Unit-5-Surface chemistry

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1 Key concepts- Unit-5-Surface chemistry The branch of the Chemistry which deals with the study of surface phenomena is called surface Chemistry. eg. Rusting of iron, Crystallization, dissolution etc. 1. Adsorption:-The accumulation of molecular species at the sur face 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.Typeof adsorption:-on the basis of interaction between adsorbent and adsorbate adsorptions are of two types: (i) Physical adsorption/physisorption:-when weak vander waal interaction involve between adsorbate and adsorbent. (ii) Chemical adsorption/chemisorption:-when chemical bonds formed between adsorbate and adsorbent results to formation of surface compound. 5.Adsorption isotherm:-the variation in the amount of gas adsorbed(x/m) 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 quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. 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 chemisorption. (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.

2 10.Promoters and poisons: Promoters are substance that enhance the activity of catalyst while poisons decrease the activity of catalyst eg. Fe=catalyst Mo= promoter CO= poison 11. Homogenous catalyst when reactants and catalyst are in same phase. 12.Heterogeneous catalyst The catalytic process in which the reactants and catalyst are in different phase. 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. 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 group11elements of the periodic table. Selectivity The selectivity of a catalyst is its ability to direct a reaction to yield a particular product. 15.Shape selective catalysis: The catalytic reaction that depends upon the pore structure of the catalyst and the size of reactant and product molecules is called shape selective catalysis. e.g. Zeolites are good shape selective catalysts. 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. Enzymes are specific to substrate. b. Enzymes are highly active under optimum temperature. c. Enzymes are specific to ph. e.g. Pepsin act in acidic medium d. Enzymes are inhabited by the presence of certain substance.

3 Mechanism of enzyme catalysis 1.Binding of enzyme to substrate to form an activated complex. E+S ES* 2.Decomposition of activated complex to form product. ES* E+ P 18.Colloid- A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) in an other substance called dispersion medium and size of dispersed phase is from 1nm-1000 nm. 19.Types of colloids (1) On the basis of nature of interaction between dispersed phase and dispersion medium. (a)lyophobic colloid (Solvent hating colloid) these colloids can not be prepared by simply mixing of dispersed phase into dispersion medium. e.g. metallic sols. (b)lyophilic colloid (Solvent loving colloid)- these colloids can be prepared by simply mixing of dispersion phase into dispersion medium. e.g. Starch sol. (2) On the basis of types of particles of the dispersed phase (a)multi-molecular colloid- on dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size in colloidal range. The species thus formed are called Multi molecular colloids. e.g.sulphur sol. (b)macromolecular colloids- macromolecules in suitable solvent form colloid, in which size of the particles are in range of colloidal range. e.g. starch sol. (c)associated colloids(micelles)- some substances in low concentration behave as normal strong electrolyte but at higher concentration exhibit colloidal behavior due to formation of aggregates. The aggregated particles are called micelles and also known as associated colloids. 20.Kraft temperature- Temperature above which formation of micelles takes places. 21.Critical micelle concentration(cmc)-concentration above which micelle formation takes place is known as CMC. 22.Preparation of colloids (a)chemical methods- By double decomposition, oxidation reaction or hydrolysis Oxidation e.g. SO 2 + 2H 2 S 3S(sol) + 2H 2 O Hydrolysis e.g. FeCl 3 + 3H 2 O Fe(OH) 3 (sol)+ 3HCl (b) Bredig s arc method- For preparation of metallic sol. It involves dispersion as well as condensation. (c) Peptization- Process of converting a precipitate into colloidal sol, by shaking it with suitable dispersion medium in the presence of a small amount of electrolyte.

4 23. Purification of colloidal solution :- (a)dialysis- It is a process of removing a dissolved impurity from a colloidal solution by membrane. (b)electrodialysis - When dialysis is carried out with an electric field applied around the membrane. (c) Ultra filtration- Use of special filters which are permeable to all ionic substances except colloidal particles. 24.Properties of colloidal solution:- (a)they show colligative properties (b)brownian movement-it is the zig-zag motion of colloidal particles (c)tyndall effect- Scattering of light by colloidal particles by which path of beam becomes clearly visible. This effect is known as tyndall effect. (d)charge on colloidal particles Colloidal particles which carry an electric charge and nature of charge is same on all particles. (e) Electrophoresis- Movement of Colloidal particles towards opposite charge electrode in presence of external electric field. (f)coagulation The process of settling of colloidal particles is called coagulation of the sol. (g)hardy-schulze Law Coagulating value of a coagulating ion is directly proportional to the charge on the ion. eg: Na + < Ca 2+ < Al 3+ for negatively charged sol. Cl - < CO < PO4 3 - < [Fe (CN)6 ] 4 for positive sol. 25.Emulsion Liquid liquid colloidal system is known as Emulsion. There are two types of Emulsion. a)o/w type - Oil dispersed in water. Eg: milk, vanishing cream. b)w/o type Water dispersed in oil. Eg: Butter & Cream. 26. Emulsifying Agent The substance which stabilizes emulsion. Graded Questions with Solution (Q.) What is meant by critical temperature of gas? (Ans) Critical temperature is the minimum temperature above which a gas cannot be liquefied howsoever high the pressure may be applied. (Q.) Give the expression for Fruendlich adsorption isother (Ans) x/m= kp 1/n (Q.) What do x and m represent in the expression x/m=kp 1/n (Ans) m is the mass of the adsorbent and x is the number of moles of the adsorbate when the dynamic equilibrium has been achieved between the free gas and the adsorbed gas. (Q.) Why is heterogeneous catalysis also known as surface catalysis Ans) In heterogeneous catalysis the reaction always starts at the surface of the catalyst. So, it is also known as surface catalysis. (Q.) What is a hydrosol? (Ans) A colloid in which the dispersion medium is water is known as hydrosol. (Q.) Define peptization? (Ans) Peptization is a process of converting a precipitate into colloidal particlesby adding suitable electrolyte. (Q.) Define Brownian movement? (Ans) Brownian movement can be defined as continuous zig- zag movement of the colloidal particles in a colloidal sol.

5 (Q.) Why is Brownian movement important? (Ans) Brownian movement opposes the force of gravity and does not allow the colloidal particles to settle down, thus making the colloidal solution stable. (Q.) Differentiate between adsorption and absorption. (Ans) Adsorption Absorption a)it occurs only at surface a) it is a bulk phenomena b)concentration on the surface b) concentration is same is more than in the bulk through out the material (Q.) What is the effect of temperature on adsorption? (Ans) Adsorption processes, being exothermic, decreases with increase in temperature. (Q.) When a finely powdered active carbon is stirred into a solution of a dye, the intensity of color in solution decreases. Why? (Ans) The intensity of color in the solution decreases because of gas adsorbed on the surface of carbon. (Q.) Why do finely divided substances have larger adsorption power? (Ans) Finely divided substances have large surface area for adsorption and hence have larger adsorption power. (Q.) What are zeolites? (Ans) Zeolites are aluminosilicates i.e. three dimensional network silicates in which some silicon atoms are replaced by aluminium atoms. (Q.) Why are zeolites called shape selective catalysts? (Ans) Zeolites are called shape selective catalysts because their catalytic action depends upon the size and shape of the reactant and the product molecules as well as on their own pores and cavities. (Q.) A small amount of silica gel and that of anhydrous CaCl 2 are placed separately in two corners of a vessel containing water vapours. What phenomena will occur in the two corners? (Ans) Adsorption would occur where silica gel is kept in the vessel where as absorption will occur in the corner where CaCl 2 is placed. (Q.) Name the substance catalysed by Zymase. (Ans) Glucose-- Zymase ->ethyl alcohol. (Q.) How can colloidal solution of ferric hydroxide be prepared by peptization? (Ans) A colloidal sol. of ferric hydroxide can be prepared by adding small quantity of ferric chloride solution to freshly prepared precipitate of ferric hydroxide. (Q.) What is the cause of Brownian movement? (Ans) Brownian movement is caused by the striking of the colloidal particles with the molecules of dispersion medium due to their kinetic energy. (Q.) Define Tyndall effect? (Ans) It is defined as the scattering of light by the colloidal particles present in a colloidal solution. (Q.) What is emulsion? What are their different types? (2 Marks) (Ans) An emulsion is the colloidal dispersion in which both the dispersed phase and the dispersion mediums are liquids. They can be of two types:- i) Emulsion of oil in water. ii) Emulsion of water in oil. (Q.) How are micelles formed in soap solution? (2 Marks) (Ans) Soap is sodium salt of fatty acids (RCOONa) which when dissolved in water dissociates to give RCOO - and Na +. The RCOO - consists of polar group COO - which is hydrophilic and stays at the surface and the non polar group R which being hydrophobic stays away from the surface. At high concentrations RCOO - ions are pulled into the solution to form spherical aggregates with R pointing to the centre COO - part remaining outward. This aggregate is known as ionic micelle.

6 (Q.) Differentiate between chemisorption and physiosorption. (2 Marks) (Ans) Physisorption: a)the forces operating are weak vander Waal s forces b)the heat of adsorption is low KJ Mol -1 c)does not require any activation energy d)forms multimoleculer layer Chemisorption: a)forces acting are similar to those of chemical bonds b) The heat of adsorption is high KJ Mol -1 c) Requires activation energy d) Forms unimolecular layer (Q.) How is the electrical charge of the colloidal particles responsible for the stability of colloidal Sols (2Marks) (Ans) The electrical charges of the particles prevent them from coming together due to electrostatic repulsion. All the dispersed particles in a colloidal solution carry the same charge while the dispersion medium has equal and opposite charge. (Q.) What is demulsification? Name two demulsifiers. (2 Marks) (Ans) The process of separation of the constituent liquids of an emulsion is called demulsification. Demulsification can be done by centrifugation or boiling. Q.) Why lyophilic colloids are called reversible sols while lyophobic sols are called irreversible sols? (3 Marks) (Ans) In the lyophilic colloids if the dispersed medium is separated from the dispersion medium the sol can be made again by simply remixing with the dispersion medium. So they are called reversible sols. In lyophobic sols if small amount of electrolyte is added, the sols are readily precipitated and do not give back the colloid by simple addition of the dispersion medium. So they are called irreversible sols. (Q.) Describe the preparation of the following colloidal solution. (a) Gold sol (b) Sulphur sol (3 Marks) (Ans) (a) Preparation of Gold sol :- By the reduction of very dilute solution of silver salts with a suitable reducing agent 2AuCl 3 + 3SnCl > 2Au + 3SnCl 4 Gold sol b) Preparation of Sulphur sol :- By the oxidation of H 2 S in the presence of suitable oxidizing agent like nitric acid, bromine water, etc. H 2 S + Br > S + 2HBr > 2H 2 O + 2NO 2 H 2 S + 2HNO S (Q.) What are macromolecular and multimolecular colloids? How are they different from associated colloids? (3 Marks) (Ans) Macromolecular colloids:- i)they are molecules of large size. ii)they have lyophobic property. Multimolecular colloids:- i) They are formed by the aggregation of large number of atomsor molecules which have diameter less than 1nm. ii) They have lyophilic property. Associated colloids:- i) They are formed by the aggregation of large number of ions in concentrated solution ii) They contain both lyophilic and lyophobic groups

7 (Q.) What are lyophilic and lyophobic solutions? Give examples for each. (3 Marks) (Ans) Lyophilic solutions are those that can be prepared by directly mixing the dispersed phase with dispersion medium. For example starch dissolved in water. Lyophobic solutions are those that can not be prepared directly but some special methods are used to prepare them. For example metal sulphides when mixed with a dispersion medium directly do not result in any colloid. (Q.) "Action of soap is due to emulsification and micelle formation". Comment. (3 Marks) (Ans) Yes, action of soap is due to emulsification and micelle formation. Soaps are sodium salt of higher fatty acids like sodium stearate, C 17 H 35 COO - Na + The anionic head of stearate ion (-COO - ) is hydrophobic in nature and has great affinity for water, while the hydrocarbon part (C 17 H 35 - ) is hydrophilic in nature and great affinity for oil, grease etc. When soap is used in water, the anions (C 17 H 35 COO - ) form micelle and due emulsification encapsulate oil or grease inside. These micelle are removed by rinsing with water; while free dirt (from oil or grease) either settle down or are washed away by water. Thus the main function of a soap is to entrap oil or grease with the micelles through emulsification, thereby freeing dirt from grease and oil. (Q). Explain What is observe when i) An electrolyte, NaCl is added to hydrate ferric oxide sol. ii) Electric current is passed through a colloidal sol. iii) When a beam of light is passed through a colloidal sol. Ans (i) The positively charged colloidal particles of Fe(OH)3 get coagulated by the positively charged Cl - ions provided by NaCl. (ii) On passing direct current, colloidal particles move towards the positively charged electrode where they lose their charge and get coagulate (iii) scattering of light by the colloidal particles takes place and the path of light becomes visible (Tyndall effect). (Q).Describes some features of catalysis by Zeolites? Ans Features of catalysis by Zeolites:- I) Zeolites are hydrated alumino silicates which have a three dimensional network structure containing water molecules in their pores.

8 II) To use them as catalysts, they heated so that water of hydration present in the pores is lost and the pores become vacant. III) The size of pores varies from 260 to 740 pm. Thus, only those molecules can be adsorbed in these pores and catalyzed whose size is small enough to enter these pores. Hence, they act as molecular sieves or shape selective catalysts. An important catalyst used in petroleum industries in zsm-5. It converts alcohols into petrol by first dehydrating them to form a mixture of hydro carbons. (Q) Write short notes on followings:- (a) Tyndall effect (b) Brownian Movement (c) Hardy Schulze Rule Ans- (a)tyndall effect-scattering of light by colloidal particles by which path of beam becomes clearly visible. this effect is known as tyndall effect (b) Brownian movement-zig-zag motion of colloidal particles. (c) Hardy Schulze Law Coagulating value of a coagulating ion is directly proportional to the charge on the ion. e.g: Na + < Ca ++ < Al 3+ for negatively changed sol. Cl - < CO3 2- < PO4 3- < [Fe (CN) 6 ] 4 for positive sol. VALUE BASED QUESTIONS : Q.1 Use of gas mask is recommended in a crowded place, a place of fire so that poisonous gases are adsorbed.does it adsorb gases like N 2 and O 2 equally? Explain your answer citing a suitable value behind this. Q.2 Milk and cold dream are examples of colloidal system. Which type of colloid they are? Can they be dilutedby adding water? Based upon your observation/ answer differentiate between them. To protect untimely curdling of milk how will you utilize the properties of colloidal solution? Extract the value associated with it. HOTS (Q.) Why the sun looks red at the time of setting? Explain on the basis of colloidal properties. (3 Marks) (Ans) At the time of setting, the sun is at the horizon. The light emitted by the sun has to travel a longer distance through the atmosphere. As a result, blue part of the light is scattered away by the dust particles in the atmosphere. Hence, the red part is visible. (Q.) Explain the reason for these: (a) Sky looks blue in colour. (b)delta is formed at the meeting place of river and sea water. (c) Blood coagulate on treatment of alum. (3 Marks) (Ans) (a) Sky looks blue in colour because colloidal particles suspended in environment scatter the light and blue light is scattered maximum. (b) The charged colloidal particles of river water neutralized by ions present in sea water so coagulation take place. (c) The charged colloidal particles present in blood are neutralized by ions of alum.

1. Chemisorption is highly specific in nature. It occurs only if there is a possibility of chemical bonding between the adsorbent and the adsorbate.

Question 5.1: Write any two characteristics of Chemisorption. 1. Chemisorption is highly specific in nature. It occurs only if there is a possibility of chemical bonding between the adsorbent and the adsorbate.