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Surface Chemistry

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Published in: Chemistry
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Revision Notes on Surface Chemistry is described in this sample note.

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  1. 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, called absorbent. The substance taken up on the surface is call adsorbate. is The common surface between the two phases where the adsorbed molecules concentrate is called the interface. Physisorption and Chemosorption: Physisorption van der Waals force are present between adsorbate and of adsorbent enthalpy of adsorption ie, in the order of 20 kjmol-l . ersible usually takes place at low temperature and does not require activation energy. i molecular layer of adsorbate are formed on the surface Chemisorption Chemical bonds are formed between adsorbate and suri of adsorbent High enthalpy of adsorption i.e, order of 200 kjmol-l . Irreversible It takes place at high temperature and require activatic energy.. Only monomolecular layers are formed. Highly specific. specific. Langmular Isotherm: If A, B & AB represents the adsorbed, absorbed — adsorbent complex then, ka = Equilibrium constant for adsorption = kd = Equilibrium constant for desorptlon = absorbent and the 10 12
  2. K = Distribution coefficient = kJkb e = Fraction of the surface of adsorbent available for adsorption. P = pressure so, KP/(I+KP) (Langmular Equation) Freundlich Isotherm: Mass of the gas adsorbed m = Mass of absorbent p = Pressure K and n = constants x/m or log x/m= log k + l/n log P Factors Affecting Adsorption: Temperature: An increase of temperature leads to a decrease in amount adsorbed and vice — versa. O 195 244 K 273K Slope. Log K (Intercept) Log p isotherm 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, HCI, C12, S02 C02 etc. are more readily adsorbed than so the called permanent gases such as 02,N2, 1-12 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 Dispersed phase or discontinuous phase. Dispersion Medium: medium is called The phase in which the scattering is done is called the dispersion medium or continuous medium. )ersed Phase Dispersion Medium Liquid Name sol Typical example Gold sol, Mud, sol,
  3. id id id solution logenous Solid Gas Solid Liquid Gas Solid Liquid Solid sol Aero sols Gel Emulsion Liquid aero sol Solid foam Foam Gems, Ruby glass, Minerals Smoke (Carbon in air) Volcanic dust Curd, Cheese, Jellies Milk, water in benzene, cream Clouds, fog (water in air) mist Lava, Pumica 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 Colloids between True Solutions , Suspension Heterogeneous Particle size more than 1000nm Settle down under the influence of gravity icle size less than Inm 't settle down Iplements cannot be separated out ltration 't show tyndrall effect Me thods of Chemical Methods: Can be filtered Show tyndrall effect preparation of Suspension & Colloid Heterogeneous Particle size between 1-1000nm Don't settle down Can be filtered using special filte papers Show tyndrall effect colloids
  4. Double decompostion As203 + 3H2S As2S3(s01) + 3H20 Oxidation 3s(s01) + 21120 S02 + 2H2S Reduction 2 Auc13 + 3 1-ICHO + 3H20 2Au(s01) + 3HCOOH + 6HC1 Viydrolvsis IFeC13 + 3H20 --------------z-------> (sol) + 3HCl Bredig's method: An 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: Medium 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 , 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. + c n H 2 1 (OCH2CH2)m OH O Hydrocabon part o Hydroxyethylene chain
  5. 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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