Surface Chemistry

Reaction rate and rate law Surface chemistry deals with the phenomena that occur at the surfaces or interfaces. Many important phenomena like corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallisation occur at interfaces. The accumulation of the molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. Desorption is the process of removing an adsorbed substance from a surface on which it is adsorbed. Absorption is a physical process in which the particles of a substance remain uniformly distributed throughout the bulk of other substance. The extent of adsorption increases with the increase in the surface area per unit mass of the adsorbent at a given temperature and pressure. Entropy of the system decreases during adsorption. When a gas is adsorbed, the freedom of the movement of its molecules becomes restricted. Adsorption is classified as physical adsorption or physisorption and chemical adsorption or chemisorption. In physisorption atoms of the gas are adsorbed on the surface of solid by weak van der Waals forces without the formation of a chemical bond between the adsorbate and adsorbent. When the gas molecules or atoms are held to the solid surface by chemical bonds (covalent or ionic), the adsorption is termed chemical adsorption or chemisorption. Adsorption isobar is a graph drawn between the amount of the gas adsorbed per gram of the adsorbent and temperature at a constant equilibrium pressure of the adsorbate gas. Adsorption isotherm is a graph that shows the variation in the amount of a gas adsorbed by the adsorbent with pressure at constant temperature. Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. Solids can also adsorb the solutes from the solutions. There are various applications of adsorption such as in gas masks, control of humidity and in chromatographic analysis. Catalysts and enzymes Catalysts are the substances which alter the rate of reaction without being consumed during the reaction. The process by which a catalyst alters the rate of a chemical reaction without being consumed or altered (chemically or quantitatively) in the process is known as catalysis. The substances that enhance the activity of a catalyst are called promoters. The substances that decrease the activity of a catalyst are known as poisons. Catalysis is categorized as homogeneous catalysis and heterogeneous catalysis. In homogeneous catalysis, the catalyst and the reactants are in the same phase. In heterogeneous catalysis, the catalyst and reactants are in different phases. Modern adsorption theory explains why the mass and chemical composition of the catalyst remains unchanged at the end of the reaction. It gives the reason for the effectiveness of the catalysts even in small quantities. This theory does not explain the action of catalytic promoters and catalytic poisons. Solids catalysts are active and selective in nature. The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product is called shape selective catalysis. Catalysts are very useful in industries. In Haber’s process, finely divided iron is used as catalyst. Enzymes are the complex nitrogenous organic compounds that are produced by living plants and animals. They are actually protein molecules of high molecular mass and form colloidal solutions in water. They are biochemical catalysts. They are highly efficient and highly specific. Colloids A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance (dispersion medium). Colloid is a system between the two extremes of suspensions and solutions. The difference between a solution and a colloid is that of the particle size. Colloids can be classified on the basis of physical state of components, nature of interaction between components and type of particles of dispersed phase. Based on the physical state of dispersed phase and dispersion medium, colloids are classified as sol, aerosol, foam, gel, emulsion etc. Based on the nature of interaction between the dispersed phase and dispersion medium, colloids are classified as lyophilic colloids and lyophobic colloids. Based on the type of particles of the dispersed phase, colloids are classified as multimolecular colloids, macromolecular colloids and associated colloids. Colloids are prepared by chemical methods, by electrical disintegration or Bredig’s Arc method and by peptisation. Colloidal solutions can be purified by Dialysis,Electro-dialysis and Ultrafiltration. Colloids show many properties such as colligative properties, electrophoresis, Tyndall effect etc. The colloidal solution of two immiscible liquids in which one of the liquids acts as the dispersed phase and the other acts as the dispersion medium is called an emulsion. There are two types of emulsions, oil in water emulsions (O/W) and water in oil emulsions (W/O). A substance that is added to an emulsion to stabilise it is known as emulsifying agent. Colloids can be observed around us, like fog and delta

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