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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Adsorption and Colloids Page 1

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Adsorption

 

The phenomenon of accumulation of one substance on the surface of another is called as adsorption.
Adsorbate is a substance adsorbed on the solid or liquid surface.
Adsorbent - adsorbs another substance on surface.

The basic difference between adsorption and absorption is as follows:

Adsorption

Absorption

1.

Surface property

Bulk property.

2.

Depends on temperature, pressure and surface area of adsorbent.

Independent of temperature, pressure and surface area.

3.

Evolution of heat occurs i.e., exothermic process.

Not accompanied by evolution of heat.

4.

Reversible process.

Irreversible process.


Adsorption can be a) Physical b) Chemical

 

 

Forces Between Adsorbate And Adsorbent

 

  1. Van der Waal's Forces: Such forces generally exist between all particles. Adsorption due to van der Waal's can be reversed by (i) reducing pressure (ii) raising temperature of solid. Such adsorption is called physical adsorption.
  2. Covalent bonds: Adsorption due to covalent bonds is called chemical adsorption.

 

Physical Adsorption

 

  1. It is due to weak van der Waal's force of attraction.
  2. It occurs at low temperatures.
  3. It is a reversible process.
  4. During physical adsorption low heat of adsorption is evolved (20-40 kJ mol-1).
  5. Formation of multimolecular layers occurs.

 

Chemical Adsorption

 

  1. It occurs due to covalent bond formation.
  2. It is a reversible process.
  3. It occurs at high temperatures.
  4. It is associated with high heat of adsorption (80-400 kJ mol-1). Energy of activation
    is high while rate of adsorption is low.

 

Differences between Physical and Chemical Adsorptions

 

 

Physical Adsorption

Chemical Adsorption

1.

Weak van der Waal's forces.

Chemical bonds hold adsorbate and adsorbent.

2.

Reversible process and a very rapid process and equilibrium is reached quickly.

Irreversible and takes place at higher temperatures.

3.

Extent of adsorption depends on surface area of adsorbent.

Extent of adsorption depends on adsorbate and adsorbent.

4.

Multilayer adsorption.

Monomolecular layer of adsorption.

5.

Heat evolved in this case is relatively very less.

Very large amount of heat is evolved in this case.

 

Factors affecting Adsorption

 

  1. Temperature: Lower temperature favours adsorption, and vice-versa.
  2. Pressure: Rate of adsorption is directly proportional to applied pressure on adsorbate, but only at low pressures. Adsorption becomes independent of pressure at high pressures.
  3. Nature of adsorbate: Gaseous substances are better adsorbents on solid surface.
  4. Nature of adsorbent: Rough and porous substances are good adsorbents.
  5. Concentration: Adsorption is directly proportional to concentration of adsorbate.
  6. Area of adsorbent: More surface of adsorbent, greater adsorption.

 

Freundlich Adsorption Isotherm

 

The equation showing the relationship between the amount of substance adsorbed by unit mass of adsorbent and the equilibrium pressure at constant temperature is called 'Freundlich adsorption isotherm'.

It is expressed as

 

= KPn


where x  = mass of adsorbate
         m  = mass of adsorbent
           P = pressure in Nm-2

K and n are constants depending on nature of adsorbate, adsorbent and temperature.

Taking logarithm of expression

 

log10  

  = log10 K + n log10 P

 

A graph of log10 against log10 p is a straight line, slope equals n and intercept

 

 

on y - axis equals log10 K.
             
n is 0 < n <1.

 

Applications Of Adsorption:

 

  1. Water Purification: Hardness causing ions like Ca+2 , Mg+2 , Cl- , SO4-2 etc. are passed over ion exchange resins and removed due to adsorption. Dissolved coloured substances are removed from water by passing over a bed of charcoal.
  2. Catalysis: Metals like Ni, Pt, Pd, Cu used for chemical reactions adsorb gases like H2, CO, O2 and help enhancing rate of reactions. Reacting gases concentrate on catalyst surface and enhance rate of reaction.
  3. Adsorption Indicators: They are used in titrations where precipitates are formed. Dyestuff such as eosin, alizarin and fluorescein are used. Adsorption of certain ions of the indicator on the precipitate produces a characteristic colour, which indicates end point.
  4. Chromatography: Separation of mixture of components is based on principle of differential adsorption of components. Mixtures when dissolved in a suitable solvent and allowed to flow through porous substances are separated at different levels according to their solubility.

 

Colloids

 


Differences between true solutiuon and colloidial solution

True solution

Colloidal solution

1.

Homogeneous.

Heterogeneous.

2.

Solute particles are not easily separated.

Solute particles are easily separated.

3.

Invisible even through powerful microscope.

Visible through ultra microscope.

4.

Do not scatter light.

Colloidal solution scatter light

5.

Size less than 10-9m.

Large size between 10-7 - 10-9 m.

 

Preparation of Colloidal Solutions:

 

  1. Mechanical Dispersion Method: The method for preparation includes dispersion method, where finely ground substance is introduced in a colloidal mill consisting of 2 metal discs rotating at a very high speed of about 7,000 rpm.
  2. Electrical Method or Bredig's Arch Method: In this method, high voltage is applied to electrodes of substance, whose colloidal solution is to be prepared. Due to electric current metal evaporates from the tip of the electrode and condenses back to form colloidal solution.
  3. Condensation Method: It includes oxidation and reduction methods.
    In oxidation, substance is converted to colloidal form using oxidising agent, for example, Solution of sulphur can be obtained.
    Example:
     
    In reduction, sols of silver, gold, Pt, etc. can be obtained.
    Example:

2 AuCl3 + 3HCHO + 3H2

  2 Au + 6 HCl + 3HCOOH

  1.                                                           colloidal form

 

Classification of Colloids

 

 

Sr.
no.

Dispersion
medium

Dispersed
phase

Common
name

Examples

1.

Gas

Liquid

Aerosol

Cloud, mist and fog

2.

Gas

Solid

Aerosol

Smoke

3.

Liquid

Gas

Foam

Foam or lather

4.

Liquid

Liquid

Emulsion

Milk, cream

5.

Liquid

Solid

Sol

Gold sol, Fe2O3 sol

6.

Solid

Gas

Solid emulsion

Pumic stone

7.

Solid

Liquid

Gel

Silica gel, jellies

8.

Solid

Solid

Solid sol

Gems, glasses

 

Lyophilic and Lyophobic Solutions

 

Lyophilic Sols: Dispersed phase passes into colloidal solution when in contact with dispersion medium. Example: gelatin, gum, etc.
Lyophobic Sols: Substances that do not give colloidal solution when brought in contact with dispersion medium. Example: metals, metal hydroxides, etc.
When water is the dispersion medium - these sols are called hydrophobic and hydrophilic sols.

Hydrophobic Solutions

Hydrophilic Solutions

1.

Surface tension and viscosity same as water.

Surface tension is lower than water while viscosity is higher.

2.

Show Tyndall effect.

Show weak Tyndall effect.

3.

Irreversible sols

Reversible sols

 

Properties of Colloidal Solution

 

General properties:

  • Colloidal solution can pass through ordinary filter paper.
  • Colloidal particles are formed by accumulation of large number of particles.
  • Colloidal particles have high molecular weight.
  • Colloidal solutions have low osmotic pressure.
  • Colour of sol depends on wavelength of scattered light.
  • Optical Properties:
    Tyndall Effect:
    Scattering is observed in all directions through which light passes. Colloidal solution has a characteristic colour.
  • Brownian Movement:
  • Zig-zag motion along straight line paths.
  • Random movement.
  • Helps to determine Avogadro's number.
  • Electrical Properties:
    Electrophoresis:
    The process of migration of colloidal particles to any one electrode under the influence of applied electric field is called electrophoresis.
  • Electro Osmosis: The migration of dispersion medium towards the electrode through a semipermeable membrane, under the influence of electric potential when dispersed phase is immobilised is called electro-osmosis.

 

Coagulation of Colloids

 


Coagulation is brought about by the following methods:

  1. Heating or Freezing
    • Heating removes the electrolyte adsorbed on sol particle and helps coagulation.
    • Freezing leads to the removal of dispersion medium, encourages sol particle coming together.
  2. Mutual Coagulation
    • Electrolyte of opposite charge when added, results in sol to be partially or completely precipitated.
  3. Addition of an Electrolyte
    • A certain minimum amount of electrolyte is necessary to bring about coagulation. The minimum amount depends on the nature of sol and valency of added ion.
    • The Hardy-Schulze rule states that: "The ion of an electrolyte responsible for coagulation has opposite charge to that of colloidal particle."
    • Coagulation power depends on valency of ions
      Sn+4 > Al+3 > Mg+2 > Na+

 

Gels

 


Gels are colloidal dispersion of solid in liquid. They are elastic or non-elastic.

  • Elastic gels can be easily regenerated by adding liquid component to solid.       
  • Inelastic gels cannot be regenerated by adding liquid to solid.



Properties of Gels:

  • Imbibition: Elastic gels generally take up large quantities of water and
    go on swelling.
  • Weeping or Synersis of Gels: Elastic as well as non-elastic gels loose water on standing.
  • Thixotropy: The phenomenon of gel turning into sol, on shaking and again getting converted into gel on standing is called thixotropy.

Uses of Gels:

  1. Silica gel is used for drying, it is used for adsorption and to support platinum catalyst.
  2. Common use articles like soap, boot polishes, various food stuff are all gels.

 

Emulsions

 


A colloidal dispersion of one liquid which is immiscible with another is called an emulsion. They are

  • Oil in water here oil is dispersed in water phase.
  • Water in Oil where water is dispersed in oil phase.
    Emulsions are stabilised by adding surface active agents or soaps.

 

Properties of Emulsions

 

  1. Emulsions exhibit properties similar to lyophobic sols.
  2. Exhibit Brownian movement and Tyndall effect.
  3. Emulsions can be coagulated by addition of substances that will destroy the
    1. emulsifier
    2. charge
  4. Boiling, freezing and centrifuging may also cause demulsification process.

 

Uses of Emulsions

 

  1. Emulsions or emulsified substances provide greater surface area of action. Hence, a lot of medicines, for example, cod liver oil, etc, are in the form of emulsions.
  2. Disinfectants like phenyl, lysol, etc. form the oil/water emulsion on mixing with water.
  3. Detergents form emulsions to provide cleaning action.
  4. Cosmetics, for example, face creams are oil/water type of emulsions while hair creams form water/oil type of emulsions.

 

Applications of Colloids

 

  1. Food: Most of the food used daily are in colloidal form. Example: Fruit jellies, butter, cream, etc.
  2. Medicines: Many drug formulations assimilated in colloidal form, show fast results. Example: Colloidal gold, manganese used as tonics.
  3. Sewage Disposal: Sewage particles containing colloidal impurities are collected and passed over electric current. Colloidal particles precipitate and pure water is collected.
  4. Precipitation of Smoke: A device called Cottrell precipitator is used to remove carbon particles in the smoke from factories. Other applications of colloids include tanning of leather, rubber plating, separation of proteins, etc.

 

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