STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
“The branch of physical chemistry, which deals the Examples :(i) Water vapours Examples : (i) Water vapours nature of surfaces and also with the chemical and physical adsorbed by silica gel. absorbed by anhydrous CaCl2 processes which takes place on the surfaces, is called surface (ii) NH3is adsorbed by charcoal. (ii) NH3is absorbed in water chemistry”. forming NH4OH
In surface chemistry, we study the phenomenon of (4) Surface forces :Only the surface atoms of an adsorption, catalysis and colloidal properties. adsorbent play an active role in adsorption. These atoms posses Adsorption unbalanced forces of various types such as, Vander Waal’s forces (1) Definition : The phenomenon of attracting and and chemical bond forces. retaining the molecules of a substance on the surface of a liquid Thus, the residual force-field on a free surface or solid resulting in to higher concentration of the molecules on which is responsible for adsorption is produced. For the surface is called adsorption. example, when a solid substance is broken into two pieces, two (2) Causes of adsorption:Unbalanced forces of new surfaces are formed and therefore, the number of attraction or free valencies which is present at the solid or liquid unbalanced forces becomes more. As a result the tendency for surface, have the property to attract and retain the molecules of adsorption become large. a gas or a dissolved substance on to their surfaces with which (5) Reversible and Irreversible adsorption :The they come in contact. adsorption is reversible, if the adsorbate can be easily removed Example:Ammonia gas placed in contact with charcoal from the surface of the adsorbent by physical methods.If the gets adsorbed on the charcoal whereas ammonia gas placed in adsorbatecan not be easily removed from the surface of the contact with water gets absorbed into water, adsorbent is called irreversible adsorption. Table : 14.1 Some basic terms used in adsorption Example for reversible adsorption: A gas adsorbed on a Interface :Any surface is a Adsorbate and Adsorbent solid surface can be completely removed in vacuum.
plane which separates any two :The substance which gets Example for irreversible adsorption: Adsorption of O2 on phases in contact with each adsorbed on any surface is called tungusten adsorbent. other. The plane which separates adsorbate for example, if a gas any two phase is generally called gets adsorbed on to the surface (6) Characteristics of adsorption an interface between the two of a solid, then the gas is termed (i) Adsorption refers to the existence of a higher phases. as the adsorbate. concentration of any particular component at the surface of a The substance on the surface of liquid or a solid phase. which adsorption takes place is called adsorbent. (ii) Adsorption is accompanied by decrease in the G (free energy change) of the system when G 0 , adsorption Desorption :The removal of Absorption :When the equilibrium is said to be established. the adsorbed substance from a molecules of a substance are (iii) Adsorption is invariably accompanied by evolution of surface is called desorption. uniformly distributed heat, i.e. it is an exothermic process. In other words, Hof throughout the body of a solid or adsorption is always negative. liquid. This phenomenon is called absorption. (iv) When a gas is adsorbed, the freedom of movement of its molecules becomes restricted. On account of it decrease in Sorption :The phenomenon in Occlusion :When adsorption of which adsorption and absorption gases occur on the surface of the entropy of the gas after adsorption, i.e. Sis negative. occur simultaneously is called metals this is called occlusion. (v) For a process to be spontaneous, the thermodynamic sorption. requirement is that G must be negative, i.e. there is Mc. Bain introduced a general decrease in free energy. On the basis of Gibb’s Helmholtz term sorption describeing both the processes, however equation, G H TS , G can be negative if H has adsorption is instantaneous i.e. a sufficiently high negative value and TS has positive value. fast process while absorption is a Classification of adsorption slow process.
Adsorption can be classified into two categories as (3) Difference between adsorption and absorption described below,
Adsorption Absorption (1) Depending upon the concentration : In It is a surface phenomenon. It concerns with the whole mass adsorption the concentration of one substance is different at the of the absorbent. surface of the other substance as compared to adjoining bulk or In it, the substance is only It implies that a substance is interior phase. retained on the surface and does uniformly distributed, through (i) Positive adsorption: If the concentration of not go into the bulk or interior of the body of the solid or liquid. adsorbate is more on the surface as compared to its the solid or liquid. concentration in the bulk phase then it is called positive In it the concentration of the In it the concentration is low. adsorbed molecules is always adsorption. greater at the free phase. Example :When a concentrated solution of KClis shaken It is rapid in the beginning and It occurs at the uniform rate. with blood charcoal, it shows positive adsorption. slows down near the equilibrium.
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
(ii)Negative adsorption: If the concentration of the (i) An increase in the pressure of the adsorbate gas adsorbate is less than its concentration in the bulk then it is increases the extent of Adsorptiontends to called negative adsorption. adsorption. reach limiting value
Example :When a dilute solution of KClis shaken with (ii) At low temperature, blood charcoal, it shows negative adsorption. the extent of adsorption (2) Depending upon the nature of force existing increases rapidly with pressure. Extent of between adsorbate molecule and adsorbent (iii) Small range of adsorption Adsorption increases pressure, the extent of rapidly at the beginning (i) Physical adsorption: If the forces of attraction adsorption is found to be directly Pressure existing between adsorbate and adsorbent are Vander Waal’s proportional to the pressure. forces, the adsorption is called physical adsorption. This type of adsorption is also known as physisorption or Vander (iv) At high pressure (closer to the saturation vapour Waal’s adsorption. It can be easily reversed by heating or pressure of the gas), the adsorption tends to achieve a limiting decreasing the pressure. value. (ii) Chemical adsorption :If the forces of attraction (4) Effect of temperature existing between adsorbate particles and adsorbent are almost (i) As adsorption is accompanied by evolution of heat, so of the same strength as chemical bonds, the adsorption is called according to the Le-Chatelier’s principle, the magnitude of chemical adsorption. This type of adsorption is also called as adsorption should decrease with rise in temperature. chemisorption or Langmuir adsorption. This type of adsorption cannot be easily reversed. P - Constant P - Constant Comparison between physisorption and chemisorption Physisorption Chemisorption x /m x/m (Vander Waal's adsorption) (Langmuir adsorption) Low heat of adsorption usually High heat of adsorption in the in range of 20-40 kJ/mol range of 50-400 kJ/mol Temperature Temperature Force of attraction areVander Forces of attraction are Physical adsorption Chemical adsorption Waal's forces. chemical bond forces. (ii) The relationship between the extent of adsorption and It is reversible It is irreversible temperature at any constant pressure is called adsorption It is usually takes place at low It takes place at high isobar. temperature and decreases temperature. (iii) A physical adsorption isobar shows a decrease in x/m with increasing temperature. (where ‘m’ is the mass of the adsorbent and ‘x’ that of adsorbate) It is related to the case of It is not related. as the temperature rises. liquefication of the gas. (iv) The isobar of chemisorption show an increase in the It forms multimolecular It forms monomolecular layers. layers. beginning and then decrease as the temperature rises. It does not require any activation It requires high activation Adsorption isotherms energy. energy. A mathematical equation, which describes the High pressure is favourable. High pressure is favourable. Decrease of pressure causes Decrease of pressure does not relationship between pressure (p) of the gaseous adsorbate and desorption cause desorption. the extent of adsorption at any fixed temperature, is called It is not very specific. It is highly specific. adsorption isotherms. The extent of adsorption is expressed as mass of the Factors which affect the extent of adsorption :The adsorbate adsorbed on one unit mass of the adsorbent. following are the factors which affect the adsorption, Thus, if x g of an adsorbate is adsorbed on m gof the (1) Nature of the adsorbate(gas) and adsorbent adsorbent, then (solid) x (i) In general, easily liquefiable gases e.g., CO2, NH3, Extent of adsorption m Cl2and SO2etc. are adsorbed to a greater extent than the Various adsorption isotherms are commonly employed in elemental gases e.g. H2, O2, N2, He etc. (while chemisorption is specific in nature.) describing the adsorption data. (ii) Porous and finely powdered solid e.g. charcoal, fullers (1) Freundlich adsorption isotherm earth, adsorb more as compared to the hard non-porous (i) Freundlich adsorption isotherm is obeyed by the materials. Due to this property powdered charcoal is used in gas adsorptions where the adsorbate forms a monomolecular masks. layer on the surface of the adsorbent. 1 (2) Surface area of the solid adsorbent x kp n (Freundlich adsorption isotherm) or (i) The extent of adsorption depends directly upon the m surface area of the adsorbent, i.e. larger the surface area of the x 1 adsorbent, greater is the extent of adsorption. log log k log p m n (ii) Surface area of a powdered solid adsorbent depends where x is the weight of the gas adsorbed by m gm of the upon its particle size. Smaller the particle size, greater is its adsorbent at a pressure p, thus x/m represents the amount of surface area. gas adsorbed by the adsorbents per gm(unit mass), k and n are (3) Effect of pressure on the adsorbate gas
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry constant at a particular temperature and for a particular (a) Langmuir’s theory of unimolecular adsorption is valid adsorbent and adsorbate (gas), n is always greater than one, only at low pressures and high temperatures. indicating that the amount of the gas adsorbed does not (b) When the pressure is increased or temperature is increase as rapidly as the pressure. lowered, additional layers are formed. This has led to the (ii) At low pressure, the extent of adsorption varies modern concept of multilayer adsorption. x linearly with pressure. p' Adsorption from solutions m (1) The process of adsorption can take place from solutions (iii) At high pressure, it becomes independent of also. x pressure. p0 (2) In any solution, there are two (or more) components m ;soluteand solvent.The solute may be present in the x (iv) Atmoderate pressure depends upon pressure molecular or ionic form. m (3) The extent of adsorption from solution depends upon 1 x the concentration of the solute in the solution, and can be raised to powers p n expressed by the Freundlich isotherm. m (4) The Freundlich adsorption isotherm for the x 1 x 0 p p 1 m m x adsorption from solution is, kc n where, x is the mass of the
m 1 slope solute adsorbed, m is the mass of the solid adsorbent, c is the x/m n x equilibrium concentration of the solute in the solution, n is a p1 / n m log(x/m) intercept = log k constant having value greater than one, kis the proportionality constant, (The value of k depends p log p upon the nature of solid, its particle size, temperature, and the Freundlich adsorption Plot of log x/m against log p for the nature of solute and solvent etc.) isotherm: plot of x/m against adsorption of a gas on a solid (2) The Langmuirp - adsorption isotherms (5) The plot of x/m againstc is similar to that Freundlich adsorption isotherm. The above equations may be written in the (i) One of the drawbacks of Freundlich adsorption x 1 isotherm is that it fails at high pressure of the gas. Irving following form, log log k log c where c, is the Langmuir in 1916 derived a simple adsorption isotherm, on m n theoretical considerations based on kinetic theory of gases. equilibrium concentration of the solute in the solution. This is named as Langmuir adsorption isotherm. Application of adsorption (a) Adsorption takes place on the surface of the solid only The phenomenon of adsorption finds a number of till the whole of the surface is completely covered with a applications. Important applications are given as follows. unimolecular layer of the adsorbed gas. (1) Production of high vacuum (b) Adsorption consists of two opposing processes, (2) In Gas masks :This apparatus is used to adsorb namely Condensationof the gas molecules on the solid surface poisonous gases (e.g. Cl , CO, oxide of sulphur etc.) and thus and Evaporation(desorption)of the gas molecules from the 2 surface back into the gaseous phase. purify the air for breathing. (3) For desiccation or dehumidification : These (c) The rate of condensation depends upon the uncovered substances can be used to reduce/remove water vapours or (bare) surface of the adsorbent available for condensation. moisture present in the air. Silica gel and alumina are used for Naturally, at start when whole of the surface is uncovered the dehumidification in electronic equipment. rate of condensation is very high and as the surface is covered (4) Removel of colouring matter from solution :(i) more and more, the rate of condensation progressively Animal charcoal removes colours of solutions by adsorbing decreases. On the contrary, the rate of evaporation depends coloured impurities. (ii) Animal charcoal is used as decolouriser upon the covered surface and hence increases as more and more in the manufacture of cane sugar. of the surface is covered ultimately an equilibrium will be set up (5) Heterogeneous catalysis : Mostly heterogeneous at a stage when the rate of condensation becomes equal to the catalytic reactions proceed through the adsorption of gaseous rate of evaporation (adsorption equilibrium). reactants on solid catalyst. For example, (d) The rate of condensation also depends upon the (i) Finely powdered nickel is used for the hydrogenation of pressure of the gas since according the kinetic theory of oils. gases,the number of molecules striking per unit area is (ii) Finely divided vanadium pentaoxide(V O ) is used in proportional to the pressure. 2 5 the contact process for the manufacture of sulphuric acid. x ap Mathematically, , where a and b are constants (6) Separation of inert gases :Due to the difference in m 1 bp degree of adsorption of gases by charcoal, a mixture of inert and their value depends upon the nature of gas (adsorbate), gases can be separated by adsorption on coconut charcoal at nature of the solid adsorbent and the temperature. Their values different low temperatures. can be determined from the experimental data. (7) Softening of hard water Limitation of Langmuir theory
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
(i) The hard water is made to pass through a column H 2SO 4 (l) C12 H 22O11 (l) H 2O(l) packed with zeolite (sodium aluminium silicate) (Sucrose solution) (ii) Ca++, Mg++ions which are responsible for hardness, C6 H12 O6 (l) C6 H12 O6 (l) get adsorbed on zeolite, exchanging sodium ions. (Glucose solution) (Fructose solution)
Na 2 Al2Si2O8 CaCl 2 CaAl 2Si2O8 2NaCl (2) Heterogeneous catalysis : The catalytic process in (iii) The exhausted zeolite is regenerated with 10% of which the reactants and the catalyst are in different phases is sodium chloride solution. known as heterogeneous catalysis.Some of the examples of heterogeneous catalysis are given below. CaAl 2Si2O8 2NaCl Na2 Al2Si 2O8 CaCl 2 (i) In contact process for H SO (8) De-ionisation of water 2 4 Pt(s) (i) Water can be de-ionised by removing all dissolved salts 2SO 2(g) O2(g) 2SO 3 (g) with the help of cation and anion-exchanger resin. BrV2O5
(ii) Cation-exchanger is an organic synthetic resin such as (ii) In Haber’s process for NH 3 polystyrene-containing a macroanion (R SO 3 etc.) which has Fe(s) N 2(g) 3H 2(g) 2 NH 3 (g) adsorbed H+ions. (iii) In Ostwald’s process for HNO 3 (iii) A resin containing a basic group (R3 N etc.) which 4 NH (g) 5O (g) Pt(s) 4 NO(g) 6 H O(g) has adsorbed OH ions acts as anion exchanger. 3 2 2 (9) In curing diseases : A number of drugs are (3) Positive catalysis : When the rate of the reaction is adsorbed on the germs and kill them or these are adsorbed on accelerated by the foreign substance, it is said to be a positive the tissues and heat them. catalystand phenomenon as positive catalysis. Some (10) Cleaning agents :Soap and detergents get adsorbed examples of positive catalysis are given below. on the interface and thus reduce the surface tension between (i) Decomposition of KClO 3 dirt and cloth, subsequently the dirt is removed from the cloth. MnO 2 (s) (11) Froth floatation process 2KClO 3 (s) 2KCl(s) 3O2(g) 270 o C A low grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method. (ii) Oxidation of SO 2
(12) In adsorption indicators V2O5 (s) 2SO 2(g) O2(g) 2SO 3 (g) Surface of certain precipitates such as silver halide, have or Pt(s) the property of adsorbing some dyes like eosin, fluorescein etc. (iii) Decon’s process (13) Chromatographic analysis CuCl 2 (s) The phenomenon of adsorption has given an excellent 4 HCl(g) O2 (g) 2Cl2(g) 2H 2O(g) 450 o C technique of analysis known as chromatographic analysis. (4) Negative catalysis : There are certain, substance (14) In dyeing :Many dyes get adsorbed on the cloth which, when added to the reaction mixture, retard the reaction either directly or by the use of mordants. rate instead of increasing it. These are called negative Catalysis catalyst or inhibitors and the phenomenon is known as “Catalyst is a substance which speeds up and speeds negative catalysis. Some examples are as follows. down a chemical reaction without itself being used up.” (i) Oxidation of sodium sulphite Berzelius (1836)introduced the term catalysis and Alcohol (l) 2Na SO (s) O (g) 2 Na SO (s) catalyst. 2 3 2 2 4 Ostwald (1895)redefined a catalyst as, “A substance (ii) Oxidation of benzaldehyde which changes the reaction rate without affecting the overall Diphenyl 2C6 H 5CHO(l) O2(g) 2C6 H 5COOH (l) energetics of the reaction is termed as a catalyst and the amine (l) phenomenon is known as catalysis.” (iii) Tetra ethyl lead (TEL) is added to petrol to retard the Types of catalysis ignition of petrol vapours on compression in an internal Catalytic reactions can be broadly divided into the combustion engine and thus minimise the knocking effect. following types, (5) Auto-catalysis : In certain reactions, one of the (1) Homogeneous catalysis : When the reactants and product acts as a catalyst. In the initial stages the reaction is the catalyst are in the same phase (i.e. solid, liquid or gas). The slow but as soon as the products come into existences the catalysis is said to be homogeneous. The following are some reaction rate increases. This type of phenomenon is known as of the examples of homogeneous catalysis. auto-catalysis. Some examples are as follows, (i) In the lead chamber process (i) The rate of oxidation of oxalic acid by acidified NO (g) 2SO 2 (g) O2 (g) 2SO 3 (g) potassium permanganate increases as the reaction progresses. (ii) In the hydrolysis of ester This acceleration is due to the presence of Mn 2 ions which are HCl (l) 2 CH 3COOCH 3 (l) H 2O(l) formed during reaction. Thus Mn ions act as auto-catalyst. CH COOH (l) CH OH(l) 3 3 5H 2C2O4 2KMnO 4 3H 2SO 4 2MnSO 4 K2SO 4 (iii) In the hydrolysis of sugar 10CO 2 8H 2O
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
(ii) When nitric acid is poured on copper, the reaction is very slow in the beginning, gradually the reaction becomes faster due to the formation of nitrous acid during the reaction which acts as an auto-catalyst. (6) Induced catalysis : When one reaction influences the rate of other reaction, which does not occur under ordinary conditions, the phenomenon is known as induced catalysis.Some examples are as follows, (i) Sodium arsenite solution is not oxidised by air. If, (5) The catalyst can not change the position of however, air is passed through a mixture of the solution of equilibrium :The catalyst catalyse both forward and backward sodium arsenite and sodium sulphite, both of them undergo reactions to the same extent in a reversible reaction and thus simultaneous oxidation. The oxidation of sodium sulphite, thus, have no effect on the equilibrium constant. induces the oxidation of sodium arsenite. (6) Catalytic promoters :Substances which themselves are not catalysts, but when mixed in small quantities with the (ii) The reduction of mercuric chloride (HgCl2) with oxalic catalysts increase their efficiency are called as promotersor acid is very slow, but potassium permanganate is reduced activators. readily with oxalic acid. If, however, oxalic acid is added to a (i) For example, in Haber’s process for the synthesis of mixture of potassium permanganate and HgCl both are 2 ammonia, traces of molybdenum increases the activity of finely reduced simultaneously. The reduction of potassium divided iron which acts as a catalyst. permanganate, thus, induces the reduction of mercuric chloride. (ii) In the manufacture of methyl alcohol from water gas (7) Acid-base catalysis : According to theArrhenius (CO H ) , chromic oxide (Cr O ) is used as a promoter with and OstwaldH+or H– ion act as a catalyst. 2 2 3 (i) For example, Hydrolysis of an ester, the catalyst zinc oxide (ZnO) . H or (7) Catalytic poisons :Substances which destroy the CH 3COOC 2 H5 (l) H 2O(l) CH 3COOH (l) C2 H5OH(l) OH activity of the catalyst by their presence are known as (ii) Inversion of cane sugar, catalytic poisons. (i) For example, the presence of traces of arsenious oxide H C12 H 22 O11 (l) H 2O C6 H12 O6 (l) C6 H12 O6 (l) (As O ) in the reacting gases reduces the activity of platinized Sugar Fructose Glucose 2 3 asbestos which is used as catalyst in contact process for the (iii) Conversion of acetone into diacetone alcohol, manufacture of sulphuric acid. OH CH 3COCH 3(l) CH 3COCH 3 (l) (ii) The activity of iron catalyst is destroyed by the
CH 3COCH 2.C(CH 3 )2 OH(l) presence of H 2S or CO in the synthesis of ammonia by Characteristics of catalysis Haber’s process. (iii) The platinum catalyst used in the oxidation of The following are the characteristics which are common to must of catalytic reactions. hydrogen is poisoned by CO . (1) A catalyst remains unchanged in mass and chemical (8) Change of temperature alters the rate of composition at the end of the reaction. catalytic reaction as it does for the same reaction in (2) A small quantity of the catalyst is generally absence of catalyst :By increasing the temperature, there is sufficient to catalyses almost unlimited reactions an increase in the catalytic power of a catalyst but after a certain temperature its power begins to decrease. A catalyst has thus, a (i) For example, in the decomposition of hydrogen particular temperature at which its catalytic activity is 8 peroxide, one gram of colloidal platinum cancatalyses 10 litres maximum. This temperature is termed as optimum of hydrogen peroxide. temperature. (ii) In Friedel craft’s reaction, anhydrous aluminium (9) A positive catalyst lowers the activation energy chloride is required in relatively large amount to the extent of (i) According to the collision theory, a reaction occurs on 30% of the mass of benzene, account of effective collisions between the reacting molecules. AlCl3 C6 H 6 C2 H5Cl C6 H5C2 H5 HCl (ii) For effective collision, it is necessary that the (3) The catalyst can not initiate the reaction: The molecules must possess a minimum amount of energy known as function of the catalyst is to alter the speed of the reaction rather activation energy (Ea). than to start it. (iii) After the collision molecules form an activated (4) The catalyst is generally specific in nature: A complex which dissociate to yield the product molecules. substance, which acts as a catalyst for a particular reaction , fails (iv) The catalyst provides a new pathway involving lower to catalyse the other reaction , different catalysts for the same amount of activation energy. Thus, reactant may for different products. Examples : Ea Ea Ea – Al2O3 C H (g) H O(g) Decreases RT RT 2 4 2 – (Dehydration) Decreases Increases C 2 H 5 OH (l ) Cu e–Ea/RT k CH 3 CHO(g) H 2 (g) Reaction speeds up Cu Increases (Dehydroge nation ) Increases Cu CO 2 (g) H 2 (g) (Dehydroge nation) HCOOH (l)
Al2O3 STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
(i) This theory is applicable to reactions between gases in the presence of a solid catalyst. Some typical examples are as follows.
larger number of effective collisions occur in the (ii) The contact process for the oxidation of SO 2 to SO 3 presence of a catalyst in comparison to effective collisions with atmospheric oxygen in the presence of platinum as the at the same temperature in absence of a catalyst. Hence catalyst. the presence of a catalyst makes the reaction to go faster. (iii) The Haber’s process for the synthesis of ammonia with iron as the catalyst. (v) Figure shows that activation energy Ea , in (iv) Adsorption results in the loosening of the chemical absence of a catalyst is higher than the activation energy bonds in the reactant molecules, so that their rupture becomes Ea, in presence of a catalyst. easier. This is confirmed by the observed lower activation energies for heterogeneous catalytic reactions in the presence of (vi) ER and E p represent the average energies of the catalysts as compared to that for the same reaction in the reactants and products. The difference gives the value of absence of the catalyst. G , i.e., G E E R P Enzyme catalysis Uncatalysed complex (1) Enzymes are complex nitrogenous substances these are actually protein molecules of higher molecular mass. Energy barrier (2) Enzymes catalyse numerous reactions, especially those connected with natural processes. Catalysed complex (3) Numerous reactions occur in the bodies of animals Ea and plants to maintain the life process. These reactions are
catalysed by enzymes. The enzymes are thus, termed as bio-
Ea chemical catalysts and the phenomenon is known as bio- chemical catalysis.
Chemical potential energy ER Reactants (4) Nitrogenase an enzyme present in bacteria on the Initial G° of (A+B) root nodules of leguminous plants such as peas and beans, state reaction EP catalyses the conversion of atmospheric N 2 to NH 3 .
Final state Products (C + (5) In the human body, the enzyme carbonic anhydrase D) Theories of catalysisReaction sequence catalyses the reaction of CO 2 with H 2 O , There are two theories of catalysis which is described as CO (aq) H O(l) follows. 2 2 H (aq.) HCO 3 (aq.) (1) Intermediate compound theory The forward reaction occurs when the blood takes up (i) This theory was proposed by Clement and CO 2 in the tissues, and the reverse reaction occurs when the Desormes in1806. According to this theory, the desired blood releases CO 2 in lungs. reaction is brought about by a path involving the formation of an unstable intermediate compound, followed by its (6) In manufacturing of ethyl alcohol Invertase decomposition into the desired end products with the (i) C12 H 22 O11 (l) H 2O(l) C6 H12O6 (l) C6H12O6 (l) regeneration of the catalyst. Glucose Fructose Zymase (ii) The intermediate compund may be formed in either of C6 H12 O6 (l) 2C2 H5OH (l) 2CO 2(l) two ways (ii) Starch (l) Diastase Maltose (l) (a) When the intermediate compound is reactive and reacts with the other reactants. Maltose Maltase Glucose Zyamase Alcohol AB X BX A Activity and Selectivity intermedia te (1) Activity : Activity is the ability of catalysts to BX C CB X .....(i) accelerate chemical reaction, the degree of acceleration can be (b) When the intermediate is unstable and decomposes to as high as 10 10 times in certain reactions. For example reaction give the final product. between H and O to form H O in presence of platinum as A B X ABX AB X .....(ii) 2 2 2 intermedia te catalyst takes place with explosive violence. Where, A, B and C are the reactant molecules and X is the In absence of catalyst, H 2 and O2 can be stored molecule of the catalyst. The first type of reaction sums up to, indefinitely without any reaction. AB C CB A (2)Selectivity :Is the ability of catalysts to direct reaction While the second to, A B AB in many cases, the to yield particular products (excluding other). intermediate compounds postulated to be formed are known Example : compounds and often their presence is detected. CH 3 (2) Adsorption theory
Toluene
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
(i) n heptane Pt Examples :Starch, gelatin, gums, silicic acid and hdemoglobin etc.
O (3) The above classification was discarded i.e., the terms || colloid does not apply to a particular class of substances but is a BiMoO 4 (ii) CH 3CH CH 2 CH 2 CHCH state of matter like solid, liquid and gas. Any substance can be Acrolein brought into colloidal state. Zeolite (Shape selective catalysis) (4) The colloidal state depends on the particle size. If is (1) Zeolite are alumino–silicates of the general formula, regarded as intermediate state between true solution and
M x / n [AlO 2 ]x .(SiO 2 )y .mH 2O , where, M may be simple cation like suspension. Na , K or Ca 2 , n is the charge on the simple cation, m is the Table : 14.2 Features of the three types of solutions number of molecules of water of crystallization. Property Suspension Colloid True (2) Some well known zeolites are as follows, solution solution Nature Heterogeneou Heterogeneou Homogeneou Erionite Na K CaMg(AlO ) (SiO ) .6H O 2 2 2 2 2 2 2 s s s
Gemelinite Na2Ca(AlO2)2(SiO2)4 .6H2O Particle size > 100 nm 1 nm – 100 nm < 1 nm Separation by Faujasite(natural) Na56 (AlO2)56 (SiO2)136 .250 H2O (i) Ordinary Possible Not possible Not possible ZSM-5 H x [(AlO2)x (SiO2)96 x ].16 H2O filtration Possible Possible Not possible (ii) Ultra- Linde-A(synthetic) [Na (AlO ) (SiO ) .27 H O] 12 2 12 2 12 2 8 filtration (3) The characteristic feature of zeolites is the openness of the Settling of Settle under Settle only on Do not settle structure, which permits cavities of different sizes. particles gravity centrifugation (4) The open structure is provided by silica in which Appearance Opaque Generally Transparent aluminium occupies x/(x+y) fraction of the telrahedral sites. transparent (5) The negative charge of the aluminosilicate framework Tyndall effect Shows Shows Does not is neutralized by the replaceable cations. show (6) The void space forms more than 50% of the total Diffusion of Does not Diffuses Diffuses volume, which is occupied by water molecules. particles diffuse slowly rapidly (7) The reaction- selectivity of zeolites depends upon the Brownian May show Shows Negligible movement size of cavities (cages), pores (apertures) and the distribution of pores in the structure. The pore size in zeolites generally varies from 260 pm to 740 pm.
(8) Zeolite have high porosity due to the presence of one, two, or three dimensional networks of interconnected channels and cavities of molecular dimensions. (9) There is a new class of highly siliceous zeolites with an optimal pore diameter of 550pm. ZSM-5 is one such zeolite Suspension Colloidal solution True solution having the formula. [H x (AlO 2 )x .(SiO 2 )96 x ].16 H 2 O Size > 100 nm Size between Size < 1 nm 1-100 nm (10) The zeolite catalyst ZSM-5 converts alcohols to Fig. 14.1 Three types solutions gasoline (petrol) by dehydrating the alcohol and producing a Phases of colloids and Their classification mixture of wide variety of hydrocarbons. (1) Phases of colloids :We know that a colloidal Colloidal state solution is of heterogeneous nature. It consists of two phases which are as follows (1) The foundation of colloidal chemistry was laid down (i) Internal phase or Dispersed phase by an English scientist, Thomas Graham, in 1861. The credit for the various advances in this field goes to eminent scientists (Discontinuous phase) : It is the component present in small like Tyndall, Hardy, Zsigmondy, N.R. Dhar, S.S. proportion and is just like a solute in a solution. For example in Bhatnagar and others. the colloidal solution of silver in water (silver acts as a dispersed (2) Thomas Graham classified the soluble substances phase) into two categories depending upon the rate of diffusion through (ii)External phase or Dispersion animal and vegetable membranes or parchment paper. medium(continuous phase) :It is generally component present (i) Crystalloids :They have higher rate of diffusion and in excess and is just like a solvent in a solution. For example, in diffused from parchment paper. the colloidal solution of silver in water.Water act as a dispersion Examples :All organic acids, bases and salts and organic medium. compounds such as sugar, urea etc. (2) Classification of colloids :The colloids are (ii) Colloids(Greek word, kolla, meaning glue-like) classified on the basis of the following criteria :They have slower rate of diffusion and can not diffused from (i) Classification based on the physical state of parchment paper. the dispersed phase and dispersion medium :Depending
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
upon the physical state of dispersed phase and dispersion takes place medium whether these are solids, liquids or gases, eight types of Examples Gum, gelatin, starch, Metals like Ag and Au, proteins, rubber etc. colloidal systems are possible. hydroxides like Al(OH )3 , Table : 14.3 Different types of colloidal systems Fe(OH )3 metal sulphides like etc. Disperse Dispersio Colloidal Examples AS 2 S 3
d phase n Medium System (iii) Classification based on types of particle of Liquid Gas Aerosol of Fogs, clouds, mists, dispersed phase :Depending upon the type of the particles of liquids fine insecticide sprays the dispersed phase, the colloids are classified as follows. Solid Gas Aerosol of Smoke, volcanic dust, (a) Multimolecular colloids solids haze When on dissolution, atoms or smaller molecules of Gas Liquid Foam or Soap lather. Lemonade substances (having diameter less than 1nm) aggregate together froth froth, foam, whipped cream, soda water to form particles of colloidal dimensions, the particles thus Liquid Liquid Emulsions Milk, emulsified oils, formed are called multimolecular colloids. medicines In these sols the dispersed phase consists of aggregates Solid Liquid Sols Most paints, starch in of atoms or molecules with molecular size less than 1 nm.
water, proteins, gold For example, sols of gold atoms and sulphur(S 8 ) sol, arsenic sulphide molecules. In these colloids, the particles are held together by sol, ink Vander Waal's forces.They have usually lyophilic Gas Solid Solid foam Pumice stone, styrene character. rubber, foam rubber (b) Macromolecular colloids Liquid Solid Gels Cheese, butter, boot polish, jelly, curd These are the substances having big size molecules Solid Solid Solid sols Ruby glass, some gem (called macromolecules) which on dissolution form size in the (coloured stones and alloys colloidal range. Such substances are called macromolecular glass) colloids. These macromolecules forming the dispersed phase are (ii) Classification based on Nature of interaction generally polymershaving very high molecular masses. between dispersed phase and dispersion medium: Naturally occurring macromolecules are starch, cellulose, Depending upon the nature of interactions between dispersed proteins, enzymes, gelatin etc. Artificial macromolecules are phase and the dispersion medium, the colloidal solutions can be synthetic polymers such as nylon, polythene, plastics, polystyrene classified into two types as (a) Lyophilic and (b) Lyophobic sols. etc. (a) Lyophilic colloids (water loving):“The colloidal They have usually lyophobic character. solutions in which the particles of the dispersed phase have a (c) Associated colloids great affinity for the dispersion medium, are called lyophilic These are the substances which on dissolved in a collodis.” medium behave as normal electrolytes at low concentration but behave, as colloidal particles at higher concentration due to the (b) Lyophobic colloids(water hateing):“The colloidal formation of aggregated particles. The aggregates particles thus solutions in which there is no affinity between particles of the formed are called micelles. dispersed phase and the dispersion medium are called Their molecules contain both lyophilicand lyophobic colloids.” lyophobicgroups. Distinction between lyophilic and lyophobic sols Micelles Property Lyophilic sols Lyophobic sols Micelles are the cluster or aggregated particles formed (suspensoid) (Emulsoid) by association of colloid in solution. Surface Lower than that of the Same as that of the The common examples of micelles are soapsand tension medium medium detergents. Viscosity Much higher than that Same as that of the of the medium medium The formation of micelles takes place above a Reversibilit Reversible Irreversible particular temperature called Kraft temperature(Tk ) and y Stability More stable Less stable above a particular concentration called critical micellization Visibility Particles can’t be Particles can be detected concentration(CMC). detected even under under ultramicroscope. They are capable of forming ions. ultramicroscope Micelles may contain as many as 100 molecules or Migration Particles may migrate Particles migrate either in either direction or do towards cathode or anode more. not migrate in an in an electric field because For example sodium stearate (C17 H 35 COONa) is a electric field because do they carry charge. not carry any charge. typical example of such type of molecules. Action of Addition of smaller Coagulation takes place When sodium stearate is dissolved in water, it gives electrolyte quantity of electrolyte Na and C H COO ions. has little effect 17 35 Hydration Extensive hydration No hydration
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
the aqueous solution of their salts, with a suitable reducing C17 H 35 COONa C17 H 35 COO Na Sodium stearate Stearate ion agent such as formaldehyde, phenyl hydrazine, hydrogen The stearate ions associate to form ionic micellesof peroxide, stannous chloride etc. colloidal size. 2 AuCl 3 3SnCl 2 3SnCl 4 2 Au Gold sol It has long hydrocarbon part of C17 H 35 radical. Which 2AuCl 3 3HCHO 3H 2O 2Au 3HCOOH 6HCl is lyophobicand COO part which is lyophilic. Gold sol In the figure, the chain corresponds to stearate ion, The gold sol, thus prepared, has a purple colour and is called purple of cassius. (C17 H35 COO ) . When the concentration of the solution is below (c) By hydrolysis :Many salt solutions are rapidly from its CMC (10 3 mol L1 ) , it behaves as normal electrolyte. hydrolysed by boiling dilute solutions of their salts. For But above this concentration it is aggregated to behave as example, ferric hydroxide and aluminium hydroxide sols are micelles. obtained by boiling solutions of the corresponding chlorides.
– + FeCl 3H O Fe(OH) 3HCl Na 3 2 3 – Na+ – Colloidal sol Na+ Similarly silicic acid sol is obtained by the hydrolysis of
Na+ sodium silicate. Na+ – – (d) By double decomposition :A sol of arsenic sulphide is obtained by passing hydrogen sulphide through a cold solution Na+ – – Na+ of arsenious oxide in water. – Na+ As 2 O3 3H 2 S As 2 S 3 3H 2 O Fig. 14.2 Aggregation of several ions to form ionic micelle The main function of a soap is to reduce oily and greasy (e) By excessive cooling:A colloidal solution of ice in an dirt to colloidal particles (an emulsion). Soap therefore, are organic solvent like ether or chloroform can be prepared by known as emulsifying agents. freezing a solution of water in the solvent. The molecules of Some other examples of micelles are sodium palmitate water which can no longer be held in solution, separately combine to form particles of colloidal size. (C15 H31COONa ), Sodium lauryl sulphate[CH 3(CH 2 )11 SO 3O Na ], (f) By exchange of solvent: Colloidal solution of certain Cetyltrimethyl ammonium bromide CH 3 (CH 2 )15 (CH 2)3 N Br etc. substances such as sulphur, phosphorus, which are soluble in General methods of preparation of colloids alcohol but insoluble in water can be prepared by pouring their alcoholic solution in excess of water. For example, alcoholic Lyophilic and lyophobic colloidal solutions (or sols) are solution of sulphur on pouring into water gives milky colloidal generally prepared by different types of methods. Some of the solution of sulphur. common methods are as follows. (g) By change of physical state : Sols of substances like (1) Preparation of Lyophilic colloids mercury and sulphur are prepared by passing their vapour’s (i) The lyophilic colloids have strong affinity between through a cold water containing a suitable stabilizer such as particles of dispersed phase and dispersion medium. ammonium salt or citrate. (ii) Simply mixing the dispersed phase and dispersion (ii) Dispersion methods :In these methods, larger medium under ordinary conditions readily forms these colloidal particles of a substance (suspensions) are broken into smaller solutions. particles. The following methods are employed. (iii) For example, the substance like gelatin, gum, starch, (a) Mechanical dispersion egg, albumin etc. pass readily into water to give colloidal In this method, Suspension solution. the substance is first Driving Belt (iv) They are reversible in nature become these can be ground to coarse particles. precipitated and directly converted into colloidal state. It is then mixed Discharge Discharge (2) Preparation of Lyophobic colloids : Lyophobic with the dispersion colloids can be prepared by mainly two types of methods. medium to get a (i) Condensation method :In these method, smaller suspension. Metal disc particles of dispersed phase are condensed suitably to be of The suspension Fig. 14.3 Colloidal mill colloidal size. This is done by the following methods. is then grinded in colloidal (a) By oxidation :A colloidal solution of sulphur can be mill. obtained by bubbling oxygen (or any other oxidising agent like It consists of two metallic discs nearly touching each HNO 3 , Br2 etc.) through a solution of hydrogen sulphide in other and rotating in opposite directions at a very high speed water. about 7000 revolution per minute. 2H S O (or any other oxidising agent) 2H O 2S The space between the discs of the mill is so adjusted 2 2 2 that coarse suspension is subjected to great shearing force giving (b) By reduction : A number of metals such as silver, gold rise to particles of colloidal size. and platinum, have been obtained in colloidal state by treating
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
Colloidal solutions of black ink, paints, varnishes, dyes (ii) It’s principle is based upon the fact that colloidal etc. are obtained by this method. particles can not pass through a parchment or cellophane (b) By electrical dispersion or Bredig’s arc method membrane while the ions of the electrolyte can pass through it. (iii) The impurities slowly diffused out of the bag leaving This method is used Metal to prepare sols of platinum, electrodes behind pure colloidal solution silver, copper or gold. (iv) The distilled water is changed frequently to avoid The metal whose sol Arc accumulation of the crystalloids otherwise they may start is to be prepared is made as diffusing back into the bag. ice two electrodes which Water (v) Dialysis can be used for removing HCl from the ferric immerged in dispersion hydroxide sol. medium such as water etc. (2) Electrodialysis The dispersion Fig. 14.4 Bredig's arc method (i) The ordinary process of dialysis is slow. medium is kept cooled by ice. (ii) To increase the process of purification, the dialysis is An electric arc is struck between the electrodes. carried out by applying electric field. This process is called The tremendous heat generate by this method and give electrodialysis. colloidal solution. (iii) The important application of electrodialysis process The colloidal solution prepared is stabilised by adding in the artificial kidney machine used for the purification of blood a small amount of KOH to it. of the patients whose kidneys have failed to work. The artificial (c) By peptisation kidney machine works on the principle of dialysis. The process of converting a freshly prepared (3) Ultra – filtration precipitate into colloidal form by the addition of suitable electrolyte is called peptisation. (i) Sol particles directly pass through ordinary filter paper The electrolyte is used for this purpose is called because their pores are larger (more than 1 or 1000 m ) than peptizing agent or stabilizing agent. the size of sol particles (less than 200 m ). Cause of peptisation is the adsorption of the ions of the (ii) If the pores of the ordinary filter paper are made electrolyte by the particles of the precipitate. smaller by soaking the filter paper in a solution of gelatin of Important peptizing agents are sugar, gum, gelatin and colloidion and subsequently hardened by soaking in electrolytes. formaldehyde, the treated filter paper may retain colloidal Freshly prepared ferric hydroxide can be converted particles and allow the true solution particles to escape. Such into colloidal state by shaking it with water containing Fe 3 or filter paper is known as ultra - filterand the process of separating colloids by using ultra – filters is known as ultra – OH ions, viz. FeCl or NH OH respectively. 3 4 filtration. 3 Fe(OH)3 FeCl3 [Fe(OH)3 Fe] 3Cl (4) Ultra – centrifugation Precipitate electrolyte Colloidal sol (i) The sol particles are prevented from setting out under + + + + + + + + + the action of gravity by kinetic impacts of the molecules of the medium. Fe(OH)3 + Fe3+ + + + + + + (ii) The setting force can be enhanced by using high speed + + + + + + From + + + centrifugal machines having 15,000 or more revolutions per Precipitate electrolyte Colloidal particles minute. Such machines are known as ultra–centrifuges. of Fe(OH)3 of Fe(OH)3 FeCl3 Properties of colloidal solutions
Fig. 14.5 Preparation of colloidal sol by peptisation A stable sol of stannic oxide is obtained by adding a The main characteristic properties of colloidal solutions small amount of dilute HCl to stannic oxide precipitates. are as follows.
Similarly, a colloidal solution of Al(OH)3 and AgCl (1) Physical properties are obtained by treating the corresponding freshly prepared (i) Heterogeneous nature :Colloidal sols are heterogeneousin nature. They consists of two phases; the precipitate with very dilute solution of HCl and AgNO 3 or dispersed phase and the dispersion medium. KCl respectively. (ii) Stable nature : The colloidal solutions are quite Purification of colloidal solution stable. Their particles are in a state of motion and do not settle The following methods are commonly used for the down at the bottom of the container. purification of colloidal solutions. (iii) Filterability : Colloidal particles are readily (1) Dialysis passed through the ordinary filter papers. However they (i) The process of separating the particles of colloid from can be retained by special filters known as ultrafilters those of crystalloid, by means of diffusion through a suitable (parchment paper). membrane is called dialysis. (2) Colligative properties (i) Due to formation of associated molecules, observed values of colligative properties like relative decrease in vapour
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry pressure, elevation in boiling point, depression in freezing point, (a) The phenomenon of movement of colloidal particles osmotic pressure are smaller than expected. under an applied electric field is called electrophoresis. (ii) For a given colloidal sol the number of particles will be (b) If the particles accumulate near the negative electrode, very small as compared to the true solution. the charge on the particles is positive. (3) Mechanical properties (c) On the other hand, if the sol particles accumulate near (i) Brownian movement the positive electrode, the charge on the particles is negative. (a) Robert Brown, a botanist discovered in 1827that (d) The apparatus consists of a U-tube with two Pt- the pollen grainssuspended in water do not remain at rest but electrodes in each limb. move about continuously and randomly in all directions. (e) When electrophoresis of a sol is carried out with out (b) Later on, it was observed that the colloidal particles stirring, the bottom layer gradually becomes more concentrated are moving at random in a zig – zagmotion. This type of while the top layer which contain pure and concentrated motion is called Brownian movement. colloidal solution may be decanted. This is called electro (c) The molecules of the dispersion medium are decanationand is used for the purification as well as for constantly colloiding with the particles of the dispersed phase. It concentrating the sol. was stated by Wienerin 1863that the impacts of the dispersion (f) The reverse of electrophoresis is called medium particles are unequal, thus causing a zig-zag motion of the Sedimentation potential or Dorn effect. The dispersed phase particles. sedimentation potential is setup when a particle is forced to (d) The Brownian movement explains the force of gravity move in a resting liquid. This phenomenon was discovered by acting on colloidal particles. This helps in providing stability to Dornand is also called Dorn effect. colloidal sols by not allowing them to settle down. (ii) Electrical double layer theory (ii) Diffusion :The sol particles diffuse from higher (a) The electrical properties of colloids can also be concentration to lower concentration region. However, due to explained by electrical double layer theory. According to this bigger size, they diffuse at a lesser speed. theory a double layer of ions appear at the surface of (iii) Sedimentation :The colloidal particles settle down solid. under the influence of gravity at a very slow rate. This (b) The ion preferentially adsorbed is held in fixed part phenomenon is used for determining the molecular mass of the and imparts charge to colloidal particles. macromolecules. (c) The second part consists of a diffuse mobile layer of ions. (4) Optical properties :Tyandall effect This second layer consists of both the type of charges. The net (i) When light passes through a sol, its path becomes visible charge on the second layer is exactly equal to that on the fixed part. because of scattering of light by particles. It is called Tyndall (d) The existence of opposite sign on fixed and diffuse effect. This phenomenon was studied for the first time by parts of double layer leads to appearance of a difference of Tyndall. The illuminated path of the beam is called Tyndall potential, known as zeta potential or electrokinetic cone. potential. Now when electric field is employed the particles move (electrophoresis) (ii) The intensity of the scattered light depends on the difference between the refractive indices of the dispersed phase (iii) Electro-osmosis and the dispersion medium. (a) In it the movement of the dispersed particles are prevented from moving by semipermeable membrane. (iii) In lyophobic colloids, the difference is appreciable and, therefore, the Tyndall effect is well - defined. But in (b) Electro-osmosis is a phenomenon in which dispersion lyophilic sols, the difference is very small and the Tyndall effect medium is allowed to move under the influence of an electrical is very weak. field, whereas colloidal particles are not allowed to move. (c) The existence of electro-osmosis has suggested that (iv) The Tyndall effect confirms the heterogeneous when liquid forced through a porous material or a capillary tube, nature of the colloidal solution. a potential difference is setup between the two sides called as (v) The Tyndall effect has also been observed by an streaming potential. So the reverse of electro-osmosis is called instrument called ultra – microscope. streaming potential. Some example of Tyndall effect are as follows Origin of the charge on colloidal particles (a) Tail of comets is seen as a Tyndall cone due to the scattering of light by the tiny solid particles left by the comet in its The origin of the charge on the sol particles in most cases is due to the preferential adsorption of either positive or path. negative ions on their surface. The sol particles acquire electrical (b) Due to scattering the sky looks blue. charge in any one or more of the following ways. (c) The blue colour of water in the sea is due to scattering (1) Due to the dissociation of the surface of blue light by water molecules. molecules :Some colloidal particles develope electrical charge (d) Visibility of projector path and circus light. due to the dissociation / ionisation of the surface molecules. The (e) Visibility of sharp ray of sunlight passing through a charge on the colloidal particles is balanced by the oppositely slit in dark room. charged ions in the sol. For example, an aqueous solution of (5) Electrical properties soap (sodium palmitate) which dissociates into ions as, (i) Electrophoresis
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
C15 H31COONa C15 H 31COO Na Stability of sols Sodium palmitate Sols are thermodynamically unstable and the dispersed + The cations (Na ) pass into the solution while the anions phase (colloidal particles) tend to separate out on long standing (C15 H31COO ) have a tendency to form aggregates due to weak due to the Vander Waal's attractive forces.However sols attractive forces present in the hydrocarbon chains. tend to exhibit some stability due to (2) Due to frictional electrification (1) Stronger repulsive forces between the (i) It is believed that the frictional electrification due to similarly charged particles the rubbing of the dispersed phase particles with that of (2) Particle-solvent interactions :Due to strong dispersion medium results in some charge on the colloidal particle-solvent (dispersion medium) interactions, the colloidal particles. particles get strongly solvated. (ii) The dispersion medium must also get some charge, Coagulation or Flocculation or Precipitation because of the friction. Since it does not carry any charge, the “The phenomenon of the precipitation of a colloidal theory does not seem to be correct. solution by the addition of the excess of an electrolyte is called (3) Due to selective adsorption of ions coagulation or flocculation.” (i) The particles constituting the dispersed phase adsorb The coagulation of the lyophobic sols can be carried out only those ions preferentially which are common with their own by following methods. lattice ions. (1) By electrophoresis :In electrophoresis the colloidal (ii) For example, when a small quantity of silver nitrate particles move towards oppositely charged electrode. When (AgNO3) solution is added to a large quantity of potassium these come in contact with the electrode for long these are iodide (KI) solution, the colloidal particles of silver iodide discharged and precipitated. adsorb I from the solution to become negatively (2) By mixing two oppositely charged sols :When oppositely charged sols are mixed in almost equal proportions, charged,(at this stage KI is in excess, and I being common their charges are neutralised. Both sols may be partially or to AgI ) completely precipitated as the mixing of ferric hydroxide (+ve AgI I (AgI)I sol) and arsenioussulphide (–ve sol) bring them in precipitated (Colloidal (in excess (Colloidal particle particle) in the becomes positively form. This type of coagulation is called mutual coagulation medium) charged) or meteral coagulation. But, when a small quantity of potassium iodide (KI) (3) By boiling :When a sol is boiled, the adsorbed layer solution is added to a large quantity of silver nitrate solution is disturbed due to increased collisions with the molecules of dispersion medium. This reduces the charge on the particles and (AgNO ) ; the colloidal silver iodide particles adsorb Ag from 3 ultimately they settle down to form a precipitate. the solution to become positively charged, (at this stage (4) By persistent dialysis :On prolonged dialysis, the AgNO 3 is in excess and Ag is common to AgI ), traces of the electrolyte present in the sol are removed almost completely and the colloids become unstable. AgI Ag (AgI)Ag (Colloidal (in excess in (Colloidal particle (5) By addition of electrolytes :The particles of the particle) the medium) becomes positively charged) dispersed phase i.e., colloids bear some charge. When an Ag+ I– electrolyte is added to sol, the colloidal particles take up ions + + – Ag – + I– carrying opposite charge from the electrolyte. As a result, their Ag – – I + + charge gets neutralised and this causes the uncharged, particles – AgI – + AgI + – – + + to come closer and to get coagulated or precipitated. For + – + – – Ag – Ag I + + I 2 example, if BaCl 2 solution is added to As 2 S 3 sol the Ba ions Ag+ I– – are attracted by the negatively charged sol particles and their Ag+ ions remain in dispersed I ions remain in dispersion + medium (Agl, I– Sol) medium (Agl, Ag Sol) charge gets neutralised. This lead to coagulation.
(iii) Depending uponFig. the 14.6nature of charge on the particles (6) Hardy schulzerule :The coagulation capacity of of the dispersed phase, the colloidal solutions are classified into different electrolytes is different. It depends upon the valency of positively charged and negatively charged colloids. Some typical the active ion are called flocculating ion, which is the ion examples are as follows carrying charge opposite to the charge on the colloidal particles. (a) Negatively charged (b) Positively charged “According to Hardy Schulze rule, greater the valency of the colloids colloids active ion or flocculating ion, greater will be its coagulating power”thus, Hardy Schulze law state: Metal sulphides : Metal hydroxides : (i) The ions carrying the charge opposite to that of sol As S ,CdS Al(OH ) , Fe(OH ) 2 3 3 3 particles are effective in causing coagulation of the sol.
Metal dispersions : Metal oxide : TiO 2 (ii) Coagulating power of an electrolyte is directly Ag, Au, Pt Basic dyes : Methylene blue proportional to the valency of the active ions (ions causing Acid dyes : Eosin, congo red Haemoglobin coagulation). Sols of starch, gums, gold, Sulphur sol gelatin etc.
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
For example to coagulate negative solof As 2 S 3 , the Table : 14.4 Gold numbers of some hydrophilic coagulation powerof different cations has been found substances todecreasein the order as, Al 3 Mg 2 Na Hydrophili Gold Hydrophilic Gold c number substance number Similarly, to coagulate a positive sol such as Fe(OH)3 , substance the coagulating powerof different anions has been found to Gelatin 0.005 - 0.01 Sodium oleate 0.4 – 1.0 decreasein the order : [Fe(CN ) ]4 PO 3 SO 2 Cl Sodium 0.01 Gum tragacanth 2 6 4 4 caseinate (7) Coagulation or flocculation value Hamoglobin 0.03 – 0.07 Potato starch 25 “The minimum concentration of an electrolyte which is Gum arabic 0.15 – 0.25 required to cause the coagulation or flocculation of a sol is known as flocculation value.” Congo rubinnumber :Ostwaldintroduced congorubin or number to account for protective nature of colloids. It is defined “The number of millimoles of an electrolyte required to as “the amount of protective colloid in milligrams which bring about the coagulation of one litre of a colloidal solution is prevents colour change in 100 ml of 0.01 % congorubin dye to which 0.16 g equivalent of KCl is added.” called its flocculation value.” Mechanism of sol protection 1 Coagulation value or flocculating value (i) The actual mechanism of sol protection is very Coagulatin g power complex. However it may be Lyophilic (8) Coagulation of lyophilic sols due to the adsorption of the protecting (i) There are two factors which are responsible for the particles protective colloid on the Lyophobic stability of lyophilic sols. lyophobic sol particles, particles (ii) These factors are the charge and solvation of the followed by its solvation. colloidal particles. Thus it stabilises the sol (iii) When these two factors are removed, a lyophilic sol Fig. 14.7 Protection of can be coagulated. viasolvation effects. colloids (iv) This is done (i) by adding electrolyte (ii) and by (ii) Solvation effects contribute much towards the stability adding suitable solvent. of lyophilic systems. For example, gelatin has a sufficiently (v) When solvent such as alcohol and acetone are added strong affinity for water. It is only because of the solvation to hydrophilic sols the dehydration of dispersed phase occurs. effects that even the addition of electrolytes in small amounts Under this condition a small quantity of electrolyte can bring does not cause any flocculation of hydrophilic sols. However at about coagulation. higher concentration, precipitation occurs. This phenomenon is called salting out. Protection of colloids and Gold number (iii) The salting out efficiency of an electrolyte depends upon Lyophilic sols are more stable than lyophobic the tendency of its constituents ions to get hydrated i.e, the tendency sols. to squeeze out water initially fied up with the colloidal particle. Lyophobic sols can be easily coagulated by the (iv) The cations and the anions can be arranged in the addition of small quantity of an electrolyte. decreasing order of the salting out power, such an arrangement When a lyophilic sol is added to any lyophobic sol, it is called lyotropic series. becomes less sensitive towards electrolytes. Thus, lyophilic Cations : colloids can prevent the coagulation of any lyophobic sol. Mg 2 Ca 2 Sr 2 Ba 2 Li Na K “The phenomenon of preventing the coagulation of a lyophobic sol due to the addition of some lyophilic colloid is NH 4 Rb Cs called sol protection or protection of colloids.” 3 2 Anions: Citrate SO 4 Cl NH 3 I CNS The protecting power of different protective (lyophilic) Ammonium sulphate, due to its very high solubility in colloids is different. The efficiency of any protective colloid is water, is oftenly used for precipitating proteins from aqueous expressed in terms of gold number. solutions. Gold number :Zsigmondyintroduced a term called (v) The precipitation of lyophilic colloids can also be gold numberto describe the protective power of different affected by the addition of organic solvents of non-electrolytes. colloids. This is defined as, “weight of the dried protective agent For example, the addition of acetone or alcohol to aqueous in milligrams, which when added to 10 ml of a standard gold gelatin solution causes precipitation of gelatin. Addition of sol (0.0053 to 0.0058%) is just sufficient to prevent a colour petroleum ether to a solution of rubber in benzene causes the change from red to blue on the addition of 1 ml of 10 % sodium precipitation of rubber. chloride solution, is equal to the gold number of that protective colloid.” Emulsion Thus, smaller is the gold number, higher is the protective “The colloidal systems in which fine droplets of one liquid action of the protective agent. are dispersed in another liquid are called emulsions the two 1 liquids otherwise being mutually immiscible.”or Protective power Gold number “Emulsion are the colloidal solutions in which both the dispersed phase and the dispersion medium are liquids.”
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
A good example of an emulsion is milk in which fat force is removed. Common examples are gelatin, agar-agar, globules are dispersed in water. The size of the emulsified starch etc. globules is generally of the order of 10 6 m. Emulsion resemble (ii) Non-elastic gels :These are the gels which are rigid lyophobic sols in some properties. and do not have the property of elasticity. For example, silica (1) Types of Emulsion :Depending upon the nature of gel. the dispersed phase, the emulsions are classified as; Application of colloids (i) Oil-in-water emulsions (O/W) :The emulsion in (1) Purification of water by alum (coagulation) :Alum which oil is present as the dispersed phase and water as the which yield Al 3 ions, is added to water to coagulate the dispersion medium (continuous phase) is called an oil-in- negatively charged clay particles. water emulsion. Milk is an example of the oil-in-water type of (2) In rubber and tanning industry (coagulation and emulsion. In milk liquid fat globules are dispersed in water. mutual coagulation) :Several industrial processes such as rubber Other examples are, vanishing cream etc. plating, chrome tanning, dyeing, lubrication etc are of colloidal (ii) Water-in-oil emulsion (W/O) :The emulsion in nature which water forms the dispersed phase, and the oil acts as the (i) In rubber platting, the negatively charged particles of dispersion medium is called a water-in-oil emulsion. These rubber (latex) are made to deposit on the wires or handle of emulsion are also termed oil emulsions. Butterand cold various tools by means of electrophoresis. The article on which creamare typical examples of this types of emulsions. Other rubber is to be deposited is made anode. examples are cod liver oil etc. (ii) In tanning the positively charged colloidal particles of (2) Properties of emulsion hides and leather are coagulated by impregnating, them in (i) Emulsions show all the characteristic properties of negatively charged tanning materials (present in the barks of colloidal solution such as Brownian movement, Tyndall effect, trees). Among the tanning agent chromium salts are most electrophoresis etc. commonly used for the coagulation of the hide material and the (ii) These are coagulated by the addition of electrolytes process is called chrome tanning. containing polyvalent metal ions indicating the negative charge (3) Artificial rains :It is possible to cause artificial rain on the globules. by throwing the electrified sand or silver iodide from an (iii) The size of the dispersed particles in emulsions in aeroplane and thus coagulating the mist hanging in air. larger than those in the sols. It ranges from 1000 Å to 10,000 Å. (4) Smoke precipitation (Coagulation) :Smoke is a However, the size is smaller than the particles in suspensioins. negative sol consisting of carbon particles dispersed in air. (iv) Emulsions can be converted into two separate liquids Thus, these particles are removed by passing through a chamber by heating, centrifuging, freezing etc. This process is also known provided with highly positively charged metallic knob. as demulsification. (5) Formation of deltas (coagulation) :River water (3) Applications of emulsions consists of negatively charged clay particles of colloidal (i) Concentration of ores in metallurgy dimension. When the river falls into the sea, the clay particles (ii) In medicine (Emulsion water-in-oil type) are coagulated by the positive Na , K , Mg 2 ions etc. present (iii) Cleansing action of soaps. (iv) Milk, which is an important constituent of our diet an in sea water and new lands called deltas are formed. emulsion of fat in water. (6) Clot formation : Blood consists of negatively (v) Digestion of fats in intestine is through charged colloidal particles (albuminoid substance). The colloidal emulsification. nature of blood explains why bleeding stops by applying a ferric Gels chloride solution to the wound. Actually ferric chloride solution causes coagulation of blood to form a clot which stops further (1) “A gel is a colloidal system in which a liquid is bleeding. dispersed in a solid.” (7) Colloidal medicine :Argyrol and protargyrol are (2) The lyophilic sols may be coagulated to give a colloidal solution of silver and are used as eye lotions colloidal semisolid jelly like mass, which encloses all the liquid present in sulphur is used as disinfectant colloidal gold, calcium and iron the sol. The process of gel formation is called gelationand the are used as tonics. colloidal system formed called gel. (8) Coating of Photographic plates :These are thin (3) Some gels are known to liquify on shaking and reset glass plates coated with gelatin containing a fine suspension of on being allowed to stand. This reversible sol-gel silver bromide. The particles of silver bromide are colloidal in transformation is called thixotropy. nature. (4) The common examples of gel are gum arabic, gelatin, processed cheese, silicic acid, ferric hydroxide etc.
(5) Gels may shrink by loosing some liquid help them.
This is known as synereises or weeping. (6) Gels may be classified into two types (i) Elastic gels :These are the gels which possess the property of elasticity. They readily change their shape on Chemisorption and physisorption both are exothermic. applying force and return to original shape when the applied Charcoal adsorbs many gases. It even adsorbs polluting
STD XII CHEMISTRY CHAPTER 5 Surface Chemistry
gases present in air in small concentration. The langmuir adsorption isotherm is restricted to the formation of unimolecular layer of gas molecules on the surface of solids. However, it was suggested that there is possibility of multimolecular layer of gas molecules on the surface of the solids rather than single layer on this basis Brunauer, Emmett and Teller proposed a new theory known as B.E.T. theory. All Bronsted acids and bases act as acid base catalysts. The principle of electrophoresis is employed for the separation of proteins from nucleic acids removing sludge from sewage waste etc. Hydrophilic sols show greater stability than hydrophobic sols. Colloidal solution of graphite is called aquadug. The phase in which the emulsifier is more soluble becomes outer phase of the emulsion that is called Ben croft rule.