Monday, November 25, 2013

Purification Of Colloids: ( Sol )

( a ) Dialysis.
( b ) Electro-dialysis.
( c ) Ultra-filtration.
     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 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.

         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.
( i ) Ferric hydroxide yields a sol by adding ferric chloride.
                Freshly prepared Fe(OH)3 is shaken with aqueous solution of  FeCl( 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.

Tuesday, November 19, 2013

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).


             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.

Monday, November 18, 2013

Catalytic Poison

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


       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.

Friday, November 15, 2013

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.
Ea  ==> Activation energy.
       ( 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.
         ( i ) Oxidation of sodium sulphite by air is retarded by alcohol.

                                          2Na2SO3    +    O2      →        2Na2SO4 .
Auto catalyst:
       In auto catalyst, the reaction is catalyzed by one of its products. This phenomenon is known as auto catalysis.
        ( 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.

Thursday, November 14, 2013

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


      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 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.

General Characteristics Of Catalytic Reactions

  • The catalyst remains unchanged in amount and chemical composition at the end of the reaction.
  • A small quantity of the catalyst is required.
  • The catalyst does not initiate the reaction. It increase the rate of chemical reaction only.
  • The catalyst does not change the equilibrium constant.
  • The catalyst is generally specific in its action.

Factors Affecting The Adsorption

 The adsorption depends upon the following character.

  • Nature of the gas.
  • Effect of temperature.
  • Effect of pressure.
  • Nature of the adsorbent.
Nature of the Gas:
       It is depends on the nature of gas . Easily liquefiable gases are adsorbed readily. The reason is inter-molecular forces which are involved in adsorption. This is valid for physical adsorption only.
       Since chemical adsorption is specific in nature, it occurs only if the gas can form a chemical bond with the solid.

Effect of temperature:
         Adsorption is a exothermic process. Therefore in accordance with Le chartelier's principle, the magnitude of adsorption increases with decrease in temperature. It is in the case of physical adsorption.
         Since chemical adsorption first increases with rise in the temperature and then stats decreases.

Effect of Pressure:
         Physical adsorption is reversible nature. Since adsorption of a gas leads to decrease of pressure, the magnitude of adsorption increases with increase in pressure.

Nature of the adsorbent:
          Adsorption is a surface phenomenon.Increase in the surface area of the adsorbent, increases the total amount of gas adsorbed.

Tuesday, November 12, 2013

Complete And Incomplete Combustion Of Alkanes

Complete Combustion:
    It gives carbon dioxide and water.
    CH4      +     2O2     →     CO2     +     2H2O

Incomplete Combustion:
    It gives carbon or carbon monoxide.
     CH4     +     H2O     →     CO     +     3H2

Changes In States Of Matter

Inter Conversion Of Solid, Liquid and Gas

Solid to Liquid: ( melting )
       A substance changes from the solid phase to liquid phase, that process is called melting. It is otherwise known as fusion.
Example: Ice cube into water.

Liquids to Solid: ( freezing )
      A substance changes from the liquid phase to solid phase when its temperature is lowered below its freezing point. This process is called freezing. It is otherwise known as solidification.
Example: Water change into ice.

Liquid to Gas: ( Boiling )
       A substance changes from the liquid phase to gas phase . This process is called boiling. The temperature at which it boils is called Boiling point.
Example: Water boiling.

Gas to Liquid: ( condensation )
        Change in the state of matter from the gas phase to liquid phase. The transition happens from the gaseous phase to solid phase directly the change is called deposition.

Solid to Gas: ( sublimation )
         Changes in the state of matter from the solid phase to gas phase, without passing through an intermediate liquid phase.This process is called sublimation. This endothermic phase transition occurs at temperature and pressure below the triple point.

States Of Matter

    Matters generally exists in the four forms of solid, liquid, gas and plasma.

  • Solids are tightly packed, usually in a regular pattern.
  • Solids has a stable, definite shape and volume.
  • Solids can only change their shape by force, as when broken or cut.
  • Example: Bar of steel and dry ice ( solid carbondioxide )
  • Liquids are close together with no regular arrangement.
  • Liquids has a definite volume, but takes the shape of its container.
  • Example: Water and Oil.
  • Gas are well separated with no regular arrangement.
  • Compressible fluid.
  • Gas has no definite volume and shape.
  • Example: Hydrogen and Oxygen.
  • Plasma does not have definite shape or volume.
  • Plasma may be formed by heating and ionizing gas.
  • Examples: Lightning, Electric sparks, and Fluorescent lights.
  • At very high temperature of stars, atoms lose their electrons. The mixture of electrons and nuclei that results is the plasma state of matter.

Deffine Element

      An element is a substance consisting of atoms which all have the same number of protons. A chemical element is a substance that cannot be broken down by chemical means.
Eg: Copper, Cesium, Iron, Neon.

Monday, November 11, 2013

Aliphatic Compounds

      Aliphatic compounds are open-chain or acyclic compounds. [ Greek - aliphos ( fat, oil ) ]                      

Acyclic meaning:        
          Acyclic compound with a linear structure, rather than a cyclic one. An open-chain compound having no side chains.
          Aliphatic compounds are otherwise known as non-aromatic compounds. Aliphatic compounds can be saturated joined by single bonds (alkanes) or unsaturated with double bonds (alkenes) or triple bonds (alkynes).
              In organic chemistry alkane or paraffin is a saturated hydrocarbon. Alkanes contain only two elements carbon and hydrogen. In alkane all of the carbon-carbon bonds are single bonds. Each carbon atom forms four bonds and each hydrogen atoms form a single bond. Alkane general chemical formula   CnH2n+2 .
Methane :
     Paraffins are the saturated hydrocarbons. Main source of paraffins is mineral oil or petroleum. The simplest paraffin is methane CH4. Methane contain one atom of carbon and four atoms of hydrogen. It is a alkane mainly occurs in natural gas. Methane is a product of organic decay in swamps and marshes, the gas obtained by the action of bacteria, this method of formation of natural gas given rise to the name "marsh-gas" for methane.
Structure of methane:

        Methane is a tetrahedral molecule with 4 equivalent C - H bonds. The molecular formula of methane is CH4.   SP3  hybridization.
Properties of methane:
  • Water insoluble.
  • Methane is a colourless, odourless and non-poisonous gas.
  • Methane is quite soluble in ethanol and ether.
  • Melting point - 90.7 K
  • Boiling point - 109 - 113 K.
  • Liquid methane does not burn unless subjected to high pressure.
  • Gas at room temperature.
  • It burns with a non-luminous flame in air or oxygen, forming carbon dioxide and water.

                            CH4     +     2O2     →     CO2     +     2H2O
Substitution reaction of methane:
                 In a substitution reaction, a functional group in a particular chemical compounds is replaced by another group.
                A good example of a substitution reaction is the photochemical chlorination of methane forming methyl chloride.
                            CH4     +     Cl2     →     CH3Cl     +     HCl
                                                              (methyl chloride)
                A mixture of  methane and chlorine is exposed to sunlight, that time substitution reaction occurs. But the reaction doesn't stop here, all four hydrogen atoms in methane are successively replaced by chlorine atoms.
                           CH4     +     Cl2     →     CH3Cl     +     HCl
                                                           (methyl chloride)
                           CH3Cl     +     Cl2     →     CH2Cl2     +     HCl
                                                            (Methylene chloride)
                           CH2Cl2     +     Cl2     →     CHCl3     +     HCl
                           CHCl3     +     Cl2     →     CCl4     +     HCl
                                                             (carbon tetrachloride)
             Methane react with chlorine, the hydrogen atoms are replaced, and chlorine atoms take their place. This type of reaction is known as substitution reaction. The atom or group that has replaced the hydrogen atom is called the substitution and when a substituent atom or group is replaced by some other atom or group, the reaction is referred to as a replacement reaction. It should be noted that in substitution or replacement reaction there is no change in structure.
             Methane reacts with bromine in the presence of UV light to form bromomethane.
                         CH4     +     Br2     →     CH3Br     +     HBr
                                                         (bromo methane)
             Methane reacts explosively with gaseous fluorine. The initial reaction is possible.
                         CH4     +     2F2     →     C     +     4HF
Step 1 :
Initiation Step :
UV light breaking a bromine molecules into free radicals.
                            Cl - Cl      →     Cl●     +     ●Cl
                            [Cl2       →     2Cl●]
Step II
Propagating Steps :
                          CH4     +     Cl●     →     CH3●     +     H-Cl 
                          CH3●     +     Cl2     →     CH3Cl     +     Cl● 
Step III
Terminating Steps:
                         Cl●     +     Cl●             →     Cl2
                         Cl●     +      CH3●        →     CH3Cl
                        CH3●     +     CH3●      →     CH3CH3