Surface chem class 12

class XII CHEMISTRY ,

1. Surface chemistry Class 12 Unit 5

2. Surface chemistry deals with phenomena that occur at the surfaces or interfaces . Surface is a layer of separation of two phases Surface chemistry Surface Chemistry is the branch of chemistry which deals with the phenomenon that occurs on the surfaces or interfaces, such phenomenon includes corrosion. catalysis, crystallisation , etc In this Unit, you will be studying some important features of surface chemistry such as adsorption,catalysis and colloids including emulsions and gels.

3. Adsorption The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption . The molecular species accumulates at the surface is termed as adsorbate The material on the surface of which the adsorption takes place is called adsorbent Adsorbate Adsorbent e.g.. ( i ) O 2 , H 2 , C1 2 , NB 3  gases are adsorbed on the surface of charcoal. (ii) Silica gels adsorb water molecules from air. Charcoal, silica gel, metals such as Ni, Cu, Ag, Pt and colloids are some adsorbents.

4. Important Characteristics of Adsorption 1. It is specific and selective in nature. 2. Adsorption is spontaneous process, therefore change in free energy (ΔG)is negative. ΔG= ΔH – TΔS, For the negative value of ΔG,in a system, in which randomness decreases, ΔH must be negative. Hence, adsorption is always exothermic. Adsorption of hydrogen over Pt is called  occlusion .

5. Desorption It is a process of removing an adsorbed substance from a surface on which it is adsorbed, is known as desorption .

6. Sorption It is a process in which both adsorption and absorption take place simultaneously, the term sorption is simply used.

7. Positive and Negative Adsorption When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk, it is called positive adsorption. On the other hand, if the concentration of the adsorbate is less relative to its concentration in the bulk, it is called negative adsorption, e.g., when a dilute solution of KCl is shaken with blood charcoal, it shows negative adsorption.

8. Distinction between Physisorption and Chemisorption

9. Factors Affecting Adsorption (a)  Nature of adsorbent  Same gas may be adsorbed to different extents on different adsorbent. (b)  Surface area of the adsorbent  Greater the surface area, greater is the extent of adsorption. (c)  Nature of the gas being adsorbed  Greater is the critical temperature of a gas, greater are the van der Waals’ forces of attraction and thus, greater is the adsorption.

10. d)  Temperature   Adsorption is an exothermic process involving the equilibrium : Gas (adsorbate) + Solid (adsorbent) ⇔ Gas adsorbed on solid + Heat Applying Le- Chatelier principle, increase of temperature decreases the adsorption and vice-versa. ( e)  Pressure  Adsorption increases with pressure at constant temperature. The effect is large if temperature is kept constant at low value. (f)  Activation of the solid adsorbent  Activation means increasing the adsorbing power of the solid adsorbent. This can be done by subdividing the solid adsorbent or by removing the gases already adsorbed by passing superheated steam.

11. Adsorption Isotherms It is the plot of the mass of gas adsorbed per gram of adsorbent (x / m) versus equilibrium pressure at constant temperature . Freundlich Adsorption Isotherm It gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. It can be expressed by the equation. x / m = kp 1/n  …( i ) Where, x is the mass of the gas adsorbed on mass m of the adsorbent at pressure p, k and n are constants which depend on the nature of the adsorbent and the gas at a particular temperature.

12. At low pressure, n = 1, i.e., x / m = kp At high pressure, n > 1, i.e., x / m = k (independent of p) Taking logarithm of Eq. ( i )

13. Freundlich Adsorption Equation for Solutions x / m = kC 1/n where, C is the equilibrium concentration. On taking logarithm of the above equation, we have

14. Langmuir Adsorption Isotherm According to Langmuir, the degree of adsorption is directly Proportional to e, i.e., the fraction of surface area occupied. x / m α θ = kθ

15. Adsorption Isobars These are plots of x / m us temperature t at constant pressure. For physical and chemical adsorption, they are shown below .

16. Applications of Adsorption For production of high vacuum. Gas masks containing activated charcoal is used for breathing in coalmines. They adsorb poisonous gases. Silica and aluminium gels are used as adsorbents for controlling humidity. Removal of colouring matter from solutions. It is used in heterogeneous catalysis. In separation of inert gas. As adsorption indicators. In chromatographic analysis. Qualitative analysis, e.g., lake test for Al 3 +.

17. Catalysis Catalyst is a chemical substance which can change the rate of reaction without being used up in that reaction and this process is known as catalysis Promoters: These are the substances which increase the activity of catalyst. Example – Mo is promoter whereas Fe is catalyst in Haber’s Process. Catalytic poisons (Inhibitors): These are the substances which decrease the activity of catalyst. Example -Arsenic acts as catalytic poison in the manufacture of sulphuric acid by ‘contact process.’

18. Types of Catalysis ( a)  Homogeneous catalysis  In this catalysis, and the catalyst reactants are in the same physical state [phase], e.g., (b)  Heterogeneous catalysis   In heterogeneous catalysis, catalyst is present in a different phase than that of reactants, e.g., (c)  Autocatalysis  When one of the product of a reaction acts as catalyst, the process is called autocatalysis.

19. Adsorption Theory of Heterogeneous Catalysis The mechanism involves five steps: ( i ) Diffusion of reactants to the surface of the catalyst (ii) Adsorption of reactant molecules on the surface of the catalyst. (ill) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate. (iv) Desorption of reaction products from t he catalyst surface. (v) Diffusion of reaction products away from the catalyst’s surface

20. Important Features of Solid Catalysts Activity of catalyst : It is the ability of a catalyst to increase the rate of a chemical reaction . Selectivity of catalyst: It is the ability of catalyst to direct a reaction to yield a particular product (excluding others). For example:  CO and H 2  react to form different products in presence of different catalysts as follows:

21. Shape – selective catalysis: It is the catalysis which depends upon the pore structure of the catalyst and molecular size of reactant and product molecules. Example – Zeolites are shape – selective catalysts due to their honey- comb structure. Enzymes : These are complex nitrogenous organic compounds which are produced by living plants and animals. They are actually protein molecules of high molecular mass. They are biochemical catalysts Steps of enzyme catalysis: i ) Binding of enzyme to substrate to form an activated complex. (ii) Decomposition of the activated complex to form product. Some examples of enzyme catalysed reactions are: Source of invertase , zymase and maltose is yeast and that of diastase is malt. Soybean is the source of urease.) Lactobacilli is used to convert milk into curd. In stomach, the pepsin enzyme converts proteins into peptides while in intestine, the pancreatic trypsin converts proteins into amino acids by hydrolysis .

22. Characteristics of enzyme catalysis: ( i ) They are highly efficient. One molecule of an enzyme can transform 106 molecules of reactants per minute. (ii) They are highly specific in nature. Example – Urease catalysis hydrolysis of urea only. (iii) They are active at optimum temperature (298 – 310 K). The rate of enzyme catalysed reaction becomes maximum at a definite temperature called the optimum temperature. (iv) They are highly active at a specific pH called optimum pH. (v) Enzymatic activity can be increased in presence of coenzymes which can be called as promoters. (vi) Activators are generally metal ions Na + , Co 2+  and Cu 2+  etc. They weakly bind to enzyme and increase its activity. (vii) Influence of inhibitors (poison): Enzymes can also be inhibited or poisoned by the presence of certain substances .

23. True solution: ( i ) It is homogeneous. (ii) The diameter of the particles is less than 1 nm. (iii) It passes through filter paper. (iv) Its particles cannot be seen under a microscope. Colloids: ( i ) It appears to be homogeneous but is actually heterogeneous. (ii) The diameter of the particles is 1 nm to 1000 nm. (iii) It passes through ordinary filter paper but not through ultra-filters. (iv) Its particles can be seen by a powerful microscope due to scattering of light.

24. Suspension: ( i ) It is heterogeneous. (ii) The diameter of the particles are larger than 1000 nm. (iii) It does not pass through filter paper. (iv) Its particles can be seen even with naked eye

25. Dispersed phase: It is the substance which is dispersed as very fine particles. Dispersion medium: It is the substance present in larger quantity .

26. Name Dispersed phase Dispersed medium Examples Solid sol solid Solid Coloured gem stones Sol Solid Liquid Paints Aerosol Solid Gas Smoke, dust Gel Liquid Solid Cheese, jellies Emulsion Liquid Liquid Hair cream, milk Aerosol Liquid Gas Mist, fog, cloud Solid sol Gas Solid Foam rubber, pumice stone Foam Gas Liquid Whipped cream Classification of colloids on the basis of the physical state of dispersed phase and dispersion medium:

27. Classification of colloids on the basis of nature of interaction between dispersed phase and dispersion medium, the colloids are classified into two types namely, Lyophobic sols Lyophilic sols Lyophobic sols: ( i ) These colloids are liquid hating. (ii) In these colloids the particles of dispersed phase have no affinity for the dispersion medium. (iii) They are not stable. (iv) They can be prepared by mixing substances directly. (v) They need stabilizing agents for their preservation. (vi) They are irreversible sols.

28. Lyophilic sols: ( i ) These colloids are liquid loving. (ii) In these colloids, the particles of dispersed phase have great affinity for the dispersion medium. (iii) They are stable. (iv) They cannot be prepared by mixing substances directly. They are prepared only by special methods. (v) They do not need stabilizing agents for their preservation. (vi) They are reversible sols.

29. Based on the Types of Particles of the Dispersed Phase- Multimolecular Colloids : Many particles(atoms or small molecules) of the dispersed phase aggregate together to form species having the size of a colloidal particle( 1-1000nm). These colloids are called multimolecular colloids. Example-gold sol, Sulphur sol   Macromolecular colloids: Substances with large molecules (macromolecules) in suitable solvents form solutions but these macromolecules might be in the colloidal range. These solutions are called macromolecular colloids and resemble true solutions in many ways. Example- Starch, Cellulose, Proteins are natural macromolecules.  Nylon , polythene, polystyrene are man-made macromolecules  

30. Associated Colloids: Some substances at high concentrations act as colloids due to the formation of aggregates. But at low concentrations they behave like normal strong electrolytes. These aggregates formed are called micelles. Such colloids are called associated colloids. Kraft Temperature- The formation of micelles takes place only above a particular temperature called Kraft’s temperature Critical Micelle Concentration(CMC) – The concentration above which micelle formation takes place Example- Soaps, synthetic detergents

31. Mechanism Of Micelle Formation- Soap is the sodium or potassium salt of fatty acid and may be represented as RCOO -  Na (e.g. sodium stearate, (CH 3 (CH 2 )16COO - Na + ]) .   When dissolved into water , it dissociates into  RCOO -  and  Na +  ions The RCOO - ion consists of two parts –  long hydrocarbon chain (also called  non – polar tail)  which is hydrophobic (water repelling ) and a polar group  COO -     (polar head) which is hydrophilic (water loving ) At higher concentrations(CMC) RCOO- ions form an aggregate of spherical shape with the hydrocarbon chains pointing towards the centre and the COO- part facing outward on the surface of the sphere. This aggregate is called ionic micelle. It may have as many as 100 ions

32. Cleansing Action of Soaps The cleansing action of soap is due to the formation of micelle by the soap molecules in such a way that the hydrophobic part is in the oil droplet(dirt) and the hydrolphilic part projects out. Since the polar groups interact with watersurrounded by soap ions is pulled from the surface and pulled into water.

33. Preparation of Colloids A few ways to prepare colloids are given below   Chemical method : Colloids can be prepared by chemical reactions leading to formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate to form sols 

34. Electrical Disintegration/ Bredig’s arc Method: This is applied to obtain colloidal sols of metals like gold, silver and platinum.  An electric arc is stuck between the electrodes of the metal immersed in the dispersion medium. The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size .

35. Peptization -   It is the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this is called peptizing agent. During peptization the precipitate absorbs the one of the ions of the electrolyte on its surface. This causes + veor – vecharge to develop on the precipitate, which ultimately break up into small particles of the size of a colloid.

36. Purification of Colloids The process used for reducing the amount of impurities to a required minimum is called purification of colloids.  Some of processes used to do so are- Dialaysis - The process of sepeartaing the particles of a colloid by diffusion through a suitable membrane . Process – An apparatus called dialyser is used. A bag with a suitable membrane containing the colloid is suspended in a vessel through which fresh water flows continuously. The impurities diffuse through the membrane into the water leaving behind the colloid

37. Electro-dialysis- Dialysis is a slow method. It is made faster by this method. In this method, an electric filed is applied using metal electrodes. These ions present in the colloidal solution migrate out to the oppositely charge to electrodes. It is possible only if the dissolved substance in the impure colloid is an electrolyte.  

38. Ultrafiltration- This is the process of separating colloidal particles from the soluble solutes(impurities) using specially prepared filters, which are permeable to all substances except the colloid. Colloidal particles can usually pass through filter papers as the pores are too large.  An ultra filter paper can be made by soaking the filter paper in a colloidal solution, hardening by formaldehyde and then finally drying it. As this is slow process, pressure or suction is applied to speed it up. The colloidal particles left on the ultra-filter paper are stirred with fresh dispersion medium(solvent) to form a pure colloid.

39. Properties of Colloids Colligative Properties- The particles in colloids are bigger aggregates than those in a true solution. So, the number of particles in a colloid is lesser than a true solution of the same concentration. The values of colligative properties (osmotic pressure, lowering of vapour pressure, depression in freezing point, elevation in boiling point) are of small order as compared to values shown by true solution at same concentration .

40. The Tyndall effect is the phenomenon in which the particles in a colloid scatter the beams of light that are directed at them. This effect is exhibited by all  colloidal solutions  and some very fine suspensions. Therefore, it can be used to verify if a given solution is a colloid. The intensity of scattered light depends on the density of the colloidal particles as well as the frequency of the incident light. When a beam of light passes through a colloid, the colloidal particles present in the solution do not allow the beam to completely pass through. The light collides with the colloidal particles and is scattered (it deviates from its normal trajectory, which is a straight line). This scattering makes the path of the light beam visible, as illustrated below. Tyndall effect-

41. Tyndall effect- Tyndall effect is the scattering of the light by the particles present in the colloidal solution when viewed at right angles to the passage of light. It is observed only when- The diameter of the dispersed particles is not much smaller than the wavelength of light used The refractive indices of the dispersed phase and dispersed medium have a large difference This effect was used to make an ultramicroscope and differentiate between true solution and colloids.

42. Colour - The colour of the colloidal solution depends on the wavelength of the light scatter by the dispersed particles, size and nature of the dispersed particles and the manner in which it is viewed.  Example- Finest gold sol is red in colour and as the size of the particle keeps increasing its colour changes to blue, then purple and finally gold. Brownian Movement- Brownian movement may be defined as continuous zigzag movement of the colloidal particles in a colloidal solution. It depends on the size of the particles and the viscosity of the colloid. Smaller the size of the particle and lesser its viscosity, faster is its motion. This movement is responsible for the stability of sols

43. Charge on Colloids- Colloidal particles always have an electric charge. The nature of this charge is the same on all the particles in a given colloidal solution and maybe either + ve   or – ve . Positively charged sols- Haemoglobin Oxides-TiO 2  solution Hydrated Metal oxides like Al 2 O 3 .xH 2 O , Fe 2 O 3 .xH 2 O Dyes-methylene blue sols Negatively charged sols- Metals- Cu, Ag, Au sols Metallic sulphides-As 2 S 3 , CdS Acid dye stuffs- Eosin Sols of starch, gelatin

44. Electrophoresis- The movement of colloidal particles under the influence of an electric field is called electrophoresis. Negatively charged particles move towards the cathode and Positively charged particles moves towards anode. When the movement of particles is prevented, it is observed that the dispersion medium starts to move in the electric field. This is called electroosmosis .

45. Coagulation-  It is process of settling of colloidal particles. Also called precipitation of sol COAGULATION OF LYOPHOBIC SOLS-    Coagulation of lyophobic sols can be done by the following methods: By electrophoresis - The colloidal particles move towards oppositely changed electrodes get discharged and precipitate. By mixing two oppositely charged sols - Oppositely charged sols when mixed together in almost equal proportion, neutralise their charges and get partially or completely precipitated. By Boiling- When a sol is boiled the adsorbed layer is disturbed due to increased number of collisions with the molecules of the dispersion medium. This reduces the charge on the particles and they ultimately settle down in the form of a precipitate. By Persistent dialysis - On prolonged dialysis, traces of the electrolyte present in the sol are removed almost completely. Colloids become unstable and coagulate. By addition of electrolyte  - When excess of electrolyte is added, colloidal particles precipitate as colloids interact with ions carrying charge opposite to that present on themselves. This causes neutralisation leading to their coagulation. Example- A negatively charged ion when added to a positively charged sol causescoagulation . The negatively charged ion is called coagulating ion/ flocculative ion as it neutralises the colloid to cause coagulation.  

46. COAGULATION OF LYOPHILLIC SOLS- Lyophilic sols are stable because of charge and solvation of colloidal particles. So we remove these two factors to coagulate them. This is done by Addition of an electrolyte Addition of a suitable solvent PROTECTION OF COLLOIDS - Lyophilic sols are more stable than lyophobic sols Lyophilic colloids have a unique ability to protect lyophobic colloids from electrolytes When a lyophilic sol is added to lyophobic sol, the lyophilic particles (colloids) form a layer around the particles of lyophobic sol Lyophilic colloids are also called protective colloids HARDY-SCHULZE RULE- The greater the valency of the flocculating ion added, the greater is its precipitation. For negative sols, when positive ions are added            Al 3+ > Ba 2+ >Na +  is the order in terms of flocculating power For positive sols, when negative ions are added            [Fe(CN) 6 ]  4- > PO 4 3- >SO 4 2- > Cl -  is the order in terms of flocculating power

47. Emulsions Emulsions are colloids where both the dispersion phase and dispersion medium are liquids. These two liquids are immiscible or partially miscible. Generally one of the liquids is water. There are two types of emulsions Oil dispersed in water (o/w type)- Water acts as dispersion medium Example –Milk, vanishing cream Water dispersed in oil (w/o type)- Oil acts as dispersion medium Example- Butter, Cream Emulsions like water and oil separate into two layers and make the emulsion unstable. So emulsions are stabilised by stabilising agents.

48. o/w emulsions are stabilised by proteins, gum, natural and synthetic soaps w/o emulsions are stabilised by metal salts of fatty acids, long chain alcohols

49. Applications of Colloids Electro precipitation of smoke – The smoke is led through a chamber containing plates having a charged opposite to that carried by smoke particles. The particles on coming in contact with these plates lose their charge and get precipitated.  The particles settle down on the floor of the chamber. The precipitator is called Cottrell precipitator.

50. Purification drinking water – Alum is added to impure water to coagulate the suspended impuritiesand  make water fit for drinking. Medicines – Most of the medicines are colloidal in nature. Colloidal medicines are bmore effective because they have a larger surface area and are more easily absorbed by the body. Eg - Argyrol is a silver sol used as an eye lotion, milk of magnesia is used to cure stomach disorders Tanning –  Animal hides are colloidal in nature. When a hide that has positively charged particles is soaked in tannin/chromium salts, which contains negatively charged particles , mutual coagulation takes place. This results in the hardening of leather. This process is termed as tanning. Photographic plates and films –  Photographic plates and films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films. Cleansing action of soaps- already explained Rubber industry-  Latex is a colloidal solution of rubber particles which are negatively charged. Rubber is obtained by coagulation of latex. Industrial products-  Paints inks, synthetic plastics, rubber, cement, graphite lubricants are all colloids