Chapter 6 Surface Chemistry free study material by TEACHING CARE online tuition and coaching classes

 

“The branch of physical chemistry, which deals the nature of surfaces and also with the chemical and physical processes which takes place on the surfaces, is called surface chemistry”.

In surface chemistry, we study the phenomenon of adsorption, catalysis and colloidal properties.

• Definition : The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or solid resulting in to higher concentration of the molecules on the surface is called adsorption.
• Causes of adsorption : Unbalanced forces of attraction or free valencies which is present at the solid or liquid surface, have the property to attract and retain the molecules of a gas or a dissolved substance on to their surfaces with which they come in

Example : (i) Ammonia gas placed in contact with charcoal gets adsorbed on the charcoal whereas ammonia

 

gas placed in contact with water gets absorbed into water, giving

NH4 OH

solution of uniform concentration.

 

(ii) If silica gel is placed in a vessel containing water vapours, the latter are adsorbed on the former. On the

 

other hand, if anhydrous

CaCl 2

is kept in place of silica gel, absorption takes places as the water vapours are

 

uniformly distributed in CaCl 2

to form hydrated calcium chloride (CaCl2 . 2H 2 O) .

Some basic terms which are used in adsorption

 

Interface : Any surface is a plane which separates any two phases in contact with each other. The plane which separates any two phase is generally called an interface between the two phases.

Adsorbate and Adsorbent : The substance which gets adsorbed on any surface is called adsorbate for example, if a gas gets adsorbed on to the surface of a solid, then the gas is termed as the adsorbate.

The substance on the surface of which adsorption takes place is called adsorbent.

Adsorbent may be a solid or a liquid metal powders. Powdered

charcoal,             animal charcoal silica powder etc. are commonly used as adsorbents.

 

 

 

Desorption : The removal of the adsorbed substance from a surface is called desorption. ×××××××       Removal of ×             × ×                ×     adsorbate ×                 × × Adsorbent × ×                  × ×                  × ×    (solid)  × ×                  × ×                 × ×               × ××× × × ×× Desorption

Absorption : When the molecules of a substance are uniformly distributed throughout the body of a solid or liquid. This phenomenon is called absorption. Substance × ×   ×    ×             molecules ×   × ×  × ×  ×   × ×  ×   ×   × ×  × ×  ×   ×             Body of ×   ×  ×   ×   × ×  ×  ×  ×  ×   ×      solid (liquid) ×  ×  ×  ×  × Absorption

Sorption : The phenomenon in which adsorption and absorption occur simultaneously is called sorption. Mc. Bain introduced a general term sorption describeing both the processes, however adsorption is instantaneous i.e. a fast process while absorption is a slow process. ×  × × × × × ×          Substance ×  × ×   ×    × × × ×  × ×   ×   × ×         molecules ×          ×  ×         × ×   ×   ×   ×   ×   ×       Body of ×  ×  × ×  ×   × × × ×   ×   ×   ×   × ×     solid (liquid) ×  ×  ×  ×  ×   × × ×  ×  ×  ×  ×  ×   × ×  ×  × ×× × × × Sorption

Occlusion :    When adsorption of gases occur on the surface of metals this is called occlusion.       Adsorption of × × × × × × ××××× ×           gas ×                           × ×                           ×  Metal surface ×                           × ×                           × ×                           × × × × × × × ×××××× occlusion

Difference between adsorption and absorption

Balanced force New unbalanced      ×   × Breaking ×  +   × ×  ×                                       ×       × No. of unbalanced No. of unbalanced

• Surface forces : Only the surface atoms of an adsorbent play an active role in These atoms posses unbalanced forces of various types such as, Vander Waal’s forces and chemical bond forces.

Thus, the residual force-field on a free surface which is responsible for adsorption is produced. For example, when a solid substance is broken into two pieces, two new surfaces are formed and therefore, the number of unbalanced forces becomes more. As a result the tendency for adsorption become large.

 

 

• Reversible and Irreversible adsorption : The adsorption is reversible, if the adsorbate can be easily removed from the surface of the adsorbent by physical If the adsorbate can not be easily removed from the surface of the adsorbent is called irreversible adsorption.
• Example for Reversible adsorption: A gas adsorbed on a solid surface can be completely removed in
• Example for Irreversible adsorption: Adsorption of O₂ on tungusten

(5)         Characteristics of adsorption

• Adsorption refers to the existence of a higher concentration of any particular component at the surface of a liquid or a solid

 

• Adsorption is accompanied by decrease in the DG

adsorption equilibrium is said to be established.

(free energy charge) of the system when

DG = 0 ,

 

• Adsorption is invariably accompanied by evolution of heat, e. it is an exothermic process. In other words,

DH of adsorption is always negative.

• When a gas is adsorbed, the freedom of movement of its molecules becomes On account of it decrease in the entropy of the gas after adsorption, i.e. DS is negative.

Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system and DG

also decreases.

DG = DH – TDS

• For a process to be spontaneous, the thermodynamic requirement is that DG must be negative, e.

 

there is decrease in free energy. On the basis of Gibb’s Helmholtz equation,

, DG

can be

 

negative if DH

has sufficiently high negative value and TDS

has positive value.

 

Note :® When adsorbents are porous, the adsorbate is actually condensed in the pores. This is called

capillary condensation.

Adsorption can be classified into two categories as described below.

• Depending upon the concentration : In adsorption the concentration of one substance is different at the surface of the other substance as compared to adjoining bulk or interior

• Positive adsorption : If the concentration of adsorbate is more on the surface as compared to its concentration in the bulk phase then it is called positive adsorption.

Example : When a concentrated solution of KCl is shaken with blood charcoal, it shows positive adsorption.

• Negative adsorption : If the concentration of the adsorbate is less than its concentration in the bulk then it is called negative adsorption.

Example : When a dilute solution of KCl is shaken with blood charcoal, it shows negative adsorption.

(2)  Depending upon the nature of force existing between adsorbate molecule and adsorbent

• Physical adsorption : If the forces of attraction existing between adsorbate and adsorbent are Vander Waal’s forces, the adsorption is called physical adsorption. This type of adsorption is also known as physisorption or Vander Waal’s adsorption. It can be easily reversed by heating or decreasing the
• Chemical adsorption : If the forces of attraction existing between adsorbate particles and adsorbent are almost of the same strength as chemical bonds, the adsorption is called chemical adsorption. This type of adsorption is also called as chemisorption or Langmuir adsorption. This type of adsorption cannot be easily

 

 

Comparison between physisorption and chemisorption

Physisorption (Vander Waal’s adsorption)	Chemisorption (Langmuir adsorption)
Low heat of  adsorption usually in range  of  20-40	High heat  of  adsorption  in  the  range  of  50-400
kJ/mol	kJ/mol
Force of attraction are Vander Waal’s forces.	Forces of attraction are chemical bond forces.
It is reversible	It is irreversible
It is  usually  takes  place  at  low  temperature  and	It takes place at high temperature.
decreases with increasing temperature.
It is related to the case of liquefication of the gas.	It is not related.
It forms multimolecular layers.	It forms monomolecular layers.
It does not require any activation energy.	It requires high activation energy.
High pressure is favourable. Decrease of pressure	High pressure is favourable. Decrease of pressure
causes desorption	does not cause desorption.
It is not very specific.	It is highly specific.

Note :®    Adsorption of gases on animal charcoal and adsorption of water vapours on silica gel is physical adsorption.

• The behavior of adsorption of N₂ on iron clearly distinguishes between physisorption and chemisorption. At 83 K, nitrogen is physisorbed on iron surface as N₂ The amount of N₂ adsorbed decreases rapidly as the temperature increases at room temperature, practically, there is no adsorption of N₂ on iron. However at 773 K and above, nitrogen is chemisorbed on the iron surface as nitrogen atoms.

• Due to formation of multilayers physical adsorption decreases after some

• Chemisorption and physisorption both are
• Whenever a clean surface is exposed to a gas, the gas molecules get adsorbed on the free surface. However, if the surface is already having a weakly held adsorbate on it, the same is displaced by the substance which has a tendency to get adsorbed more
• When the activated charcoal used in the gas masks is already exposed to the atmospheric air, the gases and the water vapours in the air are adsorbed on its surface. But when it is exposed to chlorine atmosphere, these gases are displaced by chlorine, Thus we find that the substances which get strongly adsorbed can easily displace the weakly adsorbed
• Almost all solids adsorb gases to some
• Charcoal adsorbs many It even adsorbs polluting gases present in air in small concentration.
• Gases such as H₂, N₂, O₂ and CO are adsorbed by finely divided transition metal such as Ni, Pt, Pd, Fe, Co,

Factors which affect the extent of adsorption on solid surface : The following are the factors which affect the adsorption of gases on solid surface.

• Nature of the adsorbate (gas) and adsorbent (solid)
  • In general, easily liquefiable gases e.g., CO₂, NH₃, Cl₂ and SO₂ are adsorbed to a greater extent than the elemental gases e.g. H₂, O₂, N₂, He etc. (while chemisorption is specific in nature.)

 

 

• Porous and finely powdered solid g. charcoal, fullers earth, adsorb more as compared to the hard non- porous materials. Due to this property powdered charcoal is used in gas masks.

(2)  Surface area of the solid adsorbent

• The extent of adsorption depends directly upon the surface area of the adsorbent, e. larger the surface area of the adsorbent, greater is the extent of adsorption.
• Surface area of a powdered solid adsorbent depends upon its particle size. Smaller the particle size, greater is its surface
• The surface area per gram of the adsorbent is called specific surface area of the

(3)  Effect of pressure on the adsorbate gas

• An increase in the pressure of the adsorbate gas increases the extent of
• At low temperature, the extent of adsorption increases rapidly with
• Small range of pressure, the extent of adsorption is found to be directly proportional to the
• At high pressure (closer to the saturation vapour pressure of the gas), the adsorption tends to achieve a limiting

(4)  Effect of temperature

• As adsorption is accompanied by evolution of heat, so according to the Le-Chatelier’s principle, the magnitude of adsorption should decrease with rise in
• The relationship between the extent of adsorption and temperature at any constant pressure is called adsorption
• A physical adsorption isobar shows a decrease in x/m (where ‘m’ is the mass of the adsorbent and ‘x’ that of adsorbate) as the temperature
• The isobar of chemisorption show an increase in the beginning and then decrease as the temperature

(5)  Activation of adsorbent

• Activation of an adsorbent means, increase in the adsorbing power of the
• This can be achieved by increasing the surface area of the
• This can be done by making the surface of adsorbent rough or by breaking it into small
• If the particle are made of then interparticle space will be too small hence the extent of adsorption may
• The active sites (clear surface) can be actually free from the adsorbed gases by heating in very high vacuum ( 10^-10 or 10 ^-11 mm Hg)

• A mathematical equation which describes the relationship between pressure (p) of the gaseous adsorbate and the extent of adsorption at any fixed temperature is called adsorption isotherms.
• The extent of adsorption is expressed as mass of the adsorbate adsorbed on one unit mass of the
• Thus, if x g of an adsorbate is adsorbed on m g of the adsorbent, then

 

 

 

Extent of adsorption =	x
	m

Various adsorption isotherms are commonly employed in describing the adsorption data.

(1)  Freundlich adsorption isotherm

• Freundlich adsorption isotherm is obeyed by the adsorptions where the adsorbate forms a

monomolecular layer on the surface of the adsorbent.

 

x 1 = kp n m

(Freundlich adsorption isotherm) or                             where, x is the weight of the gas

adsorbed by m gm of the adsorbent at a pressure p, thus x/m represents the amount of gas adsorbed by the adsorbents per gm (unit mass), k and n are constant at a particular temperature and for a particular adsorbent and adsorbate (gas), n is always greater than one, indicating that the amount of the gas adsorbed does not increase as rapidly as the pressure.

• At low pressure, the extent of adsorption varies linearly with

 

• At high pressure, it becomes independent of

 

x           1 µ pn m

x

 

• At moderate pressure

m

depends upon pressure raised to powers

 

 

 

 

 

 

 

 

 

 

 

 

 

Note :® Equation

log x

m

= log k + 1 log p is similar to the equation of a straight line

n

. Therefore, the

 

y = c + mx

plot of log (x/m) against log p should be a straight line with an intercept equal to log k and slope 1 .

n

(2)  The Langmuir – adsorption isotherms

• One of the drawbacks of Freundlich adsorption isotherm is that it fails at high pressure of the Irving Langmuir in 1916 derived a simple adsorption isotherm, on theoretical considerations based on kinetic theory of gases. This is named as Langmuir adsorption isotherm.
• The main points of Langmuir’s theory of adsorption are as follows,

• Adsorption takes place on the surface of the solid only till the whole of the surface is completely covered with a unimolecular layer of the adsorbed
• Adsorption consists of two opposing processes, namely Condensation of the gas molecules on the solid surface and Evaporation (desorption) of the gas molecules from the surface back into the gaseous
• The rate of condensation depends upon the uncovered (bare) surface of the adsorbent available for Naturally, at start when whole of the surface is uncovered the rate of condensation is very high and as the surface is covered more and more, the rate of condensation progressively decreases. On the contrary, the rate

 

 

of evaporation depends upon the covered surface and hence increases as more and more of the surface is covered ultimately an equilibrium will be set up at a stage when the rate of condensation becomes equal to the rate of evaporation (adsorption equilibrium).

• The rate of condensation also depends upon the pressure of the gas since according the kinetic theory of gases, the number of molecules striking per unit area is proportional to the pressure.

 

Mathematically,

x =    ap    , where a and b are constants and their value depends upon the nature of gas

m      1 + bp

 

(adsorbate), nature of the solid adsorbent and the temperature. Their values can be determined from the experimental data.

• Limitation of Langmuir theory

• Langmuir’s theory of unimolecular adsorption is valid only at low pressures and high temperatures.
• When the pressure is increased or temperature is lowered, additional layers are formed. This has led to the modern concept of multilayer adsorption.

Note :® The Langmuir adsorption isotherm is restricted to the formation of unimolecular layer of gas molecules on the surface of solids. However, it was suggested that there is possibility of multimolecular layer of gas molecules on the surface of the solids rather than single layer. On this basis, Brunauer, Emmett and Teller proposed a new theory known as B.E.T theory.

• The process of adsorption can take place from solutions
• In any solution, there are two (or more) components ; solute and The solute may be present in the molecular or ionic form.
• The extent of adsorption from solution depends upon the concentration of the solute in the solution, and can be expressed by the Freundlich isotherm.

 

• The Freundlich adsorption isotherm for the adsorption from solution is, where, x is the mass of

 

the solute adsorbed, m is the mass of the solid adsorbent, c is the equilibrium concentration of the solute in the solution, n is a constant having value greater than one,

k is the proportionality constant, (The value of k depends upon the nature of solid, its particle size, temperature, and the nature of solute and solvent etc.)

• The plot of x/m against c is similar to that Freundlich adsorption The above equations may be

 

 

written in the following form,

solution.

Note :® Adsorption Isostere : Degree

where c, is the equilibrium concentration of the solute in the

 

 

of adsorption   depends   on

 

temperature as well as on pressure. When temperature increases the extent of adsorption decreases. A linear relationship should exist between temperature and pressure with a certain amount of adsorption. The plot of temperature versus pressure for a given

 

 

amount of adsorption is called adsorption isostere.

The phenomenon of adsorption finds a number of applications. Important applications are given as follows.

• Production of high vacuum : A bulk of charcoal cooled in liquid air is connected to a vessel which has already been exhausted as for as possible by a vacuum pump. The remaining traces of air are adsorbed by the Then a very high vacuum is produced.
• In Gas masks : It is a device which consists of activated charcoal or a mixture of This

 

apparatus is used to adsorb poisonous gases (e.g. breathing.

Cl₂ , CO, oxide of sulphur etc.) and thus purify the air for

 

• For desiccation or dehumidification : Certain substances have a strong tendency to absorb water such as silica and alumina (Al ₂O₃ ) . These substances can be used to reduce/remove water vapours or moisture present in the Silica gel is also used for dehumidification in electronic equipment.
• Removel of colouring matter from solution : (i) Animal charcoal removes colours of solutions by adsorbing coloured (ii) Animal charcoal is used as decolouriser in the manufacture of cane sugar.
• Heterogeneous catalysis : Mostly heterogeneous catalytic reactions proceed through the adsorption of gaseous reactants on solid For example,
• Finely powdered nickel is used for the hydrogenation of oils.

 

• Finely divided vanadium pentaoxide sulphuric

(V2 O5 )

is used in the contact process for the manufacture of

 

• Pt, Pd are used in many industrial processes as
• Manufacture of ammonia using iron as a
• Separation of inert gases : Due to the difference in degree of adsorption of gases by charcoal, a mixture of inert gases can be separated by adsorption on coconut charcoal at different low

(7)  Softening of hard water

• The hard water is made to pass through a column packed with zeolite (sodium aluminium silicate)
• Ca⁺⁺, Mg⁺⁺ ions which are responsible for hardness, get adsorbed on zeolite, exchanging sodium

Na2 Al 2 Si2 O8 + CaCl2 ¾¾® CaAl 2 Si2 O8 + 2NaCl

• The exhausted zeolite is regenerated with 10% of sodium chloride

CaAl 2 Si2 O8 + 2NaCl ¾¾® Na2 Al 2 Si2 O8 + CaCl2

(8)  De-ionisation of water

• Water can be de-ionised by removing all dissolved salts with the help of cation and anion-exchanger
• Cation-exchanger is an organic synthetic resin such as polystyrene-containing a macroanion

3

(R – SO^– etc.) which has adsorbed H⁺ ions.

• A resin containing a basic group (R₃ N ⁺ ) which has adsorbed OH ^– ions acts as anion exchanger.

• In curing diseases : A number of drugs are adsorbed on the germs and kill them or these are adsorbed on the tissues and heat

 

 

• Cleaning agents : Soap and detergents get adsorbed on the interface and thus reduce the surface tension between dirt and cloth, subsequently the dirt is removed from the

(11)  Froth floatation process

• A low grade sulphide ore is concentrated by separating it from silica and other earthy matter by this
• The finely divided ore is added to water containing pine oil and foaming agent.
• The air is bubbled through the
• The foam formed rises to the surface on which mineral particles wetted with oil are adsorbed while earthy matter settle down at the

(12)  In adsorption indicators

• Surface of certain precipitates such as silver halide, have the property of adsorbing some dyes like eosin, fluorescein

 

• In this case of precipitation titrations (for example

AgNO3

Versus NaCl) the indicator is adsorbed at the

 

end point producing a characteristic colour on the precipitate.

(13)  Chromatographic analysis

• The phenomenon of adsorption has given an excellent technique of analysis known as chromatographic
• The technique finds a number of applications in analytical and industrial
• Chromatographic technique based on differential adsorption of different constituents of a

• In dyeing : Many dyes get adsorbed on the cloth either directly or by the use of

“Catalyst is a substance which speeds up and speeds down a chemical reaction without itself being used up.”

‘or’

“A catalyst is a foreign substance the addition of which into the reaction mixture accelerates or retards the reaction.”

• Berzelius (1836) introduced the term catalysis and catalyst.

• Ostwald (1895) redefined a catalyst as, “A substance which changes the reaction rate without affecting the overall energetics of the reaction is termed as a catalyst and the phenomenon is known as ”

Catalytic reactions can be broadly divided into the following types,

• Homogeneous catalysis : When the reactants and the catalyst are in the same phase (e. solid, liquid or gas). The catalysis is said to be homogeneous. The following are some of the examples of homogeneous catalysis.
• Oxidation of sulphur dioxide into sulphur trioxide with oxygen in the presence of oxides of nitrogen as the

 

catalyst in the lead chamber process.

2SO2 (g) + O2 (g) ¾¾^NO¾⁽¾^g) ® 2SO3 (g)

 

The reactants, products and catalyst all are in gaseous state i.e. same phase.

 

 

• Hydrolysis of methyl acetate is catalysed by H⁺ ions furnished by hydrochloric acid .

CH 3 COOCH3 (l) + H 2 O(l) ¾¾^HC¾^l¾^(l) ® CH 3 COOH(l) + CH 3 OH(l)

• Hydrolysis of sugar is catalysed by H⁺ ions furnished by sulphuric

C12 H 22 O11 (l) + H 2 O(l) ¾¾^H2S¾^O4¾^(l) ® C6 H12 O6 (l) + C6 H12 O6 (l)

 

(Sucrose solution)

(Glucose solution)         (Fructose solution)

 

• Heterogeneous catalysis : The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous Some of the examples of heterogeneous catalysis are given below.
  • Oxidation of sulphur dioxide into sulphur trioxide in the presence of platinum metal or vanadium pentaoxide as catalyst in the contact process for the manufacture of sulphuric The reactants are in gaseous

 

state while the catalyst is in solid state.

2SO2 (g) + O2 (g) ¾¾^Pt(¾^s) ® 2SO3 (g)

 

• Combination between nitrogen and hydrogen to form ammonia in the presence of finely divided iron in

Haber’s process.

N 2 (g) + 3H 2 (g) ¾¾^Fe(¾^s) ® 2NH 3 (g)

• Oxidation of ammonia into nitric oxide in the presence of platinum gauze as a catalyst in Ostwald’s

4 NH 3 (g) + 5O2 (g) ¾¾^Pt¾^(s) ® 4 NO(g) + 6H 2 O(g)

• Hydrogenation of vegetable oils in the presence of finely divided nickel as

Vagetable oils(l) + H 2 (g) ¾¾^Ni¾^(s) ® Vegetable Ghee(g)

• Positive catalysis : When the rate of the reaction is accelerated by the foreign substance, it is said to be a positive catalyst and phenomenon as positive catalysis. Some examples of positive catalysis are given

 

• Decomposition of

H 2 O2 in presence of colloidal platinum.

2H 2 O2 (l) ¾¾^Pt ® 2H 2 O(l) + O2 (g)

 

• Decomposition of

KClO3 in presence of manganese dioxide. 2KClO3 (s) ¾¾^MnO¾2¾^(s) ® 2KCl(s) + 3O2 (g)

270^o C

 

• Oxidation of ammonia in presence of platinum gauze. 4 NH3 (g) + 5O2 (g) ¾¾^Pt¾^(s) ® 4 NO(g) + 6H 2O(g)

300^o C

• Oxidation of sulphur dioxide in presence of nitric oxide. 2SO2 (g) + O2 (g) ¾¾^NO¾⁽¾^g) ® 2SO3 (g)
• Oxidation of sulphur dioxide in presence of platinised asbestos or vanadium

2SO2 (g) + O2 (g) ¾¾^V2O¾5 (¾^s) ® 2SO3 (g)

or Pt(s)

• Oxidation of hydrochloric acid into chlorine by Deacon’s process in presence of CuCl₂ .

4 HCl(g) + O2 (g) ¾¾^CuC¾^l2 ¾^(s) ® 2Cl2 (g) + 2H 2 O(g)

450^o C

• Formation of methane in presence of nickel. CO(g) + 3H 2 (g) ¾¾^Ni(¾^s) ® CH 4 (g) + H 2 O(g)
• Synthesis of ammonia by Haber’s process in presence of a mixture of iron and molybdenum.

N 2 (g) + 3H 2 (g) ¾¾^Fe(¾^s)&¾^Mo¾^(s) ® 2NH 3 (g)

450-500^o C

• Hydrogenation of vegetable oil in presence of nickel. Vegetable oil (l) + H 2 (g) ¾¾^Ni(¾^S) ®Ghee(s)

 

 

 

 

• Manufacture of methyl alcohol in presence of

ZnO / Cr₂O₃ .

CO(g) + 2H

(g) ¾¾^ZnO¾^(g)¾²⁵⁰¾0¾^C ® CH

2                 Cr2O3 (s)

₃ OH(g)

 

Note :® Positive catalyst increases the rate by lowering activation energy of reaction. Catalyst changes the mechanism by changing the intermediate i.e. intermediate of low energy is formed. It increases the rate by converting some inactive molecule into active one.

• Negative catalysis : There are certain, substance which, when added to the reaction mixture, retard the reaction rate instead of increasing it. These are called negative catalyst or inhibitors and the phenomenon is known as negative catalysis. Some examples are as
• The oxidation of sodium sulphite by air is retarded by Alcohol acts as a negative catalyst

2Na2 SO3 (s)O2 (g) ¾¾^Alco¾^hol¾^(l) ® 2Na2 SO4 (s)

• The oxidation of chloroform by air is retarded it some alcohol is added to

2CHCl3 (l) + O2 (g) ¾¾^Alco¾^hol¾^(l) ® 2COCl2 (g) + 2HCl(g)

• The oxidation of benzaldehyde is retarded if some diphenyl amine is It acts as a negative catalyst.

2C6 H5 CHO(l) + O2 (g) ¾¾^Dip¾^hen¾^yl ® 2C6 H5 COOH(l)

amine(l)

 

• Addition of small amount of acetanilide or glycerine slow down the decomposition of hydrogen
• Tetra ethyl lead (TEL) is added to petrol to retard the ignition of petrol vapours on compression in an internal combustion engine and thus minimise the knocking
• Auto-catalysis : In certain reactions, one of the product acts as a In the initial stages the reaction is slow but as soon as the products come into existences the reaction rate increases. This type of phenomenon is known as auto-catalysis. Some examples are as follows,
  • The rate of oxidation of oxalic acid by acidified potassium permanganate increases as the reaction

 

progresses. This acceleration is due to the presence of

Mn2+

ions which are formed during reaction. Thus

 

Mn²⁺ ions act as auto-catalyst. 5H 2 C2 O4 + 2KMnO4 + 3H 2 SO4 ¾¾® 2MnSO4 + K 2 SO4 + 10CO2 + 8H 2 O

(i) When nitric acid is poured on copper, the reaction is very slow in the beginning, gradually the reaction becomes faster due to the formation of nitrous acid during the reaction which acts as an auto-catalyst.

(iii) In hydrolysis of ethyl acetate, acetic acid and ethyl alcohol are formed. The reaction is initially very slow but gradually its rate increases. This is due to formation of acetic acid which acts as an auto-catalyst in this reaction.

• Induced catalysis : When one reaction influences the rate of other reaction, which does not occur under ordinary conditions, the phenomenon is known as induced Some examples are as follows,
  • Sodium arsenite solution is not oxidised by air. If, however, air is passed through a mixture of the solution of sodium arsenite and sodium sulphite, both of them undergo simultaneous The oxidation of sodium sulphite, thus, induces the oxidation of sodium arsenite.

 

• The reduction of mercuric chloride

(HgCl ₂ ) with oxalic acid is very slow, but potassium permanganate is

 

reduced readily with oxalic acid. If, however, oxalic acid is added to a mixture of potassium permanganate and

 

 

HgCl ₂ both are reduced simultaneously. The reduction of potassium permanganate, thus, induces the reduction of mercuric chloride.

• Acid-base catalysis : According to the Arrhenius and Ostwald H⁺ or H^– ion act as a

 

• For example, Hydrolysis of an ester, CH

3 COOC2 H5

(l) + H

O(l) ¾¾H +¾or  ® CH

2

6

2               OH –

3 COOH(l) + C2

H5 OH(l)

 

 

 

• Inversion of cane sugar, C12

H 22

O11

(l) + H

O ¾¾^H¾+  ® C

H12 O6

(l)+ C6

H12 O6

(l)

 

Sugar

 

• Conversion of acetone into diacetone alcohol,

Fructose

Glucose

 

 

3               3                  3               3                                 3               2              3 2

CH  COCH   (l) + CH  COCH   (l) ¾¾^OH¾-   ® CH  COCH   .C(CH   )  OH(l)

 

 

• Decomposition of nitramide,

NH   NO  (l) ¾¾^OH¾-   ® N  O(g) + H  O(l)

 

2        2                             2                   2

Note :®             All Bronsted acids and bases act as acid base catalysts.

• Catalytic converter for an automobile : The catalytic converter in the exhaust systems of cars, which converts polluting exhaust gases into non-toxic gases contains a heterogeneous catalyst. Mixtures of transition metals and their oxides embedded in inert supports act as catalyst. When the gases are passed through the catalyst bed, carbon monoxide (CO) and unburnt petrol are oxidised to carbon

 

dioxide and water while nitric oxide (NO) is reduced to

N 2 as,

 

2CO + O2  ¾¾^Cat¾^aly¾^st ® 2CO2 ;

(Unburnt petrol)

Hydrocarbons ¾¾^Cat¾^aly¾^st ® CO2  + H 2 O ;

O2

2NO ¾¾^Cat¾^aly¾^st ® N 2  + O2

 

 

The following are the characteristics which are common to must of catalytic reactions.

• A catalyst remains unchanged in mass and chemical composition at the end of the

(2)    A small quantity of the catalyst is generally sufficient to catalyses almost unlimited reactions

• For example, in the decomposition of hydrogen peroxide, one gram of colloidal platinum can catalyses

10⁸ litres of hydrogen peroxide.

• In the some reaction the rate of the reaction is proportional to the concentration of the For example the acid and alkaline hydrolysis of an ester, the rate of reaction is proportional to the concentration of

 

H ⁺ or OH ^– ions.

RCOOR^‘ (l) + H

O(l) ¾¾^H+¾^or ® RCOOH(l) + R^‘ OH(l)

2               OH ^–

 

• In Friedel – craft’s reaction, anhydrous aluminium chloride is required in relatively large amount to the

 

extent of 30% of the mass of benzene,

C6 H6  + C2 H5 Cl ¾¾^AlC¾^l3  ® C6 H5 C2 H5  + HCl

 

• In certain heterogeneous reactions, the rate of reaction increases with the increase of area of the catalytic

• The catalyst can not initiate the reaction: The function of the catalyst is to alter the speed of the reaction rather than to start

 

 

• The catalyst is generally specific in nature: A substance, which acts as a catalyst for a particular reaction , fails to catalyse the other reaction , different catalysts for the same reactant may for different

 

 

 

 

Examples :

Al 2O3

C2 H4

(g) + H

₂O(g)

Cu              CO

2

(g) + H

₂ (g)

 

C2 H

₅ OH

(l)

(Dehydration)

 

Cu

CH₃CHO(g) + H ₂ (g)

(Dehydrogenation)

HCOOH(l)

(Dehydrogenation)

 

CO(g) + H₂O(g)

(Dehydration)

 

• The catalyst can not change the position of equilibrium : The catalyst catalyse both forward and backward reactions to the same extent in a reversible reaction and thus have no effect on the equilibrium
• Catalytic promoters : Substances which themselves are not catalysts, but when mixed in small quantities with the catalysts increase their efficiency are called as promoters or activators.
  • For example, in Haber’s process for the synthesis of ammonia, traces of molybdenum increases the activity of finely divided iron which acts as a

 

• In the manufacture of methyl alcohol from water gas promoter with the catalyst zinc oxide (ZnO) .

(CO + H ₂ ), chromic oxide

(Cr2 O3 )

is used as a

 

• In the hydrogenation of oils, the activity of the catalyst nickel increases on adding small amount of

• Catalytic poisons : Substances which destroy the activity of the catalyst by their presence are known as

catalytic poisons.

• For example, the presence of traces of arsenious oxide (As2 O3 ) in the reacting gases reduces the activity of platinized asbestos which is used as catalyst in contact process for the manufacture of sulphuric

 

• The activity of iron catalyst is destroyed by the presence of Haber’s

H 2 S or CO in the synthesis of ammonia by

 

• The platinum catalyst used in the oxidation of hydrogen is poisoned by CO .

Note :®    The poisoning of the catalyst is probably due to the preferential adsorption of poison on the surface of the catalyst, thus reducing the space available for the adsorption of reacting molecules.

• Change of temperature alters the rate of catalytic reaction as it does for the same reaction in absence of catalyst : By increasing the temperature, there is an increase in the catalytic power of a catalyst but after a certain temperature its power begins to decrease. A catalyst has thus, a particular temperature at which its catalytic activity is This temperature is termed as optimum temperature.

(9)    A positive catalyst lowers the activation energy

• According to the collision theory, a reaction occurs on account of effective collisions between the reacting
• For effective collision, it is necessary that the molecules must possess a minimum amount of energy known as activation energy (E_a).
• After the collision molecules form an activated complex which dissociate to yield the product
• The catalyst provides a new pathway involving lower amount of activation Thus,

 

Þ                          Þ                        Þ

Þ                           Þ

 

 

larger number of effective collisions occur in the presence of a catalyst in comparison to effective collisions at the same temperature in absence of a catalyst. Hence the presence of a catalyst makes the reaction to go faster.

 

• Figure shows that activation energy presence of a

Ea , in absence of a catalyst is higher than the activation energy E_a, in

 

• ER

DG = ER – EP

DG , i.e.,

and

Ep represent the average energies of reactants and products. The difference gives the value of

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

There are two theories of catalysis which is described as follows.

(1)  Intermediate compound theory

• This theory was proposed by Clement and Desormes in 1806. According to this theory, the desired reaction is brought about by a path involving the formation of an unstable intermediate compound, followed by its decomposition into the desired end products with the regeneration of the
• The intermediate compund may be formed in either of two ways

• When the intermediate compound is reactive and reacts with the other

AB + X ® BX + A

 

intermediate

BX + C ® CB + X

…….(i)

 

• When the intermediate is unstable and decomposes to give the final

 

A + B + X ® ABX ® AB + X

intermediate

…….(ii)

 

Where, A, B and C are the reactant molecules and X is the molecule of the catalyst. The first type of

reaction sums up to, AB + C ® CB + A

 

While the second to,

A + B ® AB

in many cases, the intermediate compounds postulated to be formed

 

are known compounds and often their presence is detected.

(2)  Adsorption theory

• This theory is applicable to reactions between gases in the presence of a solid Some typical examples are as follows.

 

• The contact process for the oxidation of platinum as the

SO2  to

SO3

with atmospheric oxygen in the presence of

 

• The Haber’s process for the synthesis of ammonia with iron as the

 

 

• Adsorption results in the loosening of the chemical bonds in the reactant molecules, so that their rupture becomes easier. This is confirmed by the observed lower activation energies for heterogeneous catalytic reactions in the presence of the catalysts as compared to that for the same reaction in the absence of the
• The metals copper and nickel are found particularly suitable for reactions involving hydrogen These metals are known to strongly chemisorb hydrogen gas. Typical example includes the dehydrogenation of ethandol

 

vapours when passed over heated metal at

350o C .

CH3 CH

OH ¾¾Ni  ® CH

2            350o C

3 CHO + H 2

 

• Aluminium oxide in some physical forms is a good adsorbent for water It is also a useful catalyst for reactions involving dehydration processes (i.e. processes involving the removal of water from molecules). For

 

example, formation of ethene from ethyl alcohol, CH

3 CH

OH ¾¾Al2O¾3  ® CH

2              350o C                       2

= CH 2

• H 2 O

 

ethanol                 ethene

• Heterogeneous catalytic reactions generally proceed via adsorption of reactants on the surface of the
• Mechanism of such surface reactions may be explained in terms of diffusion theory of catalysis. This theory postulates the following sequence for gaseous reactions on a solid

Step: (i) Diffusion of the reactants to the surface.

Step: (ii) Adsorption of the reactant molecules onto the surface.

Step: (iii) Actual chemical reaction on the surface.

Step: (iv) Desorption of the products from the surface.

Step: (v) Diffusion of the products away from the surface.

In generally, Step (iii) determines the rate of reaction. However step (ii) and (iv) may be rate determining.

• According to Langmuir-Hinshelwood, the rate of a catalytic reaction is proportional to the concentration of the reacting species on the surface. For this, the reacting species must get adsorbed on the neighboring
• Another way in which two reacting molecules may react on a solid surface is that one of them gets adsorbed and then the adsorbed molecules reacts with a molecule in the gas phase. This mechanism is called Rideal mechanism.

• Enzymes are complex nitrogenous substances secreted by low forms of vegetable animal
• Enzymes are actually protein molecules of higher molecular
• Enzymes form colloidal solutions in water and are very effective They catalyse numerous reactions, especially those connected with natural processes.

 

 

• Numerous reactions occur in the bodies of animals and plants to maintain the life process. These reactions are catalysed by The enzymes are thus, termed as bio-chemical catalysts and the phenomenon is known as bio-chemical catalysis.
• Nitrogenase an enzyme present in bacteria on the root nodules of leguminous plants such as peas and

 

beans, catalyses the conversion of atmospheric

N 2 to

NH 3 .

 

• In the human body, the enzyme carbonic anhydrase catalyses the reaction of CO₂ with

3

CO2 (aq) + H 2 O(l)  H ⁺ (aq.) + HCO^– (aq.)

H 2 O ,

 

The forward reaction occurs when the blood takes up blood releases CO2 in lungs.

CO₂ in the tissues, and the reverse reaction occurs when the

 

Catalysts in industry

Process                                                                                                          Catalyst

 

Haber’s process for the manufacture ammonia.

 N (g) + 3H (g)  2NH (g)

Finely  divided    iron.    Molybdenum  as    promoter  and 200 atmospheric pressure and 450-500^oC temperature.

 

2              2                    3

 

 

 

 

 

 

 

 

 

 

 

acid.

Ostwald’s process for the manufacture of nitric acid.

4 NH3(g) + 5O2(g) ® 4 NO(g) + 6H2O(g)

2NO(g) + O2(g) ® 2NO2(g)

4 NO2(g) + 2H2O(l) + O2(g) ® 4HNO3(l)

Lead chamber process for the manufacture of sulphuric acid.

2SO2(g) + O2(g)  2SO3 (g)

 SO3(g) + H2O(l) ® H2SO4 (l)

Contact  process  for  the  manufacture  of  sulphuric

 

2SO2 (g) + O2 (g)  2SO3 (g)

 SO3 (g) + H 2 SO4 (l) ® H 2 S2 O7 (l)

oleum

 H 2 S2 O7 (l) + H 2 O(l) ® 2H 2 SO4 (l)

Deacon’s process for the manufacture of chlorine.

4 HCl(g) + O2 (g) ® 2H 2 O(l) + 2Cl 2 (g)

Platinised asbestos and temperature 300^o C.

 

 

 

 

Nitric oxide

 

 

 

 

Platinised    asbestos    or vanadium pentoxide Temperature 400-450⁰ C.

 

 

 

 

Cupric chloride (CuCl 2 ) . Temperature 500^o C.

 

 

 

 

 

 

(V2 O5 ) .

 

Bosch’s process for the manufacture of hydrogen.

Ferric  oxide

(Fe2O3 ) + chromic oxide as   a   promoter.

 

C1uO2+ uH32 + H2O(g) ® CO2 (g) + H2O(g) water gas

Temperature 400-600^o C.

 

 

Synthesis of methanol.

Zinc oxide

(ZnO) +chromic oxide as promoter. Pressure

 

 CO(g) + 2H 2 (g) ® CH 3 OH(l)

Hydrogenation of vegetable oils.

Oil(l) + H ₂ (g) ® Vanaspati ghee (s)

Manufacture of ethyl alcohol by fermentation of molasses (sugar solution).

 C12 H 22 O11 (l) + H 2 O(l) ¾¾^Inve¾^rta¾^se ®

200 atmopheres and temperature 250^o C.

Nickel (finely divide). Temperature 150-200^oC. High pressure.

Invertase       enzyme       and       zymase       (yeast)       enzyme. Temperature 25-30^o C. Conversion occurs in 2 or 3 days.

 

 

 C6 H12 O6 (l) ¾¾^Zym¾^a¾^se ® 2C2 H5 OH(l) + 2CO2 (l)

Manufacture of ethyl alcohol from starch.                               Germinated barley (diastase enzyme). Temperature 50-

60^o C. Yeast (maltase and zyamase enzymes).

 

 

 

• Activity : Activity is the ability of catalysts to accelerate chemical reaction, the degree of acceleration can

 

be as high as 10¹⁰ times in certain reactions. For example reaction between H 2 and O2

platinum as catalyst takes place with explosive violence.

to form

H ₂O in presence of

 

In absence of catalyst,

H ₂ and O₂ can be stored indefinitely without any reaction.

 

• Selectivity : Is the ability of catalysts to direct reaction to yield particular products (excluding other).

 

 

Example :   (i) n – heptane ¾¾^Pt ®

CH 3

toluene

• CH₃CH = CH₂

¾¾^BiM¾^oO¾4  ® CH2

O

||

= CHCH

Acrolein

 

 

 

 

• Zeolite are alumino–silicates of the general formula,

M_x / n[AlO₂]_x .(SiO₂)_y.mH₂O , where, M may be simple

 

cation like

Na ⁺ , K ⁺ or Ca ²⁺ , n is the charge on the simple cation, m is the number of molecules of water of crystallization.

 

• Some well known zeolites are as follows,

 

Erionite

¾¾® Na2 K2CaMg(AlO2 )2(SiO2 )2.6H2O

 

Gemelinite

¾¾® Na2Ca(AlO2 )2(SiO2 )4 .6H2O

 

Faujasite (natural)

¾¾® Na56 (AlO2 )56 (SiO2 )136 .250H2O

 

ZSM-5

¾¾® Hx [(AlO2 )x (SiO2 )96- x ].16H2O

 

Linde-A (synthetic)

¾¾®[Na12(AlO2 )12(SiO2 )12.27H2O]8

 

• The characteristic feature of zeolites is the openness of the structure, which permits cavities of different
• The open structure is provided by silica in which aluminium occupies x/(x+y) fraction of the telrahedral
• The negative charge of the aluminosilicate framework is neutralized by the replaceable cations.
• The void space forms more than 50% of the total volume, which is occupied by water
• The reaction- selectivity of zeolites depends upon the size of cavities (cages), pores (apertures) and the distribution of pores in the The pore size in zeolites generally varies from 260 pm to 740 pm.
• The building block of zeolites is a truncated This is also called the sodalite cage (or b – cage).
• Tetrahedral atom denoted by open circles in fig (a) are present at the corners of polygons with the oxygen atoms approximately half way between

 

 

 

 

• Zeolite have high porosity due to the presence of one, two, or three dimensional networks of interconnected channels and cavities of molecular
• Accordingly zeolite – A is formed by linking sodalite cages through double four-membered rings, Faujasite (Zeolite X and Y) is formed by linking the sodalite cages through double six-membred
• Many Zeolites occur in nature and they can be readily prepared in
• There is a new class of highly siliceous zeolites with an optimal pore diameter of 550pm. ZSM-5 is one such zeolite having the [Hx(AlO2 )x .(SiO2 )96- x ].16H 2 O
• The zeolite catalyst ZSM-5 converts alcohols to gasoline (petrol) by dehydrating the alcohol and producing a mixture of wide variety of hydrocarbons. The shape selectivity of this catalyst is demonstrated by data given in

 

 

Input stock                                                                                                  Input stock
Product (in %)	methanol	n-heptyl alcohol	Product (in %)	methanol	n-heptyl alcohol
Methane	1.0	0.0	i- Pentane	7.8	8.7
Ethane	0.6	0.3	Benzene	1.7	3.7
i-butane	18.7	19.3	Toluene	10.5	14.3
n-butane	5.6	11.3	Xylenes	17.2	11.6

• The foundation of colloidal chemistry was laid down by an English scientist, Thomas Graham, in 1861. The credit for the various advances in this field goes to eminent scientists like Tyndall, Hardy, Zsigmondy, R. Dhar, S.S. Bhatnagar and others.
• Thomas Graham classified the soluble substances into two categories depending upon the rate of diffusion through animal and vegetable membranes or parchment
• Crystalloids : They have higher rate of diffusion and diffused from parchment

Examples : All organic acids, bases and salts and organic compounds such as sugar, urea etc.

• Colloids (Greek word, kolla, meaning glue-like) : They have slower rate of diffusion and can not diffused from parchment Examples : Starch, gelatin, gums, silicic acid and hdemoglobin etc.
• The above classification was discarded e., the terms colloid does not apply to a particular class of substances but is a state of matter like solid, liquid and gas. Any substance can be brought into colloidal state.

 

 

• The colloidal state depends on the particle If is regarded as intermediate state between true solution and suspension.
• True solution : In true solutions the size of the particles of solute is very small and thus, these can not be

detected  by  any  optical  means  and   freely  diffuse  through membranes. It is a homogenous system.

Size < 1 nm Size between 1-100 nm Size > 100 nm

• Suspension : The size of particles is large and, thus it can be seen by naked eye and do not pass through

filter paper. It is a heterogeneous system.

The size of different solutions are sometimes expressed in other units also as given below :

Size (diameter) of particles in particles in different units

True solutions	Colloids	Suspensions	Relation
<10–9m	10–9 m to 10–7m	>  10 –7 m
<1nm	1 nm – 100 nm	> 100 nm	1 nm = 10–9 m
<10 Å	10 Å – 1000 Å	> 1000 Å	1 Å = 10–10 m
<1000 pm	1000 pm –105 pm	>105 pm	1 pm = 10–12 m

The important distinguishing features of the three types of solutions

 

Property	Suspension	Colloid solution		True solution
Nature	Heterogeneous	Heterogeneous		Homogeneous
Particle size	> 100 nm	1 nm – 100 nm		< 1 nm
Separation by
(i) Ordinary filtration	Possible	Not possible		Not possible
(ii) Ultra- filtration	Possible	Possible		Not possible
Settling of particles	Settle under gravity	Settle              only centrifugation	on	Do not settle
Appearance	Opaque	Generally transparent		Transparent
Tyndall effect	Shows	Shows		Does not show
Diffusion of particles	Does not diffuse	Diffuses slowly		Diffuses rapidly
Brownian movement	May show	Shows		Negligible

• Roughly speaking the colloidal state is a heterogeneous dispersion of solute particles of size between true solution and

Note :® Colloidal particles do not settle down under the force of gravity even an long keeping.

• The surface area of colloidal particle is very large in comparison to

• Phases of colloids : We know that a colloidal solution is of heterogeneous nature. It consists of two phases which are as follows
  • Internal phase or Dispersed phase (Discontinuous phase) : It is the component present in small proportion and is just like a solute in a solution. For example in the colloidal solution of silver in water (silver acts as a dispersed phase)

 

 

• External phase or Dispersion medium (continuous phase) : It is generally component present in excess and is just like a solvent in a For example, in the colloidal solution of silver in water. Water act as a dispersion medium.

Note :® When dispersion medium is a gas, the colloidal system is called aerosol. When it is a liquid system is called solution (hydrosol for water, alcosol for alcohol, benzosol for benzene)

Colloidal system = Dispersed phase + Dispersionmedium

• Colloidal system is a two phase

• Classification of colloids : The colloids are classified on the basis of the following criteria
  • Classification based on the physical state of the dispersed phase and dispersion medium Depending upon the physical state of dispersed phase and dispersion medium whether these are solids, liquids or gases, eight types of colloidal systems are

Different types of colloidal systems

 

Dispersed phase	Dispersion Medium	Colloidal System	Examples
Liquid	Gas	Aerosol of liquids	Fogs, clouds, mists, fine insecticide sprays
Solid	Gas	Aerosol of solids	Smoke, volcanic dust, haze
Gas	Liquid	Foam or froth	Soap lather. Lemonade froth, foam, whipped cream, soda water
Liquid	Liquid	Emulsions	Milk, emulsified oils, medicines
Solid	Liquid	Sols	Most paints, starch in water, proteins, gold sol, arsenic sulphide sol, ink
Gas	Solid	Solid foam	Pumice stone, styrene rubber, foam rubber
Liquid	Solid	Gels	Cheese, butter, boot polish, jelly, curd
Solid	Solid	Solid sols (coloured glass)	Ruby glass, some gem stones and alloys

Note :® A colloidal system of gas in gas is not possible as gases are completely miscible and always form homogenous true solution.

• Classification based on Nature of interaction between dispersed phase and dispersion medium: Depending upon the nature of interactions between dispersed phase and the dispersion medium, the colloidal solutions can be classified into two types as (a) Lyophilic and (b) Lyophobic

• Lyophilic colloids (water loving) : Substances such as proteins, starch and rubber whose molecules are large enough to be close to the lower limit of colloidal range pass readily into colloidal state whenever mixed with suitable Thus these colloids have strong interaction with the dispersion medium and are called lyophilic colloids (hydrophilic when continuous phase is water)

“or”

“The colloidal solutions in which the particles of the dispersed phase have a great affinity for the dispersion medium, are called lyophilic collodis.”

• Lyophobic colloids (water hateing) : Substance such as arsenic sulphide, ferric hydroxide, gold and other metals, which are sparingly soluble and whose molecules are much smaller than the lower colloidal limit, change

 

 

into colloidal state by aggregation of many individual molecules. “These substances; therefore, do not pass into colloidal state readily and are called lyophobic colloids (hydrophobic when continuous phase is water).”

“or”

“The colloidal solutions in which there is no affinity between particles of the dispersed phase and the dispersion medium are called lyophobic colloids.”

Distinction between Lyophilic and Lyophobic Sols

 

Property	Lyophilic (suspensoid)	Lyophobic Sols (Emulsoid )
Surface tension	Lower than that of the medium	Same as that of the medium
Viscosity	Much higher than that of the medium	Same as that of the medium
Reversibility	Reversible	Irreversible
Stability	More stable	Less stable
Visibility	Particles can’t  be  detected  even  under	Particles can be detected under ultramicroscope.
	ultramicroscope
Migration	Particles may migrate in either direction	Particles  migrate   either   towards   cathode   or
	or do  not  migrate  in  an  electric  field	anode in  an  electric  field  because  they  carry
	because do not carry any charge.	charge.
Action of electrolyte	Addition       of      smaller      quantity       of	Coagulation takes place
	electrolyte has little effect
Hydration	Extensive hydration takes place	No hydration
Examples	Gum, gelatin, starch,  proteins,  rubber	Metals like Ag and Au, hydroxides like  Al(OH)3 ,
	etc.	Fe(OH)3 metal sulphides like AS2S3  etc.

• Classification based on types of particle of dispersed phase : Depending upon the type of the particles of the dispersed phase, the colloids are classified as

• Multimolecular colloids
• When on dissolution, atoms or smaller molecules of substances (having diameter less than 1nm) aggregate together to form particles of colloidal dimensions, the particles thus formed are called multimolecular colloids.
• In these sols the dispersed phase consists of aggregates of atoms or molecules with molecular size less than 1

 

• For example, sols of gold atoms and sulphur

(S₈ ) molecules. In these colloids, the particles are held

 

together by Vander Waal’s forces. They have usually lyophilic character.

• Macromolecular colloids
• These are the substances having big size molecules (called macromolecules) which on dissolution form size in the colloidal Such substances are called macromolecular colloids.
• These macromolecules forming the dispersed phase are generally polymers having very high molecular
• Naturally occurring macromolecules are starch, cellulose, proteins, enzymes, gelatin
• Artificial macromolecules are synthetic polymers such as nylon, polythene, plastics, polystyrene

 

 

• Their solutions are quite stable and resemble with true solution in many
• They have usually lyophobic
• The molecules are flexible and can take any
• Associated colloids
• These are the substances which on dissolved in a medium behave as normal electrolytes at low concentration but behave, as colloidal particles at higher concentration due to the formation of aggregated The aggregates particles thus formed are called micelles.
• Their molecules contain both lyophilic and lyophobic
• The colloidal behaviour of such substances is due to the formation of aggregates or clusters in solutions. Such aggregated particles are called

Micelles

• Micelles are the cluster or aggregated particles formed by association of colloid in
• The common examples of micelles are soaps and detergents.
• The formation of micelles takes place above a particular temperature called Kraft temperature (Tk ) and above a particular concentration called critical micellization concentration (CMC).
• They are capable of forming
• Micelles may contain as many as 100 molecules or
• For example sodium stearate (C17 H35 COONa)is a typical example of such type of

 

• When sodium stearate is dissolved in water, it gives

Na+ and C

H35

COO–

ions.

 

17

C17

H35

COONa  C17

H35

COO– + Na+

 

Sodium stearate                               Stearate ion

The stearate ions associate to form ionic micelles of colloidal size.

 

• It has long hydrocarbon part of

C17 H35 radical. Which is

 

lyophobic and COO– part which is lyophilic.

 

• In the   figure,   the   chain   corresponds  to   stearate  ion,

17     35

(C H COO– ) . When the concentration of the solution is below from its CMC

(10 -3 mol

L-1 ) , it

 

behaves as normal electrolyte. But above this concentration it is aggregated to behave as micelles.

• The main function of a soap is to reduce oily and greasy dirt to colloidal particles (an emulsion). Soap therefore, are known as emulsifying agents.
• Some other examples of micelles are sodium palmitate (C15 H31 COONa) , Sodium lauryl sulphate

3          2 11        3                                                                                                         3          2 15          2 3

[CH (CH ) SO O– Na + ] , Cetyl trimethyl ammonium bromide CH (CH ) (CH ) N + Br – etc.

Note :®  Polydisperse and Monodisperse colloids : In multimolecular colloids, the colloidal particles consist of aggregates of atoms or small molecules with diameters less than 10 -9 m of 1 nm Colloidal solutions in which colloidal particles are of different sizes are called polydisperse colloids. For example, a gold sol may contain particles of various sizes having several atoms of gold. The colloidal solutions in which all the colloidal particles are more or less of identical size are monodisperse colloids.

 

 

 

Lyophilic and lyophobic colloidal solutions (or sols) are generally prepared by different types of methods.

Some of the common methods are as follows.

(1)  Preparation of Lyophilic colloids

• The lyophilic colloids have strong affinity between particles of dispersed phase and dispersion
• These colloidal solutions are readily formed by simply mixing the dispersed phase and dispersion medium under ordinary
• For example, the substance like gelatin, gum, starch, egg, albumin pass readily into water to give colloidal solution.
• They are reversible in nature become these can be precipitated and directly converted into colloidal

• Preparation of Lyophobic colloids : Lyophobic colloids can be prepared by mainly two types of
• Condensation method : In these method, smaller particles of dispersed phase are condensed suitably to be of colloidal This is done by the following methods.
• By oxidation : A colloidal solution of sulphur can be obtained by bubbling oxygen (or any other oxidising

 

agent like

HNO3 , Br2 etc.) through a solution of hydrogen sulphide in water.

2H 2 S + O2 (or any other oxidising agent) ¾¾® 2H 2 O + 2S

 

• By reduction : A number of metals such as silver, gold and platinum, have been obtained in colloidal state by treating the aqueous solution of their salts, with a suitable reducing agent such as formaldehyde, phenyl

 

hydrazine, hydrogen peroxide, stannous chloride etc.

2AuCl3 + 3SnCl2 ¾¾® 3SnCl4 +

2Au

Gold sol

 

2AuCl3 + 3HCHO + 3H 2 O ¾¾® 2Au + 3HCOOH + 6HCl

Gold sol

The gold sol, thus prepared, has a purple colour and is called purple of cassius.

• By hydrolysis : Many salt solutions are rapidly hydrolysed by boiling dilute solutions of their For example, ferric hydroxide and aluminium hydroxide sols are obtained by boiling solutions of the corresponding

 

chlorides.

FeCl3 + 3H 2 O ¾¾® Fe(OH)3 + 3HCl

Colloidal sol

 

Similarly silicic acid sol is obtained by the hydrolysis of sodium silicate.

• By double decomposition : A sol of arsenic sulphide is obtained by passing hydrogen sulphide through a

 

cold solution of arsenious oxide in water.

As 2 O3  + 3H 2 S ¾¾® As 2 S3  + 3H 2 O

 

• By excessive cooling : A colloidal solution of ice in an organic solvent like ether or chloroform can be prepared by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution, separately combine to form particles of colloidal
• By exchange of solvent : Colloidal solution of certain substances such as sulphur, phosphorus, which are soluble in alcohol but insoluble in water can be prepared by pouring their alcoholic solution in excess of For example, alcoholic solution of sulphur on pouring into water gives milky colloidal solution of sulphur.
• By change of physical state : Sols of substances like mercury and sulphur are prepared by passing their vapour’s through a cold water containing a suitable stabilizer such as ammonium salt or

 

 

• Dispersion methods : In these methods, larger particles of a substance (suspensions) are broken into smaller The following methods are employed.
• Mechanical dispersion
  • In this method, the substance is first ground to coarse
  • It is then mixed with the dispersion medium to get a
  • The suspension is then grinded in colloidal
  • It consists of two metallic discs nearly touching each other and rotating

in opposite directions at a very high speed about 7000 revolution per minute.

• The space between the discs of the mill is so adjusted that coarse suspension is subjected to great shearing force giving rise to particles of colloidal
• Colloidal solutions of black ink, paints, varnishes, dyes etc. are obtained by this

• By electrical dispersion or Bredig’s arc method : This method is used to prepare sols of platinum, silver, copper or
• The metal whose sol is to be prepared is made as two electrodes which immerged in dispersion medium such as water
• The dispersion medium is kept cooled by ice.
• An electric arc is struck between the
• The tremendous heat generate by this method and give colloidal
• The colloidal solution prepared is stabilised by adding a small amount of KOH to
• By peptisation
• The process of converting a freshly prepared precipitate into colloidal form by the addition of suitable electrolyte is called peptisation.
• The electrolyte is used for this purpose is called peptizing agent or stabilizing agent.
• Cause of peptisation is the adsorption of the ions of the electrolyte by the particles of the
• Important peptizing agents are sugar, gum, gelatin and
• Freshly prepared  ferric   hydroxide   can   be converted into colloidal state by shaking it with

 

water containing

Fe 3+ or

OH – ions, viz.

FeCl3 or

 

NH ₄ OH respectively.

Fe(OH)3  + FeCl3  ¾¾®[Fe(OH)3 Fe]   + 3Cl

3+                    –

 

Precipitate electrolyte                              Colloidal sol

• A stable sol of stannic oxide is obtained by adding

a small amount of dilute HCl to stannic oxide precipitates.

 

• Similarly, a colloidal solution of

Al(OH)3

and AgCl are obtained by treating the corresponding freshly

 

prepared precipitate with very dilute solution of HCl and

• Gelatin stabilises the colloidal state of ice-cream.
• Lamp black is peptised by gums to form Indian

AgNO3

or KCl respectively.

 

 

 

• If precipitate of

CuS, BaSO4

or Prussian blue are washed continuously with water, after sometime the

 

precipitate are converted into colloidal state which thus pass through the fitter paper and thus can be detected in wash water.

The colloidal solutions prepared by the above methods usually contain impurities especially electrolytes which can destabilize the sols. These impurities must be eliminated to make the colloidal

solutions stable. The following methods are commonly used for the purification of colloidal solutions.

(1)  Dialysis

• The process of separating the particles of colloid from those of crystalloid, by means of diffusion through a suitable membrane is called
• It’s principle is based upon the fact that colloidal particles can not pass

through a parchment or cellophane membrane while the ions of the electrolyte can pass through it.

• The colloidal solution is taken in a bag (parchment paper).
• The bag is suspended in fresh
• The impurities slowly diffused out of the bag leaving behind pure colloidal solution
• The distilled water is changed frequently to avoid accumulation of the crystalloids otherwise they may start diffusing back into the
• Dialysis can be used for removing HCl from the ferric hydroxide

(2)  Electrodialysis

• The ordinary process of dialysis is
• To increase the process of purification, the dialysis is carried out by applying electric This process is called electrodialysis.
• Kidneys in the human body act as dialysers to purify blood which is colloidal in
• The important application of dialysis process in the artificial kidney machine used for the purification of blood of the patients whose kidneys have failed to The artificial kidney machine works on the principle of dialysis.

(3)  Ultra – filtration

• Sol particles directly pass through ordinary filter paper because their pores

are larger (more than 1m or 1000mm ) than the size of sol particles (less than 200mm ).

• If the pores of the ordinary filter paper are made smaller by soaking the filter paper in a solution of gelatin of colloidion and subsequently hardened by soaking in formaldehyde, the treated filter paper may retain colloidal particles and allow the true solution particles to escape. Such filter paper is known as ultra – filter and the process of separating colloids by using ultra – filters is known as ultra – filtration.

(4)  Ultra – centrifugation

• The sol particles are prevented from setting out under the action of gravity by kinetic impacts of the molecules of the
• The setting force can be enhanced by using high speed centrifugal machines having 15,000 or more revolutions per Such machines are known as ultra–centrifuges.

 

 

 

 

The main characteristic properties of colloidal solutions are as follows.

(1)  Physical properties

• Heterogeneous nature : Colloidal sols are heterogeneous in They consists of two phases; the dispersed phase and the dispersion medium.
• Stable nature : The colloidal solutions are quite stable. Their particles are in a state of motion and do not settle down at the bottom of the
• Filterability : Colloidal particles are readily passed through the ordinary filter papers. However they can be retained by special filters known as ultrafilters (parchment paper).

(2)  Colligative properties

• Due to formation of associated molecules, observed values of colligative properties like relative decrease in vapour pressure, elevation in boiling point, depression in freezing point, osmotic pressure are smaller than
• For a given colloidal sol the number of particles will be very small as compared to the true

(3)  Mechanical properties

• Brownian movement

• Robert Brown, a botanist discovered in 1827 that the pollen grains suspended in water do not remain at rest but move about continuously and randomly in all
• Later on, it was observed that the colloidal particles are moving at random in a zig – zag This type of motion is called Brownian movement.
• The molecules of the dispersion medium are constantly colloiding with the particles of the dispersed phase. It was stated by Wiener in 1863 that the impacts of the dispersion medium particles are unequal, thus causing a zig-zag motion of the dispersed phase
• When a molecule of dispersion medium colloids with a colloidal particle, it is then displaced in one Then another molecules strikes it, displacing it to another direction and so on. This process give rise to zig- zag motion.
• This can be confirmed by the fact that the suspensions do not show any such movement due to large molecular
• Brownian movement provides a direct demonstration of the ceaseless motion of molecules as postulated by kinetic
• The Brownian movement explains the force of gravity acting on colloidal particles. This helps in providing stability to colloidal sols by not allowing them to settle
  • Diffusion : The sol particles diffuse from higher concentration to lower concentration However, due to bigger size, they diffuse at a lesser speed.
  • Sedimentation : The colloidal particles settle down under the influence of gravity at a very slow rate. This phenomenon is used for determining the molecular mass of the
• Optical properties : Tyandall effect
  • When light passes through a sol, its path becomes visible because of scattering of light by It is called Tyndall effect.

 

 

This phenomenon was studied for the first time by Tyndall. The illuminated path of the beam is called Tyndall cone.

• In a true solution, there are no particles of sufficiently large diameter to scatter light and hence no Tyndall effect is
• The intensity of the scattered light depends on the difference between the refractive indices of the dispersed phase and the dispersion
• In lyophobic colloids, the difference is appreciable and, therefore, the Tyndall effect is well – But in lyophilic sols, the difference is very small and the Tyndall effect is very weak.
• The Tyndall effect confirms the heterogeneous nature of the colloidal solution.
• The Tyndall effect has also been observed by an instrument called ultra –

Note :® The smoke is colloidal, so when it is viewed at an angle to the source of light, it appears blue due to Tydnall effect.

• Dust in the atmosphere is often colloidal. When the sun is low down on the horizon, light from it has to pass through a great deal of dust to reach your eyes. The blue part of the light is scattered away from your eyes and you observe red part of the Thus red sunsets are Tyndall effect on a large scale.
• Tail of comets is seen as a Tyndall cone due to the scattering of light by the tiny solid particles left by the comet in its
• Due to scattering the sky looks
• The blue colour of water in the sea is due to scattering of blue light by water
• Visibility of projector path and circus
• Visibility of sharp ray of sunlight passing through a slit in dark room.

• Electrical properties : Colloidal particles carry an electric charge and the dispersion medium has an opposite and equal charge, the system as a whole being electrically neutral. The presence of equal and similar charges on colloidal particles is largely responsible in giving stability to the system because the mutual forces of repulsion between similarly charged particles prevent them from coalescing and coagulating when they come closer to one

• Electrophoresis
• The phenomenon of movement of colloidal particles under an applied electric field is called

electrophoresis.

• If the particles accumulate near the negative electrode, the charge on the particles is
• On the other hand, if the sol particles accumulate near the positive electrode, the charge on the particles is
• The apparatus consists of a U-tube with two Pt– electrodes in each

 

• Take a sol of

As2 S3 in the U-tube.

 

• The intensity of the colour of the sol in both the arms is Now pass the current through the sol.
• After some time it is observed that the colour of sol near the positive electrode become This indicates

 

that the

As 2 S3

particles are negatively charged and they move towards oppositely charged electrodes.

 

• Similarly, when an electric current is passed through positively charged they move towards negatively charged electrode and get accumulated

Fe(OH)₃

sol, it is observed that

 

 

• Thus, by observing the direction of movement of the colloidal particles, the sign of the charge carried by the particles can be
• When electrophoresis of a sol is carried out with out stirring, the bottom layer gradually becomes more concentrated while the top layer which contain pure and concentrated colloidal solution may be This is called electro decanation and is used for the purification as well as for concentrating the sol.
• The reverse of electrophoresis is called Sedimentation potential or Dorn effect. The sedimentation potential is setup when a particle is forced to move in a resting This phenomenon was discovered by Dorn and is also called Dorn effect.
• Electrical double layer theory
• The electrical properties of colloids can also be explained by electrical double layer According to this theory a double layer of ions appear at the surface of solid.
• The ion preferentially adsorbed is held in fixed part and imparts charge to colloidal particles.
• The second part consists of a diffuse mobile layer of This second layer consists of both the type of charges. The net charge on the second layer is exactly equal to that on the fixed part.
• The existence of opposite sign on fixed and diffuse parts of double layer leads to appearance of a difference of potential, known as zeta potential or electrokinetic potential. Now when electric field is employed the particles move (electrophoresis)
• Electro-osmosis
• In it the movement of the dispersed particles are prevented from moving by semipermeable
• Electro-osmosis is a phenomenon in which dispersion medium is allowed to move under the influence of an electrical field, whereas colloidal particles are not allowed to
• The existence of electro-osmosis has suggested that when liquid forced through a porous material or a capillary tube, a potential difference is setup between the two sides called as streaming potential. So the reverse of electro-osmosis is called streaming

Note :®Distance traveled by colloidal particles in one second under a potential gradient of one volt per cm is called electrophoretic mobility of the colloidal particles.

• The principle of electrophoresis is employed for the separation of proteins from nucleic acids,

removing sludge from sewage waste etc.

The origin of the charge on the sol particles in most cases is due to the preferential adsorption of either positive or negative ions on their surface. The sol particles acquire electrical charge in any one or more of the following ways.

• Due to the dissociation of the surface molecules : Some colloidal particles develope electrical charge due to the dissociation / ionisation of the surface molecules. The charge on the colloidal particles is balanced by the oppositely charged ions in the For example, an aqueous solution of soap (sodium palmitate) which

 

dissociates into ions as,

C15 H 31 COONa  C15 H 31COO^– + Na ⁺ . The cations (Na⁺) pass into the solution while the

Sodium palmitate

 

anions (C15 H 31COO^– ) have a tendency to form aggregates due to weak attractive forces present in the hydrocarbon chains.

(2)  Due to frictional electrification

 

 

• It is believed that the frictional electrification due to the rubbing of the dispersed phase particles with that of dispersion medium results in some charge on the colloidal
• The dispersion medium must also get some charge, because of the Since it does not carry any charge, the theory does not seem to be correct.

(3)  Due to selective adsorption of ions

• The particles constituting the dispersed phase adsorb only those ions preferentially which are common with their own lattice

 

• For example, when a small quantity of silver nitrate

(AgNO3 ) solution is added to a large quantity of

 

potassium iodide

(KI) solution, the colloidal particles of silver iodide adsorb

I –  from the solution to become

 

negatively charged, (at this stage KI is in excess, and I ^– being common to AgI )

 

But, when a small quantity of potassium iodide

(KI)

solution is added to a large quantity of silver nitrate

 

solution

(AgNO3 ) ; the colloidal silver iodide particles adsorb

Ag +

from the solution to become positively

 

charged, (at this stage

AgNO3 is in excess and

Ag + is common to AgI ),

 

 

 

 

 

• Similarly, the ferric hydroxide colloidal particles develop positive charge due to the adsorption of ions from the

Fe 3+

 

Ferric hydroxide colloidal particles develop negative charge due to adsorption of either OH ^– or Cl ^–

• Depending upon the nature of charge on the particles of the dispersed phase, the colloidal solutions are classified into positively charged and negatively charged Some typical examples are as follows

 

 

 

Note :®

SnO2

forms positively charged colloidal sol in acidic medium and negative charged colloidal

 

in basic medium this is due to

SnO2 is amphoteric reacting with acid and base both. In acidic medium (say

 

HCl)Sn⁴⁺ ion is formed which is preferentially adsorbed on SnO2

giving positively charged colloidal sol.

 

SnO2

• 4 HCl ¾¾® SnCl4

• 2H

2 O ;

SnO2

• SnCl4

¾¾®[SnO ]Sn4 + : 4Cl –

2

3

Positively charged

 

In basic medium,

colloidal sol.

SnO2- is formed which is preferentially adsorbed on

SnO2 giving negatively charged

 

2NaOH + SnO

¾¾® Na  SnO   + H  O ;    SnO   + Na  SnO   ¾¾®[SnO  ]SnO2-  : 2Na +

 

2                      2          3          2                    2             2          3                         2            3

(Negatively charged)

Sols are thermodynamically unstable and the dispersed phase (colloidal particles) tend to separate out on long standing due to the Vander Waal’s attractive forces. However sols tend to exhibit some stability due to

• Stronger repulsive forces between the similarly charged particles : All colloidal particles in any sol possess similar Therefore, due to the electrostatic repulsion these are not able to come closer and form aggregates. Thus stronger repulsive forces between the similarly charged particles in a sol promote its stability.

(2)  Particle-solvent interactions

• Due to strong particle-solvent (dispersion medium) interactions, the colloidal particles get strongly
• Due to solvation, the effective distance between the colloidal particles increases, and therefore, the Vander Waal’s force of attraction As a result, the particles are not able to form aggregates.

• Lyophilic sols are mainly stabilized by solvation effects due to strong interactions between the sol particles and the dispersion

“The phenomenon of the precipitation of a colloidal solution by the addition of the excess of an electrolyte is called coagulation or flocculation.” or

“The stability of the lyophobic sol is due to the presence of charge on colloidal particles. If the charge is removed, the particles will come nearer to each other and thus, aggregate or flocculate and settle down under the force of gravity. This phenomena is known as coagulation or flocculation.”

The coagulation of the lyophobic sols can be carried out by following methods.

• By electrophoresis : In electrophoresis the colloidal particles move towards oppositely charged When these come in contact with the electrode for long these are discharged and precipitated.
• By mixing two oppositely charged sols : When oppositely charged sols are mixed in almost equal proportions, their charges are Both sols may be partially or completely precipitated as the mixing of ferric hydroxide (+ve sol) and arsenious sulphide (–ve sol) bring them in precipitated form. This type of coagulation is called mutual coagulation or meteral coagulation.
• By boiling : When a sol is boiled, the adsorbed layer is disturbed due to increased collisions with the molecules of dispersion medium. This reduces the charge on the particles and ultimately they settle down to form a
• By persistent dialysis : On prolonged dialysis, the traces of the electrolyte present in the sol are removed almost completely and the colloids become
• By addition of electrolytes : The particles of the dispersed phase e., colloids bear some charge. When an electrolyte is added to sol, the colloidal particles take up ions carrying opposite charge from the electrolyte. As a result, their charge gets neutralised and this causes the uncharged, particles to come closer and to get coagulated or

 

 

 

 

precipitated. For example, if

BaCl ₂ solution is added to

 

As 2 S3

sol the

Ba ²⁺ ions are attracted by the negatively

 

charged sol particles and their charge gets neutralised. This lead to coagulation.

• Hardy schulze rule : The coagulation capacity of different electrolytes is It depends upon the valency of the active ion are called flocculating ion, which is the ion carrying charge opposite to the charge on the colloidal particles. “According to Hardy Schulze rule, greater the valency of the active ion or flocculating ion, greater will be its coagulating power” thus, Hardy Schulze law state:

• The ions carrying the charge opposite to that of sol particles are effective in causing coagulation of the
• Coagulating power of an electrolyte is directly proportional to the valency of the active ions (ions causing coagulation).

 

Al 3+ > Mg 2+ > Na +

For example to coagulate negative sol of found to decrease in the order as,

Similarly, to coagulate a positive sol such as

found to decrease in the order :

(7)  Coagulation or flocculation value

As ₂S₃ , the coagulation power of different cations has been

 

Fe(OH)3 , the coagulating power of different anions has been

 

“The minimum concentration of an electrolyte which is required to cause the coagulation or flocculation of a sol is known as flocculation value.”

or

“The number of millimoles of an electrolyte required to bring about the coagulation of one litre of a colloidal solution is called its flocculation value.”

Thus , a more efficient flocculating agent shall have lower flocculating value.

Flocculation values of some electrolytes

 

Sol	Electrolyte	Flocculation value (mM)	Sol	Electrolyte	Flocculation value (mM)
As2S3	NaCl	51.0	Fe(OH)3	KCl	9.5
(-vely charged	KCl	49.5	(+ vely	BaCl 2	9.3
)			charged)
	CaCl2	0.65		K 2 SO4	0.20
	MgCl2	0.72		MgSO4	0.22
	MgSO4	0.81
	AlCl3	0.093
	Al2(SO4 )3	0.096
	Al(NO3 )3	0.095

Note :®             Coagulating value or flocculating value µ           1           .

coagulating power

(8)  Coagulation of lyophilic sols

• There are two factors which are responsible for the stability of lyophilic
• These factors are the charge and solvation of the colloidal
• When these two factors are removed, a lyophilic sol can be
• This is done (i) by adding electrolyte (ii) and by adding suitable
• When solvent such as alcohol and acetone are added to hydrophilic sols the dehydration of dispersed phase Under this condition a small quantity of electrolyte can bring about coagulation.

 

 

Note: ®         Hydrophilic sols show greater stability than hydrophobic sols.

• Lyophilic sols are more stable than lyophobic
• Lyophobic sols can be easily coagulated by the addition of small quantity of an
• When a lyophilic sol is added to any lyophobic sol, it becomes less sensitive towards Thus, lyophilic colloids can prevent the coagulation of any lyophobic sol.

“The phenomenon of preventing the coagulation of a lyophobic sol due to the addition of some lyophilic colloid is called sol protection or protection of colloids.”

• The protecting power of different protective (lyophilic) colloids is The efficiency of any protective colloid is expressed in terms of gold number.

Gold number : Zsigmondy introduced a term called gold number to describe the protective power of different colloids. This is defined as, “weight of the dried protective agent in milligrams, which when added to 10 ml of a standard gold sol (0.0053 to 0.0058%) is just sufficient to prevent a colour change from red to blue on the addition of 1 ml of 10 % sodium chloride solution, is equal to the gold number of that protective colloid.”

Thus, smaller is the gold number, higher is the protective action of the protective agent.

Gold numbers of some hydrophilic substances

 

Hydrophilic substance	Gold number	Hydrophilic substance	Gold number
Gelatin	0.005 – 0.01	Sodium oleate	0.4 – 1.0
Sodium caseinate	0.01	Gum tragacanth	2
Hamoglobin	0.03 – 0.07	Potato starch	25
Gum arabic	0.15 – 0.25

The protective colloids play very significant role in stabilisation of the non–aqueous dispersions, such as paints, printing inks etc.

• Congo rubin number : Ostwald introduced congo rubin number to account for protective nature of It is defined as “the amount of protective colloid in milligrams which prevents colour change in 100 ml of

0.01 % congo rubin dye to which 0.16 g equivalent of KCl is added.”

(6)    Mechanism of sol protection

• The actual mechanism of sol protection is very However it may be due to the adsorption of the protective colloid on the lyophobic sol particles, followed by its solvation. Thus it stabilises the sol via solvation effects.
• Solvation effects contribute much towards the stability of lyophilic For example, gelatin has a sufficiently strong affinity for water. It is only because of the solvation effects that even the addition of electrolytes in small

amounts does not cause any flocculation of hydrophilic sols. However at higher concentration, precipitation occurs. This phenomenon is called salting out.

• The salting out efficiency of an electrolyte depends upon the tendency of its constituents ions to get hydrated e, the tendency to squeeze out water initially fied up with the colloidal particle.
• The cations and the anions can be arranged in the decreasing order of the salting out power, such an arrangement is called lyotropic

 

 

 

 

Cations :

Mg ²⁺ > Ca ²⁺ > Sr ²⁺ > Ba ²⁺ > Li ⁺ > Na ⁺ > K ⁺ > NH 4 +   > Rb ⁺ > Cs ⁺

 

Anions : Citrate ^3–  > SO4 2- > Cl ^–  > NH 3 –   > I ^– > CNS ^–

Ammonium sulphate, due to its very high solubility in water, is oftenly used for precipitating proteins from aqueous solutions.

• The precipitation of lyophilic colloids can also be affected by the addition of organic solvents of non- For example, the addition of acetone or alcohol to aqueous gelatin solution causes precipitation of gelatin. Addition of petroleum ether to a solution of rubber in benzene causes the precipitation of rubber.

“The colloidal systems in which fine droplets of one liquid are dispersed in another liquid are called emulsions the two liquids otherwise being mutually immiscible.”      or

“Emulsion are the colloidal solutions in which both the dispersed phase and the dispersion medium are liquids.”

A good example of an emulsion is milk in which fat globules are dispersed in

 

water. The size of the emulsified globules is generally of the order of Emulsion resemble lyophobic sols in some properties.

10 ^-6 m.

 

• Types of Emulsion : Depending upon the nature of the dispersed phase, the emulsions are classified as;

• Oil-in-water emulsions (O/W) : The emulsion in which oil is present as the dispersed phase and water as the dispersion medium (continuous phase) is called an oil-in-water Milk is an example of the oil-in- water type of emulsion. In milk liquid fat globules are dispersed in water. Other examples are, vanishing cream etc.
• Water-in-oil emulsion (W/O) : The emulsion in which water forms the dispersed phase, and the oil acts as the dispersion medium is called a water-in-oil These emulsion are also

termed oil emulsions. Butter and cold cream are typical examples of this types of emulsions. Other examples are cod liver oil etc.

Note :®             The emulsion can be inter converted by simply changing the ratio of the dispersed phase and dispersion medium. For example, an oil-in- water emulsion can be converted to water in oil emulsion by simply adding excess of oil in the first case.

(2)  Preparation of Emulsions

(i) Emulsions are generally prepared by vigorously agitating a mixture of the relevant oil and water by using either a high speed mixer or by using ultrasonic vibrators.

(ii) The emulsions obtained by simple mechanical stirring are unstable. The two components (oil and water) tend to separate out.

(iii) To obtain a stable emulsion, a suitable stabilizing substance is generally added.

(iv) The stabilizing substance is called emulsifier of emulsifying agent. The emulsifier is added along with the oil and water in the beginning. For Examples : substances which can act as emulsifiers are soaps, detergents, long chain sulphonic acid, lyophilic colloids like gelatin, albumin, casein etc.

• Nature of emulsifier : Different emulsifiers may act differently in the case of a particular emulsion. For example,

• Sodium oleate is used to prepare oil-in-water (O/W)

 

 

• Magnesium and calcium oleates are used to prepare water-in-oil (W/O) When calcium oleate is added to an emulsion stabilized by sodium oleate, the stability of the system decreases. At a certain ratio of

Na ⁺ : Ca ²⁺ , the oil-in-water emulsion becomes unstable. If the Ca ²⁺ ions concentration is increased further very

quickly, then the reversal of the emulsion type occurs, that is the oil-in-water emulsion gets converted into a water- in-oil type.

• Identification of emulsions : Several methods are available to find out whether an emulsion is of the oil-in-water type or of the water-in-oil type emulsion. An emulsion can be identified as

(i) Dilute test : Add water to the emulsion. If the emulsion can be diluted with water this means that water acts as the dispersion medium and it is an example of oil-in-water emulsion. In case it is not diluted, then oil acts as dispersion medium and it is an example of water-in-oil emulsion.

(ii) Dye test : An oil soluble suitable dye is shaken with the emulsion. If colour is noticed on looking at a drop of the emulsion, it is oil-in-water type emulsion. In case the entire background is coloured, it is an example of water- in-oil type.

(iii) Conductivity test : Add small amount of an electrolyte (e.g. KCl) to the emulsion. If this makes the emulsion electrically conducting , then water is the dispersion medium. If water is not the dispersed phase.

(5)  Properties of emulsion

(i) Emulsions show all the characteristic properties of colloidal solution such as Brownian movement, Tyndall effect, electrophoresis etc.

(ii) These are coagulated by the addition of electrolytes containing polyvalent metal ions indicating the negative charge on the globules.

(iii) The size of the dispersed particles in emulsions in larger than those in the sols. It ranges from 1000 Å to 10,000 Å. However, the size is smaller than the particles in suspensioins.

(iv) Emulsions can be converted into two separate liquids by heating, centrifuging, freezing etc. This process is also known as demulsification.

(6)  Applications of emulsions

(i) Concentration of ores in metallurgy

(ii) In medicine (Emulsion water-in-oil type)

(iii) Cleansing action of soaps.

(iv) Milk, which is an important constituent of our diet an emulsion of fat in water.

(v) Digestion of fats in intestine is through emulsification.

(1) “A gel is a colloidal system in which a liquid is dispersed in a solid.”

(2) The lyophilic sols may be coagulated to give a semisolid jelly like mass, which encloses all the liquid present in the sol. The process of gel formation is called gelation and the colloidal system formed called gel.

(3) Some gels are known to liquify on shaking and reset on being allowed to stand. This reversible sol-gel transformation is called thixotropy.

(4) The common examples of gel are gum arabic, gelatin, processed cheese, silicic acid, ferric hydroxide etc.

 

 

(5) Gels may shrink by loosing some liquid help them. This is known as synereises or weeping.

(6) Gels may be classified into two types

(i) Elastic gels : These are the gels which possess the property of elasticity. They readily change their shape on applying force and return to original shape when the applied force is removed. Common examples are gelatin, agar-agar, starch etc.

(ii) Non-elastic gels : These are the gels which are rigid and do not have the property of elasticity. For example, silica gel.

 

(1) Purification of water by alum (coagulation)  : Alum which yield coagulate the negatively charged clay particles.

Al ³⁺ ions, is added to water to

 

(2) In rubber and tanning industry (coagulation and mutual coagulation) : Several industrial processes such as rubber plating, chrome tanning, dyeing, lubrication etc are of colloidal nature

(i) In rubber platting, the negatively charged particles of rubber (latex) are made to deposit on the wires or handle of various tools by means of electrophoresis. The article on which rubber is to be deposited is made anode.

(ii) In tanning the positively charged colloidal particles of hides and leather are coagulated by impregnating, them in negatively charged tanning materials (present in the barks of trees). Among the tanning agent chromium salts are most commonly used for the coagulation of the hide material and the process is called chrome tanning.

(3) Artificial rains : It is possible to cause artificial rain by throwing the electrified sand or silver iodide from an aeroplane and thus coagulating the mist hanging in air.

(4) Smoke precipitation (Coagulation) : Smoke is a negative sol consisting of carbon particles dispersed in air. Thus, these particles are removed by passing through a chamber provided with highly positively charged metallic knob.

(5) Formation of deltas (coagulation) : River water consists of negatively charged clay particles of colloidal

 

dimension. When the river falls into the sea, the clay particles are coagulated by the positive etc. present in sea water and new lands called deltas are formed.

Na ⁺ , K ⁺ , Mg ²⁺

ions

 

(6) Blood consists of negatively charged colloidal particles (albuminoid substance). The colloidal nature of blood explains why bleeding stops by applying a ferric chloride solution to the wound. Actually ferric chloride solution causes coagulation of blood to form a clot which stops further bleeding.

(7) Colloidal medicine : Argyrol and protargyrol are colloidal solution of silver and are used as eye lotions colloidal sulphur is used as disinfectant colloidal gold, calcium and iron are used as tonics.

(8) Photographic plates : These are thin glass plates coated with gelatin containing a fine suspension of silver bromide. The particles of silver bromide are colloidal in nature.

Note : ®  Isoelectric point of the colloid : The hydrogen ion concentration at which the colloidal particles are neither positively charged nor negatively charged (neutral) is known as isoelectric point of the colloids. At this point, the lyophilic colloids are expected to have minimum stability because at this point particles have no charge or equal quantum of positively and negatively charge. For example,

 

 

isoelectric point of gelatin is 4.7 (at pH 4.7 gelatin has no electrophoretic motion; at pH<4.7, gelatin moves towards anode)

• Colloidal solution of graphite is called Aqua
• Ultrasonic dispersion : Various substances such as mercury, oils, sulphur, sulphides and oxide of metals can be dispersed into colloidal state very easily with the help of ultrasonic
• Stem technology: Colloidal particles in a sol are very small and most of them are not visible through an ultramicroscope or light Recently, new techniques have been developed to determine the size and shape of the colloidal particles. These are

(i) Scanning Electron Microscope (SEM), (ii) Transmission Electron Microscope (TEM)

A modified form of the above methods has also been developed. It is called Scanning Transmission Electron Microscope (STEM). All these techniques are superior to the light microscope because they have greater resolving power.

• Bencroft rule : The phase in which the emulsifier is more soluble becomes outer phase of the

 

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