Chapter-5 SURFACE CHEMISTRY
The branch of the Chemistry wich deals with the study of surface phenomena is called surface Chemistry.
POINTS TO BE REMEMBERED: --1. Adsorption: - The accumulation of molecules species at the surface rather in the bulk of a solid or liquid is termed adsorption. 2. Desorption:-Removal of adsorbate from the surface of adsorbent is known as Desorption. 3. Sorption:-When adsorption and absorption both takes place simultaneously. 4. Type of adsorption: - On the basis of interaction between adsorption and absorption, adsorbate are of two types: (i)Physical adsorption/physisorption: - When weak vander waal interaction involve between adsorbate and adsorbent. (ii) Chemical adsorption/chemisortion:-When chemical bonds form between adsorbate and adsorbent. 5. Adsorption isotherm:-The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm. 6. Application of adsorption:(a) Removal of colouring matter from solution using animal charcoal. (b)Chromatographic analysis is based on adsorption. 7. Freundlich adsorption isotherm:-It is a graph which shows relationship between the quality of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature.
195k x/ m x/ m 244k
L o g K Log P 8 .Factors affecting adsorption:(i)Surface area: - Adsorption increases with increases of surface area of adsorbent. (ii)Nature of adsorbate:- Easily liquefiable gases are readily adsorbed. (iii)Temperature:-Low temperature is favorable for physical adsorption and High temperature for chemisortion. (iv)Pressure: - Pressure increases, adsorption increases. 9. CATALYSIS:- Substances which alter the rate of chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction are known as catalyst and the phenomenon is known as catalysis. 10. PROMOTERS AND POISONS Promoters are substance that enhance the activity of catalyst while poisons decrease the activity of catalyst. Fe N 2 + 3H 2 Fe =catalyst 2NH3 (g)
Mo= promoter 11. Homogenous catalyst – when reactants and catalyst are in same phase. e.g. 12. e.g.
2 SO2 (g)
O 2 (g)
Heterogeneous catalyst – the catalytic process in which the reactants and catalyst are in different phase. SO2 + O2 Pt 2SO3 (g)
13. Adsorption theory of Heterogeneous catalysis – It explains the mechanism of heterogeneous catalyst. The mechanism involves 5 steps:a. Diffusion of reactants to the surface of catalyst. 2
b. Adsorption of reactant molecules on the surface of catalyst. c. Chemical reaction on the catalyst surface through formation of an intermediate. d. Desorption of reaction product from the catalyst surface. e. Diffusion of reaction product away from the catalyst surface. 14. IMPORTANT FEATURES OF SOLID CATALYST Activity - The activity of a catalyst depend on the strength of chemisorption. Catalytic activity increases from group 5 to group 11 elements of the periodic table. Pt 2H2 + O2 2H2O Selectivity – The selectivity of a catalyst is its ability to direct a reaction to yield a particular product. Ni 1. CO + 3H2 CH4 + H2O Cu 2. CO + H2 HCHO 15. SHAPE SELECTIVE CATALYSIS The catalytic reaction that depends upon the pure substance of the catalyst and the size of reactant and product molecules is called shape selective catalysis. e.g. Zeolites are good shape selective catalyst. 16. ENZYME CATALYSIS Enzymes are protein molecules of high molecular mass which catalyse the biochemical reaction. e.g. Inversion of cane sugar by invertase enzyme. 17. Characteristic of enzyme catalysis – a. b. c. d. Enzymes Enzymes Enzymes Enzymes are are are are specific to substrate. highly active under optimum temperature. specific to pH. e.g. Pepsin act in acidic medium inhabited by the presence of certain substance.
Mechanism of enzyme catalysis...
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