SURFACE CHEMISTRY
I.
Adsorption
II.
The accumulation of molecular species at the surface rather than in the bulk of
a solid or liquid is known as adsorption. It is surface phenomena.
Difference between and adsorption and adsorbs.

Absorption
1.It is the phenomenon in which the particles of
gas or liquid get uniformly distributed throughout
the body of the solid.
2. The concentration is the same throughout the
material. Therefore, it is a bulk phenomenon.
3. Absorption occurs at uniform rate.
Adsorption
1.it is the phenomenon of higher concentration of
particles of gas or liquid on the surface than in the
bulk of the solid.
2. The concentration on the surface of the
adsorbent is different from that in the bulk.
3. Adsorption is rapid in the beginning and its rate
slowly decreases.
III. Thermodynamics behind adsorption




Adsorption of a gas liquid is a ways exothermic process,  H=-Ve.
Adsorption lowers the degree of randomness of adsorbate (adsorbed substance
gas/liquid).
So  S=-Ve.
*For a process to be spontaneous.  G=-Ve.
As the adsorption proceeds,  H becomes less –Ve and slowly  H becomes
equal to T  S, which leads to zero value of  G, at this state equilibrium is attend.
IV. Difference between physisorption and Chemisorptions
Physisorption
Chemisorption
1) It arises because of Vander Waals’forces.
2) It is not specific in nature.
3) It is reversible in nature
4) It depends on the nature of gas. More easily
liquefiable gases are adsorbed readily
1)It is caused by chemical bond formation
2.it is highly specific in nature
3).It is irreversible
4).It also depends on the nature of gas. Gases
which can react with the adsorbent show
chemisorptions.
5.Enthalpy of adsorption is high (80-240kJ mol-1
In this case.
6). High temperature is favorable for adsorption.
It increases with the increase of temperature.
7) High activation energy is sometimes needed.
8) It also depends on the surface area. It too
increases with an increase of surface area.
9. It results into unimolecular layer.
5) Enthalpy of adsorption is low (20-40) kJ mol-1) in
this case.
6) Low temperature is favorable for adsorption. It
decreases with increases of temperature.
7) No appreciable activation energy is needed.
8) It depends on the surface area. It increases with
an increase of surface are
9) It results into multimolecular layers on adsorbent
surface under high pressure.
V. Important features of solid catalysis:Activity of a catalyst: the ability of a
catalyst to increase the rate of a reaction is called activity of a catalyst. The catalytic activity
increases we move from group 5 to group11.Maximum activity is shown by groups 7 -9
elements of modern periodic table
Selectivity
The selectivity of a catalyst is its ability to direct a reaction to yielda particular product. For e
example, starting with H2 and CO, andusing different catalysts, we get different products.
CH  CH
+
H2
Pt


CH  CH
+
H2
Linderscatalyst






CH3 –CH3
CH2 = CH2
Zeolites as catalysts:

Zeolites are micro porous aluminosilicates with 3D net work of silicates in
which some silica (Si) atoms are replaced by aluminum (Al) atoms.

ZSM-5, Zeolite is used to convert alcohols to gasoline by dehydration of
alcohols.
Characteristics of enzyme catalysis:
 Highly efficient
 Highly specific
 Highly active under optimum temperature, pH
 Activated by co-enzymes
 Affected by inhibitors and poisons
Mechanism of enzyme catalysis: (Lock and Key Mechanism):
Various steps involved in enzyme catalysis are
E
+
S
→
ES
ES
→
EP
EP
→
E
+
E = Enzyme.
S = Substrate.
P
P = Product.
Catalyst


Catalyst is a substance which alters the rate of reaction(may increase or
decrease the rate of reaction)
If the catalyst, reactants and products all are in the same phase, then that is
homogeneous catalyst. Examples are as follows
2SO2(g)

+
NO( g)

O2(g) 
2SO3(g)
If the catalyst, reactants and products all are in different phase, then that is
heterogeneous catalyst. Examples are as follows



If the catalyst, reactants and products all are in different phase, then that is
homogeneous catalyst. Examples are as follows
2SO2(g)
+
N2(g)
+
Pt (s)


Fe(s)

3H (g) 
O2(g)
2
2SO3(g)
2NH3
A substance which enhances the efficiency of a catalyst is known as
promoter.
Mo (molybdenum) acts as promoter in Haber’s manufacture of Ammonia.
F
ec
( atlyst)






N2(g)
+
3H2(g)
2NH3
M
o(prom
oter)
A substance which lowers the efficiency of a catalyst is known as catalytic
poison.
Arsenic and its compounds like As2O5 act as poison contact processes for
manufacture of sulphuric acid.
COLLOIDS
 Colloid is heterogeneous system in which dispersed phase particles with 1nm
to 1000 nm size are distributed in dispersion medium.

Colloids are classified on the basis of interaction between dispersion medium
and dispersion phase are
a) Lyophilic and lyophobic colloids.
Lyophilic means solvent loving-can be directly prepared by mixing a
suitable dispersion phase with dispersion medium-these are reversibleexamples-starch sol etc.
Lyophobic means solvent-hating. Can be prepared by indirect methods.
Not so stable and ireversible. Eg. Ferric hydroxide sol, aluminium hydroxide
sol, arsenic sulphide sol, sulphur sol and all metal sols.
Peptization
The process of converting precipitate into colloidal solution by shaking it with
small
Amount of electrolyte and sufficient amount of dispersion medium is known
as peptization. Generally colloidal particles adsorbs common ion from added
electrolyte either positive or negative.
Examples:
Precipitate of Fe(OH)3 + FeCl3 gives Fe(OH)3 ferric hydroxide sol.
Precipitate of As2S3 + H2S gives As2S3arsenius sulphide sol.
Precipitate of AgI + AgNO3 gives AgI Silver iodide sol.
Tyndal effect: The scattering of light by colloidal particles is known as Tyndall
effect, the scattered light is in the form of a cone so it is called tyndal cone.
Zigmondy design the first microscope based on tyndal effect.
Tyndal effect is observed only when the following conditions are satisfied.
i)
The diameter of the dispersed particles is not much smaller than the
wavelength of the light used.
ii)
The refractive indices of the dispersed phase and the dispersion
medium differ greatly in magnitude.
Brownian movement: the zig-zag motion of colloidal particles in dispersion
medium is known as Brownian movement. Which was observed by British Botanist
Robert Brown when pollein grains are left in water.
The primary reason for Brownian movement is constant collision between dispersion
medium particles and dispersion phase particles, the secondary is that in general all
colloidal particles have similar charge and they may repel with each other.
Electrophoresis: the movement of colloidal particles in eclectic field is know as
electrophoresis. If they move towards anode than it is called anaphoresis. If the they
move towards cathode it is called cataphoresis.
Electrophoresis is totally different from electro osmosis.
. COAGULATION.
The process of separation of dispersion phase from dispersion medium is known as
coagulation or precipitation or flocculation. This can be done by any of the following
base.
 By Electrophoresis
 By mixing two oppositely charged sols
 By boiling
 By persistent Dialysis
 Addition of electrolyte
HARDY-SCHULZE RULES OF COAGULATION.
i)
coagulation is cause by ion which is having opposite charge to that of charge
present on
colloidal particles. This ion which causes coagulations is known as effective
ions.
Example:
Fe(OH)3 sol can be coagulated by adding NaCl, in this contest Cl- is effective
ion which causes coagulation of positive Fe(OH)3 sol .
The coagulating power of effective ion is directly proportional to its charge
(valency).
Example:
Fe(OH)3 sol can be coagulated more effectively by Na2SO4 than NaCl.
Because the effective ion of Na2SO4 is SO-2 which has more charge.
The coagulating power of Al3+ , Ba2+,Na+ in coagulation negative sols like
As2S3 is in the order of Al3+ > Ba2+>Na+
.
ii)
EMULSION.
Emulsions are special type of colloidal solutions in which both dispersion
medium and
dispersion phase are in liquid state. There are two types of emulsions
a) Oil dispersed in water (O/W) type.
b) Water dispersed in oil (W/O) type.
The above two emulsions are distinguished are by using dye test or dilution test.

If an emulsion is diluted by water then it will be oil in water type
emulsion otherwise it is water in oil type.


The conductivity of oil in water type emulsion increases by adding any
ionic compound which is soluble in water.
If a dye is added to oil in water type emulsion color droplets are
noticed because added dye is soluble only in oil but not in water.
Very Short Answer Questions of one mark
Q1.
Which of the two: absorption or adsorption is a surface phenomenon?
Solution:
Adsorption.
Q2.
Compare the heat of adsorption for physical and chemical adsorption?
Solution:
The heat of adsorption for chemical adsorption is high (of the order of 200-400 J/mol)
while the heat of adsorption for physical adsorption is low(of the order of 20-40/mol).
Q3.
Why are lyophilic colloidal sols. More stable than lyophobic colloidal sols?
Solution:
The Lyophilic colloidal sols. Are more stable because3 they are highly hydrated in
Solution.
Q5.
What is colloidon ?
Solution:
Colloidon is a sol. Of cellulose nitrate in ethyl alcohol.
Q6. Why is adsorption exothermic ?
Q7.Powdered Catalsyt is more effective over solid Catalyst .Why?
Q8.Why is chemisorptions takes place at higher temperature ?
SA- I of two marks
Q1. How the nature of adsorbate affects the adsorption of gases on solids?
Solution:
Easily liquefiable gases like NH3, CO2, HCl get adsorbed to greater extent than
permanent gases like H2,O2,N2 etc,. the ease of liquefaction of a gas depends upon its
critical temperature. Higher is the critical temperature more easily will be adsorbed on
solid.
Q2.
The extent of physical adsorption decreases with rise in temperature why?
Solution :
i)The physisorption shows the features of physical process .As physisorption is
generally exothermic, the extent of physisorption decreases with rise in temperature.
ii) The physisorption is due to weak wander wall bond between absorbent and
adsorbate which starts breaking with rise in temperature.
Q3.
Explain the peptization.
Solution:
It is the process of conversion of precipitate in to a colloidal solution by shaking the
Precipitate with suitable electrolyte. Electrolyte used is also called as peptizing agent.
Example: Shaking c Precipitate with FeCl3 solution gives + sol
Shaking As2S3 Precipitate with H2S solution gives – sol
Q4.
Delta is generally formed when river meets the Ocean. Why?
Solution:
River water is a colloidal solution of negatively charged silicates, clay and other
earthly
Impurities. Sea water consists of Ca2+ ions and Mg2+ ions. So coagulation takes place
at
the place where Sea water and river water meets each other which leads to delta
formation.
Q5. What phenomenon will occur when silica gel and anhydrous calcium chloride
are placed separately in two corners of a vessel containing water vapour.
Solution:
Adsorption of water takes place in the case of silica gel. Whereas absorption of water
takes place in the case of anhydrous calcium chloride.
Q6. Differentiate between homogeneous and heterogeneous catalysis. Give
one example for each.
Solution:
Homogeneous catalysis:-A catalysis in which the catalysts are present in the
same phase as the reactants. For example- oxidation of CO gas with O2 gas in the
presence of NO gas as catalyst.
Heterogeneous catalysis:-A catalysis in which a catalyst is present in different
phase than that of the reactants. For example- manufacture of ammonia from
nitrogen and hydrogen gases using solid iron as catalyst.
Q7. What is emulsion? What are their different types?
An emulsion is the colloidal dispersion in which both the dispersed phase and the
dispersion mediums are liquids. They can be of two types:i) Emulsion of oil in water.
ii) Emulsion of water in oil.
8
9.
Explain the following terms giving a suitable example in each case:
(i) Emulsification
(ii) Homogeneous catalysis.
Complete the following:
i)
The liquid-liquid colloidal dispersions are called…….
ii)
The enthalpy of chemisorption is…… than the enthalpy of
physisorption.
iii)
The scattering of light by colloidal dispersion is called……
iv)
The zig-zag motion of colloidal particles is called……
Solution:
i) Emulsions ii) more
iii) Tyndall effect
iv) Brownian movement.
SA-II of three marks
1.
Illustrate with examples:
i) Lyophilic and Lyophobic sols.
ii) Multimolecular and Macromolecular colloids.
iii) Homogeneous and Heterogeneous catalysis.
2.
Explain the following observations:
i) A beam of light passing through a colloidal solution has a visible path.
ii) Passing an electric current through a colloidal solution removes colloidal
sulphate.
iii) Ferric hydroxide sol coagulates on addition of a solution
potassiumsulphate .
3.
of
Explain the following observations:
(a) Lyophilic colloid is more stable than lyophobic colloid.
(b) Coagulation takes place when sodium chloride solution is added to a colloidal
solution of ferric hydroxide.
(c) Sky appears blue in color.
4. What are micelles? How do they differ from ordinary colloidal particles? Give two
examples of micelles forming substances.
5.
(a) In which of the following does adsorption take place and why?
i) Silica gel placed in the atmosphere saturated with water.
ii) Anhydrous CaCl2 placed in the atmosphere saturated with water.
(b) How does BF3 act as a catalyst in industrial process?
(c) Give an example of shape-selective catalysis.
Higher Order Thinking Skill
Q1.
Which of the following more effective in coagulating Fe(OH)3 and why ?
Na2PO4,Na2SO4, NaCl.
Solution:i) Fe(OH)3 sol is +vely charged so it will be coagulated by –ve ionic of
added electrolyte. ii) Coagulating power is directly proportional to charge , so PO-4 is
more effective.
Q2.
Which will be adsorbed more readily on the surface of charcoal and why NH3 or
CO2?
Solution:
NH3 because it is more easily liquefiable. Hence, NH3 has greater intermolecular
force
and will be adsorbed more readily.
Q3. What is meant by Kraft temperature (Tk) and critical micelle concentration
(CMC)?
Solution:
The optimum temperature from which micelle formation starts is Kraft temperature
(Tk).
The optimum concentration from which micelle formation starts is critical micelle
concentration (CMC).
Q4.
Q5.
Give any two reasons for the origin of electrical charge on the colloidal
particles.
Solution:
The two reasons are:
i) Due to electron capture by sol particles during electro dispersion of metals, due
to preferential adsorption of ions from solution
ii) Dissociation of colloidal sols.
How do size of particles of adsorbent, pressure of gas and prevailing temperature
influence the extent of adsorption of a gas on a solid?