PHYSICAL CHEMISTRY
By: Shailendra Kumar
Surface Chemistry
I. INTRODUCTION
Many chemical reactions take place at the surfaces of solids, e.g., at the surface of solid catalysts.
Corrosion of metals take place at their surfaces means the topmost layer on the very exterior of
a solid, Now-a-days, there are weys of studying the top 0.5 to 2 nm, i.e., two to five of the topmost
layers of atoms of the solid surface.
In this Unit, we shall discuss phenomenon occurring at surface of solids. We shall also study
colloidal state, emulsions and catalysts.
II. IMPORTANT TERMS AND DEFINITIONS
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Adsorption. The existence of a substance at a surface in different concentration than in the
adjoining bulk is called adsorption. It is selective absorption.
Absorption. It is the phenomenon in which a substance is uniformly distributed all over the
surface.
Adsorbent. The substance present in bulk, on the surface of which adsorption is taking place is
called adsorbent.
Adsorbate. The substance which is being adsorbed on the surface of another substance is
called adsorbate.
Desorption. The process of removing an adsorbed substance from a surface on which it is
adsorbed is called desorption.
Physical adsorption or physisorption. If the adsorbate is held on a surface of adsorbent by
weak Vander Waal’s forces, the adsorption is called physical adsorption or physisorption.
Chemical adsorption or chemisorption. If the forces holding the adsorbate are as strong as in
chemical bonds, the absorption process is known as chemical adsorption or chemisorption.
Enthalpy or heat of adsorption. Adsorption generally occurs with release in energy, i.e., it is
exothermic in nature. The enthalpy change for the adsorption of one mole of an adsorbate on the
surface of adsorbent is called enthalpy or heat of adsorption.
Adsorption isotherm. When extent of adsorption (x/m; where ‘x’ is amount of adsorbate, ‘m’ is
mass of adsorbent) is plotted against pressure at constant temperature, curve obtained is known
as adsorption isotherm.
Specific surface area of solid. It is defined as the surface area (in square metre) per gram of the
adsorbent.
Adsorption isobar. A plot between amount adsorbed (x/m) vs. temperature at constant pressure
is called an adsorption isobar.
Colloidal solution. The solution in which the diameter of particles may range from 1 to 100 nm
The colloidal particles do not settle down under the force of gravity even on long standing. A
colloidal system is always heterogeneous, e.g., glue , starch, albumin, gelatin, blood,
soap solution , ink, clouds, smoke, etc.
Crystalloids. Those substances whose aqueous solution can pass through semi-permeable
membrane are called crystalloids, e.g., NaCl, KCl, H2SO4, NaOH, etc.
Disperse phase. The phase of colloidal particles is called disperse phase.
Dispersion medium. The medium in which these particles are dispersed is called dospersion
medium, e.g., water is dispersion medium in hydrosol.
Aerosols. When dispersion medium is gas and dispersed phase is either solid or liquid, the
colloidal system is called aerosols, e.g., fog, clouds, fine insecticide sprays, smoke, etc. Colloidal
suspension in air are also called aerosols.
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PHYSICAL CHEMISTRY
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Sols. When dispersed phase is solid and dispersion medium is either solid or liquid, the
Colloidal system is known as sols, e.g., paints, starch dispersed in water, gold sol, muddy
water, ruby glass, gem stones, etc.
Gels. When dispersed phase is liquid and dispersion medium is solid, such colloidal systems
are called gels, e.g., cheese, butter, boot polish, etc.
Emulsion. Liquid-liuid colloidal dispersions are called emulsion are called emulsion, e.g.,
milk, emulsified oils and medicines.
Luophobic sols. Those colloids which are solvent hating are called ‘lyophobic sols’. In these
colloids, the particles of dispersed phase has no affinity for the dispersion medium. They are
not easily prepared. They need stabilising agent to preserve them. They are irreversible.
Example of lyophobic sols are colloidal solutions of gold, silver, Fe(OH)3, As2S3, etc.
Lyophilic sols. These are defined as the colloidal solutions in which the particles of dispersed
phase have great affinity for the dispersion medium. They are self-stabilised because of strong
attractive forces operating between the suspended particles and the dispersion medium. They
are reversible sols, The solids obtained after their evaporation may be recovered to the colloidal
solution simply by simple agitating them with the dispersion medium. For example gums,
gelatine,starch, albumin and other proteins in water solution are lyophilic.
Hydrosols. Colloids in water are called hydrosols.
Alcosols. Colloids in alcohol are called alcosols.
Benzosols. Colloids in benzene are called benzosols.
Hydrophobic sols. If dispersion medium is water in lyophobic sols, they are called hydrobic
sols. They are also called ‘suspensoids’.
Hydrophilic sols. If dispersion medium is water in lyophilic sols, they are called hydropilic
sols.
Intrinsic colloids. Some substances which readily pass into colloidal solution on warming
with suitable liquids are called intrinsic colloids, e.g., gum-arabic, glue, starch and gelatin. The
colloids solutions thus obtained are called lyophilic sols.
Extrinsic colloids. Substance which do not pass into colloidal solution even on heating are
called extrinsic colloids. They are referred to as lyophobic sols, e.g., silver and gold sol.
Multi-molecular colloids. The colloids in which the colloidal particles of aggregates of atoms
or small molecules with diameters of less than 1 nm are called multi-molecular colloids. For
example, a gold sol may contain particles of various sizes having several atoms of gold.
Similarly, a sulphur sol consists of particles containing about a thousand of S8 molecules.These
are held together by Vander Waal’s forces.
Macromolecular colloid. These are the colloids in which the dispersed particles are
themselves large molecules (usually polymers). Since these molecules have dimensions
comparable to those of colloidal particles, their dispersions are called macromolecular colloids,
e.g., proteins starch and cellulose form macromolecular colloids.
Miscelles. Those colloids which behave as normal, strong electrolytes at low concentrations,
but show colloidal properties at higher concentrations due to the formation of aggregated
particles of colloidial dimensions. Such sunstances are also referred to as associated colloids.
Surface active agents like soaps and synthetic detergents belong to this class. They form
ions also. Molecules of soaps and detergents consist of lyophilic as wall as lyopobic parts
which associate together to form miscelles. Miscelles may contain 100 molecules or more.
Surface-active agents. Those substances when tend to concentrate on the surface of solution
and, therefore, lowers the surface tension, are called surface-active agents. e.g., soaps and
detergents.
Peptization. The process of passing of a precipitate into colloidal solution on adding an
electrolyte is called peptization. In this process, dispersion medium (liquid) break up the
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PHYSICAL CHEMISTRY
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By: Shailendra Kumar
precipitate into colloidal state. Lyophilic sols are prepared by bringing into contact or warming the
substance with a dispersion medium, e.g., gelatin, gum arabic, starch and soaps are easily peptized
by water. Cellulose nitrate is peptized by organic solvents such as ethyl alcohol. The product
obtained is commercially known as collodion.
Dialysis. It is a process of purification of sols containing electrolyte by keeping the sol in a bag
made of parchment or cellophane and suspending the bag in pure water. Small molecules or ions
can pass through the membrane, but the sol is retained. When electric field is applied during
dialysis, it is called electrodialysis.
Brownian movement. Colloidal particales move in zig-zag path. This typeof motion is due to
moving molecules of dispersion medium constantly with colloidal particles. During collisiion,
molecules of dispersion medium impart momentum to colloidal particales . You must have seen
dust particles in a beam of sunlight coming through a slit in a dark room, which keep on moving
in a zig-zag path. It is used to determine Avogadro’s number.
Tyndall effect. When light passes through a sol, its path because visible just as a beam of
sunlight is seen as it enters a dark room through a small hole or slit because of scattering of light
by colloidal particles. On the basis of dust Tyndall effect we can explain that sky is blue in day
light due to scattering of blue light dust particles. Scattering of light depends upon the difference
between the refractive indices of dispersed phase and dispersion medium.
Electrophoresis. The movement of colloidal particles towards positive or negative electrode in
electric field is called electrophoresis. It occurs due to the presence of positive or negative charge
on colloids. It is also called cataphoresis.
Emulsification. The process of making an emulsion from an oil is termed as emulsification.
Emulsifying agent. The substance which stabilise emulsions are called emulsions are called
emulsifying agents.
Homogeneous catalysis. When the catalyst mixes homogeneously with the reactant(s) and
forms a simple phase, the catalyst is said to be homogeneous and this kind of catalysis is known
as homogeneous catalysts, e.g., catalytic oxidation of SO2 to SO3 in presence of NO is an example
of homogeneous catalysis.
Heterogeneous catalysis. When the catalyst forms a separate phase (usually a solid phase), it
is said to be heterogeneous and the catalysis. Example of heterogeneous catalysis is manufacture
of H2SO4 in contact process using V2O5 as catalyst. Usually, in a heterogeneous catalysis, the
reactants are gases, and reaction starts from the surface of the solid catalyst. This is the reason
why heterogeneous catalysis is also called ‘surface catalysis’.
Activity of catalyst. It is the ability of catalyst to accelerate chemical reaction, e.g., H2 and O2 in
presence of platinum react to form water explosively.
Selectivity of catalyst. It is ability of catalyst to direct reaction to yield particular products (excluding
others). For example, n-heptane selectively gives toluene in presence of a platinum catalyst.
Gold number. It is defined as number of milligrams of protective colloids which must be added to
10 ml of gold solution to prevent coagulation when 1 ml of 10% solution of NaCl is added.
Shape selective catalysis. The catalysis which depends upon the pore-structure of the catalyst
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PHYSICAL CHEMISTRY
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By: Shailendra Kumar
and molecular sizes of reactants and product molecules is called shape selective catalysis,
e.g., Zeolites are shape selective catalysts due to their honey-comb structure ZSM-5 is used to
convert methanol into gasoline.
Coagulation. The process of converting a colloidal solution into precipitate is called coagulation.
Coagulation value or flocculation value. It is defined as minimum number of millimoles of
electrolyte required to coagulate 1 litre of colloidal solution.
Sturation pressure. The pressure beyond which extent of adsorption becomes independent of
pressure.
Critical micellisation concentration (CMC). It is the lowest concentration of the surfactant
(e.g.,soap solution) at which miscelle formation takes place.
Syneresis. The shrinkage in volume by gels when allowed to stand is called syneresis.
Aqua-dag. It is colloidal solution of graphite in water.
COMMIT TO MEMORY
Differences between True Solution, Colloidal solution and Suspension
Property
True solution
Colloidal solution
Suspension
Nature
Homogeneous
Appears to be homogeneous
Heterogeneous
but actually heterogeneous
Particle size
< 1 nm
1 nm to 100 nm
> 100nm
Visibility
Particles are not
Visible through ultra-microscope Visible with naked
visible even with
eye
microscope
Molar mass
Low
High
Moderate
Colour
Depends upon nature Depends upon size of particles Depends upon
of ions
nature of particles
Osmotic
High
Low
Very low
pressure
Diffusion
Fast
Slow
Do not diffuse
Tyndall effect
No scattering of light Light is scattered
Light is scattered
Sedimentation Do not settle down
Settle on centrifugation
May settle due may
not
to gravity
Brownian
Do not show
Show brownian movement
May or may not
movement
show
Electrophoresis Not shown
Exhibited
Not exhibited
Coagulation
Cannot be coagulated Can be coagulated
The may settle
under gravity.
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Types of colloidal solution
Dispersed
Dispersion medium
phase
Solid
Gas
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Solid
Liquids
Sols
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Solid
solid
Solid sols
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Liquid
Solid
Gels
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Name
Examples
Aerosols
Smok dust
particles
As2S3, Gold sol,
starch, gum, muddy
water
Coloured gem stones,
some alloys, pearls,
ruby glass
Jellies, cheese, shoe
polish
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PHYSICAL CHEMISTRY
By: Shailendra Kumar
5.
Liquid
Liquid
Emulsions
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Liquid
Gas
Gas
Solid
Aerosols of liquids
Solid foam
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Gas
Liquid
Foam
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Methods of Preparation of Colloidal solution
Dispersion Methods. In these methods bigger particles are
disintegrated into particles of colloidal dimensions by suitable
methods, e.g.,
(a) Mechanical despersion. A suspension of a coarsely ground
particles prepared in dispersion medium is fed into colloidal
mill and speed of rotation is adjusted so as to get particles of
colloidal size.
(b) Electro-dispersion (Bredig’s Arc Method). In this method two
rods of the metal (Au, Cu, Pt, Ag,) to be dispersed are kept
immersed in cold water and a direct electric arc us struck
between them. In this way vapours of metal are formed which
then immediately condense to form particles of colloidal size.
(c) Oxidation.Solutions of non-metals are prepared by this method, e.g., colloidal solution of
sulphur
2H2S + (O)
S + 2H2O
(d) Reduction. Metal sols can be prepared by this method, e.g., Gold sol
2AuCl3 + 3SnCl2 → 2Au + 3SnCl4
(e) Hydrolysis. Hydroxides sols are prepared by this methode.g., Fe(OH)3, Al(OH)3
FeCl3 + 3H2O → Fe (OH)3 + 3HCl
(f) Double decomposition. This method is used to prepare colloids from inorganic salts,e.g.,
As2O3 +3H2S → As2S3 + 3H2O
(g) Exchange of solvent. Some substances which form true solution in one solvent form colloidal
solution in another due to lowering of solubility, e.g., sulphur dissolved in alcohol forms
colloidal solution in water and phenolphthalein dessolved in alcohal forms colloidal solution
in water.
(h) Peptization. The conversion of precipite into colloidal solution in presence of peptizing agent
is called peptization. Peptizing agent is generally an electrolyte.
Hardy-Schulze Rules :
1. Oppositely charged ions are effective for coagulation.
2. The coagulating power of electrolyte increases with increases in charge on the ions used
for coagulation, e.g., Al3+ > Ba2+ > Na+ for negatively charged colloids.
[Fe (CN)6]4- > PO43- > SO42- > Cl- for positively charged colloids.
The reciprocal of coagulation value is called coagulating power i.e., lower the coagulation value,
higher will be coagulating power.
Origin of charge on colloidal solution :
(a) By self-dissociation.
(b) By ionisation of surface molecule.
(c) Capture of electron in Bredig’s arc method.
(d) pH of solution.
(e) By preferential adsorption ions from solution.
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Milk, hair cream, cod
liver oil
Fog, mist, cloud
Pumic stone, foam
rubber, cork
Whipped cream,
lemonade forth
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PHYSICAL CHEMISTRY
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(f) Frictional rubbing.
Oil in water emulsions. When oil is dispersed in water, e.g., Milk, Vanishing cream, etc.
Water in oil emulsions. When water is dispersed in oil, e.g., Butter, cold cream, etc.
Smaller the gold number, greater will be protective action of the given lyophilic colloid e.g., Gelatin
has least value of Gold number (0.006 to .01) while starch has highest (10 to 25).
When lyophillic sol is added to lyophobic sol so as to make it stable and prevents its coagulation,
it is called protective colloid.
Multual coagulation. When positively charged colloid like Fe(OH)3 is mixed with negatively
charged colloid like As2S3, mutual coagulation takes place.
Kraft temperature. Miscells form ionic surfactants only above a certain temperature called kraft
temperature.
Hydrophillic lipophillic balance (HLB) is a numerical scale to describe the effectiveness of a
particular surfactant to produce a particular type of emulsion.
Thioxotropy. Some gels like gelatin liquify on shaking into sol. The sol on standing becomes gel
again. This property is called Thioxotropy.
Oildag is colloidal solution of graphite in oil.
Bancroft’s Rule. Hydrophillic emulsifiers dissolve better in water than in oil, hence help in formation
of oil in water emulsion. Hydrophobic emulsifiers dissolve better in oil than in water, hence form
water in oil emulsions.
Anionic Surfactants. Sodium palmitate (C15H31COONa), sodium stearate (C17H35COONa) and
sodium oleate (C17H33COONa) are anionic surfactants. They are called soaps. Detergents are
also anionic surfactants.
Cationic Surfactants. Quaternary ammonium salts of long chain tertiary amines form synthetic
detergents which are cationic surfactants e.g., Octadecyl ammonium chloride C18H37H+H3ClCetyltri methyl ammonium chloride C16H33(CH3)3N+Cl-.
Non-inorganic Surfactants. Alcohols react with epoxides to form non-inorganic surfactants.
e.g.,
CnH2n+1 —OH + nCH
— 2—CH2 → CnH2n+1 (—O —CH2 —CH2 - —)n OH
Surfactant
O —
Tyndall cone. The bright cone shaped illuminated path observed in case of colloidal solution
under microscope is called Tyndall cone.
The intensity of scattered light varies inversely as fourth power of wavelenght. Scince blue light
has smaller wavelenght, therefore blue Tyndall cone is formed.
Zeta-potential. The surface of a colloidal particles acquires a +ve or -ve charge by selective
adsorption of ions. This layer attracts counter ions from the medium which forms second layer
having opposite charge and this combination of layers is called Helmoholtz electric double layer.
The first layer is firmly held and called fixed layer and second layer being diffused is called mobile
layer. The potential difference between these two layers is called Zeta-potential.
Elastic gels change into solid mass on dehydration and changed back to original form on addition
of water. They absorb water when placed in it with simultaneous swelling and this phenomenon is
called imbibition.
Non-elastic gels change into solid mass on dehydration which do not change back to original
form by addition of water. They do not imbibe.
Adsorption indicator. A substance eosin which gets adsorbed on the precipitate formed in a
reaction and which undergoes a colour change at the end point in a titration is called an adsorption
indicator.
Determination of size of colloidal particle.
n1 4 3
RT
log e
= πr (h 2 - h1)(ρ - ρ' )
No
n2 3
No. is Avogadro’s number, R = gas constant, T = Temperature in Kelvin, n1 and n2 are number of
particles at depth h1 and h2 respectively, ρ and ρ’ = density of colloidal particles and dispersion
medium respectively, ‘ r’ is radius of the particles which can be determind easily.
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