class XII CHEMISTRY ,

1. Surface chemistry
Class 12
Unit 5

2. Surface chemistry deals with phenomena that occur
at the surfaces or interfaces.
Surface is a layer of separation of two phases
Surface chemistry
Surface Chemistry is the branch of chemistry which
deals with the phenomenon that occurs on the
surfaces or interfaces, such phenomenon includes
corrosion. catalysis, crystallisation, etc
In this Unit, you will be studying some important features of
surface chemistry such as adsorption,catalysis and colloids
including emulsions and gels.

3. Adsorption
The accumulation of molecular species at the surface rather than
in the bulk of a solid or liquid is termed adsorption.
The molecular species accumulates at the surface is
termed as adsorbate
The material on the surface of which the adsorption
takes place is called adsorbent
Adsorbate
Adsorbent
e.g..
(i) O2, H2, C12, NB3 gases are adsorbed on the surface of charcoal.
(ii) Silica gels adsorb water molecules from air.
Charcoal, silica gel, metals such as Ni, Cu, Ag, Pt and colloids are some
adsorbents.

4. Important Characteristics of Adsorption
1. It is specific and selective in nature.
2. Adsorption is spontaneous process, therefore change
in free energy (ΔG)is negative.
ΔG= ΔH – TΔS,
For the negative value of ΔG,in a system, in which
randomness decreases, ΔH must be negative. Hence,
adsorption is always exothermic.
Adsorption of hydrogen over Pt is called occlusion.

5. Desorption
It is a process of removing an adsorbed substance from a
surface on which it is adsorbed, is known as desorption.

7. Sorption
It is a process in which both adsorption and
absorption take place simultaneously, the term
sorption is simply used.

8. Positive and Negative Adsorption
When the concentration of the adsorbate is more on the surface of the adsorbent
than in the bulk, it is called positive adsorption.
On the other hand, if the concentration of the adsorbate is less relative to its
concentration in the bulk, it is called negative adsorption, e.g., when a dilute
solution of KCl is shaken with blood charcoal, it shows negative adsorption.

9. Distinction between Physisorption and Chemisorption

10. Factors Affecting Adsorption
(a) Nature of adsorbent Same gas may be adsorbed to different extents on
different adsorbent.
(b) Surface area of the adsorbent Greater the surface area, greater is the extent
of adsorption.
(c) Nature of the gas being adsorbed Greater is the critical temperature of a
gas, greater are the van der Waals’ forces of attraction and thus, greater is the
adsorption.

11. d) Temperature Adsorption is an exothermic process involving the
equilibrium :
Gas (adsorbate) + Solid (adsorbent) ⇔ Gas adsorbed on solid + Heat
Applying Le-Chatelier principle, increase of temperature decreases
the adsorption and vice-versa.
(e) Pressure Adsorption increases with pressure at constant
temperature. The effect is large if temperature is kept constant at low
value.
(f) Activation of the solid adsorbent Activation means increasing
the adsorbing power of the solid adsorbent. This can be done by
subdividing the solid adsorbent or by removing the gases already
adsorbed by passing superheated steam.

12. Adsorption Isotherms
It is the plot of the mass of gas adsorbed per gram of
adsorbent (x / m) versus equilibrium pressure at constant
temperature.
Freundlich Adsorption Isotherm
It gave an empirical relationship between the quantity of
gas adsorbed by unit mass of solid adsorbent and
pressure at a particular temperature. It can be expressed
by the equation.
x / m = kp1/n …(i)
Where, x is the mass of the gas adsorbed on mass m of
the adsorbent at pressure p, k and n are constants which
depend on the nature of the adsorbent and the gas at a
particular temperature.

13. At low pressure, n = 1, i.e., x / m = kp
At high pressure, n > 1, i.e., x / m = k (independent of p)
Taking logarithm of Eq. (i)

14. Freundlich Adsorption Equation for Solutions
x / m = kC1/n
where, C is the equilibrium concentration. On taking
logarithm of the above equation, we have

15. Langmuir Adsorption Isotherm
According to Langmuir, the degree of adsorption is directly Proportional to e, i.e., the
fraction of surface area occupied.
x / m α θ = kθ

16. Adsorption Isobars
These are plots of x / m us temperature t at constant pressure.
For physical and chemical adsorption, they are shown below.

17. Applications of Adsorption
1.For production of high vacuum.
2.Gas masks containing activated charcoal is used for
breathing in coalmines. They adsorb poisonous gases.
3.Silica and aluminium gels are used as adsorbents for
controlling humidity.
4.Removal of colouring matter from solutions.
5.It is used in heterogeneous catalysis.
6.In separation of inert gas.
7.As adsorption indicators.
8.In chromatographic analysis.
9.Qualitative analysis, e.g., lake test for Al3+.

18. Catalysis
Catalyst is a chemical substance which can change the rate of
reaction without being used up in that reaction and this process
is known as catalysis
Promoters: These are the substances which increase the
activity of catalyst. Example – Mo is promoter whereas Fe
is catalyst in Haber’s Process.
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.’

19. Types of Catalysis
(a) Homogeneous catalysis In this catalysis, and the catalyst
reactants are in the same physical state [phase], e.g.,
(b) Heterogeneous catalysis In heterogeneous catalysis,
catalyst is present in a different phase than that of reactants, e.g.,
(c) Autocatalysis When one of the product of a reaction acts as
catalyst, the process is called autocatalysis.

20. Adsorption Theory of Heterogeneous Catalysis
The mechanism involves five steps:
(i) Diffusion of reactants to the surface of the
catalyst
(ii) Adsorption of reactant molecules on the surface
of the catalyst.
(ill) Occurrence of chemical reaction on the
catalyst’s surface through formation of an
intermediate.
(iv) Desorption of reaction products from t he
catalyst surface.
(v) Diffusion of reaction products away from the
catalyst’s surface

21. Important Features of Solid Catalysts
Activity of catalyst: It is the ability of a catalyst to increase the rate
of a chemical reaction.
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:

22. Shape – selective catalysis: It is the catalysis which depends upon the pore
structure of the catalyst and molecular size of reactant and product molecules.
Example – Zeolites are shape – selective catalysts due to their honey- comb
structure.
•Enzymes: These are complex nitrogenous organic compounds which are produced by living plants
and animals. They are actually protein molecules of high molecular mass. They are biochemical
catalysts
•Steps of enzyme catalysis:
i) Binding of enzyme to substrate to form an
activated complex.
(ii) Decomposition of the activated complex
to form product.
Some examples of enzyme catalysed reactions are:
Source of invertase, zymase and maltose is yeast and
that of diastase is malt. Soybean is the source of urease.)
Lactobacilli is used to convert milk into curd.
In stomach, the pepsin enzyme converts proteins into
peptides while in intestine, the pancreatic trypsin
converts proteins into amino acids by hydrolysis.

23. •Characteristics of enzyme catalysis:
(i) They are highly efficient. One molecule of an enzyme can transform 106
molecules of reactants per 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.
(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 increase its activity.
(vii) Influence of inhibitors (poison): Enzymes can also be inhibited or poisoned
by the presence of certain substances.

24. •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.
•Colloids:
(i) It appears to be homogeneous but is actually
heterogeneous.
(ii) The diameter of the 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.

25. •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

26. •Dispersed phase: It is the substance which is dispersed as very
fine particles.
•Dispersion medium: It is the substance present in larger quantity.

27. 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
•Classification of colloids on the basis of the physical state of dispersed
phase and dispersion medium:

28. •Classification of colloids on the basis of nature of interaction
between dispersed phase and dispersion medium, the colloids are
classified into two types namely,
1.Lyophobic sols
2.Lyophilic sols
•Lyophobic sols:
(i) These colloids are liquid 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.

29. •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.

30. Based on the Types of Particles of the Dispersed Phase-
•Multimolecular Colloids: Many particles(atoms or small molecules) of
the dispersed phase aggregate together to form species having the size of
a colloidal particle( 1-1000nm). These colloids are called multimolecular
colloids. Example-gold sol, Sulphur sol
•Macromolecular colloids: Substances with large molecules
(macromolecules) in suitable solvents form solutions but these
macromolecules might be in the colloidal range. These solutions are called
macromolecular colloids and resemble true solutions in many ways.
Example- Starch, Cellulose, Proteins are natural macromolecules. Nylon,
polythene, polystyrene are man-made macromolecules

31. •Associated Colloids: Some substances at high concentrations act as
colloids due to the formation of aggregates. But at low concentrations
they behave like normal strong electrolytes. These aggregates formed
are called micelles. Such colloids are called associated colloids.
Kraft Temperature- The formation of micelles takes place only above a
particular temperature called Kraft’s temperature
Critical Micelle Concentration(CMC) – The concentration above which
micelle formation takes place
Example- Soaps, synthetic detergents

32. Mechanism Of Micelle Formation-
•Soap is the sodium or potassium salt of fatty acid and may be represented as RCOO- Na (e.g. sodium
stearate, (CH3(CH2)16COO-Na+]) . When dissolved into water , it dissociates into RCOO- and Na+ ions
•The RCOO-ion consists of two parts – long hydrocarbon chain (also called non – polar tail) which is
hydrophobic (water repelling ) and a polar group COO- (polar head) which is hydrophilic (water loving )
At higher concentrations(CMC) RCOO- ions form an aggregate of spherical shape with the hydrocarbon
chains pointing towards the centre and the COO- part facing outward on the surface of the sphere. This
aggregate is called ionic micelle. It may have as many as 100 ions

33. Cleansing Action of Soaps
The cleansing action of soap is due to the formation of micelle by the soap molecules in such
a way that the hydrophobic part is in the oil droplet(dirt) and the hydrolphilic part projects out.
Since the polar groups interact with watersurrounded by soap ions is pulled from the surface
and pulled into water.

34. Preparation of Colloids
A few ways to prepare colloids are given below
• Chemical method : Colloids can be prepared by chemical
reactions leading to formation of molecules by double
decomposition, oxidation, reduction or hydrolysis. These molecules
then aggregate to form sols

35. Electrical Disintegration/ Bredig’s arc Method:
This is applied to obtain colloidal sols of metals like gold, silver and
platinum. An electric arc is stuck between the electrodes of the
metal immersed in the dispersion medium. The intense heat
produced vapourises the metal, which then condenses to form
particles of colloidal size.

36. 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. The electrolyte
used for this is called peptizing agent. During peptization the precipitate absorbs
the one of the ions of the electrolyte on its surface. This causes +veor –vecharge to
develop on the precipitate, which ultimately break up into small particles of the size
of a colloid.

37. Purification of Colloids
The process used for reducing the amount of impurities to a required minimum is called
purification of colloids.
Some of processes used to do so are-
Dialaysis- The process of sepeartaing the particles of a colloid by diffusion through a
suitable membrane .
Process – An apparatus called dialyser is used. A bag with a suitable membrane
containing the colloid is suspended in a vessel through which fresh water flows
continuously. The impurities diffuse through the membrane into the water leaving behind
the colloid

38. Electro-dialysis- Dialysis is a slow method. It is made faster by
this method. In this method, an electric filed is applied using metal
electrodes. These ions present in the colloidal solution migrate out to
the oppositely charge to electrodes. It is possible only if the dissolved
substance in the impure colloid is an electrolyte.

39. Ultrafiltration-
This is the process of separating colloidal particles from the soluble solutes(impurities)
using specially prepared filters, which are permeable to all substances except the colloid.
Colloidal particles can usually pass through filter papers as the pores are too large.
An ultra filter paper can be made by soaking the filter paper in a colloidal solution,
hardening by formaldehyde and then finally drying it. As this is slow process, pressure or
suction is applied to speed it up. The colloidal particles left on the ultra-filter paper are
stirred with fresh dispersion medium(solvent) to form a pure colloid.

40. Properties of Colloids
Colligative Properties-
The particles in colloids are bigger aggregates than those in a true
solution. So, the number of particles in a colloid is lesser than a true
solution of the same concentration. The values of colligative properties
(osmotic pressure, lowering of vapour pressure, depression in freezing
point, elevation in boiling point) are of small order as compared to values
shown by true solution at same concentration.

41. The Tyndall effect is the phenomenon in which the particles in a
colloid scatter the beams of light that are directed at them. This effect
is exhibited by all colloidal solutions and some very fine suspensions.
Therefore, it can be used to verify if a given solution is a colloid. The
intensity of scattered light depends on the density of the colloidal
particles as well as the frequency of the incident light.
When a beam of light passes through a colloid, the colloidal particles
present in the solution do not allow the beam to completely pass
through. The light collides with the colloidal particles and is scattered
(it deviates from its normal trajectory, which is a straight line). This
scattering makes the path of the light beam visible, as illustrated
below.
Tyndall effect-

42. Tyndall effect-
Tyndall effect is the scattering of the light by the particles present in the colloidal
solution when viewed at right angles to the passage of light.
It is observed only when-
•The diameter of the dispersed particles is not much smaller than the wavelength of
light used
•The refractive indices of the dispersed phase and dispersed medium have a large
difference
•This effect was used to make an ultramicroscope and differentiate between true
solution and colloids.

43. Colour-
The colour of the colloidal solution depends on the wavelength of the
light scatter by the dispersed particles, size and nature of the
dispersed particles and the manner in which it is viewed. Example-
Finest gold sol is red in colour and as the size of the particle keeps
increasing its colour changes to blue, then purple and finally gold.
Brownian Movement-
Brownian movement may be defined as continuous zigzag movement of the
colloidal particles in a colloidal solution. It depends on the size of the particles
and the viscosity of the colloid. Smaller the size of the particle and lesser its
viscosity, faster is its motion. This movement is responsible for the stability of
sols

44. Charge on Colloids-
Colloidal particles always have an electric charge. The nature of this charge
is the same on all the particles in a given colloidal solution and maybe either
+ve or –ve.
Positively charged sols-
•Haemoglobin
•Oxides-TiO2 solution
•Hydrated Metal oxides like Al2O3.xH2O , Fe2O3.xH2O
•Dyes-methylene blue sols
Negatively charged sols-
•Metals- Cu, Ag, Au sols
•Metallic sulphides-As2S3, CdS
•Acid dye stuffs- Eosin
•Sols of starch, gelatin

45. Electrophoresis-
The movement of colloidal particles under
the influence of an electric field is called
electrophoresis. Negatively charged
particles move towards the cathode and
Positively charged particles moves towards
anode.
When the movement of particles is
prevented, it is observed that the dispersion
medium starts to move in the electric field.
This is called electroosmosis.

46. Coagulation-
It is process of settling of colloidal particles. Also called precipitation of sol
COAGULATION OF LYOPHOBIC SOLS-
Coagulation of lyophobic sols can be done by the following methods:
•By electrophoresis - The colloidal particles move towards oppositely changed
electrodes get discharged and precipitate.
•By mixing two oppositely charged sols - Oppositely charged sols when mixed
together in almost equal proportion, neutralise their charges and get partially or
completely precipitated.
•By Boiling- When a sol is boiled the adsorbed layer is disturbed due to increased
number of collisions with the molecules of the dispersion medium. This reduces the
charge on the particles and they ultimately settle down in the form of a precipitate.
•By Persistent dialysis - On prolonged dialysis, traces of the electrolyte present in the
sol are removed almost completely. Colloids become unstable and coagulate.
•By addition of electrolyte - When excess of electrolyte is added, colloidal particles
precipitate as colloids interact with ions carrying charge opposite to that present on
themselves. This causes neutralisation leading to their coagulation.
Example- A negatively charged ion when added to a positively charged sol
causescoagulation. The negatively charged ion is called coagulating ion/flocculative
ion as it neutralises the colloid to cause coagulation.

47. COAGULATION OF LYOPHILLIC SOLS-
Lyophilic sols are stable because of charge and solvation of colloidal particles. So
we remove these two factors to coagulate them. This is done by
•Addition of an electrolyte
•Addition of a suitable solvent
PROTECTION OF COLLOIDS -
•Lyophilic sols are more stable than lyophobic sols
•Lyophilic colloids have a unique ability to protect lyophobic colloids from
electrolytes
•When a lyophilic sol is added to lyophobic sol, the lyophilic particles (colloids)
form a layer around the particles of lyophobic sol
•Lyophilic colloids are also called protective colloids
HARDY-SCHULZE RULE-
•The greater the valency of the flocculating ion added, the greater is its
precipitation.
•For negative sols, when positive ions are added
Al3+> Ba2+>Na+ is the order in terms of flocculating power
•For positive sols, when negative ions are added
[Fe(CN)6] 4-> PO4
3->SO4
2->Cl- is the order in terms of flocculating power

48. Emulsions
•Emulsions are colloids where both the dispersion phase and dispersion
medium are liquids. These two liquids are immiscible or partially miscible.
Generally one of the liquids is water.
•There are two types of emulsions
1.Oil dispersed in water (o/w type)- Water acts as dispersion medium
Example –Milk, vanishing cream
2.Water dispersed in oil (w/o type)- Oil acts as dispersion medium
Example- Butter, Cream
•Emulsions like water and oil separate into two layers and make the
emulsion unstable. So emulsions are stabilised by stabilising agents.

49. •o/w emulsions are stabilised by proteins, gum, natural
and synthetic soaps
•w/o emulsions are stabilised by metal salts of fatty acids,
long chain alcohols

50. Applications of Colloids
1.Electro precipitation of smoke – The smoke is led through a chamber
containing plates having a charged opposite to that carried by smoke
particles. The particles on coming in contact with these plates lose their
charge and get precipitated. The particles settle down on the floor of the
chamber. The precipitator is called Cottrell precipitator.

51. 2.Purification drinking water – Alum is added to impure water to coagulate the
suspended impuritiesand make water fit for drinking.
3.Medicines – Most of the medicines are colloidal in nature. Colloidal
medicines are bmore effective because they have a larger surface area and
are more easily absorbed by the body. Eg- Argyrol is a silver sol used as an
eye lotion, milk of magnesia is used to cure stomach disorders
4.Tanning – Animal hides are colloidal in nature. When a hide that has
positively charged particles is soaked in tannin/chromium salts, which
contains negatively charged particles , mutual coagulation takes place. This
results in the hardening of leather. This process is termed as tanning.
5.Photographic plates and films – Photographic plates and films are prepared
by coating an emulsion of the light sensitive silver bromide in gelatin over
glass plates or celluloid films.
6.Cleansing action of soaps- already explained
7.Rubber industry- Latex is a colloidal solution of rubber particles which are
negatively charged. Rubber is obtained by coagulation of latex.
8.Industrial products- Paints inks, synthetic plastics, rubber, cement, graphite
lubricants are all colloids