Types Of Colloidal Dispersion

Solute	Solvent	Dispersion	Name	Example
Solid	Solid	Solid In Solid	Solid Sol	Pearl, Gemstone
Solid	Liquid	Solid In Liquid	Sol	Ink
Solid	Gas	Solid In Gas	Solid Aerosol	Dust, Smoke
Liquid	Solid	Liquid In Solid	Gel	Cheese, Butter
Liquid	Liquid	Liquid In Liquid	Emulsion	Milk
Liquid	Gas	Liquid In Gas	Liquid Aerosol	Cloud, Fog
Gas	Solid	Gas In Solid	Solid Foam	Cork
Gas	Liquid	Gas In Liquid	Foam	Soda water, Soap

Combined Gas Law

         It is a combinations of three laws. Boyle's Law, Charles' Law, Gay-Lussac's Law. The ratio of the product of pressure and volume and the temperature of a system remains constant.
Written by mathematically,
PV/T  = k
P  --> pressure
V --> volume
T --> temperature
k --> constant

Charles' Law

               At constant pressure the volume of a gas is directly proportional to the temperature.The law was stated by the scientist Jacques Charles.
Written by mathematically,
V/T = k
V  --> volume of the gas
T  --> temperature of the gas (kelvin)
k  --> constant

Boyle's Law

                    At constant temperature the volume of ideal gas is inversely proportional to its pressure. The law was named after scientist Robert Boyle.
Written by mathematically,
    PV = k
P  --> pressure of the gas
V  --> volume of the gas
k   --> constant 

Avogadro's Law

                Equal volume of same gases, at the same temperature and pressure, contains the equal number of molecules. It is otherwise known as Avogadro's principle. This law was stated by the Italian chemist Ameden Avogadro.
Written by mathematically,
        V/n =  k  
V --> volume of the gas
n  --> amount of substance of the gas (moles)
k  --> proportionality constant

Properties Of Colloids

• Kinetic property
• Optical property
• Electrical property

Kinetic Property: ( Brownian movement )

          In 1827 botanist Robert Brown observe the movement of plant spores floating in water.The molecules of the dispersion medium constantly colloid with the colloidal particles thereby passing kinetic energy to them. The continuous rapid zig-zag motion of the colloidal particle in the dispersion medium is called Brownian movement. This is due to the bombardment of colloidal particles by molecules of the dispersion medium. the particle motion depends on the size of the particle and viscosity of the dispersion medium. This phenomenon was latterly found in the case of colloidal solution, when observed ultra-microscopically. It is otherwise known as Brownian motion or Pedesis.

Optical property: ( Tyndall effect )

           A British physicist John Tyndall in 1869 first observed this phenomenon and hence it is known as Tyndall effect.

           A beam of light is passed through a true solution, the light beam is not visible in this case. But in the case of colloidal solution light beam is visible. This is due to the colloidal particles absorb light energy and then emit it in all directions. It illuminates the path of the beam. This phenomenon of illuminated beam formed by the particles is called Tyndall effect. The amount of scattering depends on the frequency of the light and density of particles. It is otherwise known as Tyndall scattering.

Example:

• Illumination of light beam from projector in smoke filled theater's.
• Visible beam of headlights in fog. 

Electrical Properties:

Electrophoresis:

        The movement of the colloidal particles under the influence of an applied electric potential is called as Electrophoresis. If the sol particles are negatively charged, they migrate towards the positive electrodes. this is called Cataphoresis. If the sol particles are positively charged, they migrate towards the negative electrode. This is called Anaphoresis.

Process:

• Placing the colloidal solution in 'U' tube.
• Two platinum electrodes are dipped into deionized water.
• Electric current is passed, the charged colloidal particles move towards the positive electrode side.

Electro-osmosis:

       The dispersion medium itself begins to move under the influence of an electric current. This phenomenon is known as Electro-osmosis. 

Process:

• Placing the colloidal solution in 'U' tube.
• Two platinum electrodes are dipped into deionized water.
• Electric current is applied, the movement of the medium towards the negative electrode.

Purification Of Colloids: ( Sol )

( a ) Dialysis.
( b ) Electro-dialysis.
( c ) Ultra-filtration.
Dialysis:
     Dialysis is the removal of soluble impurities from sols by the use of semipermeable membrane. The ordinary process of dialysis is slow. The apparatus used for this purpose is called Dialyser.
      Cellophane bag is filled with impure colloidal solution and placed in fresh water ( distilled water ). The cellophane bag is not allow the colloidal particles to pass through, but particles of true solution can pass through it. The molecules and ions diffuse through the membrane into the outer water and pure colloidal solution is left behind. The water is renewed frequently to avoid accumulation of the crystalloid as otherwise they may start diffusing back into the bag.

Electro-dialysis:
       Electro dialysis is the removal of soluble impurities from sols by the use of semipermeable membrane in the presence of electric field.
       Cellophane bag is filled with impure colloidal solution and placed in fresh water ( distilled water ). The cellophane bag is not allow the colloidal particles to pass through, but particles of true solution can pass through it. Now the electric field is applied. The soluble particles passed through the membrane and migrate towards the oppositely charged electrodes. Leaving behind the pure colloidal solution. The most important application of dialysis process in purification of the artificial kidney mechaine, used for the purification of blood.

Ultra filtration:
       In this method, colloidal solutions are purified by carrying out filtration through special type of filters called ultra-filters. Sols pass through ordinary filter paper, So that use special filter paper. The filters are permeable to all substances except the colloidal particles. This process is very slow. Pressure are suction is applied to speed it up.

Preparation Of Colloids

Preparation of lyophilic sols:
         Lyophilic sols prepared by simply warming the solid with liquid dispersion medium.
Example: Starch with water.

Preparation of lyophobic sols:
         Lyophobic sols have to be prepared by special method. These methods fall into two categories.

• Dispersion methods.
• Condensation methods.

Dispersion Method:
        Large particles of substances are broken, into smaller particles in the presence of dispersion medium . They are stabilized  by adding some suitable stabilizer. There are 4 ways using dispersion method.

1. Mechanical Dispersion:
        In this method making a colloid by milling a given solid and the powder produced is dispersed into a given dispersing medium.

• The colloidal mill consists of two steel plates nearly touching to each other and rotating in opposite directions with high speed.
• The solid particles are turn off to colloidal size and dispersed in liquid to give sol.
• The colloidal graphite and printing inks are made by this method.

2. Electro-dispersion Method: ( Bredig's arc method )

           In this method, two metallic wires functioning as electrodes are immersed into water. Both the end of the wires gives a strong electric current to be evaporated and then it is dispersed into water to form a metallic sol.

• This method is suitable for the preparation of colloidal solution of metals like gold, silver, platinum etc.
• An electric arc is struck between the electrodes of metal immersed in the dispersion medium [ such as water and alkali ].
• The intense heat of the arc vaporizes some of the metal, which then condenses to form particles of colloidal size.
• The water is cooled by immersing the container in a cold bath.

3.Peptisation:

         In this method converting a precipitate into a colloidal sol by shaking it with the dispersion medium, in the presence of small amount of electrolyte. The electrolyte used is called as peptizing agent.

Example:

( i ) Ferric hydroxide yields a sol by adding ferric chloride.

                Freshly prepared Fe(OH)₃ is shaken with aqueous solution of  FeCl₃ ( peptising agent ) it adsorbs Fe₃⁺  ions and breaks up into small sized particles of type Fe(OH)₃ / Fe₃⁺ .

( ii ) Silver chloride can be converted into a sol by adding hydrochloric acid.

4. Ultra-sonic Dispersion.

         The sound waves of high frequency are usually called ultra-sonic waves. Ultrasonic waves are passed through the solution containing large particles are breaks down to form colloidal solution.

Condensation Method:

           In this method molecules of the dispersed phase appearing first and aggregate to form colloidal particles.

1. Change Of Physical State:

          Colloidal solution of  certain elements such as mercury and sulfur are obtained by passing their vapor through cold water containing a suitable stabilizer such as ammonium salt or citrate.

2. Chemical Methods:

( i ) Double Decomposition:

               A sloe steam of hydrogen sulfide gas is passing through cold solution of aresenious oxide to form an arsenic sulfide sol. This is continued till the yellow color of the sol attains maximum intensity.

                                  As₂O₃     +     3H₂S     →     As₂S₃     +     3H₂O

                                                                         ( Yellow sol )

                                                                                              (Arsenic sulphide)

( ii ) Hydrolysis:

               This method is used to prepare hydroxides and oxides of weakly electropositive metals like Fe, Al, Cr, Sn etc. A colloidal solution of ferric hydroxide is obtained by boiling a dilute solution of ferric chloride.

                                    FeCl₃     +     3H₂O     →     Fe(OH)₃     +     3HCl

                                                                                (Red sol)

                                                                          ( Ferric hydroxide )

( iii ) Oxidation:

                Passing hydrogen sulfide into a solution of sulfur dioxide to form a colloidal solution of sulfur

                                    2H₂S     +     SO₂      →     2H₂O     +     3S

( iv ) Reduction:

                 silver sol can be obtained by treating dilute solution of silver nitrate with organic reducing agents like tannic acid.

                            AgNO₃      +      Tannic acid     →     Ag sol.

[ Reducing agents- stannous chloride, tannic acid, formaldehyde, hydrazine. ] Use suitable reducing agents.

Similar to Gold.

                            AuCl₃     +     Tannic acid      →     Au sol.

3. Exchange Of Solvent:

               A solution of sulfur in alcohol is poured into water, a colloidal solution of sulfur is obtained due to low solubility in water. similar to phosphorus also.

Reason: Sulfur and phosphorous are soluble in alcohol but insoluble in water. So that first prepare the alcoholic solution and then pouring into the solution in excess of water.

Lyophobic and Lyophilic Colloids

Lyophobic Colloids:

• Colloidal solution in which the dispersed phase has very little affinity for the dispersion medium are termed as lyophobic colloids. 
• Irreversible in nature and once precipitation can not easily pass into colloidal state because particles are solvent hating.
• They are unstable and the particles are aggregates of thousands of molecules.
• Low concentration of dispersed phase are possible. Example. Sulphur in water.

Lyophilic Colloids:

• Colloidal solution in which are dispersed phase has considerable affinity for the dispersion medium are called lyophilic colloids.
• Reversible in nature and once precipitated can readily pass into colloidal state because particles are solvent loving.
• They are stable and self stabilized.
• Particles are true molecule and bigger size.
• High concentration of dispersed phase are possible. Example: Gelatin, Starch and protein.

Classification Of Colloids

• Hydrophilic colloids.
• Hydrophobic colloids.

Hydrophilic colloids:

         Hydrophilic colloids are water loving colloids. The colloid particles are attracted towards water. This type of colloids are also called reversible sols.

Hydrophobic colloids:

          Hydrophobic colloids are opposite in nature to hydrophilic colloids. The colloid particles repelled by water. The colloid particles are further away from the water. This type of colloids are called irreversible sols. 

Types Of Colloids

      A colloidal system is made up of two phases.

• Dispersed phase.
• Dispersion medium.

Dispersed phase:

          The substances distributed as the colloidal particles is called the dispersed phase. ( the substance found in the lesser extent).

Dispersion Medium:

           The colloidal particles are dispersed in that phase is called dispersion medium. ( the substance in a colloid found in the greater extent).

Colloid

             A colloid is a substance microscopically dispersed throughout another substance. This field of study was introduced in 1861 by Thomas Graham. Certain substances such as sugar, salt, and acids diffused readily through a parchment membrane, other substances such as gelatin, albumen and glue diffused at very slow rate. The substances belonging to the second category were given the name colloids.
            A type of homogeneous mixture in which the dispersed particles do not settle out.
Example: Butter, milk, fog, smoke, paint.

Catalytic Poison

             A substance which destroy the activity of catalyst this process is called Catalytic poisoning.

Promoter

       The activity of the catalyst increased by the addition of small quantity of a second material. That substance not act as a catalyst, promotes (increase) the activity of a catalyst so that it is  called Promoter.
        ( i ) Synthesis of ammonia in Haber's process, small amount of molybdenum increase the activity of finely divides iron. ( catalyst ).

Catalytic Reactions And Catalyst

( i ) Process:   Haber's process for the manufacture of ammonia.
      Catalyst:   Finely divided Iron. Molybdenum as promoter.

( ii ) Process:   Ostwald's process for the manufactures of nitric acid.
       Catalyst:    Platinished asbestos.

( iii ) Process:  Lead chamber process for the manufactures of sulphuric acid.
        Catalyst:  Nitric oxide.

( iv ) Process:   Contact process for the manufacture of sulphuric acid.
        Catalyst:   Vanadium pentoxide.

( v ) Process:    Decon's process for the manufactures of chloride.
       Catalyst:   Cupric chloride.

( vi ) Process:   Bosch's Process for the manufactures of hydrogen.
        Catalyst:   Ferric oxide. Chromic oxide as a promoter.

( vii ) Process:   Hydrogenation of vegetable oil, manufactures of vanaspathi.
        Catalyst:   Finely divided nickel.

( viii ) Process:   Bergius process for the synthesis of petrol from coal.
         Catalyst:    Ferric oxide.

Theories Of Catalysis

• Intermediate compound formation theory.
• Adsorption theory.

Intermediate compound formation theory:

          This theory was proposed by Clement and Desormes in 1806. In general , the intermediate compound formation theory applies to homogeneous catalyst reactions. According to this theory, the catalyst form an intermediate with one of the reactants. But the intermediate compound is unstable. So that the intermediate compound combines with other reactant to form the desired product and the catalyst is regenerated. 

Example: ( Reaction Type )

Example :

                                       2NO     +     O₂     →     2NO₂

                                    (catalyst)                      (Intermediate compound)

                                      NO₂     +SO₂     →     SO₃     +     NO

                                                                      (product)       (catalyst)

Adsorption Theory:

             In general adsorption theory applies to heterogeneous catalytic reactions. The catalyst functions by the adsorption of the reacting molecules on its surface.

The adsorption reaction undergoes four types of steps.

( i) Adsorption of reactant molecule:

                 The reactant molecules A and B strike the surface of the catalyst. The reaction molecules held up by the partial chemical bond.

( ii ) Formation of intermediate complex:

                 The reactant molecule adjacent one another join to form an intermediate complex ( A-B ). The intermediate complex is unstable.

( iii ) Decomposition of intermediate complex:

                The intermediate complex breaks to form the products C and D. The product molecules hold to the catalyst surface by partial chemical bond.

( iv ) Release of product:

                The product particles are released from the surface.

Types Of Catalysts

Catalysts are divided into different types.

• Positive catalyst
• Negative catalyst.
• Auto catalyst.
• Induced catalyst.

Positive catalyst:

          A catalyst which enhances the speed of reaction is called positive catalyst. This phenomenon is known as positive catalysis.

R   ==> Reactant.

P   ==> Product.

E_a  ==> Activation energy.

Example:

       ( i ) Decomposition of hydrogen peroxide in presence of colloidal platinum.

      ( ii ) Decomposition of  KClO₃  in presence of manganese dioxide.

Negative catalyst:

            There are certain substances which, when added to the reaction mixture, retard the reaction rate instead of increase it. This is called negative catalysts, otherwise known as inhibitors. This phenomenon is known as negative catalysis.

Example:

         ( i ) Oxidation of sodium sulphite by air is retarded by alcohol.

                                                                            C₂H₅OH

                                          2Na₂SO₃    +    O2      →        2Na₂SO_{4 .}

Auto catalyst:

       In auto catalyst, the reaction is catalyzed by one of its products. This phenomenon is known as auto catalysis.

Example:

        ( i ) Oxidation of oxalic acid by potassium permanganate, one of the products MnSO₄  act as a auto catalyst. It increase the rate of the reaction.

Induced catalyst:

        In induces catalyst one reactant influences the rate of other reaction, which does not occur under ordinary condition. This phenomenon is known as induced catalysis.

                          Na₃AsO₃      +       Na₂SO₃      →      Na₃AsO₄      +     Na₂SO₄

                                                                                (sodium arsenate)

          Air is passed through a mixture of solution of sodium arsenite and sodium sulphite, both of them undergo simultaneous oxidation. Thus sulphite has induced the arsenite. Because sodium sulphite solution readily oxidises in air, but sodium arsenite solution does not oxidise by air. So  Na₂SO₃ act as a induced catalyst.

Types Of Catalytic Reactions

• Homogeneous catalysis.
• Heterogeneous catalysis.

Homogeneous catalysis:

       In homogeneous catalysis the catalyst in the same phase as the reactants. Typically everything will be present as a gas or contained in a single liquid phase.

        ( i ) Homogeneous catalysis involves the influence of  H⁺ on the esterification of carboxylic acids, such as the formation of methyl acetate from acetic acid and methanol.

         ( ii ) Oxidation of  SO₂  to SO₃ with oxygen in the presence of nitric oxide as the catalyst in the lead chamber process.

Heterogeneous catalysis:
      The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Typical examples involve a solid catalyst with the reactants as either liquids or gases.
       ( i ) The Haber process finely divided iron serves as a catalyst for the synthesis of ammonia from nitrogen and hydrogen. The reactants are in the gaseous phase while the catalyst is in solid phase.

                                                                    Fe

                                       N₂     +     3H₂     →     2NH₃

        ( ii ) Another example of heterogeneous catalyst is applied in the oxidation of sulfur dioxide on vanadium (V) oxide for the production of sulfuric acid.

                                                                   V₂O₅

                                       2SO₂     +     O2     →     2SO₃
Hydrogenation of ethene on a solid surface.

Explanation.

     

      Ethene  molecule is adsorbed by the surface of the platinum.

       

        Hydrogen molecule adsorbed and broken into atoms. 

        Hydrogen atom form a bond with one of the carbon atom of ethene.

        Another hydrogen atom form a bond with another carbon atom of ethene.

         Now the product molecule is free.

Differences Between Physical Adsorption And Chemical Adsorption

Catalysis

        Catalysis is the increase rate of a chemical reaction without itself undergoing any chemical change. This phenomenon is known as catalysis. The participation of a substance is called catalyst.
Example: