Surface Chemistry (Ch. 12, p 467, G. R. Ch.) (Ch. 23, p 675, B S Bahl)

3. Surface Chemistry (Ch. 12, p 467, G. R. Ch.) (Ch. 23, p 675, B S Bahl) Adsorption: It is the process in which molecules, atoms or ions of one substance are attached to the surface of another. • Molecules of one substance are attracted and retained on the surface of the other substance which may be liquid or solid • Adsorbate:It is that substance which is adsorbed on the surface of other substance. • Adsorbent:It is that substance on which other substance is adsorbed. • Absorption: It involves passing of the particles of one substance into the bulk of the liquid or solid. • Ink is absorbed by the chalk and gases are adsorbed by charcoal. • Ammonia is adsorbed by charcoal but absorbed by water.

4. Adsorbent Absorbent Adsorbate Absorption Adsorption Examples of Adsorption • Adsorption of a dye by charcoal • Adsorption of a gas by charcoal

5. Sorption In many cases the initial rapid adsorption is followed by a slow process of absorption of the substance into the interior of the solid called as SORPTION. In this case adsorption can not be distinguished from absorption. For example the uptake of gases by zeolites • Physical Adsorption • or Van der Waals Adsorption • Chemical Adsorption • or Chemisorption

6. Characteristics of Adsorption • It is a spontaneous process. It occurs at the surface. • It occurs due to attractive forces (?) • Generally an exothermic process. • Heats of physical adsorption are around 20 kJ/mol or less and never more than 40 kJ/mol. and for chemisorption vary between 70-420 kJ/mol being around 200 kJ/mol near room temperature. • Physical adsorption is reversible process. • Physical adsorption is non-specific and depends only upon the nature of adsorbent • Chemical adsorption is highly specific and depends on the nature of both adsorbent and adsorbate.

7. Factors Influencing Adsorption • Nature and surface area of the adsorbent • Nature of adsorbate • Temperature: physical occurs at low temp. chemisorption increases with temperature. • Pressure: It shows its effect on adsorption of gases. • Contact time of adsorbent and adsorbate • Presence of foreign cations and anions • Nature of medium • pH of the adsorption medium Difference between physical and chemical adsorption (see Ch. 12, p 471, G. R. Ch.)

8. Applications of Adsorption (p 475, G. R. Ch.) • In the process of dyeing, dyes are adsorbed on fibre • Production of high vacuum • Gas masks: These contain suitable adsorbents • Chromatographic analysis: Selective adsorption is used for separation of the components of a mixture. • Removal of colouring matter from solutions: • Adsorption indicators: Various dyes work as indicators in precipitation titrations. • The catalysis: It works on the theory of adsorption. • Removal of gases from paints by suitable adsorbents • In softening of hard water Mg2+ and Ca2+ ions are adsorbed by zeolites releasing Na+ ions. • In curing of diseases some drugs are adsorbed on germs.

9. Chromatography (Ch 1, Org Chem by G. R. Ch.) It is a technique for separation of the components of a mixture of solutes brought about by distribution of dissolved or dispersed materials between two immiscible phases, one of which is moving past the other. • TYPES: (i) Partition chromatography (ii) adsorption chromatography • Partition Chromatography: mixture is separated by means of partition between a moving solvent and a stationary liquid which is held on suitable solid support. • Mobile Phase: Moving component • Stationary Phase:The component which does not move. • If mobile phase is liquid, liquid-liquid chromatography • If mobile phase is gas, gas-liquid chromatography

10. Liquid-Liquid Chromatography • PC (Paper chromatography) • TLC (Thin layer Chromatography) • ColumnChromatography (if moist silica gel is used as stationary phase) Adsorption Chromatography: mixture is separated by means of small difference in the adsorption behaviour of substances between a moving solvent (liquid or gas) and a stationary solid phase. • If mobile phase is liquid, liquid-solid chromatography • If mobile phase is gas, gas-solid chromatography

11. PC (Paper chromatography) • (a) Paper Partition Chromatography (b) Adsorption paper Chromatography Rf value = distance moved by sample distance moved by solvent • Rf depends upon (i) temp (ii) quality of paper (iii) technique applied (iv) chemical reaction between the substances (v) distance traveled (vi) pH of solution • Development of Chromatogram: may be by • Ascending Technique • Descending Technique • Radial Technique • Drying of chromatogram • Location of the compound (i) from colour But for colourless compounds by (i) fluorescence (ii) radioactivity (iii) by chemical methods

12. TLC (Thin Layer chromatography) • Adsorbents: alumina, silica gel, cellulose powder A binding agent such as plaster of Paris is added to hold the adsorbent on plate. • polyamide powder, magnesium silicate, calcium sulphate are also used as adsorbent • Activation: removal of water at 100-105oC • Selection of solvent • Sample application • Development of chromatogram (by ascending technique) • Location of compounds

13. Applications of TLC • It is a quantitative and qualitative technique • Separation and identification of dyes. • Separation and identification of a large number of organic and inorganic compounds • Identification of by-products of a reaction • Isolation of pure compounds from a mixture • Analysis of amino acids, peptides, carbohydrates, steroids, hormones, alkaloids, pesticides, insecticides, vitamins, pigments etc.

14. Column Chromatography • The column and its packing. • Adsorbents: silica gel, alumina, charcoal, starch, calcium sulphate etc • Solvents and its selection • Identification of compound • Applications: • separation of dyes • purification of the products of a reaction • identification of unknown compounds • separation and identification of inorganic compounds

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