Surface Chemistry SO 4

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1 Surface Chemistry I. INTRODUCTION Many chemical reactions take place at the surfaces of solids, e.g., at the surface of solid catalysts. Corrosion of metals take place at their surfaces means the topmost layer on the very exterior of a solid, Now-a-days, there are weys of studying the top 0.5 to 2 nm, i.e., two to five of the topmost layers of atoms of the solid surface. In this Unit, we shall discuss phenomenon occurring at surface of solids. We shall also study colloidal state, emulsions and catalysts. II. IMPORTANT TERMS AND DEFINITIONS 1. Adsorption. The existence of a substance at a surface in different concentration than in the adjoining bulk is called adsorption. It is selective absorption. 2. Absorption. It is the phenomenon in which a substance is uniformly distributed all over the surface. 3. Adsorbent. The substance present in bulk, on the surface of which adsorption is taking place is called adsorbent. 4. Adsorbate. The substance which is being adsorbed on the surface of another substance is called adsorbate. 5. Desorption. The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption. 6. Physical adsorption or physisorption. If the adsorbate is held on a surface of adsorbent by weak Vander Waal s forces, the adsorption is called physical adsorption or physisorption. 7. Chemical adsorption or chemisorption. If the forces holding the adsorbate are as strong as in chemical bonds, the absorption process is known as chemical adsorption or chemisorption. 8. Enthalpy or heat of adsorption. Adsorption generally occurs with release in energy, i.e., it is exothermic in nature. The enthalpy change for the adsorption of one mole of an adsorbate on the surface of adsorbent is called enthalpy or heat of adsorption. 9. Adsorption isotherm. When extent of adsorption (x/m; where x is amount of adsorbate, m is mass of adsorbent) is plotted against pressure at constant temperature, curve obtained is known as adsorption isotherm. 10. Specific surface area of solid. It is defined as the surface area (in square metre) per gram of the adsorbent. 11. Adsorption isobar. A plot between amount adsorbed (x/m) vs. temperature at constant pressure is called an adsorption isobar. 12. Colloidal solution. The solution in which the diameter of particles may range from 1 to 100 nm The colloidal particles do not settle down under the force of gravity even on long standing. A colloidal system is always heterogeneous, e.g., glue, starch, albumin, gelatin, blood, soap solution, ink, clouds, smoke, etc. 13. Crystalloids. Those substances whose aqueous solution can pass through semi-permeable membrane are called crystalloids, e.g., NaCl, KCl, H 2 SO 4, NaOH, etc. 14. Disperse phase. The phase of colloidal particles is called disperse phase. 15. Dispersion medium. The medium in which these particles are dispersed is called dospersion medium, e.g., water is dispersion medium in hydrosol. 16. Aerosols. When dispersion medium is gas and dispersed phase is either solid or liquid, the colloidal system is called aerosols, e.g., fog, clouds, fine insecticide sprays, smoke, etc. Colloidal suspension in air are also called aerosols. Page No.: 1

2 17. Sols. When dispersed phase is solid and dispersion medium is either solid or liquid, the Colloidal system is known as sols, e.g., paints, starch dispersed in water, gold sol, muddy water, ruby glass, gem stones, etc. 18. Gels. When dispersed phase is liquid and dispersion medium is solid, such colloidal systems are called gels, e.g., cheese, butter, boot polish, etc. 19. Emulsion. Liquid-liuid colloidal dispersions are called emulsion are called emulsion, e.g., milk, emulsified oils and medicines. 20. Luophobic sols. Those colloids which are solvent hating are called lyophobic sols. In these colloids, the particles of dispersed phase has no affinity for the dispersion medium. They are not easily prepared. They need stabilising agent to preserve them. They are irreversible. Example of lyophobic sols are colloidal solutions of gold, silver, Fe(OH) 3, As 2, etc. 21. Lyophilic sols. These are defined as the colloidal solutions in which the particles of dispersed phase have great affinity for the dispersion medium. They are self-stabilised because of strong attractive forces operating between the suspended particles and the dispersion medium. They are reversible sols, The solids obtained after their evaporation may be recovered to the colloidal solution simply by simple agitating them with the dispersion medium. For example gums, gelatine,starch, albumin and other proteins in water solution are lyophilic. 22. Hydrosols. Colloids in water are called hydrosols. 23. Alcosols. Colloids in alcohol are called alcosols. 24. Benzosols. Colloids in benzene are called benzosols. 25. Hydrophobic sols. If dispersion medium is water in lyophobic sols, they are called hydrobic sols. They are also called suspensoids. 26. Hydrophilic sols. If dispersion medium is water in lyophilic sols, they are called hydropilic sols. 27. Intrinsic colloids. Some substances which readily pass into colloidal solution on warming with suitable liquids are called intrinsic colloids, e.g., gum-arabic, glue, starch and gelatin. The colloids solutions thus obtained are called lyophilic sols. 28. Extrinsic colloids. Substance which do not pass into colloidal solution even on heating are called extrinsic colloids. They are referred to as lyophobic sols, e.g., silver and gold sol. 29. Multi-molecular colloids. The colloids in which the colloidal particles of aggregates of atoms or small molecules with diameters of less than 1 nm are called multi-molecular colloids. For example, a gold sol may contain particles of various sizes having several atoms of gold. Similarly, a sulphur sol consists of particles containing about a thousand of S 8 molecules.these are held together by Vander Waal s forces. 30. Macromolecular colloid. These are the colloids in which the dispersed particles are themselves large molecules (usually polymers). Since these molecules have dimensions comparable to those of colloidal particles, their dispersions are called macromolecular colloids, e.g., proteins starch and cellulose form macromolecular colloids. 31. Miscelles. Those colloids which behave as normal, strong electrolytes at low concentrations, but show colloidal properties at higher concentrations due to the formation of aggregated particles of colloidial dimensions. Such sunstances are also referred to as associated colloids. Surface active agents like soaps and synthetic detergents belong to this class. They form ions also. Molecules of soaps and detergents consist of lyophilic as wall as lyopobic parts which associate together to form miscelles. Miscelles may contain 100 molecules or more. 32. Surface-active agents. Those substances when tend to concentrate on the surface of solution and, therefore, lowers the surface tension, are called surface-active agents. e.g., soaps and detergents. 33. Peptization. The process of passing of a precipitate into colloidal solution on adding an electrolyte is called peptization. In this process, dispersion medium (liquid) break up the Page No.: 2

3 precipitate into colloidal state. Lyophilic sols are prepared by bringing into contact or warming the substance with a dispersion medium, e.g., gelatin, gum arabic, starch and soaps are easily peptized by water. Cellulose nitrate is peptized by organic solvents such as ethyl alcohol. The product obtained is commercially known as collodion. 34. Dialysis. It is a process of purification of sols containing electrolyte by keeping the sol in a bag made of parchment or cellophane and suspending the bag in pure water. Small molecules or ions can pass through the membrane, but the sol is retained. When electric field is applied during dialysis, it is called electrodialysis. 35. Brownian movement. Colloidal particales move in zig-zag path. This typeof motion is due to moving molecules of dispersion medium constantly with colloidal particles. During collisiion, molecules of dispersion medium impart momentum to colloidal particales. You must have seen dust particles in a beam of sunlight coming through a slit in a dark room, which keep on moving in a zig-zag path. It is used to determine Avogadro s number. 36. Tyndall effect. When light passes through a sol, its path because visible just as a beam of sunlight is seen as it enters a dark room through a small hole or slit because of scattering of light by colloidal particles. On the basis of dust Tyndall effect we can explain that sky is blue in day light due to scattering of blue light dust particles. Scattering of light depends upon the difference between the refractive indices of dispersed phase and dispersion medium. 37. Electrophoresis. The movement of colloidal particles towards positive or negative electrode in electric field is called electrophoresis. It occurs due to the presence of positive or negative charge on colloids. It is also called cataphoresis. 38. Emulsification. The process of making an emulsion from an oil is termed as emulsification. 39. Emulsifying agent. The substance which stabilise emulsions are called emulsions are called emulsifying agents. 40. Homogeneous catalysis. When the catalyst mixes homogeneously with the reactant(s) and forms a simple phase, the catalyst is said to be homogeneous and this kind of catalysis is known as homogeneous catalysts, e.g., catalytic oxidation of SO 2 to SO 3 in presence of NO is an example of homogeneous catalysis. 41. Heterogeneous catalysis. When the catalyst forms a separate phase (usually a solid phase), it is said to be heterogeneous and the catalysis. Example of heterogeneous catalysis is manufacture of H 2 SO 4 in contact process using V 2 O 5 as catalyst. Usually, in a heterogeneous catalysis, the reactants are gases, and reaction starts from the surface of the solid catalyst. This is the reason why heterogeneous catalysis is also called surface catalysis. 42. Activity of catalyst. It is the ability of catalyst to accelerate chemical reaction, e.g., H 2 and O 2 in presence of platinum react to form water explosively. 43. Selectivity of catalyst. It is ability of catalyst to direct reaction to yield particular products (excluding others). For example, n-heptane selectively gives toluene in presence of a platinum catalyst. 44. Gold number. It is defined as number of milligrams of protective colloids which must be added to 10 ml of gold solution to prevent coagulation when 1 ml of 10% solution of NaCl is added. 45. Shape selective catalysis. The catalysis which depends upon the pore-structure of the catalyst Page No.: 3

4 and molecular sizes of reactants and product molecules is called shape selective catalysis, e.g., Zeolites are shape selective catalysts due to their honey-comb structure ZSM-5 is used to convert methanol into gasoline. 46. Coagulation. The process of converting a colloidal solution into precipitate is called coagulation. 47. Coagulation value or flocculation value. It is defined as minimum number of millimoles of electrolyte required to coagulate 1 litre of colloidal solution. 48. Sturation pressure. The pressure beyond which extent of adsorption becomes independent of pressure. 49. Critical micellisation concentration (CMC). It is the lowest concentration of the surfactant (e.g.,soap solution) at which miscelle formation takes place. 50. Syneresis. The shrinkage in volume by gels when allowed to stand is called syneresis. 51. Aqua-dag. It is colloidal solution of graphite in water. III. COMMIT TO MEMORY 1. Differences between True Solution, Colloidal solution and Suspension Property True solution Colloidal solution Suspension 1. Nature Homogeneous Appears to be homogeneous Heterogeneous but actually heterogeneous 2. Particle size < 1 nm 1 nm to 100 nm > 100nm 3. Visibility Particles are not Visible through ultra-microscope Visible with naked visible even with eye microscope 4. Molar mass Low High Moderate 5. Colour Depends upon nature Depends upon size of particles Depends upon of ions nature of particles 6. Osmotic High Low Very low pressure 7. Diffusion Fast Slow Do not diffuse 8. Tyndall effect No scattering of light Light is scattered Light is scattered 9. Sedimentation Do not settle down Settle on centrifugation May settle due may not to gravity 10. Brownian Do not show Show brownian movement May or may not movement show 11. Electrophoresis Not shown Exhibited Not exhibited 12. Coagulation Cannot be coagulated Can be coagulated The may settle under gravity. 2. Types of colloidal solution Dispersed Dispersion medium Name Examples phase 1. Solid Gas Aerosols Smok dust particles 2. Solid Liquids Sols As 2, Gold sol, starch, gum, muddy water 3. Solid solid Solid sols Coloured gem stones, some alloys, pearls, ruby glass 4. Liquid Solid Gels Jellies, cheese, shoe polish Page No.: 4

5 5. Liquid Liquid Emulsions Milk, hair cream, cod liver oil 6. Liquid Gas Aerosols of liquids Fog, mist, cloud 7. Gas Solid Solid foam Pumic stone, foam rubber, cork 8. Gas Liquid Foam Whipped cream, lemonade forth 3. Methods of Preparation of Colloidal solution Dispersion Methods. In these methods bigger particles are disintegrated into particles of colloidal dimensions by suitable methods, e.g., (a) Mechanical despersion. A suspension of a coarsely ground particles prepared in dispersion medium is fed into colloidal mill and speed of rotation is adjusted so as to get particles of colloidal size. (b) Electro-dispersion (Bredig s Arc Method). In this method two rods of the metal (Au, Cu, Pt, Ag,) to be dispersed are kept immersed in cold water and a direct electric arc us struck between them. In this way vapours of metal are formed which then immediately condense to form particles of colloidal size. (c) Oxidation.Solutions of non-metals are prepared by this method, e.g., colloidal solution of sulphur 2H 2 S + (O) S + 2H 2 O (d) Reduction. Metal sols can be prepared by this method, e.g., Gold sol 2AuCl 3 + 3SnCl 2 2Au + 3SnCl 4 (e) Hydrolysis. Hydroxides sols are prepared by this methode.g., Fe(OH) 3, Al(OH) 3 FeCl 3 + 3H 2 O Fe (OH) 3 + 3HCl (f) Double decomposition. This method is used to prepare colloids from inorganic salts,e.g., As 2 O 3 +3H 2 S As 2 + 3H 2 O (g) Exchange of solvent. Some substances which form true solution in one solvent form colloidal solution in another due to lowering of solubility, e.g., sulphur dissolved in alcohol forms colloidal solution in water and phenolphthalein dessolved in alcohal forms colloidal solution in water. (h) Peptization. The conversion of precipite into colloidal solution in presence of peptizing agent is called peptization. Peptizing agent is generally an electrolyte. 4. Hardy-Schulze Rules : 1. Oppositely charged ions are effective for coagulation. 2. The coagulating power of electrolyte increases with increases in charge on the ions used for coagulation, e.g., Al 3+ > Ba 2+ > Na + for negatively charged colloids. [Fe (CN) 6 ] > PO 4 > SO 4 > Cl - for positively charged colloids. 5. The reciprocal of coagulation value is called coagulating power i.e., lower the coagulation value, higher will be coagulating power. 6. Origin of charge on colloidal solution : (a) By self-dissociation. (b) By ionisation of surface molecule. (c) Capture of electron in Bredig s arc method. (d) ph of solution. (e) By preferential adsorption ions from solution. Page No.: 5

6 (f) Frictional rubbing. 7. Oil in water emulsions. When oil is dispersed in water, e.g., Milk, Vanishing cream, etc. Water in oil emulsions. When water is dispersed in oil, e.g., Butter, cold cream, etc. 8. Smaller the gold number, greater will be protective action of the given lyophilic colloid e.g., Gelatin has least value of Gold number (0.006 to.01) while starch has highest (10 to 25). 9. When lyophillic sol is added to lyophobic sol so as to make it stable and prevents its coagulation, it is called protective colloid. 10. Multual coagulation. When positively charged colloid like Fe(OH) 3 is mixed with negatively charged colloid like As 2, mutual coagulation takes place. 11. Kraft temperature. Miscells form ionic surfactants only above a certain temperature called kraft temperature. 12. Hydrophillic lipophillic balance (HLB) is a numerical scale to describe the effectiveness of a particular surfactant to produce a particular type of emulsion. 13. Thioxotropy. Some gels like gelatin liquify on shaking into sol. The sol on standing becomes gel again. This property is called Thioxotropy. 14. Oildag is colloidal solution of graphite in oil. 15. Bancroft s Rule. Hydrophillic emulsifiers dissolve better in water than in oil, hence help in formation of oil in water emulsion. Hydrophobic emulsifiers dissolve better in oil than in water, hence form water in oil emulsions. 16. Anionic Surfactants. Sodium palmitate (C 15 H 31 COONa), sodium stearate (C 17 H 35 COONa) and sodium oleate (C 17 H 33 COONa) are anionic surfactants. They are called soaps. Detergents are also anionic surfactants. 17. Cationic Surfactants. Quaternary ammonium salts of long chain tertiary amines form synthetic detergents which are cationic surfactants e.g., Octadecyl ammonium chloride C 18 H 37 H + H 3 Cl - Cetyltri methyl ammonium chloride C 16 H 33 (CH 3 ) 3 N + Cl Non-inorganic Surfactants. Alcohols react with epoxides to form non-inorganic surfactants. e.g., C n H 2n+1 OH + n CH 2 CH 2 CnH 2n+1 ( O CH 2 CH 2 - ) n OH O Surfactant 19. Tyndall cone. The bright cone shaped illuminated path observed in case of colloidal solution under microscope is called Tyndall cone. The intensity of scattered light varies inversely as fourth power of wavelenght. Scince blue light has smaller wavelenght, therefore blue Tyndall cone is formed. 20. Zeta-potential. The surface of a colloidal particles acquires a +ve or -ve charge by selective adsorption of ions. This layer attracts counter ions from the medium which forms second layer having opposite charge and this combination of layers is called Helmoholtz electric double layer. The first layer is firmly held and called fixed layer and second layer being diffused is called mobile layer. The potential difference between these two layers is called Zeta-potential. 21. Elastic gels change into solid mass on dehydration and changed back to original form on addition of water. They absorb water when placed in it with simultaneous swelling and this phenomenon is called imbibition. Non-elastic gels change into solid mass on dehydration which do not change back to original form by addition of water. They do not imbibe. 22. Adsorption indicator. A substance eosin which gets adsorbed on the precipitate formed in a reaction and which undergoes a colour change at the end point in a titration is called an adsorption indicator. 23. Determination of size of colloidal particle. RT N log n 1 n = 4 3 e πr (h2 -h 1)( ρ- ρ') o 2 3 N o. is Avogadro s number, R = gas constant, T = Temperature in Kelvin, n 1 and n 2 are number of particles at depth h 1 and h 2 respectively, ρ and ρ = density of colloidal particles and dispersion medium respectively, r is radius of the particles which can be determind easily. Page No.: 6

COLLOID CHEMISTRY MD. KHAIRUL ISLAM

COLLOID CHEMISTRY MD. KHAIRUL ISLAM HISTORICAL BACKGROUND Thomas Graham (1861) observed that crystalline substances such as sugar, urea, and sodium chloride passed through the membrane, while others like