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Surface Chemistry

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Surface Chemistry SURFACE CHEMISTRY Adsorption Adsorption is due to the presence of unbalanced forces, believed to have developed either during crystallisation of solids or due to presence of unpaired, electrons or free valencies in solids having d–orbital. In liquids it is due to surface tension. Difference between adsorption and absorption Sr. No. Adsorption Absorption 1 Adsorption is a surface phenomenon. The adsorbing It is a bulk phenomenon. In substance is called adsorbate and is only concentrated absorption the substance on the surface of adsorbent. penetrates into the bulk of the other substance. 2 The rate of adsorption is rapid to start with and its rate Absorption occurs at a uniform slowly decreases. rate The amount of heat evolved when one mole of an adsorbate gets adsorbed on the surface of an adsorbate is called molar heat (enthalpy) of adsorption. Adsorption in Terms of Gibb’s Helmholtz Equation Adsorption is an exothermic reaction. Therefore, adsorption is accompanied by release of energy or H is always negative and favours the process. Also absorbate molecules get lesser opportunity to move about on the surface of adsorbent. Thus, entropy factor opposes the process. According to Gibb’s Helmholtz equation, G = H – T S Since adsorption does actually take place, H is greater than T S G is negative. As the adsorption continues, the difference between the two opposing tendencies becomes lesser and lesser till they are equal i.e., H = TS or G = 0. At this stage, equilibrium called adsorption equilibrium gets established and there is no net adsorption taking place at this stage. Types of Adsorption There are two types of adsorption. a) Physical adsorption (or Vander waal’s adsorption) or physiosorption. b) Chemisorption Characteristics of Physical and Chemisorption Physio–sorption Chemisorption 1. Adsorption by weak vander waal’s forces. By chemical force (covalent or ionic bond) 2. Multimolecular layer may be formed Unimolecular layer will be formed 3. Low heat of adsorption viz. about 20–40 High heat of adsorption viz. about (200–400 kJ/mol kJ/mol) 4. Easily reversed Not reversed 5. Molecular state of adsorbate on Molecular state may be different. Surface adsorbent is same. No surface compounds are found. compounds are found. 6. Usually occurs rapidly at low It occurs at high temperature initially but then temperature and decreases with increase decreases. in temperature. 7. It increases with pressure Change in pressure will have less effect on chemisorptions 8. Not specific Highly specific 9. Extent of adsorption depends on ease of There is no relative between extent of adsorption depends on ease of adsorption and ease of liquefaction of gas. liquefaction i.e. surface area, critical e.g. Adsorption of H2 on Pt; decomposition of temperature, inversion temperature NH3 in presence of tungsten, decomposition of Vanderwaal’s constant ‘a’, e.g. Adsorption Hl on gold (zero order) of gases on charcoal Application of Adsorption 1. In gas mask: 1 Finally divided coconut charcoal is used as gas masks for absorbing toxic gases like CH4, CO, COCl2. It is usually used for breathing in coal mines to adsorb poisonous gases. 2. In preserving Vacuum: In Dewar flasks, activated charcoal is placed between the walls of the flask so that any gas which enters into annular space either due to glass imperfection of diffusion through glass is adsorbed. 3. As dehumidizer e.g silica gel 4. Fe(OH)3 antidote adsorbent in Arsenic poisoning. 5. In clarification of sugar: Sugar decolorized by treating sugar solution with charcoal powder. The latter adsorbs the undesirable colours present. 6. Heterogeneous catalysis: Adsorption of reactants on the solid surface of the catalysts increases the rate of reaction. There are many gaseous reactions of industrial importance involving solid catalysts. Manufacture of ammonia using iron as a catalyst, manufacture of H2SO4 by contact process and use of finely divided nickel in the hydrogenation of oils are excellent examples of heterogeneous catalysis. 7. In Chromatography: The different chromatographic techniques such as adsorption, paper or column chromatography which are used for the purification and the separation of the substances available in small amounts, are based upon the theory of selective adsorption. Catalysis Catalyst is a substance which changes the speed of a reaction, and usually, can be recovered completely unchanged at the end of a reaction. However it may take part in a reaction consumed in one step and regenerated in another. This phenomenon is known as Catalysis. A substance is termed a positive catalyst or simply as catalyst if it accelerates the rate of chemical reaction. On the other hand, the added substance is termed as negative catalyst if it retards the rate of a chemical reaction. Example of Negative Catalysts: i) H3PO4 or acetanilide in the decomposition of H2O2. ii) Alcohol in the oxidation of chloroform leading to the formation of phosgene. iii) Tertraethyl lead or nickel carbonyl acting as antiknock material in internal combustion engines. iv) Antifreezes like glycerol which retard the rusting of the machines. Catalytic Promoters There are certain substances which when added only in small quantity to a catalyst enhance its activity. This substance itself may not be catalyst. Such substances which enhance the activity of a catalyst are called catalytic promoters. E.g. Molybdenum is used as Promoter for Fe catalyst in Haber’s process. Catalytic Inhibitors or poisons The rates of some reactions are reduced considerably by the presence of small amounts of other substances called inhibitors. The substance whose presence decreases or destroy the activity of a catalyst are called poison. CO or H2S in hydrogen gas acts as a poison for Fe catalyst in Haber process. Types of Catalysis Broadly, two types of catalysis are known: (a) Homogeneous catalysis (b) Heterogeneous catalysis (a) Homogeneous catalysis If the catalyst is present in the same phase as the reactants, it is called a homogeneous catalyst and this type of catalysis is called homogeneous catalysis. Ex: H 2SO 4 C12H22O11 (aq) H2O(l) C6H12O6 (aq)C6H12O6 (aq) Sucrose Glu cose Fructose NO(g) 2SO 2 (g)O2 (g) 2SO 3(g) HCl() CH 3COOCH3()H2O() CH 3COOH(aq) CH 3OH(aq) b) Heterogeneous catalysis: In this type of catalysis the catalyst is present in a difference phase than that of the reactants. In heterogeneous catalysis, catalyst is generally a solid and the reactants are generally gases but sometimes liquid reactants are also used. It is also known as surface catalysis. Ex: i) Synthesis of methyl alcohol (CH3OH) from CO and H2 using a mixture of copper, ZnO and Cr2O3 as catalyst. Cu , ZnO Cr2O3 CO(g) + 2H2(g) CH3OH(l) ii) Manufacture of ammonia from N2 and H2 by Haber’s process using iron as catalyst. Fe N2(g) + 3H2(g) 2NH3(g) Theories of Catalysis Intermediate Compound Formation Theory This theory explains homogeneous catalysis mainly. 2 According to this theory, the catalyst combines with one of the reactants to give an intermediate compound. This compound intermediately reacts with the other reactants and given the product and regenerates the catalyst in its original form. Thus the reactants do not directly combine with each other, instead they react through the catalyst which provides an alternative pathway which involves lesser energy of activation. For example, the function of nitric oxide [NO] as a catalyst in the formation of SO3 is explained as follows: 2NO + O2 2NO2 Catalyst Reactant Intermediate NO2 + SO2 SO3 + NO Intermediate Reactant Product Catalyst regenerated Adsorption Theory: This theory explains the heterogeneous catalysis. The role of a solid catalyst in enhancing the reaction rate is explained on the basis of this theory in the following steps: (i) The reactant molecules are adsorbed on the surface of the catalyst at adjacent point. Adsorption leads to higher concentration of the adsorbed reactant on the surface of a catalyst. (ii) As adsorption is an exothermic process, the heat of adsorption provides the necessary activation energy for the chemical reaction to proceed and enhance rate of greater. (iii) The product molecules rapidly leave the catalyst surface to make room for the other reactant molecules to get adsorbed. Thus the chemical combination between reactant molecules occurs at the surface of the catalyst at a must faster rate. e.g. Hydrogenation of ethane in presence of Ni H – H + 2M 2M – H C2H4 + 2M – H C2H6 + 2M Modern Adsorption Theory of Heterogeneous Catalysis The modern adsorption theory is the combination of intermediate compound formation theory and the old adsorption theory. The catalytic activity is localized on the surface of the catalyst. The mechanism involves five steps: Adsorption of O O O reacting molecules O O O A + A + B O O O Reacting molecuels O O O B Catalyst surface having free valencies Desorption of product O O O A O O O molecules | + A - B Product O O O B O O O Intermedaite Catalyst Adsorption of reacting molecules, formation of intermediate and desorption of products i) Diffusion of reactants to the surface of the catalyst. ii) Adsorption of reactant molecules on the surface of the catalyst. iii) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate. iv) Desorption of reaction products from the catalyst surface, and thereby, making the surface available again for more reaction to occur. v) Diffusion of reaction products away from the catalyst’s surface. The surface of the catalyst unlike the inner part of the bulk, has free valencies which provide the basis for chemical forces of attraction. When a gas comes in contact with such a surface, its molecules are held up there due to loose chemical combination. If different molecules are adsorbed side by side, they may react with each other resulting in the formation of new molecules.
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