Revision questions from Surface chemistry Answer the following questions.
1. Which will be adsorbed more readily on charcoal : Ammonia or carbon dioxide? Why? A. Ammonia because it has higher critical temperature and is more easily liquefiable. Hence it has greater intermolecular force of attraction and is adsorbed e asily. 2. Why a finely divided substance like charcoal is more e ffective as an adsorbent? A. Finely divided substance has a larger surface ar ea and hence extent of adsorption is gr eater. 3. Which of the following is most effective in coagulating ferric hydroxide sol and why? KCl, FeCl3 ,Na2SO4 , K3[Fe(CN)6] A. Fe(OH)3 sol is positively charged and according to Hardy-Schulz rule, anion having the highest charge will be 3-
more effective in coagulating. Hence [Fe(CN)6] has most coagulating power. 4. What happens when a freshly precipitated Fe(OH) Fe(OH)3 is shaken with a little little amount of dil. solution of FeCl3? 3+
A. Peptisation takes place. Fe ions from FeCl3 are adsorbed on the surface of t he precipitate. These positively charged particles repel eachother and form colloidal solution. 5. What is the difference between a sol and a gel? A. In sol, dispersion medium is liquid and the dispersed phase is solid. In gel, the dispersion medium is solid and dispersed phase is liquid. 6. What happens when a colloidal solution of Fe(OH) 3 and As2O3 are mixed? A. Mutual precipitation takes place. This is because Fe(OH) 3 is is positively charged and As 2O3 is negatively charged. When they are mixed , the particles get neutralised and get precipitated. 7. FeCl3 is more effective than KCl in coagulating blood. blood. Why? A. Fe
3+
+
ion has more charge than K ion. Hence it has more coagulating power and is more effective in
coagulating blood. 8. How is adsorption of gas related to its critical temperature? A. Higher the critical temperature, greater is ease of liquification of gas and hence larger is the vander Waal’s force of attraction. 9. What is observed when : i.
A beam of light is passed through a colloidal solution of As2O3
ii.
An emulsion is subjected to heating / c entrifugation. A. i. Tyndall effect occurs i.e the path of light becomes visible due to the scattering of light by colloidal particles. ii.De-emulsification takes place and the emulsion will get separated into its constituent liquids. Page 1
10. The enthalpy of adsorption of chemisorption is higher than that of physisorption . Why? A. In case of Chemisorption the adsorbent and the adsorbate molecules are held together by strong chemical bonds. Where as in case of Physisorption, there is only weak van der waal’s force of attraction between
these molecules. 11. Why is the colour of sky blue? A. Dust particles along with water suspended in air scatter blue light which reaches our eyes and it appears blue. 12. In Chemisorption, adsorption first increases and then decreases with temperature. Why? A. In chemisorption, for the formation of chemical bonds between the adsorbate and adsorbent molecules, activation energy is required . The r ate of adsorption first increases with temperature as more molecules will gain energy and possess energy greater than activation energy. On further increasing the temperature , it will increase the energy of the molecules and desorption will take place. 13. Sun looks red at the time of sun set. Why? A. The sun is at the horizon at t he time of sunset. The light emitted by the sun has to travel a relatively longer distance through the atmosphere. Due to t his the blue part of the light is scattered away by the particulate matter in the atmosphere causing red part to be visible. 14. Physisorption is multimolecular while chemisorption is monomolecular. Why? A. Chemisorption occurs as a result of the reaction between adsorbent and adsorbate. When the surface of the adsorbent is covered with one layer ,no further reaction can take place. Physical adsorption involves van der Waal’s forces. So any number of layers may be formed one over the other on the surface of the
adsorbent. 15. What causes Brownian movement in a colloidal solution? A. It is caused due to unbalanced bombardment of the colloidal particles by the molecules of the dispersion medium. 16. Explain what is observed when: i.
An electrolyte , KCl is added to hydrated ferric oxide sol.
ii.
An electric current is passed through a colloidal solution.
iii.
Persistant dialysis of a colloidal sol is carried out. A. i.Coagulation takes place as ferric hydroxide is precipitated. ii.Electrophoresis takes place. i.e movement of colloidal particles towards oppositely charged electrodes . iii.The electrolyte is almost completely removed which leads to coagulation of sol.
17. What is autocatalysis? A. It is a phenomenon in which one of the product formed during the chemical reac tion act as a catalyst and speeds upt he reaction. Eg. Mn
2+
ions produced during oxidation of oxalic acid by acidified KMnO 4 act as
catalyst and speed up the chemical reaction as it progresses. Page 2
18. How are the following colloids different from each other in respect of their dispersion medium and dispersed phase. Give 1 eg of each.Aerosol, emulsion, hydrosol A. i. Aerosol: D.P – solid or liquid , D.M-Gas
eg. Smoke, dust, fog , mist
ii.Emulsion: D.P – liquid , D.M- liquid.
eg. milk , cream
iii.Hydrosol : D.P – solid D.M- water
eg. salt in water, sugar in water
19. Explain how the phenomenon of adsorption finds application in each of the following processes: i.
Production of vacuum: The remaining traces of air can be adsorbed by charcoal from a vessel to c reate a vacuum.
ii.
Heterogeneous catalysis: Adsorption of reactants on the solid surface of t he catalysts increases the rate of reaction.
iii.
Froth flotation process: In this process, sulphide ore is concentrated by using pine oil which adsorbs t he ore particles and impurities are wetted by water which settle at the bottom.
Important terms: 1. Physisorption: The adsorbent and adsorbate molecules are held together by weak van der Waal’s forces. eg. adsorption of water vapour on silica gel 2. Chemisorption: The adsorbent and adsorbate molecules are held together by strong chemical bonds. Eg. When oxygen is adsorbed on metals, m etal oxide is formed. 3. Adsorption isotherm: The variation in the amount of gas adsorbed by adsorbent with pressure at constant temperature can be expressed by a curve termed as adsorption isotherm. 4. Homogeneous catalyst : The catalyst is in the same phase as the reactants. Eg.
2SO2(g) + O2
2SO3(g)
NO(g)
5. Heterogeneous catalyst: eg. 2SO 2(g) + O2(g)
Pt(s)
The catalytic reaction in which reactants and catalysts are in different phases. 2SO3(g)
6. Promotors: Substances that enhance the activity of a catalyst are called promoters.eg. molybdenum acts as a promoter for catalyst iron in the Haber’s process for manufacture of ammonia.
7. Activity : It is the ability of a catalyst to increase the rate of chemical reaction. Activity depends upon the strength of chemisorptions. Eg. 2H2(g) + O2(g )
Pt__
2H2O(l)
8. Selectivity: It is the ability of the catalyst to direct a reaction to give a particular product. Eg. i. CO(g) + 3H2(g) ___Ni___ ii. CO(g) + 2H2(g)
CH4(g) + H2O
Cu-ZnO-Cr2O3
iii. CO(g) + 2H2(g) __ Cu___
CH3OH(g) HCHO(g)
9. Shape selective catalysts: The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules eg. Zeolites
10. Lyophillic colloids: The colloidal solution in which the particles of the dispersed phase have greater affinity for dispersion medium. Eg . gelatin 11. Lyophobic colloids: The colloidal solution in which there is no affinity between the particles of dispersed phase and dispersion medium.eg. Fe(OH) 3 12. Multimolecular colloids: In this colloid, large number of atom/molecules with diameter less than 1nm aggregate together to form species having size in the colloidal range. Eg. sulphur sol, gold sol 13. Macro molecular colloids: Macro molecules on dissolution in a suitable solvent form a colloid in which the size of the macromolecules may be in colloidal range. Eg. starch 14. Associated colloids: These substance which behave as normal strong electrolytes at low concentration, but at higher concentration exhibit colloidal behaviour due to formation of aggregates called mice lles. Eg. soap solution 15. Peptisation : The process of converting a freshly prepared precipitate into colloidal form by the addition of suitable electrolyte. 16. Dialysis : The process of separating the particles of colloids from those of crystalloids by means of diffusion through a semi permeable membrane. 17. Electrodialysis: The process of separating the particles of colloids from those of crystalloids by means of diffusion through a semi permeable membrane is called dialysis. This process is slow. To increase the process of purification , dialysis is carried out by applying electric field. This is called e lectrodialysis. 18. Ultra filtration : The process of removing the impurities from t he colloidal solution by passing it through ultra filter paper which does not allow the c olloidal particles to flow through it. 19. Brownian movement : It is the zig zag movement of the colloidal particles due to the unbalanced bombardment between the molecules of dispersed phase and dispersion medium. 20. Tyndall effect : The phenomenon of scattering of light by colloidal particles as a result of which the path of the light becomes visible. 21. Electrophoresis : It is the phenomenon of movement of particles towards anode or cathode under an applied field. Once these particles reach the electrode , they lose their charge and coagulate (precipitate) 22. Coagulation / Precipitation : It is the process of settling of colloidal particles when converted to prec ipitate. 23. Electrosmosis: The phenomenon of movement of molecules of dispersion medium under the influence of electric field, whereas the colloidal particles are not allowed to move. 24. Hardy-Schulz rule : Coagulating power of an electrolyte is directly proportional to the valency of the flocculating ion. 25. Emulsification: It is the process of preparing an emulsion by mixing two liquids and stabilizing it by means of an emulsifier. 26. De-emulsification: It is the process of separation of emulsion into constituent liquids by freezing , heating or centrifugation.
