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Premium member Presentation Transcript SURFACE CHEMISTRY: SURFACE CHEMISTRY MUKUL PATHAK 2011Slide 2: INTRODUCTION Surface chemistry is the study of processes that occur at the interface of two bulk phases. The bulk phases can be of the type : Solid - gas Solid - liquid Liquid - gas Liquid - liquid Mukul pathakSlide 3: In this process, accumulation or concentration of a substance takes place at the surface or interface as compared to the bulk phases This happens because the molecules of the bulk phase at the surface are attracted only from below and from the sides as compared to the molecule inside the bulk of the phase. In the interior, molecules experience forces of attraction from all sides Adsorption is a surface phenomenon Mukul pathakSlide 4: Types of Adsorption Depending on the nature of attractive forces existing between the adsorbate and adsorbent, adsorption can be classified as: Physical Adsorption Chemical Adsorption Mukul pathakSlide 5: In physical adsorption, the forces of attraction between the molecules of the adsorbate and the adsorbent are of the weak van der Waals' type. Since the forces of attraction are weak, the process of physisorption can be easily reversed by heating or decreasing the pressure of the adsorbate (as in the case of gases). Physical Adsorption - PHYSISORPTION Mukul pathakSlide 6: Chemical adsorption - CHEMIOSORPTION In chemisorption , the forces of attraction between the adsorbate and the adsorbent are very strong; the molecules of adsorbate form chemical bonds with the molecules of the adsorbent present in the surface. Mukul pathakSlide 7: Forces of attraction are vander Waals’ forces Forces of attraction are chemical bond forces Low enthalpy of adsorption (20 - 40 k.J /mole) High enthapy of adsorption (200 - 400 k.J /mole) This process is observed under conditions of low temperature This process takes place at high temperatures It is not specific It is highly specific Multi-molecular layers may be formed Generally, monomolecular layer is formed This process is reversible This process is irreversible Difference between Physisorption Chemisorption Mukul pathakSlide 8: A catalyst is a substance that increases the rate of a reaction by providing an alternate path of lower activation energy. Since catalysts are not consumed in a reaction, very small quantities of catalyst are required to enhance a reaction rate CATALYSTS Mukul pathak Mukul pathakSlide 9: The activity of a catalyst depends on the strength of chemisorption . To be active, the surface of the catalyst should be extensively covered by the adsorbate , that is, the chemisorption should be strong. However , if the strength of the adsorbent- adsorbate bond becomes too strong then the activity of the catalyst declines because other reactant molecules cannot react with the adsorbate or because the adsorbate molecules become immobilized on the surface. It has been observed that the catalytic activity increases from group 5 metals to group 11 with maximum activity shown by group 7-9 elements of the periodic table. Properties of catalysts Mukul pathakSlide 10: Zeolites - Shape-Selective Catalysis by Zeolites Shape-selective catalysis are those reactions that depend on the pore structure of the catalyst and the size of the reactant and product molecules. In such reactions, zeolites are used as catalysts. Zeolites are microporous aluminosilicates of the general formula M x /n [(AlO 2 ) x (SiO 2 ) y ].z H 2 O where n is the charge of the metal cation , M n + . M is usually Na + , K + or Ca 2+ and z is the number of moles of water of hydration, which is highly variable. The characteristic of zeolites is the openness of the [( Al 2 )O 2 ] n framework. In this framework, some of the silicon atoms are replaced by aluminium atoms. Zeolites are found in nature and they are also synthesized for catalytic selectivity. Because of the three dimensional cage like structure, zeolites can be used as ion-exchange materials and selective adsorbents. Mukul pathakSlide 11: Zeolites as ion-exchange materials have been replaced by synthetic cationic and anionic exchange resins, but their use as selective adsorbents for gases or liquids is extensive. The zeolites act as molecular sieves for the reactants. To achieve this, zeolites are heated in vacuum so that the water of hydration is lost. Once the water is removed, the zeolite structure becomes porous. The pore size in zeolites is generally in the range 260 - 740 pm. The size of the porous cavities influences the catalytic activity of the catalyst. Only those molecules can be adsorbed in the pores whose sizes are small enough. This facilitates the molecules to enter the cavities and leave easily. Hence, zeolites act as selective adsorbents Mukul pathakSlide 12: Depending on the size of the reactants and product molecules relative to the sizes of the cavities of zeolites , reactions occur in a specific manner. For e.g., alcohols are converted into gasoline using a zeolite catalyst ZSM-5 ( Zeolie Sieve of Molecular Porosity-5). The formula of ZSM-5 is H x [(AlO 2 ) x (SiO 2 ) 96-x ].16 H 2 O. The alcohol is dehydrated in the cavities by the protons and this results in gasoline. Zeolites are used in cracking long-chain hydrocarbons in the petrochemical industry. Mukul pathakSlide 13: Enzymes are macromolecules, usually proteins, produced in living systems, which act as catalysts in physiological reactions. The striking characteristics of enzymes are their catalytic power and specificity. Enzymes have immense catalytic power; they accelerate reactions by factors of at least a million. Most reactions in living systems do not occur at perceptible rates in the absence of enzymes. A simple reaction like hydration of CO 2 is catalyzed by the enzyme carbonic anhydrase . ENZYME CATALYSIS Mukul pathakSlide 14: The transfer of CO 2 from tissues into the blood and then to the alveolar air would be very slow in the absence of this enzyme. The enzyme can hydrate 10 5 molecules of CO 2 per second, which is 10 7 times faster than the unanalyzed one . Mukul pathakSlide 15: It is well known, that solutions are homogenous systems while suspensions are heterogeneous systems, i.e., they consist of more than one phase. In between the extremes of suspensions and solutions lies the colloidal system. COLLOIDS Mukul pathakSlide 16: Colloids are characterized as a) according to the state of the dispersed phase and the state of dispersion medium. b) nature of interaction between dispersed phase and dispersion medium . c) types of particles of the dispersed phase . Types of colloids Mukul pathakSlide 17: Emulsions are colloidal solutions in which both the dispersed phase and dispersion medium are liquids. Emulsions are broadly classified into two types. EMULSIONS Oil-in-water emulsion Water-in-oil emulsion Mukul pathakSlide 18: Emulsification Emulsification is the process of making emulsions. Emulsions are made by shaking the dispersed phase and dispersion medium vigorously and then stabilizing the emulsion with an emulsifier. Most often soaps and detergents are added to stabilize emulsions. Stabilization is obtained by the coating of the drops of an emulsion by the stabilizer. This prevents the drops of the emulsion from combining together and separating out as a separate layer. Other common stabilizing agents are proteins, gum and agar Preparation of emulsions Mukul pathakSlide 19: Colloids have uses in our daily life as well as in various industrial processes. Some of the applications where colloids are present are listed below. Pharmaceutical industry makes use of colloidal solution preparation in many medicines. A wide variety of medicines are emulsions. An example is Cod Liver Oil Paint industry also uses colloids in the preparation of paints. Applications of colloids Mukul pathakSlide 20: THANK YOU Mukul Anand Pathak Mukul pathak You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.