Unit V: Surface chemistry

PREPARED BY: ARUNESH GUPTA
PGT (CHEMISTRY)
KENDRIYA VIDYALAYA BARRACHPORE (AFS)

ADSORPTION
Adsorption:
(i) The accumulation of molecular species at the surface rather than in the
bulk of a solid or liquid is termed as adsorption.
(ii) It is a surface phenomenon
(iii) The concentration of adsorbate increases only at the surface of the
adsorbent.
Absorption:
(i) It is the phenomenon in which a substance is uniformly distributed throughout the bulk
of the solid.
(ii) It is a bulk phenomenon.
(iii) The concentration is uniform throughout the bulk of solid.

Adsorbate, Adsorbent, Desorption, Sorption
Adsorbate: It is the substance which is being adsorbed on
the surface of another substance. Example: gas molecules
etc.
Adsorbent: It is the substance present in bulk, on the
surface of which adsorption is taking place. Examples:
activated charcoal, alumina gel, clay, silica sol etc.
Air is dried by silica gel (adsorbent) which adsorbs water
molecules (adsorbate) on its surface.
Desorption: It is the process of removing an adsorbed
substance from a surface on which it is adsorbed.
Sorption: When adsorption and absorption take place
simultaneously, it is called sorption.

Enthalpy or heat of adsorption: Since, adsorption 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 is a natural phenomenon so, there is always a decrease in residual
forces of the surface, i.e. there is the decrease in surface energy which appears as
heat.
So, ΔH= -ve, ΔS= -ve & ΔG= -ve, on the basis of ΔG= ΔH-TΔS.

Types of adsorption: There are two types of adsorption:
(a) Physical adsorption (or physisorption) (b) Chemical adsorption (or chemisorption)
(a) Physical adsorption (or Physisorption)
(i) If the adsorbate is held on a surface of adsorbent by weak van der Waals’ forces, the adsorption is
called physical adsorption or physisorption.
(ii) It is non-specific.
(iii) It is reversible.
(iv) The amount of gas depends upon nature of gas, i.e., easily liquefiable gases like NH3, CO2, gas
adsorbed to greater extent than H2 and He. Higher the critical temperature of gas, more will be the
extent of adsorption.
(v) The extent of adsorption increases with increase in surface area, e.g. porous and finely divided
metals are good adsorbents.
(vi) There are weak van der Waals’ forces of attraction between adsorbate and adsorbent.
(vii) It has low enthalpy of adsorption (20 – 40 kJ mol-1).
(viii) Low temperature is favourable.
(ix) No appreciable activation energy is needed.
(x) It forms multimolecular layers.

Chemical adsorption or chemisorption:
(i) If the forces holding the adsorbate are as strong as in chemical bonds, the
adsorption process is known as chemical adsorption of chemisorption.
(ii) It is highly specific.
(iii) It is irreversible.
(iv) The amount of gas adsorbed is not related to critical temperature of the gas.
(v) It also increases with increase in surface area.
(vi) There is strong force of attraction similar to chemical bond.
(vii) It has enthalpy heat of adsorption (80 – 240 kJ mol-1).
(viii) High temperature is favourable.
(ix) High activation energy is sometimes needed.
(x) It forms unimolecular layers.
At low temperature 83 K, N2 is physisorbed on iron on its surface whose adsorption
decreases with the increase in temperature but at 773 K & above N2 is chemisorbed on iron
surface forming chemical bonds as N atoms.

Comparison between physisorption & chemisorption
Physical Adsorption (Physisorption)
1. It arises due to weak van der Waal’s force
2. It is not specific in nature
3. It is reversible in nature
4. It depends on nature of a gas, Gases with
high critical temperature are adsorbed more
5. Enthalpy of adsorption is low (20 – 40 kJ/mol)
6. It is favoured by low temperature
7. No appreciable activation energy needed.
8. It results into multi molecular layers on
adsorbent surface under high pressure.
Chemical Adsorption (Chemisorption)
1. It arises due to chemical bond formation
2. It is highly specific in nature.
3.Irreversible in nature
4.Depends on nature of gases. Gases which can
react with the adsorbent show chemisorption.
5.Enthalpy of adsorption is high. (80-240kJ/mol)
6.It is favoured by high temperature.
7. High activation energy is sometimes needed.
8. It results into unimolecular layer due to
formation of chemical bond.

Factors affecting adsorption of gases on solids:
Nature of adsorbate: Physical adsorption is non-specific in nature and therefore every gas gets
adsorbed on the surface of any solid to a lesser or greater extent. However, easily liquefiable gases like NH3,
HCl, CO2, etc. which have higher critical temperatures are absorbed to greater extent whereas H2, O2, N2
etc. are adsorbed to lesser extent. The chemical adsorption being highly specific, therefore, a gas gets
adsorbed on specific solid only if it enters into chemical combination with it.
Nature of adsorbent: Activated carbon (activated charcoal), metal oxides like Al2O3, silica gel and
clay are commonly used adsorbents. They have their specific adsorption properties depending upon pores.
Specific area of the adsorbent: The greater the specific area, more will be the extent of
adsorption. That is why porous or finely divided forms of adsorbents adsorb larger quantities of adsorbate.
The pores should be large enough to allow the gas molecules to enter.

Pressure of the gas: Physical adsorption increases with increase in pressure & is explained by Adsorption
isotherm.
Adsorption isotherm: The relation between the amount of substance adsorbed by the adsorbent & the
equilibrium gas pressure ( or molar concentration for solutions) at constant temperature is called adsorption
isotherm.
Freundlich Adsorption isotherm: The relationship between rate of adsorption x/m and pressure (P) of the
gas at constant temperature is called adsorption isotherm and is given by
𝒙
𝒎
= k 𝑃
1
𝑛 (n > 1) . . . . . . . . . . . ..(1)
where x is the mass of the gas adsorbed on mass m of the adsorbent and the gas at a particular temperature
k and n depends upon the nature of gas. The x/m denotes the extent of adsorption.
(a) At low pressure P, n ≈1 So, x/m = k P or x/m α P
(b) At high pressure, P, n ≈0 So, x/m = k. So, we conclude, adsorption of a gas on solid first increases with
increase in pressure at low pressure but becomes independent of pressure at high pressure.
At saturation pressure rate of adsorption & rate of desorption are equal & the graph becomes parallel to P
axis.

Freundlich Adsorption isotherm:
𝒙
𝒎
= k 𝑃
1
𝑛 (n > 1), Graphically, we get
If we plot a graph between log
𝑥
𝑚
and log P, we get a straight line.
The slope of the line is 1/n and intercept will be equal to
log k.
Freundlich adsorption holds good over a limited range of
pressure where 1/n lie between 0 to 1
Freundlich adsorption isotherm fails at high pressure of the
gas.

Adsorption from solutions:
The process of adsorption can takes place for a solutions on solids.
Example: when solution of acetic acid in water is shaken with charcoal, acetic acid is
adsorbed on charcoal & concentration of acetic acid in water decreases due to
adsorption.
Freundlich’s adsorption isotherms has been found to be approximately applicable to the
adsorption of solutes from solution on solid. It is mathematically given as
𝒙
𝒎
= k 𝐶
1
𝑛 𝑤ℎ𝑒𝑟𝑒 𝑛 𝑙𝑖𝑒 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 1 𝑎𝑛𝑑 0.
& taking log we get, log
𝒙
𝒎
= log k + 1/n log C
where C is the molar concentration of solution.

Temperature:
In case of physical adsorption, at constant pressure, increasing temperature will decrease the
rate of adsorption. KE of adsorbate increases & desorption takes place.
In case of chemical adsorption, at low temperature rate of adsorption is less as less chemical
bonds are formed. With the increase in temperature, more activation energy is supplied to form more
chemical bonds & rate of adsorption increases & on further increase in temperature when chemical bonds
are formed all along the surface of adsorbent, desorption takes place & rate of adsorption decreases.
Graphically,

Applications of adsorption:
(i) In gas masks: Activated charcoal is generally used to adsorb harmful gases like CO, CH4, SO2 etc.
(ii) In dyeing of clothes: Mordants such as alums are used in dyeing of cloth. Mordant helps to adsorb
dye particles strongly for fast colour on cloth.
(iii) In dehumidizers for control of humidity: Silica & alumina gels are commonly used to adsorb
moisture from air.
(iv) Removal of colouring matter from solutions: Animal charcoal decolourises & de-odourises
sugarcane juice to form sugar, activated charcoal or fuller’s earth are used to remove impurities from
juices, vegetable oils etc.
(v) In heterogeneous catalysis:
(vi) In ion exchange resins.
(vii) In qualitative analysis: In ‘blue lake test’ of test of presence of Al3+, litmus pigments are adsorbed
by Al(OH)3 to form floating blue particles in basic solution.
(viii) In production of high vacuum using activated charcoal.
(ix) Separation of inert gases using coconut charcoal at different temperature.
(x) In froth floatation process:
(xi) In chromatographic analysis:

Catalyst: These are substances which alter the rate of a chemical reaction and themselves remain
chemically and quantitatively unchanged after the reaction and the phenomenon is known as catalysis. A
catalyst increases the rate of a reaction by decreasing its activation energy.
(i) A positive catalyst increases the rate of a chemical reaction.
Example: 2SO2(g) + O2(g)
𝑃𝑡 𝑠
2SO3(g)
(ii) A negative catalyst decreases the rate of a chemical
reaction. Example: The rate of decomposition of H2O2 decreases
in the presence of negative catalyst like, acetanilide, urea or
phosphoric acid.
(iii)Promoters: These are the substances which increase the
activity of catalyst. Example – Mo is promoter whereas Fe isthe
catalyst in Haber’s Process.
N2(g) + 3H2(g)
𝐹𝑒 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 ,𝑀𝑜(𝑝𝑟𝑜𝑚𝑜𝑡𝑒𝑟)
2NH3(g)
(iv) Catalytic poisons (Inhibitors): These are the substances
which decrease the activity of catalyst.
Example -Arsenic acts as catalytic poison in the manufacture of
sulphuric acid by ‘Contact Process.

Types of catalysis: There are two types of catalysis namely,
(i) Homogeneous catalysis: When the catalyst and the reactants are in the same phase (liquid or gas),
this kind of catalytic process is known as homogeneous catalysis.
Example: (1) In Lead chamber process: 2SO2(g) + O2(g)
𝑁𝑂( 𝑔)
2SO3(g) [reactants & catalyst are gases]
(2) C12H22O11(aq) + H2O(l)
𝐻2 𝑆𝑂4
C6H12O6(aq) (glucose) + C6H12O6(aq) (fructose)
(ii) Heterogeneous catalysis: When the catalyst and the reactants are in different phases, the catalytic
process is said to be heterogeneous catalysis.
Examples: (1) In Contact Process: 2SO2(g) + O2(g)
𝑃𝑡( 𝑠)
2SO3(g)
(2) 𝑁2(𝑔) + 3𝐻2(𝑔)
𝐹𝑒 ( 𝑠)
2𝑁𝐻3(𝑔)

Adsorption Theory of Heterogeneous Catalysis:
It explains the mechanism of heterogeneous catalysis.
It explains the adsorption of gaseous reactants on the solid surface
as catalyst.
There in the increase in concentration on catalyst surface & rate of
reaction increases. Also Adsorption being exothermic, the heat of
adsorption enhances the rate of the reaction. (ΔadsH = negative)
The mechanism involves five steps.
(i) Diffusion of reactants to the surface of the catalyst.
(ii) Adsorption of reactant molecules on the catalyst surface.
(iii) Chemical reaction occurs on catalyst surface through the
formation of an intermediate.
(iv) Desorption of products formed from catalyst surface& making
the surface available again for more reactants to get adsorbed for
reaction.
(v) Diffusion of the reaction products away from the surface of
catalyst.
This explains that the catalyst remains unchanged in mass &
chemical composition at the end of the reaction & is even small
quantities.
This theory cannot explain the action of catalytic promoters or
poisons in a chemical reaction

Activity of catalyst: It depends upon the strength of chemisorption to a large extent. The
reactants must get adsorbed strongly on the surface but not so strong that they are immobilised &other
reactants are left with no space on the catalyst surface.
Example: 2H2(g) + O2(g)
𝑃𝑡(𝑠)
2H2O (l)
Selectivity of catalyst: It is the ability of catalyst to direct a reaction to yield a particular product
(excluding others).
For example: CO and H2 react to form different products in presence of different catalysts as follows:
(i) CO(g) + H2(g)
𝑃𝑡 (𝑠)
HCHO(g)
(ii) CO(g) + 2H2(g)
𝐶𝑢/𝑍𝑛𝑂−𝐶𝑢𝑂
CH3OH(l)
(iii) CO(g) + 3H2(g)
𝑁𝑖 𝑠
CH4(g) + H2O(l)
So, a substance which acts as a catalyst in one reaction may fail to catalyse other reaction.

Shape – selective catalysis by Zeolites:
It is the catalysis which depends upon the pore structure of the catalyst
and molecular size of reactant and product molecules.
Zeolites are shape – selective catalysts due to their honey- comb structure having
Al-O-Si frameworks of aluminosilicates with 3 dimensional network structure.
Example – In petroleum industry, alcohols are directly converted into gasoline or
petrol using ZSM-5 as shape selective catalyst. (ZSM-5 means zeolite sieve of
molecular porosity 5)

Enzymes:These are complex nitrogenous organic compounds which are produced by living
plants and animals. They are actually globular protein molecules of high molecular mass. They are
biochemical catalysts. Examples:
(i) C12H22O11(aq) (sucrose) + H2O(l)
𝐼𝑛𝑣𝑒𝑟𝑡𝑎𝑠𝑒
C6H12O6(aq) (glucose) + C6H12O6(aq) (fructose)
(ii) C6H12O6(aq)
𝑍𝑦𝑚𝑎𝑠𝑒
2C2H5OH(aq) + 2CO2(g)
(iii) NH2CONH2(aq) [urea] + H2O(l)
𝑈𝑟𝑒𝑎𝑠𝑒
2NH3(g) + CO2(g)
Steps of enzyme catalysis:
Binding of enzyme to substrate to form
an activated complex.
E +S ⇌ [E-S]#
(ii) Decomposition of the activated
complex to form product.
[E-S]# → E + P

Characteristics of enzyme catalysis:
(i) They are highly efficient. One molecule of an enzyme can transform 106 reactant
molecules / minute.
(ii) They are highly specific in nature. Example – Urease catalysis hydrolysis of urea only.
(iii) They are active at optimum temperature (298 – 310 K). The rate of enzyme catalysed
reaction becomes maximum at a definite temperature called the optimum temperature.
(iv) They are highly active at a specific pH called optimum pH (generally between 5 to 7)
(v) Enzymatic activity can be increased in presence of coenzymes which can be called as
promoters.
(vi) Activators are generally metal ions Na+, Co2+ and Cu2+ etc. They weakly bind to
enzyme and promote the activity of enzyme.
(vii) Influence of inhibitors and poisons: Enzymes can also be inhibited or poisoned by the
presence of certain substances e.g. certain drugs inhibits enzyme.

Some Industrial Catalytic Processes:
(1) Heber’s Process:
N2(g)+ 3H2(g)
𝐹𝑒 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 ,𝑀𝑜 𝑝𝑟𝑜𝑚𝑜𝑡𝑒𝑟 200𝑏𝑎𝑟, 723−773𝐾
2NH3(g)
(2) Ostwald’s Process:
(i) 4NH3(g) + 5 O2(g)
𝑃𝑙𝑎𝑡𝑖𝑛𝑖𝑧𝑒𝑑 𝑎𝑠𝑏𝑒𝑠𝑡𝑜𝑠, 573𝐾
4NO(g) + 6 H2O(g)
(ii) 2NO(g) + O2(g)  2NO2(g)
(iii) 4NO2(g) + 2H2O(l)  4HNO3(aq)
(3) Contact Process:
(i) 2SO2(g) + O2(g)
𝑉2 𝑂5, 673 −723𝐾
2SO3(g)
(ii) SO3(g) + H2SO4(aq)  H2S2O7(l)
(iii) H2S2O7(l) + H2O(l)  2H2SO4 (aq)

True solution:
(i) It is homogeneous.
(ii) The diameter of the particles is less than 1 nm.
(iii) It passes through filter paper.
(iv) Its particles cannot be seen under a microscope.
Colloidal solution (Colloid):
(i) It appears to be homogeneous but is actually heterogeneous.
(ii) The diameter of the colloidal particles is 1 nm to 1000 nm.
(iii) It passes through ordinary filter paper but not through ultra-filters.
(iv) Its particles can be seen by a powerful microscope due to scattering of light.
Suspension:
(i) It is heterogeneous.
(ii) The diameter of the particles are larger than 1000 nm.
(iii) It does not pass through filter paper.
(iv) Its particles can be seen even with naked eye.

Colloidal solution has dispersed phase & dispersion medium.
Dispersed phase: It is the substance which is dispersed as very fine particles of size 1-1000nm.
Dispersion medium: It is the substance present in larger quantity.
Classification of colloids
(1) On the basis of the physical state of dispersed phase and dispersion medium:
Name Dispersed phase Dispersed medium Examples
Solid sol Solid Solid Coloured gem stones
Sol Solid Liquid Paints
Aerosol Solid Gas Smoke, dust
Gel Liquid Solid Cheese, jellies
Emulsion Liquid Liquid Hair cream, milk
Aerosol Liquid Gas Mist, fog, cloud
Solid sol Gas Solid Foam rubber, pumice stone
Foam Gas Liquid Whipped cream

(2) on the basis of nature of interaction between dispersed phase and dispersion medium:
The colloids are classified into two types namely (a) Lyophobic sols (b) Lyophilic sols
(a) Lyophobic sols:
(i) These colloids are liquid(dispersion medium) hating.
(ii) In these colloids the particles of dispersed phase have no affinity for the dispersion medium.
(iii) They are not stable.
(iv) They can be prepared by mixing substances directly.
(v) They need stabilizing agents for their preservation.
(vi) They are irreversible sols.
(b) Lyophilic sols:
(i)These colloids are liquid loving.
(ii) In these colloids, the particles of dispersed phase have great affinity for the dispersion medium.
(iii) They are stable.
(iv) They cannot be prepared by mixing substances directly. They are prepared only by special methods.
(v) They do not need stabilizing agents for their preservation.
(vi) They are reversible sols.

Classification of colloids:
(3) on the basis of types of particles of the dispersed phase:
There are three types of colloids based on the type of dispersed phase, namely,
(a) Multimolecular colloids: The colloids in which the colloidal particles
consist of aggregates of atoms or small molecules. The diameter of species of the
colloidal particle formed is less than 1 nm.
Example: gold sol, sulphur sol has a thousand or more S8 molecules.
(b) Macromolecular colloids: These are the colloids in which the dispersed
particles are themselves large molecules (usually polymers). Since these
molecules have dimensions comparable to those of colloids particles, their
dispersions are called macromolecular colloids, e.g., proteins, starch and cellulose
form macromolecular colloids.

(c) Associated colloids (Micelles): 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 colloidal dimensions. Such substances are
also referred to as associated colloids or micelles.
Kraft Temperature (Tk): Micelles are formed, only
above a certain temperature called Kraft temperature.
Critical Micelle Concentration (CMC): Micelles
are formed, only above a particular concentration called
critical micelle concentration. For soap CMC value is 10-4
to 10-3 mol L-1.
e.g. Soaps are sodium or potassium salts of higher fatty
acids e.g., sodium stearate CH3(CH2)16COO-Na+
It forms an ionic micelles.

Methods of preparation of colloids:
(a) Chemical methods: Colloids can be prepared by chemical reactions leading to the
formation of molecules. These molecules aggregate leading to formation of sols.
(i) Arsenic sulphide sol: As2O3 + 3H2S
𝐷𝑜𝑢𝑏𝑙𝑒 𝑑𝑒𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛
As2S3(sol) + 3H2O(l)
(ii) Sulphur sol : SO2 + 2H2S
𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛
3S (sol) + 2H2O (l)
(iii) Ferric hydroxide sol: FeCl3 + 3H2O
𝐻𝑦𝑑𝑟𝑜𝑙𝑦𝑠𝑖𝑠
Fe(OH)3 (sol) + 3HCl
(iv) Gold Sol: 2AuCl3 + 3HCHO + 3H2O
𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛
2Au (sol) + 3HCOOH + 6HCl

(b) Electrical disintegration
or Bredig’s Arc method:
This process involves dispersion as well
as condensation. In this method,
electric arc is struck between
electrodes of the metal immersed in
the dispersion medium. The intense
heat produced vaporizes the metal
which then condenses to form particles
of colloidal size. Example: gold sol,
silver sol etc.
Methods of preparation of colloids:

Peptization:
It is the process of converting a precipitate into colloidal sol by shaking it with
dispersion medium in the presence of a small amount of electrolyte having one ion
common. The electrolyte used for this purpose is called peptizing agent.
The common ion is adsorbed on the surface of colloid as preferential
adsorption.
Thus it results into the formation of positive colloid or negative colloid.
Example:
AgI (precipitate) + KI (aq) (peptizing agent)  AgI/I- (-ve colloid) + K+(aq) (crystalloid)

Purification of colloids:
Dialysis:
It is a process of removing a
dissolved substance from a colloidal
solution by means of diffusion through a
suitable membrane.
The ions or smaller molecules can
pass through the animal membrane or
parchment paper or cellophane sheet,
but colloids can not. So fresh water is
passed to remove impurities.
It is a slow process.

Purification of colloids:
Electro dialysis:
Dialysis can be made faster by
applying an electric field if the
dissolved substance in the impure
colloidal solution is only an
electrolyte. The ions present as
crystalloids migrate towards
oppositely charged electrode & speed
up the process of dialysis. On
prolonged electro-dialysis, all the ions
are removed, the colloidal particles
are precipitated & coagulation takes
place.

Ultrafiltration:
It is the process of separating the colloidal particles from the
solvent and soluble solutes present in the colloidal solution by specially
prepared filters, which are permeable to all substances except the colloidal
particles.
It is a slow process. Its speed can be increased by using pressure or suction.
The pores of filter paper can be reduced in size by impregnating with
collodion solution to stop the flow of colloidal particles.
The usual collodion is a 4% solution of nitrocellulose in a mixture of
alcohol & ether. An ultrafilter paper is prepared by soaking the filter paper in
a collodion solution, hardened by formaldehyde and then finally drying it.

Ultra centrifugation:
In this process, the colloidal solution is taken in a
tube which is placed in ultracentrifuge.
On rotating the tube at very high speed, the colloidal
particles settle down at the bottom of the tube and the
impurities remain in solution.
The settled particles are mixed with dispersion
medium to regenerate the sol.

Properties of colloids:
Colloidal particles are heterogeneous particles, stable
in nature & can pass through an ordinary filter paper but can not pass through
animal & plant membranes, ultrafilter papers etc. Colloidal particles diffuse at a
very slower speed.
Colligative properties:
Colloids are larger particles of high molar masses. Its no. of
particles per litre are less & it behaves like true solution & shows colligative
properties particularly osmotic pressure, used to find molar masses of colloidal
particles.

Colour:
The colour of colloidal solution depends upon the
wavelength of light scattered by the colloidal particles
which in turn depends upon the nature and size of
particles.
The colour also depends upon the manner in
which light is received by the observer.
Example: Very fine gold is red in colour, as the size
increases, the gold colloid colour changes to purple,
then blue and finally golden yellow.

Brownian movement:
Colloidal particles move in continuous
zig – zag path. This type of motion is due to
colliding molecules of dispersion medium
constantly with colloidal particles.
It is independent on the nature of the colloids
but depends on the size of the colloid particles &
viscosity of the solution.
Because of Brownian movement, colloidal
particles do not settle down & this explains the
stability of colloidal solution.

Tyndall effect: The scattering of a beam of light by colloidal particles as a result of which
the path of beam becomes visible is called Tyndall effect. The bright cone of light is called
the Tyndall cone. It confirms the heterogeneous nature of colloidal solution.
Tyndall effect is observed only on the following
conditions when
(1) The diameter of the dispersed phase particles is
not much smaller than the wave length of the light
used.
(2) The refractive indices of the dispersed phase & the
dispersion medium differ largely in magnitude.
Example: It is observed during the projection of
picture in a cinema hall by scattering of light by dust
& smoke particles.
[ Zsigmondy in 1903 used Tyndall effect to set up an
apparatus known as ultra-microscope.]

Charge on colloidal particles:
Colloidal particles always carry an electric charge. The nature of this charge is the same
on all the particles in a given colloidal solution and may be either positive or negative.
Positively charged colloidal particles:
(i) These include hydrated metallic oxides such as Fe2O3.xH2O, Cr2O3.xH2O, Al2O3.xH2O.
(ii) Basic dye stuff like malachite green, methylene blue sols.
(iii) Example – Haemoglobin (blood).
Negatively charged colloidal particles:
(i) Metallic sulphides like As2S3, Sb2S3 sols.
(ii) Acid dye stuff like eosin, methyl orange, Congo red sols.
(iii) Examples – Starch sol, gum, gelatin, clay, charcoal, egg albumin, etc.

Charge on colloidal particles: Examples:
(a) When silver nitrate solution is added in KI solution in a test tube,
AgI is formed which in presence of excess KI solution, I- ion being
excess common ion, is preferentially adsorbed on AgI forming negative
sol: AgI/I-.
(i) AgNO3(aq) + KI(aq)  AgI(s)+ KNO3(aq),
(ii) (ii) AgI(s) + KI(aq. Excess)  AgI/I-
(colloid) + K+
(crystalloid)
(b) When KI solution is added in silver nitrate solution in a test tube,
AgI is formed which in presence of excess AgNO3 solution, Ag+ being
excess common ion, is adsorbed on AgI forming positive sol: AgI/Ag+.
(i) AgNO3(aq) + KI(aq)  AgI(s)+ KNO3(aq),
(ii) AgI(s) + AgNO3(aq. Excess)  AgI/Ag+
(colloid) + NO3
-
(crystalloid)
(c) If FeCl3 is added to excess of hot water, a positively charged sol of
hydrated ferric oxide is formed due to adsorption of Fe3+ ions:
Fe2O3.xH2O/Fe3+.
(d) When FeCl3 is added in NaOH solution, a negatively charged sol is
obtained with the adsorption of OH- ions.: Fe2O3.xH2O/OH-.

Qu. What is the cause of charge on the sol particles?
Ans. The charge on the sol particles is due to
(i) electron capture by sol particles during electro dispersion
of metals.
(ii) due to preferential adsorption of ions from solution or
(iii) due to the formation of electrical double layer.

Helmholtz electrical double layer:
When the colloidal particles acquire negative or
positive charge by selective adsorption of one of
the ions, it attracts counter ions from the
medium forming a second layer. The
combination of these two layers of opposite
charges around colloidal particles is called
Helmholtz electrical double layer.
Electrokinetic potential or zeta potential:
The potential difference between the fixed layer
and the diffused layer of opposite charges is
called electrokinetic potential or zeta potential.

Electrophoresis:
The movement of colloidal particles under an
applied electric potential is called
electrophoresis. After electrophoresis, the
the colloidal particles coagulate at oppositely
charged electrode.
Electro-osmosis: When the movement of
colloidal particles is prevented by some
suitable means, it is observed that the
dispersion medium begins to move in an
electric field. This phenomenon is called
electro-osmosis.

Coagulation or precipitation:
The process of settling of colloidal particles as precipitate is called
coagulation. If somehow the charge on the surface of colloidal particles are
removed, the particles come nearer to each other to form aggregate & settle
down as precipitate due to force of gravity.
The coagulation can be carried out by the following ways.
(i) by electrophoresis
(ii) by mixing & shaking oppositely charged sol,
(iii) by boiling
(iv) by persistent dialysis
(v) by adding & shaking with electrolyte etc.
[ The minimum concentration of electrolytes in millimoles per litre required to
precipitate a sol in 2 hours is called coagulating value.]

Hardy – Schulze rule:
(i) The ions charge opposite to that of the sol particles are effective in
causing the coagulation of the sol. These ions are called active ions.
ii) The coagulating power of electrolyte increases with increase in charge on
the active ions used for coagulation. Examples –The negatively charged
colloids has active ions are Al3+> Ba2+> Na+ and
Fe (CN)6]4-> PO4
3-> SO4
2->Cl– for positively charged colloids.
COAGULATION OR PRECIPITATION

Protection of Colloids:
Lyophilic colloids are highly solvated & are
highly stable but lyophobic colloids are not.
So lyophilic colloids are used to stabilise the
lyophobic colloids. This is called protection of
colloid.
Lyophilic colloids are called protective
colloids.
When a lyophilic sol is added to a lyophobic
sol, it forms a layer around lyophobic sol &
protect it from coagulation from electrolytes.

Protection of colloid (contd.
The protective action of different lyophilic colloids is expressed
in terms of ‘Gold Number’.
It is defined as the no. of milligrams of protective colloid which
is added to 10 ml of standard red gold sol (having 53 x 10-4 to 58 x 10-4 %
gold) so that no coagulation of gold sol takes place ( colour of gold sol
changes from red to blue) when 1 ml of 10% NaCl solution is rapidly added
to it.
Smaller value of Gold number of a protective colloid means it is a
better protective colloid having maximum protection power.
Example: Gold no. of gelatin is 0.005 to 0.01 is the best protective colloid.
Other protective colloids are, casein. Albumin, potato starch etc.

Emulsion:It is colloidal solution having liquid as
dispersed phase & liquid as dispersion medium. It is
generally prepared by vigorously shaking two
immiscible or partially miscible liquids.
For stabilization of an emulsion, another component
s added called emulsifying agent or emulsifier.
Types of emulsions:
Water dispersed in oil (or W/O Type): When water is the dispersed phase and oil is the
dispersion medium. E.g. butter
Oil dispersed in water (or O/W Type): When oil is the dispersed phase and water is the
dispersion medium. E.g. milk.

Test of types of emulsion:
Dye test:
An oil soluble dye is added to O/W & W/O types of emulsions separately. In
Water in oil type the solution is appeared fully coloured & dispersion medium is oil, but
in oil in water type, fine coloured droplets are seen in oil as dispersed phase (oil) &
dispersion medium (water) is colourless.

Emulsification: It is the process of stabilizing an emulsion by means of an emulsifier.
Emulsifying agent or emulsifier: These are the substances which are added to
stabilize the emulsions. Examples: for O/W type– soaps, gum, proteins etc & for W/O
type: heavy metal salts, fatty acids, long chain alcohols, lamp black etc.
Demulsification: It is the process of breaking an emulsion into its constituent liquids
by freezing, boiling, centrifugation, by adding a certain amount of electrolyte or some
chemical methods.
Example: Separation of cream from milk by centrifugation.
**[ Syneresis: When a gel is allowed to stand for a long time, it shrinks & loses entire
liquid held by it. This shrinking of gel on long standing is called syneresis.
Thixotropy: Some gel liquify on shaking & reset on being allowed to stand. This
reversible sol-gel transformation is known as thixotropy.]

Colloids arounds us:
1. Blue colour of sky & sea:
2. Fog, mist & rain:
3.Food articles like milk, ice creams, jellies, fruit juices etc are colloidal
solutions.
4. Dyeing of fabric by mordants (a colloidal solution e.g. aluminium hydroxide
etc)
5. Varnishes, paints, enamels, resins, cellulose gums, glues, soaps, detergents
etc.
6.Aspalth emulsified with water is used for building roads etc.
7.Formation of delta at the junction of river & sea waters.
8. Tail of comets
9. Blood is a colloidal solution of an albuminoid substances.

Application of colloids:
1.Electrical precipitation of smoke using Cottrell smoke precipitator.
2.Sewage disposal:
3. Purification of drinking water by adding small amount of alums [K2SO4.Al2(SO4)3.24H2O]
4. Colloidal medicines: (i) Argyrol is a silver sol used as eye lotion
(ii) Colloidal antimony is used to cure disease kala azar.
(iii) Milk of magnesia, an emulsion of magnesium hydroxide is used as an antacid.
(iv) Colloidal gold is used as intra muscular injection.
5. Tanning of animal hides.
6. As disinfectants: Dettol & Lysol are emulsions of oil in water.
7. Photographic plates & films: contains fine particles of AgBr as colloids with gelatine.
8. Artificial rain is carried our by spraying charged colloidal dust or fine sand particles
over a cloud of opposite charge where bigger water droplets coagulates to cause rain.
9. Rubber industry: Latex s a negatively charged colloidal rubber particles. These are
precipitated as rubber by electrophoresis on wares, handles of tools etc.
10. Metallurgical processes: in froth floatation process- concentration of bauxite ore.

Some conceptual questions:
Qu.1) Why are substances like Pt, Pd etc often used for carrying out electrolysis of aqueous solution?
Ans: Pt, Pd etc are chosen as these are inert electrodes& not attacked by the ions of electrolytes or the
products of electrolysis.
Qu.2) Why does physisorption decrease with the increase in temperature?
Ans. Physisorption is an exothermic process. According to the Le Chatelier’s principle, with the increase in
temperature of an exothermic process, the equilibrium shifts towards the backward direction. Here gas is
released from the surface of the adsorbent as shown below:-
Solid(adsorbent) + gas (adsorbate) ⇌gas adsorbed on solid surface + heat.
Qu.3) Why are powdered substances more effective adsorbents than their crystalline form?
Ans. Powdered substances have greater surface area as compared to their crystalline forms. Hence with
greater surface area, more adsorption takes place in case of powered substances.
Qu.4) Why is it necessary to remove CO when ammonia is obtained from Haber’s process?
Ans. because CO acts as a poison for Fe catalyst in Haber’s process. ( forms stable complex [Fe(CO)5] )

Some conceptual questions: (Contd.)
Qu.5) What is the role of desorption in the process of catalysis?
Ans. Desorption makes the surface of solid catalyst free for the fresh adsorption of reactants, regenerating
free valencies on catalyst surface.
Qu.6) Why is the ester hydrolysis slow in the beginning & become faster after sometime?
Ans. R-COOR’ (ester) + H2O ⇌ R-COOH (Carboxylic acid) + R’-OH (alcohol)
Ester catalysis is an acid catalysed reaction. Here, first acid is first produced on hydrolysis of ester, after
which H3O+ formed acts as a catalyst. R-COOH + H2O ⇌RCOO-(aq) + H3O+ (catalyst).
So, the reaction is slow in the beginning & with the formation of H3O+, gradually the reaction becomes fast.
Qu.7) How is a delta formed?
Ans. River water is a colloidal solution of clay & sea water containing different electrolytes in the form of
ions. When river water meets the sea water, the electrolytes present in sea water coagulate the colloidal
solution of clay resulting in its precipitation or coagulation & deposition of clay to form delta at the
junction point of river & sea.

Some conceptual questions (Contd.)
Qu.8) How does alums purify drinking water?
Ans. Water from natural resources have like well water etc have suspended impurities as colloidal particles.
Alums have Al3+ & SO4
2- ions which coagulate these colloidal particles to make water fit to drink.
Qu.9) How is tanning of animal hides (leather) done?
Ans. Animal hides have positively charged colloidal particles & is soaked in tannin or in solution of chromium
salt which are negatively charged colloids. Due to mutual coagulation, leather gets hardened. This process is
called tanning.
Qu.10) What happens when ferric hydroxide sol & arsenic sulphide sol are mixed in proportion?
Ans. Ferric hydroxide forms a +ve sol: Fe(OH)3/Fe3+ & arsenic sulphide forms a negative sol: As2S3/S2-When
these two are mixed in proper proportion, due to mutual coagulation, both the sols are coagulated.
Qu.11) What modification can you suggest in the Hardy Schulze rule?
Ans. The Hardy Schulze may be modified as “ when oppositely charged sols are mixed in proper proportions to
neutralize the charges of each other, coagulation of both the sols occur.”

Some conceptual questions: (Contd.)
Qu.12) Why is it essential to wash the precipitate with water before estimating it quantitatively?
Ans. Some amount of electrolytes or other impurities are adsorbed on the surface of the particles of
precipitate, so ….
Qu.13) Why is adsorption always exothermic?
Ans. We know, ΔG = ΔH – TΔ. During adsorption of a gas on solid surface, entropy decreases so ΔS = -ve.
Adsorption is a spontaneous process so, its ΔG = -ve. As TΔS is -ve, -TΔS is positive.
So to have ΔG = - ve or < 0, ΔH has to be always -ve, adsorption is always exothermic.
Qu.14) Explain What is observed:
(a) when a beam of light is passed through a colloidal solution
Ans. Tyndall effect takes place. i.e. scattering of light takes place through colloidal solution & the path of light
becomes visible.
(b) The sodium chloride crystals are added in Fe(OH)3 sol or hydrated ferric oxide sol in water.
Ans. Ferric hydroxide is a positive sol Fe(OH)3/Fe3+. It gets coagulated by the oppositely charged Cl- ions of
NaCl crystal.
(c) electric current is passed through a colloidal solution.
Ans: When electric current is passed through a colloidal solution, the colloidal particles mive towards the
oppositely charged electrode & gets coagulated due to electrophoresis..

Some conceptual questions: (contd.)
Qu.15) Comment on the statement: “ Colloid is not a substance but state of a substance.”
Ans. This statement is true because colloid is formed only when a certain charge is adsorbed on a given
substance & as a result the particles develop some charge on it & size of 1 to 1000nm.
Example: Soap behaves as crystalloid (electrolyte) in water at low concentration but at high concentration, it
behaves as a colloid called associated colloid or micelles.
Crystalloid has size < 1 nm but colloid has the size 1 to 1000 nm having +ve or -ve charge on its surface.
Hence…..
Qu.16) Why is the colour of sky blue?
Ans: This is due to the scattering of blue light of white light of sun by colloidal dust particles present in air.
(Tyndall Effect)
Qu.17) What is tail of a comet?
Ans. It is seen as a Tyndall cone due to scattering of light by tiny solid particles left by the comet in its path.
Qu.18) How is fog or mist formed?
Ans. When a large mass of air containing dust particles is cooled below its dew point, the moisture from the
air condenses on the surfaces of these particles forming fine droplets. These droplets are colloidal in nature
& floats in air to form fog or mist.

Qu.19) How can we cause artificial rain?
Ans. Clouds are aerosols having small droplets of water suspended in air. So, it is possible
to cause artificial rain by throwing electrified sand or spraying a sol carrying charge
opposite to the one on clouds from an aeroplane.
Qu.20) Name some medicines colloidal in nature.
Ans. (i) Argyrol is a silver sol used as an eye lotion.
(ii) Colloidal antimony is used in curing kalazar.
(iii) Colloidal gold (called purple of Cascius) is used for intra muscular injection.
(iv) Milk of magnesia, an emulsion mainly of magnesium hydroxide is used for stomach
disorder.
Qu.21 Why are the colloidal form of medicines more effective?
Ans. It is because colloidal particles have large surface area & are therefore easily
assimilated.

Qu. 22) Which inert gas is adsorbed to (i) the least extent & (ii) maximum extent on coconut
charcoal surface & Why?
Ans. (i) Least adsorbed = He having least critical temperature & (ii) Xe is maximum adsorbed as it has
maximum Tc value among inert gases.
Qu.23) What is the principle on which Cottrell’s smoke precipitator work?
Ans. It works on electrophoresis. The colloidal smoke particles from a furnace enters in Cottrell’s
smoke precipitator where a high voltage electrode of charge opposite to that of smoke particles are
fitted. The smoke particles move towards the electrodes, loose its charge & precipitate as ash mostly
contain carbon dust particles.

Try:
Qu. No.1) Adsorption of a gas on the surface of solid is generally accompanied by the
decrease in entropy, still it is a spontaneous process. Explain.
Qu.2) Rough catalyst surface is more effective than smooth surface. Why?
Qu.3 Why do different colloidal solutions have different colours?
Qu.4) Why is gelatin added to ice cream?
Qu.5) Explain why a yellow turbidity is formed when H2S gas is passed through an aqueous
solution of SO2 gas.
Qu.6) You are given two different test tubes A & B. Test tube A contains the solution of KI
solution & test tube B contains AgNO3 solution. What will you observe when a few drops of
AgNO3 solution is added in ‘A’ & a few drops of KI solution is added in ‘B’ and shaken well?
Explain.
Qu.7) Explain:
(a) all adsorptions can be reversed by heating.
(b) Enthalpy of adsorption of chemisorption is more than that of physisorption.
(c) Can the same substance act both as colloid & crystalloid?
Qu.8) Why can not we prepare a colloidal solution of a gas in gas as dispersion medium?

Unit V: Surface chemistry

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Unit V: Surface chemistry