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Revision Notes on Surface Chemistry:

Adsorption

Reversible and irreversible adsorption

The adsorption is reversible, if the adsorbent can be easily removed from the surface of the adsorbent by physical methods. It is called irreversible adsorption, if the adsorbate can not be removed from the surface of the adsorbent.

A gas adsorbed on a solid surface can be completely removed in vacuum. It is, therefore, reversible adsorption. Examples of irreversible adsorption are adsorption of oxygen on tungsten adsorbate and adsorption of CO on tungsten surface

Adsorbent, Adsorbate and Interface

  • The substances upon whose surface the change of concentration occurs, is called absorbent.

  • The substance taken up on the surface is call adsorbate.

  • The common surface between the two phases where the adsorbed molecules concentrate is called the interface.

Physisorption and Chemosorption:

Physisorption

Chemisorption

Only van der Waals force are present between adsorbate and surface of adsorbent

 

Chemical bonds are formed between adsorbate and surface of adsorbent

Low enthalpy of adsorption  ie, in the order of 20 kjmol-1.

 

High enthalpy of adsorption  i.e, order of 200 kjmol-1.

Reversible

Irreversible

It is usually takes place at  low temperature  and does not require any activation energy.

It takes place at  high temperature and require activation energy..

Multi molecular layer of adsorbate are formed on the surface

Only  monomolecular layers are formed.

Not specific.

Highly specific.

Langmular Isotherm:

Langmular EquationIf A, B & AB represents the adsorbed, absorbent and the absorbed – adsorbent complex then,

A + B ↔AB

ka = Equilibrium constant for adsorption = [AB]/[A][B]

kd = Equilibrium constant for desorption = [A][B]/[AB]

K = Distribution coefficient = ka/kb

Θ = Fraction of the surface of adsorbent available for adsorption.

P = pressure

So,

Θ= KP/(1+KP)     (Langmular Equation)

Freundlich Isotherm:

Freundlich Isothermx= Mass of the gas adsorbed

m = Mass of absorbent

p = Pressure

K and n = constants 

x/m =k.p(1/n)                 [n >1]

or

log x/m = log k + 1/n log P

Factors Affecting Adsorption:

  • Temperature: An increase of temperature leads to a decrease in amount adsorbed and vice – versa.

  • Pressure or concentration: It has been found that in most cases,  the adsorption is reversible and an increased pressure of a gases vapour or an increase in concentration of a solute causes increased adsorption.

  • Nature of Adsorbate and Adsorbent: The amount of the gas adsorbed depends upon the nature of adsorbent and the gas (adsorbate), which is to be adsorbed. It has been found that easily liquifiable gases such as NH3, HCl, Cl2 , SO2 CO2  etc. are more readily adsorbed than so the called permanent gases such as O2,N2, H2 etc. This is because that molecules of the former type of gases have greater Vander waal’s or molecular force of attraction.

Colloids

Dispersed Phase:

The phase which is dispersed or scattered through the dispersion  medium is called Dispersed phase or discontinuous phase.

Dispersion Medium:

The phase in which the scattering is done is called the dispersion medium or continuous medium.

Dispersed Phase

Dispersion Medium

Name

Typical example

Solid

Liquid

Sol

Gold sol, Mud, Fe(OH)3 sol,

Solid

Solid

Solid sol

Gems, Ruby glass, Minerals 

Solid

Gas

Aero sols

Smoke (Carbon in air) Volcanic dust

Liquid

Solid

Gel

Curd, Cheese, Jellies

Liquid

Liquid

Emulsion

Milk, water in benzene, cream

Liquid

Gas

Liquid aero sol

Clouds, fog (water in air) mist

Gas

Solid

Solid foam

Lava, Pumica

Gas

Liquid

Foam

Froth on beer , whipped cream

Lyophobic and Lyophilic Colloids:

Those substance whose colloidal solution cannot be prepared by bringing them in  contact with a solvent are called Lyophobic  (disliking, fearing or hating a liquid). On the other hand those substances whose colloidal solutions can be prepared by bringing them in contact with a liquid solvent are  called lyophilic colloids (loving a solvent).

Emulsions:

  • Emulsion of oil in water: Those emulsions in which the dispersed phase is oil and water is the dispersion medium. These emulsion are generally represented as O in W emulsions. Examples are milk, vanishing cream etc.

  • Emulsions of water in oil: Those emulsion in which the dispersed phase is water while oil is the dispersion medium. These emulsion are generally represented as W in O emulsions. Examples are butter, ice cream etc.

Difference between True Solutions, Suspension & Colloids

True solution 

Suspension 

Colloid

Homogenous

Heterogeneous

Heterogeneous

Particle size less than 1nm

Particle size more than 1000nm

Particle size between 1-1000nm

Don’t settle down

Settle down under the influence of gravity

Don’t settle down

Complements cannot be separated out by filtration

Can be filtered

Can be filtered using special filter papers

Don’t show tyndrall effect

Show tyndrall effect

Show tyndrall effect

Methods of preparation of colloids

Chemical Methods: 

Bredig's method:

Bredig's methodAn electric arc is struck between two metallic electrodes immersed in dispersion medium. The arc produced vapourises the metal which on further condensation produces particles of colloidal size.

  • Peptization: 

Process of converting a precipitate into colloidal sol by shaking it with electrolyte

in dispersion medium.

Hardy Schulze Rule:

  • Ion carrying charge opposite to the colloidal particle has capacity to coagulate the colloid.

  • Greater the valency of ion, greater will be  the coagulating power.

  • Gold Number:  The minimum amount of lyophilic colloid in milligrams which can prevent the coagulation of 10 ml gold sol against 1 ml of 10% NaCl solution. 

Surfactants

substances which gets preferentially adsorbed at the air – water and solid – water interfaces forming an oriented monolayer where the hydrophilic groups point towards the aqueous phase and the hydrocarbon chain point towards the air or towards the oil phase.

  • Anionic surfactants : NSodium salts of higher fatty acids such as sodium palmitate (C15H31COONa), sodium stereate (C17H35COONa) and sodium Oleate (C17H33COONa).

  • Catiuonic Surfactants: Those which dissociates in water to yield positively charged ions  examples: C18H37 , C16H33(CH3)3  etc.

  • Non ionogenic: Those whose molecules cannot undergo dissociation when an alcohol having a higher molecular weight reacts with several molecules of ethylene oxide, a non – ionogenic surfactant is produced.

Surfactants

Micelle: 

Aggregates formed when the surfactant molecules in the water air interface become so packed in the monolayer that no more molecules can be accumulated with ease they accumulate in the bulk of the solution.

At a given temperature and concentration, a micelle of a surfactant of monodispersed i.e., they contain same number of molecules usually between 25 to 100.

Critical concentration for micelle formation decreases as the molecular weight of hydrocarbon chain of surfactant grows because in this case true solubility diminishes and the tendency of surfactant molecules to associate increases.

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