PHYSICS-15-10- 11th (PQRS) SOLUTION

https://docs.google.com/document/d/1f1ar1mFlQs0G1BBu-W0cmJlNBMu-dOmV/edit?usp=sharing&ouid=109474854956598892099&rtpof=true&sd=true Surface Chemistry Adsorption- Physisorption and chemisorption and their characteristics, factors affecting adsorption of gases on solids - Freundlich and Langmuir adsorption isotherms, adsorption from solutions. Catalysis - Homogeneous and heterogeneous, activity and selectivity of solid catalysts, enzyme catalysis and its mechanism. Colloidal state - distinction among true solutions, colloids and suspensions, classification of colloids - lyophilic, lyophobic; multi molecular, macromolecular and associated colloids (micelles), preparation a nd properties of colloids - Tynda ll effec t, Brownia n movement, electrophoresis, dialysis, coagulation and flocculation; Emulsions and their characteristics. ADSORPTION The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or a solid resulting into higher concentration of the molecules on the surface is called adsorption There are two types of adsorption Physical adsorption Chemical adsorption Physical adsorption or Physisorption Chemical adsorption or Chemisorption 1. Enthalpy of adsorption usually is of the order of –20 kJ mol–1 i.e. exothermic Molecule of adsorbate and adsorbent are held by Van der Waal interaction It usually takes place at low temperature and decreases with increasing temperature It is not very specific i.e. all gases are adsorbed on all solids to some extent Multimolecular layers may be formed on adsorbent Enthalpy of adsorption is of the order of –200 kJ mol–1. Molecules of adsorbate and adsorbent are held by chemical bonds It takes place relatively at high temperature It is highly specific and take place when there is some possibility of compound formation between adsorbate and adsorbent molecule. Usually monomolecular layer is formed on the adsorbent. ADSORPTION ISOTHERM A relation or graph between x/m (x are number of moles of adsorbate and m is the mass of adsorbent) and the pressure (P) of the gas at a constant temperature is called adsorption isotherm. C H A P T E R CHAPTER INCLUDES Adsorption Adsorption isotherm Freundlich adsorption Isotherm Langmuir adsorption isotherm Catalysis Homogeneous a nd Hetrogeneous catalysis Shape selective catalysis by zeolites Enzyme catalysis Colloids Classification of colloids Based on physical state of dispersed phase and dispersion medium Based on nature of interaction between dispersed phase and dispersion medium Emulsions and its type Preparation of colloids General properties of colloids Purifications of colloids Freundlich Adsorption Isotherm Freundlich gave an equation x/m = KP1/n (n > 1) to explain the effect of pressure on amount of gas adsorbed where K and n are parameters of the equations depending upon the nature of the gas and solid. x increases with increase in pressure. Since n > 1, so x/m does not increase, as rapidly as 'P' m log x/m = logK + 1 log P (Taking log on both sides) n log P → From this graph it is possible to find out value of K and n Langmuir Adsorption Isotherm Langmuir considered that adsorption to consist of the two opposing processes i.e. adsorption and desorption both take place and dynamic equilibrium established between the above two processes. He also assumed that the layer of the adsorbed gas was only one molecule thick i.e. unimolecular and for chemisorption. The Langmuir adsorption isotherm is represented by the relation. x = ap m 1+ bp where a and b are two Langmuir parameters. x = a (At very high pressure 1 + bP −~ bP) m b and x = aP (At very low pressure 1 + bP ≅ 1) m So at high pressure of the gas x remains constant and nature of graph is linear at very high pressure m Pressure → In order to determine the parameters a and b, we may write m = 1+ bP = b + 1 x aP a aP A plot of m x against 1 P 1 gives a straight line with slope and intercept equal to a b and a respectively m x CATALYSIS 1 P The process of changing the rate of a chemical reaction by addition of a foreign substance (catalyst) is called catalysis. The catalyst is specific in nature and change the rate of a particular reaction by providing an alternate path of different activation energy. When catalyst increases the rate of reaction, it is called positive catalyst and decreases the ratio of reaction is called negative catalyst. Ex. 2KClO3 ⎯⎯MnO⎯2 (⎯s) → 2KCl(s) + 3O2 (g) MnO2 (s) act as positive catalyst and phenomenon is known as positive catalysis 2H2O2 (l) ⎯⎯ace⎯tan⎯ilide⎯(⎯s) → 2H2O (l) + O2 (g) Acetanilide (s) act as negative catalyst and process is known as negative catalysis. HOMOGENEOUS AND HETEROGENEOUS CATALYSIS A reaction in which the catalyst and the reacting substances are present in the same phase, is called homogeneous catalysis Ex. 2SO2 (g) + O2 (g) ⎯⎯NO⎯(⎯g) → 2SO3 (g) CH COOCH (aq) + H O (l) H+(aq) CH COOH(aq) + CH OH(aq) 3 3 2 ⎯⎯ ⎯→ 3 3 A reaction in which the catalyst is present in a phase different from that of the reacting substances is called heterogeneous catalysis. All the reactions in this case occur at the surface of the catalyst Ex. N2(g) + 3H2(g) Finely divided Fe (s) 2NH3 (g) 2SO2 (g) + O2(g) P(s) or V2O5(s) 2SO3 (g) SHAPE SELECTIVE CATALYSIS BY ZEOLITES Zeolites are the most important metal oxide or aluminosilicates catalysts. These catalysts are widely used in the petrochemical industries for (i) cracking of hydrocarbons and (ii) for aromatisation. Zeolites are microporous silicates having the general formula Mx / n[(AlO2 )x (SiO2 )y ] . mH2O Shape-selectivity is the most remarkable feature of zeolite catalysts. The reactions depend on the size of the cavities (Cages) and pores (apertures) present on the surface of zeolite. The pore size varies between 260pm and 760pm. Hence depending upon the size of the reactant and product molecules, as compared to the size and the shape of the cavities and the pores of the zeolite, the reactions can move in a specified manner. A special catalyst, called ZSM-5, is used to convert alcohols such as methanol, ethanol directly to gasoline or petrol. xCH3OH ⎯⎯ZSM⎯−⎯5 → (CH2 )x + xH2O Methanol Catalyst Gasoline Zeolites are also used in water softening. ENZYME CATALYSIS All biological reactions are catalysed by special catalysts called enzymes. Thus enzymes are defined by biogical catalyst. All biological reactions require a different kind of enzyme. Only small amount of enzymes can be highly efficient, speed up the reaction by factors of upto 1020 Enzyme Reaction catalysed Invertase or sucrase Maltase Lactase L – Amylase Pepsin Trypsin Urease Sucrose → Glucose + Fructose Maltose → Glucose + Glucose Lactase → Glucose + Galactose Starch → n × Glucose Proteins → L – Amino acid Proteins → L – Amino acid NH2CONH2 + H2O→ CO2 + 2NH3 Urea COLLOIDS Colloidal state of matter is a state in which the size of particles is such (10Å – 1000Å) that they can pass through filter paper but not through animal or vegetable membrane. CLASSIFICATION OF COLLOIDS Based on physical state of dispersed phase and dispersion medium–The physical state of dispersed phase and dispersion medium may be solids, liquids or gases, eight types of colloidal system are possible. Dispersed Phase Dispersion Medium Name Examples Solid Solid Solid Sol Some coloured glasses, gem stones Solid Liquid Sol Some paints, cell fuids, muddy water Solid Gas Aerosol Smoke, dust Liquid Solid Gel Cheese, butter, jellies Liquid Liquid Emulsion Milk, hair cream Liquid Gas Aerosol Fog, mist, cloud Gas Solid Solid foam Pumice stone, foam rubber Gas Liquid Foam Froth, whipped cream, soap lather Based on nature of interaction between dispersed phase and dispersion medium– Divided into two types namely Lyophilic and Lyophobic Colloids Lyophilic Sols Lyophobic Sols Nature Preparation Stability Hydration Nature & substances Viscosity Surface tension Reversible Prepared by direct mixing with liquid dispersion medium i.e. solvent loving Quite stable and not easily precipitated or coagulated Highly hydrated These sols are usually formed by the organic substances like starch, gum, proteins etc. Much higher than that of medium Lower than that of dispersion medium Irreversible Cannot be prepared directly but by special method i.e. solvent hating Precipitated by adding small amount of suitable electrolyte Not much hydrated These are usually formed by the inorganic materials like metals, their sulphides etc. Almost same as that of medium Nearly same as that of dispersion medium EMULSIONS AND ITS TYPE Emulsions are colloids in which both dispersed phase and dispersion medium are liquids and broadly classified into two types Oil in water emulsion – In this type of emulsion oil act as dispersed phase and water act as dispersion medium. Ex.– Milk, vanishing cream etc. Water in oil emulsion – In this type of emulsion water act as dispersed phase and oil act as dispersion medium. Ex.– Cold cream, butter, cod liver oil etc. PREPARATION OF COLLOIDS Prepared by two types of methods (a) Condensation method (b) Dispersion method Condensation method Condensation Method Physical Method Chemical method As O + 3H S Double As S (sol) + 3H O 2 3 2 decomposition 2 3 2 Exchange of solvent When a true solution is mixed with an excess of other solvent in which the solute is insoluble but Excessive cooling A sol of ice in CHCl3 or ether by freezing the water in the SO + 2H S Oxidation 2AuCl3 + 3HCHO + 3H2O 3S(sol) + 2H2O Reduction 2Au (sol) +3HCOOH+6HCl solvent is miscible. Ex– when a solution of sulphur in alcohol is poured in excess of water sol of S is obtained solvent FeCl3 + 3H2O Hydrolysis Fe(OH)3+3HCl (sol) Dispersion method In this method large particles of the substance are broken into particles of colloidal dimensions in the presence of dispersion medium and stabilised by adding some suitable stabilizer. Some methods given below: Mechanical dispersion – In this method, the course suspension of the substance is brought into a colloidal state in dispersion medium by grinding it in a colloidal mill, ball mill or ultrasonic disintegrator. Electrical disintegration or Bredig's arc method This process involves dispersion as well as condensation colloidal sols of metals such as gold, silver, platinum etc. can be prepared by this method. Electric arc is struck between electrodes of metals immersed in the dispersion medium. The intense heat produced vaporises the metal, which then condenses to form particles of colloidal size. + Electrodes – Dispersion Medium Ice-Bath Peptization – It is a process of converting the precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte (Peptising agent) During peptization the precipitate adsorb one of the ion (cation or anion) which is common ion. Ex – When freshly precipitated Fe(OH)3 is shaken with aqueous solution of FeCl3 (Peptizing agent) it adsorbs Fe+3 ion (common ion) and there by breaks up into small sized particles. GENERAL PROPERTIES OF COLLOIDS Colligative properties – Due to high average molecular masses of colloidal particles, mole fraction of dispersed phase is very low and the value of colligative properties observed experimentally are very small. Optical properties–Tyndall effect – Tyndall (1869) observed that if a strong beam of light is passed through a colloidal sol placed in the dark place the path of the beam gets illuminated. This phenomenon is called Tyndall effect, which is due to scattering of light by the colloidal particles. Tyndall effect not found in true solution. Mechanical properties–Brownian movement – Colloidal particles are found to be in continuous zig–zag motion called Brownian movement, it arises because of the impact of the molecules of the dispersion medium with the colloidal particles. + Electrical Properties-Electrophoresis. The particles of colloids are electrically charged and are positively or negatively charge. The dispersion medium has an equal and opposite charge making the – Electrode Decrease the system neutral as a whole. The existance of charge is shown by the phenomenon of electrophoresis. It involves the movement of colloidal particles either towards the cathode or anode under the influence of electric field. Coagulated sol particles length of tube As2S3 sol (negatively charged) Positively charged collidal particles Negatively charged colloidal particles Metal hydroxides [Fe(OH)3, Al(OH) 3, Ca(OH) 2, Cr(OH) 3], Oxides (TiO2) haemoglobin, basic dye (Methyl blue, Prussian blue) etc. Metal, starch, clay, metal sulphides (As2S3, CdS), Acidic dye (congo red) etc. Coagulation of colloids – The phenomenon of precipitation of a colloidal solution by the addition of excess of an electrolyte is called coagulation or flocculation. At lower concentration of electrolytes, the aggregation of particles is called flocculation that can be reversed on shaking while at higher concentration of electrolyte coagulation takes place and the same cannot be reversed simply by shaking. Hardy–Schulze Rule The ions carrying charge opposite to that of sol particles are effective in causing the coagulation of the sol Coagulating power of an electrolyte is directly proportional to the fourth power of the valency of the ions causing coagulation. The minimum concentration of an electrolyte which is required to cause coagulation of a sol is known as coagulation value and expressed in millimoles / litre. PURIFICATION OF COLLOIDS Following methods are used Dialysis – A bag made-up of parchment paper or cellophone membrane is filled with colloidal solution and is then suspended in fresh water, the electrolyte particles passout leaving behind the colloidal sol Movement of ions across the membrane can be expedited by applying electric potential through two electrodes. This method is faster than simple dialysis and is known as electrodialysis Ultrafiltration – In this method the colloidal solution is filtered through ultra-filters which allow to pass electrolytes only. Ultra–centrifugation – In this method colloidal sol is taken in a tube which is placed in an ultra-centrifuge. On rotation of the tube with high speeds the colloidal particles settle down at the bottom of the tube and the impurities remain in solution called the centrifugate. The settle colloidal particles are mixed with an appropriate dispersing medium to regenerate the sol. ❑ ❑ ❑