For high pressure,
x/m α P°
 For intermediate pressure,
x/m α P1/n
or x/m = KP1/n
n=integer, k=const.
Log x/m = log K + 1/n log P
Adsorption of solutes from solution by solid adsorbents.
x/m = Kc1/n
Catalysts:- A substance which can change the speed
of a chemical reaction without being used up
in the reaction .

Types of catalysts: Homogenous catalyst
 Heterogenous catalyst
 Homogenous catalyst: When catalyst present
is in the same phase as the reactants.
 Eg:o Lead chamber process for the
manufacture H2SO4.
2SO2 + O2 → (cat.=NO) 2SO3
o Hydrolysis of ethyl acetate in the
presence of dil.H2SO4 acid.
2CO + O2 → (cat.=NO)2CO2
CH3COOC2H5 +H2O →(cat.=H2SO4) CH3COOH + C2H5OH
 Heterogenous Catalyst:-When catalyst is
present in different phase than that of reactants.
 Eg:o Manufacture of NH3 using Fe as a catalyst.
(by haber’s process)
N2+3H2 → (cat. =Fe) 2NH3
CH2=CH2+H2 →(cat.=Pt)CH3-CH3
o Contact process:(Manufacture of H2SO4)
SO2+O2→ (cat. =V2O5) SO3
o Polymerization of Ethene using
Zeingler-Natta catalyst
CH2=CH2→(cat.=TiCl4&Tri.Alkylaluminium)-(CH2-CH2-)n
Properties of catalyst: Activity-Activity of a catalyst depends on the
strength of chemisorption. The reactant must
adsorb reasonably strong strongly for the catalyst
to the active.
 Selective-The ability of a catalyst to direct a
reaction to yield a particular product.
CO + 3H2 →(cat.=Ni) CH4 + H2O
CO + 2H2 →(cat.=Cu/ZnO-Cr2O3)CH3OH
CO + H2 →(cat.=Cu)HCHO
i.e. a given substance can act as a catalyst only for a
particular reaction, not for all the reactions.
Eg:-
Shape selective Catalysis by Zeolites: Catalytic reaction that depends upon the
pore structure of catalysts & the size of
reactant & product molecules.
 Zeolites are good shape selective catalysts.
 Zeolites are aluminosilicates.
 Zeolites are 1st heated before used as catalyst
to remove water of hydration and create
cavities.
 Size generally varies from 260pm to 740pm
this only some molecules can be adsorbed.
 Eg ZSM-5 converts alcohols to petrol.
Enzyme Catalysis: Biochemical catalysts are called enzymes.
 Enzymes are proteins with high molecular
mass.
 They increase rate by 108 to 1020 times.
 They are extremely specific.
 E.g:- Only urease catalyses the hydrolysis of
urea & none of the several thousand other
enzymes present in the cell catalyses that
reaction.
NH2CONH2 + H2O → (cat.=urease) 2NH3+ CO2
Enzyme Action:- Two models:
 Lock & key model
 Induced fit model or hand in glove model.

Lock & key model: Although enzyme is a very
large molecule the reaction is
catalyzed at a very specific
location in the enzyme called
active site.
 The substances that undergo
reaction at this site are called
substrates.
 The substrate fits into the active
site forming enzyme-substrate
complex.
 Once reaction occurs products
depart, allowing another
substrate to enter.

Induced fit model:The enzyme changes shape when substrate lands at
active site.
 This may be pictured as hand
and glove in which the glove
(active site) does not attain its
functional shape until hand
(substrate) moves onto place.
COLLOIDS: A colloid is a heterogenous system in which one
substance is dispersed (dispersed phase) as every fine
particle in another substance called dispersion
medium. (Dispersed phase is discontinuous phase
whereas dispersion medium is continuous).
 Colloid particles range in diameter from
approximately 10 to 2000Ao.
 The two phases, dispersed phase & dispersion
medium can be solid, liquid or gas.
o Sols-solid in liquid colloids. E.g.: starch.
o Emulsions- liquids in liquids. E.g.: milk.
o
Gels- liquids in solids. E.g.: butter,
jellies. Aquasol- dispersion medium is
water. (Hydrosol)
o Alcosol-dispersion medium is alcohol.
o Aerosols- dispersion medium is gases.
o Benzosols- dispersion medium is benzene.
Classification based on interaction between
dispersed phase & dispersion medium:
LYOPHILIC COLLOIDS: Particles of dispersed
phase have a great affinity for the dispersion medium.
 These are quite stable & cannot be easily coagulated.
 The solids obtained after evaporation may be
reconverted to sol state by simply agitating with
dispersion medium. Therefore they are called
reversible sol.
 If water is dispersion medium, it is called hydrophilic.

LYOPHOBIC SOLS. :-Particles of dispersed phase
have very little or no affinity for dispersion medium.
 These are less stable & irreversible.
 Lyophobic sols need stabilizing agents for their
preservation.
 If water is dis. medium, it is called hydrophobic.
CLASSIFICATION BASED ON TYPE OF
DISPERSED PHASE PARTICLES
1. Multimolecular
2. Macromolecular
3. Associated colloids
 MULTIMOLECULAR: They are formed by aggregation of large number
of atoms or molecules which generally have
diameters less than 1nm.
 E.g.: sols of gold, sulphur, etc.
 Atom or molecules are held together by weak
Wander Waals forces.
 MACROMOLECULAR:o They are molecules of large size.
o E.g. polymers like rubber, nylon, starch etc. when
dissolved in suitable solvent from colloidal solution.
o They are quite stable.
 ASSOCIATED COLLOIDS: They are formed by the aggregation of a large no.
of ions due to attraction towards oppositely
charged ions in concentrated soln.
 E.g. soap & detergent solns.
 They behave as normal electrolytes in low conc.
but at higher conc. exhibit colloidal behaviour.
 The aggregate particles thus formed are called
micelles.
 Formation of micelles takes place above a
particular temp. called Kraft temperature &
above a particular conc. are called critical micelle
concentration (CMC).
 These colloids have both lyophilic & lyophobic
parts.
MECHANISM OF FORMATION OF
MICELLES: E.g. soap.
 Soap is sodium salt of higher fatty acid
RCOO-Na+.
 E.g. sodium stearate CH3(CH2)16COONa.
 When dissolved in water it dissociate to form
RCOO- & Na+ ion.
 COO- consists of 2 parts:-
 Long hydrocarbon chain(R) or non-polar tail
which is hydrophloeic &
 COO- or polar head which is hydrophilic.
 At higher conc. they aggregate in a spherical
form with their R-chain pointing towards
centre & COO- part on outward on the surface.
PREPARATION OF COLLOIDAL
SOLUTIONS: LYOPHOLIC SOLUTIONS: CONDENSATION
METHODS
 DISPERSION
METHODS
CONDENSATION METHODS (CHEMICAL &
PHYSICAL METHODS):
Particles of atomic & molecular sizes are
induced to combine to form aggregates with
colloidal size.
o CHEMICAL METHODS:
As2O3+3H2S→As2S3+3H2O – double
decomposition

SO2+2H2S→3S+2H2O – oxidation

2AuCl3+3HCHO+3H2O→2Au+3HCOOH+6HCl
– reduction
 FeCl3+3H2O→Fe(OH)3+3HCl – hydrolysis
o PHYSICAL METHODS: Exchange of solvent
 Excessive cooling
1. Exchange of solvent:- When a true sol. is mixed
with excess of other solvent in which the solute
is insoluble but solvent is miscible, a colloidal
sol is obtained.
E.g. solution of sulphur in alcohol is poured in
excess of water. – Colloidal sol of sulphur is
obtained.
2. Excessive cooling:- The colloidal sol of ice in
organic solvent CHCl3 or ether can be obtained
by freezing solution of water in the solvent.
DISPERSION METHODS:Mechanical dispersion – The substance is brought
into a colloidal state in the dispersion medium by
grinding it in colloidal mill. or ultrasonic
disintegrator.
Electrical disintegrator or Bredig’s arc method:In this method electric arc is stuck between
electrodes of metal immersed in dispersion medium.
The intense heat produced vaporizes the metal which
then condenses to form particles of colloidal size.
E.g. gold, silver, etc. can be prepared by this method.
PEPTIZATION:-In this method precipitate is
converted to colloidal sol by shaking it with
dispersion medium in the presence of small
amount of electrolyte.
o The electrolyte used is called peptizing
agent.
E.g. When Fe(OH)3 is shaken with FeCl3 it
absorbs Fe3+ ions & thereby breaksup to
smaller sized particles.
 LYOPHILIC SOLUTIONS:Prepared by shaking lyophilic material with
dispersion medium.
PURIFICATION OF COLLOIDS:Impurities in sols may destabilize colloids so they have to
be removed.
 METHODS:DIALYSIS OR ELECTROLYSIS
ULTRA FILTRATION
ULTRA CENTRIFUGATION
 DIALYSIS: A bag made up of parchment paper is filled
with colloidal solution & is then
suspended in fresh water.
 The electrolytes pass out leaving behind
the colloidal sol.
 Only particles of true solution pass through
parchment paper not sol particles.
 ELECTROLYSIS:In this method the movement of ions across
the membrane is made faster by applying
electric current through two electrodes.
 ULTRA-FILTRATION: This is the process of separating the
particles of colloids from electrolytes
by filtration through ultra filter papers.
 Ultra filter papers allow only
electrolytes to pass through.
 It is prepared by treating ordinary filter
paper with gelatine solution to narrow
down the pores suitably.
 ULTRA CONFIGURATION: In this method the colloidal solution is placed
in a high speed centrifugal machine.
 The colloidal particles settle down while
impurities remain in the centrifugate.
PROPERTIES OF COLLOIDAL SOLUTIONS;o TYNDALL EFFECT:- If a strong beam of
light is passed through a colloidal sol the
path of the beam gets illuminated. This
phenomenon is called TYNDALL
EFFECT.
o This is due to scattering of light by colloidal
particles.
BROWNIAN MOVEMENT: The continuous zig-zag movement of colloidal
particles in colloidal sol.
 This is due to unequal impact of particles of dispersion
medium on colloidal particles.
 This phenomenon was observed by Robert Brown in
1827.
ELECTROPHORESIS: All the dispersed phase particles in a colloidal solution
carry same charge (either +ve or –ve) while dispersion
medium has equal & opp charge.
 Particles therefore repel each other & do not come
together to form large particles.
 E.g.As2S3, Au, Ag, Pt are –vely charged. Fe(OH)3 ,
Al(OH)3 are +vely charged.
 The charge on colloidal particles in due to preferential
adsorption of ions.
 E.g. Fe(OH)3 adsorbs Fe3+ ions from FeCl3 soln. AgCl
by adsorption of Cl- ions.
 Existence of charge can be shown by electrophoresis
which involves the movement of colloidal particles
either towards cathode or anode under the influence of
electric field.
COAGULATION VALUES:-The minimum
amount of an electrolyte that must be added to
one liter of colloidal soln. so as to cause its
complete coagulation is called coagulation value.
HARDY-SCHULZE RULE: The ions carrying charge opposite to that of
sol particles are effective in causing the
coagulation of sol.
 Coagulating power of an electrolyte is
directly proportional to the fourth power of
valency of ions causing coagulation.
 E.g.Fe3+ is more effective than B2+ for
coagulation of As2S3 soln.
 PO43- is more effective than SO 42- or d-ions
for coagulation of Fe(OH)3 sol.
EMULSIONS:- are colloids in which both
dispersion phase & dispersion medium
are liquids.
o OIL IN WATER EMULSIONS:-Oil acts
as dispersion phase and water acts as
dispersion medium.
o E.g. old cream, butter.
IDENTIFICATION OF EMULSIONS: TESTS:I. DILUTION TESTS:-If emulsion can be
diluted with water, this indicates that
dispersion medium is water i.e. it is oil in
water type.
If added water forms a separate layer it is
water in oil type.
II. DYO TEST:- Some oil soluble dye is added
to the emulsion if the background becomes
colored, emulsion is water-in oil type, & if
colored droplets are seen emulsion is oil in
water type.
 PREPARATION OF EMULSIONS :EMULSIFICATION: Process of making an
emulsion is known as emulsification. To
stabilize emulsion substances added are
called emulsifying agents or emulsifier.
E.g. soap & detergents.

DEMULSIFICATION:Separation of an emulsion into its constituent
liquids is called demulsification.
o Methods used are – boiling, freezing,
centrifugation, etc.
APPLICATION:a) Rubber plating
b) Sewage disposal
c) Cottrell smoke precipitator
d) Preparation of nano materials
e) In medicines
f) In disinfectants
g) In metallurgical operations.