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CBSE Class 12th Chemistry Notes: Surface Chemistry (Part – II)

Sep 30, 2016 13:00 IST

    In this article, you will get revision notes on CBSE Class 12 Chemistry: Chapter 5 - Surface Chemistry.

    The purpose of these notes is to give you a quick glance of the chapter. This article is a continuation of the revision notes on Class 12 Chemistry: Chapter 5- Surface Chemistry, Part-I. In Part-I you discovered about the concepts of Surface Chemistry, Adsorption, Absorption, Catalysis etc.

    Now in this Part - 2 of the revision notes on CBSE Class 12 Chemistry: Chapter 5 - Surface Chemistry you will study the following topics:

    o    Colloids

    o    Classification of colloids

         •    Physical state of dispersed phase and dispersion medium

         •    Nature of interaction between dispersed phase and dispersion medium

         •    Particles of dispersed phase

    o    Preparation of colloids

    o    Purification of colloids

    o    Properties of colloids

    o    Emulsions

    o    Applications of colloids

    The notes of the chapter are as follow:

    Colloid:

    Depending upon the size of the solute particles the solutions can be categorised as true solution, suspension and colloids. The type of solution in which the size of the solute particle is in the range of 1 to 100 nm, is called a colloid. A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase or colloidal particles) in another substance which is called dispersion medium.

    Classification of Colloids:

    On the basis of the physical state of the dispersed phase and the dispersion medium the colloidal solutions can be classified into eight types:

    Dispersed phase

    Dispersion medium

    Type of colloid

    Examples

    Solid

    Solid

    Solid sol

    Gem stones, Coloured glasses

    Solid

    Gas

    Aerosol

    Dust, Smoke

    Solid

    Liquid

    Sol

    Paint

    Liquid

    Liquid

    Emulsion

    Milk

    Liquid

    Solid

    Gel

    Butter, Cheese

    Liquid

    Gas

    Aerosol

    Fog, Cloud

    Gas

    Liquid

    Foam

    Froth

    Gas

    Solid

    Solid sol

    Rubber, Foam

    In short, if the dispersion medium is solid then the colloid is known as gel. If the dispersion medium is liquid or gas then the colloid is called sol. A colloid in which both the dispersed phase and dispersion medium are in the liquid form is known as emulsion.

    On the basis of nature of interaction between dispersed phase and dispersion medium, colloids can be divided into two forms:

    (a) Lyophilic Colloids: As the name lyophilic (liquid-loving or solvent attracting) indicates, lyophilic colloids are the colloids exhibiting a strong interaction between the two phases.
    The substances like gum, gelatin, starch, when mixed with a suitable liquid as the dispersion medium, directly form the colloidal sol which is also named as lyophilic sol.

    (b) Lyophobic Colloids: Lyophobic' means 'liquid hating', which indicates that in these sols there is little or no interaction between the two phases. Substances like metals and their sulphides, when simply mixed with the dispersion medium do not form colloidal sol. They cannot be prepared by simply mixing the two phases. Such substances are called Lyophobic Sols.

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    Difference between Lyophilic sol and Lyophobic sol:

    Lyophilic Sol

    Lyophobic Sol 

    These are reversible sols.

     These are irreversible sols.

    They are quite stable and are not easily coagulated by electrolytes.

    They are less stable and get coagulated by electrolytes, by heating or by agitating.

    They are obtained by simple solution method, e.g. starch solution.

    They are prepared by indirect methods which are not so easy.

    They are obtained from organic material such as starch, gum, gelatin etc.

    They are obtained from inorganic materials such as metals, sulphides, metal oxides etc.

    The particles are hydrated.

    The particles are not hydrated.

    Preparation of Lyophilic and Lyophobic sols:

    Lyophilic sols are prepared simply by the stirring dispersed phase with dispersion medium. Examples include sol of starch, gelatin, egg albumin.

    Methods of preparation of Lyophobic sols can be prepared by two types of methods:

    Condensation

    Dispersion

    Condensation methods are in turn of four different types

    Hydrolysis

    Reduction

    Oxidation

    Double decomposition method

    Dispersion method involves breaking down of large particles of a substance into particles of colloidal size. There are three such methods:

    Mechanical dispersion

    Bredig's arc method (to prepare metal sol)

    Peptisation method (to convert precipitate into particles of colloidal size using suitable peptising agent). The peptising agent used is usually an electrolyte.

    On the basis of type of particles of the dispersed phase, colloids can be classified into following three types:

    Multimolecular colloids: They contains a large number of molecules in the form of aggregates, e.g., sulphur sol.

    Macromolecular colloids: These colloids consist of one single large size molecule as a dispersed phase, e.g., polythene.

    Associated colloids: These are the colloids which behave as normal electrolytes at low concentration but as a colloid at higher concentration. This is due to the formation of aggregates which are called micelles, e.g. soaps and detergents.

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    Purification of colloids:

    Colloidal contains a number of electrolytic impurities. The following method are used to purify colloids:

    Dialysis (by using semipermeable membrane)

     

    Ultra-filtration (by using ultra fine quality filter papers)

     

    Ultra-centrifugation

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    Properties of colloidal solution:

    The colloidal solution shows the following properties:

    Colligative properties: The properties of a solution which depends on the number of moles of solute particles present in the solution are called colligative properties like osmotic pressure, elevation in boiling point etc.

    Tyndall effect: The scattering of light by colloidal particles is known as Tyndall effect. True solutions do not show Tyndall effect.

    Brownian movement: The zigzag motion of the colloidal particles is termed as Brownian movement. This is due to the impact of the molecules of the dispersion medium on the molecules of the dispersed phase.

      Electrophoresis: The movement of colloidal particles towards their respective electrodes in the presence of electric field is known as electrophoresis. This is also known as cataphoresis. This helps in determining the charge present on the colloid.

    Coagulation and Floculation:

    The process of forming aggregates from colloidal particles by the addition of suitable electrolyte is called coagulation. The addition of an electrolyte to a lyophobic colloid results in its coagulation. At lower concentration of electrolyte, the aggregation of particles is called flocculation. Flocculation is reversible while coagulation is irreversible.

    Hardy-Schulze' s rules:

    The precipitation or coagulating power of an electrolyte is determined by using Hardy-Schulze' s rules:

    The effective ions of the electrolyte in bringing about coagulation are those which carry charge opposite to that of the colloidal particles. These ions are called coagulating ions.

    Coagulating power increases as the charge on the ion increases. Addition of AgNO3 to excess of KI results in the formation of yellow precipitate of AgI. AgI then adsorbs I from the excess of KI preferentially. This adsorption of  I break down the precipitate of AgI into colloid particles acquiring a negative charge AgI/I. This layer of negatively charged particles is balanced by the counter K+ ion. As a result the electric potential is developed. This is called Zeta potential or double layer potential or electro kinetic potential.

    CBSE Class 12th Chemistry Notes: Solutions

    Emulsion:

    Emulsions are colloids in which both the dispersed phase and the dispersion medium are in the liquid states.

    Types of Emulsion:

    Oil in water: It is the emulsion in which dispersed phase is oil and dispersion medium is water. For example: Milk, vanishing cream.

    Water in oil: It is the emulsion in which dispersed phase is water and dispersion medium is oil.  For example: Cold cream. butter, cod liver oil.

    Emulsification:

    The process of making emulsion is called emulsification.

    Emulsifier or Emulsifying agent:

    The emulsions are generally prepared by shaking strongly the mixture of two colloids these emulsions are generally unstable, e.g., oil and water are immiscible and form unstable emulsions. Thus a substance is added to stabilize the emulsions which named as emulsifiers or emulsifying agents.
    For example: Protein casein is present in milk as an emulsifying agent.

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    Demulsification:

    The process of converting the emulsion back into two distinct components, oil and water is called demulsification. This can be carried out by :

    Boiling

     

    Freezing

     

    Changing pH

     

    Electrostatic precipitation

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    Application of colloids:

    Sewage disposal: Colloidal particles of the dirt, mud etc. carry electric charge, hence when sewage water is passed through the plates kept at a high potential, the colloidal particles are coagulated due to electrophoresis and the suspended matter gets removed.

    Cleansimg action of soap: Soap solution is colloidal in nature. It removes the dirt particles either by adsorption or by emulsifying the greasy matter sticking to the cloth.

    Rubber plating: Rubber plated articles are prepared by depositing negatively charged rubber particles over the article to be rubber plated by making that article an anode in a rubber plating bath.

    Medicines: Number of medicines are emulsions. Actually, medicines in colloidal form are easily adsorbed by the body tissues and hence are more effective.

    Artificial rain: Artificial rain can be caused by spraying oppositely charged colloidal dust or sand particles over a cloud. The colloidal water particles present in the cloud will be neutralized and coagulate to from bigger water drops causing artificial rain.

    Disinfectant: Certain disinfectants like Dettol and Lysol are formed of oil-in-water type emulsions.

     Froth floatation process: In the metallurgical processes, the concentration of ore by forth floatation process is based upon the treatment of the powdered ore with oil emulsion. The valuable particles of the ore form foam which comes to the surface and is skimmed off.

    CBSE Class 12th Chemistry Notes: Surface Chemistry (Part – I)

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