Surface Chemistry Notes

SURFACE CHEMISTRY SURFACE CHEMISTRY Adsorption: The existence of a substance at a surface in different concentration than in the adjoining bulk is called adsorption, The substance so Adsorbed is called Adsorbate and the substance upon which adsorption takes place is called adsorbent Desorption: The process of removing an adsorbed substance from a surface on which it is absorbed. Sorption: When adsorption and absorption take place simultaneously Physiosorption Chemisorption Specific Irreversible In creases with increase in surface area Chemical bond formation 6. 7. 8. Non specific Reversible Increases with increase in surface area Weak vander waals forces between absorbate and adsorbent. Low heat of adsorption (20 - 40 KJ mol ) Occurs at low temperature Decreases with increase of T Increases with increase of P 9. Forms multimolecular layer 1. 2. 3. 4. 5. High –1 At moderate temperature It first increases, then decreases with T It increases with increase in P at low P but is independent of P at high pressure Forms unimolecular layer. Adsorption isotherm: The graph plotted between x/m and p at constant T. 195 K 244 K x m 273 K O P Frendulich adsorption isotherm x  kp1/n m log Adsorption isotherm b n 1 h x 1  logk  logp m n When log x m 1 x =0, = Constant, the adsorption is independent of pressure. n m log k (intercept) O Log P 1 x x When =1, = kp, i.e.,,  p, the adsorption varies directly with pressure. n m m Adsorption from solution phase When litmus solution is shaken with charcoal becomes colorless.  x   Extent of adsorption  m x  KC1/ n m  log 1  S.A.  [Solute] and also on nature of adsorbate and adsorbent T x 1 = log K + logC m n SURFACE CHEMISTRY 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Applications of adsorption To create vacuum. Gas masks to adsorb poisonous gases Controlling humidity (Silica and Al gels) Removing colouring matter from solutions. Heterogeneous catalysis (eg. Fe catalyst to make NH3) Seperation of inert gases. Drugs used to kill germs by getting adsorbed on it Froth floatation process. Adsorption indicators eg. Ag halides adsorbs eosin and fluorosin and give characteristic colour at end point. Chromatographic analysis. Catalysis : The process in which catalyst is used to increase the rate of reaction. 2KClO3 MnO2 473 – 633 K 2KCl + 3O2 *Promoters are substances that inhance the activity of a catalyst while poisons decrease the activity of a catalyst. Fe(s) Mo(s) N2(g) + 3H2(g) 2NH3(g) Homogenous catalysis : When the reactant and the catalyst are in the same phase. 2SO2(g) + O2(g) NO(g) 2SO3(g) H2SO4(l) C12H22O11(aq) + H2O(l) C6H12O6(g) + C6H12O6 Heterogeneous catalysis : The catalytic process in which the reactants and the catalyst are in different phases. O2 + 2SO2(g) Pt(s) 4NH3(g) + 5O2(g) Pt(s) 2SO3(g) 4NO(g) + 6H2O(g) + A + B Reacting molecules B Catalyst surface having free valencies Adsorption of reacting molecules + A– B Product fromed Catalyst A Adsorption of reacting molecules A Desoption of reacting molecules B Intermediate Adsorption of reacting molecules, formation of intermediate and desorption of products Shape selective catalysis : The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape selective catalysis. Zeolites are good shape–selective catalysts because of their honey comb structure . They are microporous. Aluminosilicates. A zeolite catalyst ZSM – 5 converts alcohol to gasolene. SURFACE CHEMISTRY Enzymes (biocatalysts): Nitrogenous organic compounds produced by living cells that catalyse biochemical reactions in living organisms. Examples of enzyme catalysed reactions. 1. Inversion of cane sugar. Invertase C12H22O11(aq) + H2O(l) Sucrose C6H12O6(aq) + C6H12O6(aq) Glucose Fructose Zymase 2. C6H12O6(aq) Glucose 3. 2(C6H10O5)n + nH2O Starch 4. C12H22O11(aq)+ H2O(l) Maltose 5. NH2CONH2 + H2O Urea 6. Milk 7. Protein 2 C2H5OH(aq) + 2CO2(g) Diastase Maltase Urease Lacto bacilli enzyme nC12H22O11(aq) Maltose 2C6H12O6(aq) Glucose 2NH3 + CO2 curd Pepsin Amino acid. Characteristics of enzyme-catalysis 1. Highly efficient -1 molecule of enzyme may transform one million molecules of reactant per minute. 2. Specific for given reaction. 3. Highly active under optimum temperature (298 -310K) and optimum pH (b/w 5–7) 4. Increasing activity in presence of activators and co-enzymes (non protein part) eg. Amylase in presence of NaCl i.e., Na+ are catalytically very active. 5. Influence of inhibitors and poisoned. [E – S] E+S E–P Active Site P + E S E S E + P Enzyme (catalyst) Substrate (reactants) Enzyme-Substrate complex Enzyme Products Mechanism of enzyme catalysed reaction Step I E + S  ES Step II ES + + E+P SURFACE CHEMISTRY Industrial catalytic processes H2SO4 H2S2O7(l) + H2O(l) m 2 H2SO4(aq) Colloids : colloidal solutions are intermediate between true solution and suspension * Heterogenous mixture * Size of colloidal particles varies from 1 nm to 1000 nm Dispersed phase (DP) : The component present in a small proportion (like solute in solution) Dispersion medium (DM): The component present in large proportion. Types of colloids DP Foam G Solid sol G or S Aerosol S or L Emulsion L Gel L Sol S DM L S G L S L example Soap lather Pumice stone, Gems fog, cloud, Smoke milk Cheese Paints Lyophilic Colloids : The colloidal solution in which the particles of the dispersed phase have great affinity for the dispersion medium * Liquid loving in nature. * Reversible sols. (reconstituted by remixing) * Cannot be easily coagulated. Lyophobic colloids : The colloidal solutions in which the particles of the dispersed phase have no affinity for the dispersion medium are called lyophobic colloids. * liquid hating in nature * Irreversible sols * Need stabilising agent for their preservation * prepared by special methods. Multimolecular colloids : On dissolution , a large number of atoms or smaller molecules aggregate together to form species having size in the colloidal range. The colloid so formed is called multimolecular colloid eg gold, sulphur sol. Macromolecular colloids : If the size of particles of substance (dispersed phase) is in colloidal range. eg starch cellulose, protein, enzymes, polythene, nylon, rubber. SURFACE CHEMISTRY Associated colloids : Some substances (D.P) at low concentration behave as electrolytes, but at higher concentration behave as colloids. (Due to formation of aggregates – micelles) CMC -(Critical micelleconcentration) ( Tk ) Kraft temperature – micelle formation above particular temperature. Soap – Na or K salt of higher fatty acids eg sodium stearate C17H35COO–Na+ O CH2 CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 C CH2 _ + O Na _ + Sodium stearate (C17H35COO Na) O CH2 CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 O CO _ OOC Water _ OO C (a) CO _ O (b) Low concentration of Soap (a) _ COO _ COO _ OC O Ionic micelle _ CO O _ _ _ OO C stearate ion Higher concentration of Soap (b) (c) (a) Grease on cloth (b) Stearate ions arranging around the grease droplet and (c) Grease droplet surrounded by stearate ions (micelle formed) Methods of preparation of colloids : (1) Chemical methods 2H2S + SO2 As2O3 + 3H2S Oxidation 3S(sol) + 2H2O double decomposition 2AuCl3 + 3HCHO + 3H2O FeCl3 + 3H2O Hydrolysis As2S3(sol) + 3H2O reduction 2Au(Sol) + 3HCOOH + 6HCl Fe(OH)3(Sol) + 3HCl CH2 _ O Hydrophilic head COO Hydrophobic tail Stearate ion C SURFACE CHEMISTRY 2. Bredig's Arc method (Electrical disintegration): Electric spark is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size. 3. + Electrodes _ Dispersion medium Peptization: The process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte (peptizing agent) Ice - bath Bredig's Arc method Purification of colloidal solution Dialysis: The process of removing a dissolved substance from a colloidal solution by means of diffusion. through a suitable membrane [animal membrane (bladder), parchment Paper or cellophane sheet.) Dialysing membrane Dialysing membrane Water + Crystalloid Sol particle Anode _ + Water + Electrolyte Cathode Water Crystalloid Dialysis Sol particle Crystalloid Electro-dialysis Ultrafiltration : Separating the colloidal particles by specially designed filters (prepared by soaking the filter paper in a collodion solution). To speed up the process pressure or suction is applied Collodion - 4% solution of nitrocellulose in mixtures of alcohol and ether. Properties of colloidal solution 1. Colour - depends on  of light scattered eg (1) Mixture of milk and water appears blue when viewed by reflected light and red when viewed by transmitted light (2) Finest gold sol is red in colour: as the size increases, it appears purple, their blue and finallty golden. 2. Colloidal particles are always charged: either + or – ) Positive charged sols Negative charged sols 1 Hydrated metal oxides eg. Al2O3. xH2O, CrO3. xH2O Fe2O3 . xH2O etc. Metals eg Cu, Ag, Au Sols 2. Basic dye stuffs eg. methylene blue Sol. *Acid dye stuffs eg eosin,congo red Sols. 3. Haemoglobin (blood) Metallic sulphides eg As2S3,Sb2S3,CdS Sols 4. Oxides eg. TiO2 Sol. *Sols of starch, gum, clay, Charcoal, Helmholtz electrical double layer : When the colloidal particles acquire (–) or (+) charge by selective adsorption of one of the ions it attracts counter ions from the medium forming a second layer as shown below AgI/ I– K+ AgI/Ag+ I– The combination of these 2 layers of opposite charge around colloidal particles is called helm holtz electrical double layer SURFACE CHEMISTRY Electrokinetic potential (Zeta potential): The potential difference between the fixed layer and the diffused layer of colloidal solution having opposite charges is called zeta potential. 3. Colligative properties : (RLVP, Osmotic pressure, depression in freezing pt, elevation in boiling point) are small or lower as compared to true solution 4. Heterogeneous, stable solution 5. Show Tyndall effect : 6. Brownian movement 7. Electrophoresis : The movement of colloidal particles under an applied electric field is called electrophoresis. 8. Electro-osmosis : The movement of molecules of dispersion medium in an electric field is called electroosmoses. 9. Coagulation or Precipitation : The process of settling of colloidal partcles. When an electrolyte is added to the solution, the colloidal particles take up the ions having opposite charge from the electrolyte and get neutralised and thus, coagulated. * Hardy-Schulze rule : It states that the coagulating power of an electrolyte is directly proportional to the fourth power of the valency of the oppositively charged ion of the electrolyte. Coagulating power  ( valency )4 For example : Negative solution As2S3 is coagulated faster by AlCl3 than MgCl2 In coagulation of negative sol, the flocculating power is in the order: Al3+ > Ba+2 > Na+ Similarly, in the coagulation of a positive sol, the flocculating power is in the order [Fe(CN)6]4– > PO43– >SO43– > Cl– Coagulating value : Min conc of an electrolyte in millimoles per litre required to cause precipitation of a sol i n 2 hours. *The smaller the quantity needed, the higher will be the Coagulating power of the ion. Coagulation of lyophilic sols: By adding (i) electrolyte or (ii) suitable solvent. Protection of colloids: When lyophilic colloid is added to lyophobic sol, the lyophilic particles form a layer around lyophobic particles and thus protect the latter from electrolytes. Lyophilic colloids used for this purpose are called protective colloids. *Protective power of different colloids is expressed in gold number. Protecting Power  1 Gold number SURFACE CHEMISTRY Emulsions : The colloidal system in which both the dispersed phase and dispersion medium are in liquid state is called emulsion. eg. Milk, cod liver oil. Emulsification: The process of making emulsion Emulsions are of 2 types 1. (O/W) Oil - in - Water : Emulsion in which, oil acts as the dispersed phase and water act as dispersion medium eg. Vanishing cream. Fat or oil (D.P) eq. Milk Water (D.M) 2. (W/O) Water - in - Oil: dispersion medium Emulsion in which water acts as the dispersed phase and oil act as eg. Butter, Cream cod liver oil Water (D.P) Oil (D.M) Emulsifying agent : The emulsions are stabilized by adding a stabilizing substance called emulsifying agent eg soaps, detergents etc. *Emulsifying agent for O/W emulsion are proteins gums, soaps etc. *Emulsifying agent for W/O emulsion are lamp black heavy metal salts of fatty acids, long chain alcohols, etc. Demulsification : The separation of an emulsion into it components * It is done by centrifugation, heating, freezing etc. *Soil is a colloid in which humus act as protective colloids *Delta : when river water meets sea water the electrolytes present in sea water coagulate the colloidal solution of clay resulting in its deposition. Applications of colloids 1. Precipitation of smoke - By contrell precipitator. 2. Purification of drinking water - by Alum. 3. Medicines -eg(1) colloidal antimony is used in kalaazar (ii) argyrol is silver sol. used as eye lotion (iii) Milk of magnesia for acidity in stomach 4. Tanning -The process of hardening leather. 5. Cleansing action of soap. 6. Photo graplic films are coated with emulsions of light sensitive silver bromide in gelatin over cellulold film. 7. Rubber is obtained by coagulation of latex. 8. Paint, ink, synthetic plastic, cement etc are colloidal solutions.