+2 UNIT 7 PAGE- 2
CATALYSIS
A catalyst is a substance that increases the rate of reaction without being consumed in the reaction. The phenomenon of increase in the rate of reaction with the help of a catalyst is known as catalysis. Since catalysts are not consumed in a reaction, very small, non-stoichiometric quantities are generally all that are required. Catalysts are employed in a number of industrial processes (Table)
Some modern processes based on catalysis
Reactants | Catalyst | Product |
HomogeneousPropylene,oxygen Methanol,CO Butadiene, HCN a-olefin, CO, H2 | Mo(VI) complexes [Rh(CO)2I2]- Ni/Pd compounds Rh/Pd compounds | Propylene oxide Acetic acid Adiponitrile Aldehydes |
Heterogeneous Ethylene, O2 Propylene, NH3,O2 Ethylene | Ag,CsCl on alumina Bismuth molybdates Organo chromium and titanium | Ethylene oxide Acrylonitrile High density polythene |
Nature is the master designer of catalysts. Even the simplest bacterium employs thousands of biological catalysts, known as enzymes , to speed up its cellular reactions. Every organisms relies on enzymes to sustain life.
Each catalyst has its own specific way of functioning , but in general, a catalyst functions by lowering the energy of activation which in turn makes the rate constant larger and hence the rate of reaction is higher.
Comparison of the activation energies of a catalysed and uncatalysed reaction
Two important points stand out in Fig.
(i) A catalyst speeds up the forward and reverse reactions to the same extent and therefore does not affect the equilibrium constant.
(ii) A catalyst lowers the energy of activation by providing a different mechanism for the reaction
Positive and Negative Catalysis
Depending upon the nature of catalyst, catalysis can be classified as positive and negative catalysis.
Positive Catalysis
When a catalyst increases the rate of reaction , it is termed as positive catalyst and the process is known as positive catalysis. Some examples are as follows :
(i) Oxidation of ammonia in the presence of platinum gauze. Here, platinum acts as a positive catalyst.
(ii) Oxidation of sulphur dioxide into sulphur trioxide is enhanced in the presence of vanadium pentoxide.
(iii) Decomposition of MnO2 is facilitated in the presence of MnO2.
Negative catalysis
When a catalyst decreases the rate of a reaction, it is termed as a negative catalyst and the process is called negative catalysis. Some examples of negative catalysis are as follows.
(i) The oxidation of chloroform by air is retarded in the presence of ethyl alcohol. Here ethyl alcohol acts as a negative catalyst.
(ii) Decomposition of hydrogen peroxide gets retarded when some glycerine is added to it.
2 H2O2 ® 2 H2O + O2
(retarded in presence of glycerol)
Types of Catalysis
There are two broad categories of catalysis– homogeneous and heterogeneous depending on whether the catalyst is in the same phase as the reaction mixture or not.
1. Homogeneous catalysis
If the catalyst is present in the same phase as the reactants, it is called a homogeneous catalyst and this type of catalysis is called homogeneous catalysis. It is believed that in homogeneous catalysis , a catalyst enters into chemical combination with one or more of the reactants forming intermediate compound which then decomposes or combines with one of the reactants to produce the product and the catalyst is generated.
The Table given above shows industrial processes that employ these catalysts.
A throughly studied example of homogeneous catalysis is the hydrolysis of an organic ester (RCOOR’).
where R and R’ are alkyl groups, RCOOH is a carboxylic acid and ROH is an alcohol. The reaction rate is low at room temperature but can be increased by adding a small amount of a strong inorganic acid which provides H+ ion, that acts as a catalyst in the reaction.
Some other examples of homogeneous catalysts are :
(i) Catalytic decomposition of ozone by Cl atoms in the gas phase.
(ii) Oxidation of CO by O2 in the presence of NO as catalyst.
(iii) Hydrolysis of cane sugar solution in the presence of dilute sulphuric acid.
Theory of Homogeneous Catalysis
(Intermediate Compound Theory)
The role of a catalyst in homogeneous catalysis can be understood in the light of intermediate compound theory.
According to intermediate compound theory, the homogeneous catalyst combines with a reactant to form an unstable intermediate compound, the formation of which occurs at a lower activation energy. The intermediate compound thus formed either reacts with other reactant(s) or decomposes to give the desired end-products and the catalyst is finally regenerated. Thus, catalyst enhances the rate of reaction by providing an alternate path of lower activation energy.
For example, in the lead chamber process used for the manufacture of sulphuric acid, sulphur dioxide is oxidised to sulphur trioxide in the presence of nitric oxide as catalyst. In the absence of catalyst, the oxidation of SO2 into SO3 is very slow.
2 SO2(g) + O2(g) ® 2 SO3(g)
When nitric oxide is used as a catalyst, the rate of reaction increases considerably. It is believed that the catalyst provides an alternate path of low activation energy by combining with O2 to form an intermediate NO2 which subsequently reacts with SO2 to yield the required product, i.e., SO3 and the catalyst gets regenerated as shown below.
HETEROGENEOUS CATALYSIS
In this type of catalysis the catalyst is present in a different phase than that of the reactants.
In heterogeneous catalysis , catalyst is generally a solid and the reactants are generally gases but some times liquid reactants are also used. It is also known as surface catalysis as the reaction starts at the surface of solid catalyst. These catalysts has enormous surface areas between 1 to 500 m2 g-1 for contact. Interestingly , many reactions that occur on the metal surface such as decomposition of HI on gold and decomposition of N2O on platinum, are zero order because the rate determining step occurs on the surface itself. Thus despite an enormous surface area , once the reactant gas covers the surface, increasing the reactant concentration cannot increase the rate. Some industrial processes that employ heterogeneous catalysts are given below.
Reactants | Catalyst | Product |
Ethylene, O2 Propylene, NH3,O2 Ethylene | Ag,CsCl on alumina Bismuth molybdates Organo chromium and titanium | Ethylene oxide Acrylonitrile High density polythene |
One of the most important examples of heterogeneous catalysis is the addition of H2 to C=C bonds of organic compounds to form C-C bonds. The petroleum . plastics and food industries frequently use catalytic hydrogenation. The conversion of vegetable oil into margarine is one example.
The simplest hydrogenation process converts ethylene to ethane :
CH2=CH2 (g) + H2(g) ® CH3-CH3 (g)
In the absence of a catalyst , the reaction occurs very slowly. At high H2 pressure in presence of finely divided nickel, palladium or platinum the reaction becomes rapid at ordinary temperatures. The catalysed reaction is believed to take place through the following consecutive steps :
(i) Diffusion of C2H4 and H2 towards the surface.
(ii) Adsorption of C2H4 and H2 at the active sites.
(iii) (a) Dissociation of H2 into H atoms (b) Linking of the H-atom to C2H4 to form C2H6.
(iv) Diffusion of C2H6 from the surface.
(v) Diffusion of C2H6 away from the surface.
Some other examples of heterogeneous catalysis are :
(i) Manufacture of ammonia from nitrogen and hydrogen in
presence of spongy iron.
(ii) Synthesis of methanol from CO and H2 using a mixture of copper, ZnO and Cr2O3 as catalyst.
Nature of Solid Catalyst
Solid catalysts may be metals, metal oxides , metal sulphides , clays etc. These materials may be used in their pure form or in the form of their mixtures. Further they may be crystalline , microcrystalline (in the form of fine particles) or amorphous. Two significant aspects of heterogeneous catalyst are :
(i) Activity
(ii) Selectivity
Activity : The activity a solid catalyst depends upon the strength of chemisorption to a large extent. The reactant must adsorb reasonably strongly for the catalyst to be active must not adsorb so strongly that they are immobilised and other reactants are left with no space on the catalyst surface for adsorption. It has been found that for hydrogenation the catalytic activity increases as we go from group 5 metals to group11 with maximum activity shown by group 7 – 9 elements in the periodic table.
The ability of catalysts to accelerate chemical reaction is called activity. In certain cases, the degree of acceleration shall be as high as 1010 times. For example, in the absence of a catalyst, a mixture of H2 and O2 (both pure) can be stored indefinitely without any reaction. However, in presence of a catalyst such as platinum, they (H2 and O2) combine explosively to form water.
(ii) Selectivity : It is the ability of catalysts to direct a reaction to yield a particular product (preventing side reactions), i.e., a particular catalyst cannot be used for all types of reactions.
Example,
(a) Ethanol can undergo both dehydration and dehydrogenation.
i) In the presence of Ni catalyst, only dehydrogenation of ethanol occurs.
(ii) In the presence of alumina, only dehydration of ethanol occurs.
(b) MnO2 can catalyse the decomposition of KClO3 but not KNO3.
Action of a catalyst is highly specific (selective) in nature i.e., a given substance can act as a catalyst in a particular reaction and not for all reactions. It means a substance which acts as a catalyst in one reaction may fail to catalyze other reaction i.e., a catalyst is highly selective in nature.
SHAPE SELECTIVE CATALYSIS BY ZEOLITES
Zeolites are microporous aluminosilicates of the general formula Mx/n [ (AlO2)x(SiO2)y ] m H2O, where M represents a metal cation (e.g. Na+, K+, or Ca2+) having valency n. They may be natural as well as synthetic. Some common zeoltes are :
Fujasite Na56[(AlO2)56(SiO2)136] 250 H2O
Linde A {Na2[(AlO2)12(SiO2)12} 27 H2O
ZSM-5 Hx[ (AlO2)x (SiO2)96-x ] 16 H2O
Zeolites may be considered as open structures of silica in which x / (x + y) fraction of tetrahedral sites is occupied by aluminium. The net negative charge of aluminosilicate frame work is neutralised by exchangeable cations Mn+ . The void space present in the unit cell (which may be greater than 50% of the volume) is occupied by water molecules. Due to openness of the structure, zeolites have high porosity. The high porosity is mainly due to one, two or three-dimensional networks of interconnected channels and cavities of molecular dimensions.
The catalytic behaviour of zeolite depends on the size of the cavities (cages) and pores(apertures) present in them. In zeolites, the size of the pores varies between 260 pm to 740 pm. Through these pores , the reactant molecules of a particular size and shape can enter, fit in and get adsorbed. The adsorbed molecules form an activated complex which on decomposition yields the products. If the reactant molecules are too large, they will not enter these pores. In case, if they are too small, they will slip through the pores of the catalyst without being adsorbed. In both these cases, the catalyst cannot influence the rate of reaction. Thus zeolites are shape-selective catalysts. Zeolites are widely used in petrochemical industries for cracking of hydrocarbons and isomerization.
Recently, a synthetic zeolite ZSM-5 has been used to convert alcohols to gasoline. The alcohol is dehydrated in the cavities by protons and hydrocarbons are formed. Shape-selectivity in this reaction can be seen from the data given below for the conversion of methanol and 1-heptanol to hydrocarbon mixtures.
Product | starting from methanol (%) | Starting from n-heptanol (%) |
Methane | 11.0 | 0.0 |
Ethane | 0.6 | 0.3 |
Isobutane | 18.7 | 19.3 |
n-butane | 5.6 | 11.0 |
Isopentane | 7.8 | 8.7 |
Benzene | 1.7 | 14.3 |
Xylenes | 17.2 | 11.6 |
The nature of the product formed depends upon the ability of pores to accommodate linear and iso-alkanes as well as benzene derivatives.
ENZYME CATALYSIS
Living organisms carry out thousands of chemical reactions which take place in dilute solution at ordinary temperature and pressure. For example, they can use small molecules to assemble complex biopolymers such as proteins and DNA. Organisms can produce molecules that combat bacterial invaders. They can break down large , energy-rich molecules in many steps to extract chemical energy in small proportions to drive their many activities.
Most of these reactions are catalysed by biochemical catalysts called enzymes. Enzymes are proteins with high molecular mass ranging from 15,000 to 1,000,000 g/mol. Enzymes are efficient catalysts. They increase rate by 108 to 1020 times. Enzymes are also extremely specific , each reaction is generally catalysed by a particular enzyme. Urease for example, catalyses only the hydrolysis of urea and none of the several thousand other enzymes present in the cell catalyses that reaction :
The remarkable specificity of enzymes results from the fact that each enzyme has a specific site on its surface. When the reactant molecules , called the substrates of the reaction, bind at the active site, a chemical change is initiated. In most cases, the substrates bind to the active site through intermolecular forces : H-bonds, dipole forces and other weak attractions.
Two models of enzyme action have been proposed (Fig ).
A.. Lock and key model
B. Induced-fit model
Two models of enzyme action.
In lock-and-key model, the active site is thought to be an exact fit for the substrate shape.
In the induced-fit-model , the active site is thought to change shape to fit the substrate.
Most enzyme-catalysed reactions proceed through a fast, reversible formation of an enzyme-substrate complex, followed by a slow conversion to product and free enzyme.
According to the lock-and-key model , when the ‘key’ (substrate) fits the lock (active site) , the chemical change begins. However, modern X-ray crystallographic methods show that in many cases, the enzyme changes shape when substrate lands at all the active site. Thus induced-fit model of enzyme action pictures the substrate inducing the active site to adopt a perfect fit, rather than a rigidly shaped lock and key. Therefore, we might picture a hand in a glove, in which ‘glove’ (active site) does not attain its functional shape until the ‘hand’ (subtrate) moves into place.
The kinetics of enzyme catalysis has many features in common with ordinary catalysis. In the enzyme catalysed reaction, substrate (S) and enzyme (E) form an intermediate-enzyme complex (ES) whose concentration determines the rate of product (P) formation. The steps common to virtually all enzyme catalysed reactions are :
The rate of enzyme-catalysed reaction changes from first order to zero-order as the concentration of the substate is increased.
Chemically, all enzymes are globular proteins. However, some enzymes are associated with non-protein components called prosthetic group. The prosthetic groups could be either metal ions such as Zn2+ , Mg2+, Co2+, K+ , Na+ etc or small organic molecules. When the prosthetic group is a metal ion, it is called a cofactor. In case the prosthetic group is a small organic molecule, it is referred to as a coenzyme. Many of the coenzymes for biological processes are derived from vitamins like thiamine, riboflavin, niacin etc. In such enzymes, the protein part of the enzyme is called apoenzyme. Neither apoenzyme nor coenzyme is able to catalyse the reaction alone. The two must combine together before acting as a catalyst.
Since the action of enzyme is highly specific, every biological reaction requires a particular type of enzyme. To date, more than 3000 different kinds of enzymes have been identified in living systems, each catalysing a different biological reaction. Some important enzymes and their functions are listed in TABLE.
Enzyme | Reaction caalysed |
Invertase | Sucrose ® Glucose + Fructose |
Amylase | Starch ® n Glucose |
Maltase | Maltose ® 2 Glucose |
Lactase | Lactose ® Glucose + Galactose |
Urease | Urea ® CO2 + NH3 |
Carbonic anhydrase | H2CO3 ® H2O + CO2 |
Pepsin | Proteins ® Amino acids |
Trypsin | Proteins ® Aminoacids |
Nucleases | DNA, RNA ® Nucleotides |
DNA polymerase | Deoxynucleotide triphosphate ® DNA |
RNA polymerase | Ribonucleotidetriphosphates ® RNA |
COLLOIDAL STATE
Based on the size of particles, solutions can be classified into three :
(i) True solution or molecular solution : It is homogeneous solution which contains solute particles having size (diameter) less than 1000 pm (i.e., 1 nm), dispersed throughout the solvent , e.g., the solution of KCl in water. The particles (molecules or ions ) present in a true solution are invisible under microscope. Moreover they pass through ordinary filter paper and animal membrane.
(ii) Suspension : It is a heterogeneous mixture which contains particles having size greater than 100,000 pm (i.e., 100 nm). Eg. AgCl in water, dust particles in water etc. The particles of a suspension are visible even for naked eye. These particles do not pass through the pores of ordinary filter paper or animal membrane.
(iii) Colloidal solution : It is a heterogeneous solution which contains particles having size in between 1000 – 100,000 pm (i.e., 1 nm - 100 nm), i.e., particles of colloidal solutions are intermediate in size between the particles of true solutions and supensons. In other words, colloidal state is an intermediate state between true solutions and suspensions. e.g. starch in water, milk etc. Collidal particles cannot be seen with a naked eye, but light reflected by them can be seen under an ultramicroscope. The particles pass through ordinary filter paper but they are retained when filtered through an animal membrane.
The three types of systems containing dispersed particles of different size.
Distinction between True solution, Colloidal solution and suspension
Property | True solution | Colloids | Suspension |
1. particle size | < 103 pm | 103 - 105 pm | > 105 pm |
2. Appearance | transparent | generally transparent | opaque |
3. Separation a. ordinary filtration b. ultra filtration | not possible not possible | not possible possible | possible possible |
4. settling of particles | do not settle | settle only on centrifugation | settle under gravity |
5. Nature | homogeneous | heterogeneous | heterogeneous |
6. Tyndall effect | does not show | shows | may show |
7. Brownian movement | may or may not show | shows | shows |
Phases of colloids and their classification
A colloidal solution is of heterogeneous in nature. It consists of two phases , i.e., a dispersed phase and a dispersion medium.
(i) Dispersed phase : It is the component present in small proportion and is just like a solute in a solution. For example, in colloidal solution of silver in water, the former acts as the dispersed phase.
(ii) Dispersion medium : It is generally the component present in excess and is just like a solvent in a solution. In the above example, water acts as the dispersion medium.
Classification of Colloidal Systems
a) Based on states of dispersed and dispersion medium
The dispersed phase need not always be a solid. It may be a solid, liquid or a gas. Similarly, the dispersion medium shall be a gas or a liquid or even a solid. Thus based on the physical state of dispersed phase and dispersion medium, nine different colloidal systems are possible. However, a gas dispersed in another gas does not give a colloidal solution, since gases are completely miscible. Therefore, the number of colloidal systems reduces to eight, which are listed below.
Types of colloidal solutions
Dispersed phase | Dispersion medium | Examples |
1. gas | liquid | soap lather, whipped cream, soda water |
2. gas | solid | pumice stone, foam rubber |
3. liquid | gas | mist, fog, cloud, insecticide spray |
4. liquid | liquid | milk, emulsified oils, cod liver oil |
5. liquid | solid | jelly, butter, cheese, boot polish |
6. solid | gas | smoke, dust storm, haze |
7. solid | liquid | paints, starch dispersed in water, gold sol |
8. solid | solid | alloys, coloured glass, gem stones, ruby glass. |
1. A gas dispersed in liquid is called foam.
2. A gas dispersed in solid is called solid foam
3. A liquid dispersed in gas is called aerosol
4. Liquid dispersed in liquid is called emulsion
5. Liquid dispersed in solid is called gel.
6. Solid dispersed in gas is called aerosol.
7. Solid dispersed in liquid is called sol.
8. Solid dispersed in a solid is called solid sol.
b) Based on the nature of dispersion medium
The colloidal systems are classified as follows on the basis of nature of dispersion medium.
Dispersion medium | Name of colloidal solution |
water | hydrosol |
alcohol | alcosol |
benzene | benzosol |
air | aerosol |
(c) Based on nature of interaction between Dispersed phase and dispersion medium.
The colloids can be classified into two on the basis of affinity of phases.
(i) Lyophilic colloids : The colloidal systems in which the particles of dispersed phase have a great affinity towards the dispersion medium are called lyophilic colloids. Such colloidal solutions are readily formed when the dispersed phase and dispersion medium are brought together. These are reversible colloids. When water is the dispersion medium, then the lyophilic colloids are called hydrophilic colloids. Examples of lyophilic colloids are glue, gelation, starch etc.
(ii) Lyophobic colloids : The colloidal solutions in which the particles of dispersed phase have no affinity for dispersion medium are called lyophobic collids. Such colloidal solutions are formed with difficulty. They are irreversible colloids. Example of lyophobic colloids are As2S3 sol, gold sol, Fe(OH)3 sol etc. In case the dispersion medium is water, then lyophobic colloids are hydrophobic colloids.
Comparison of lyophilic colloids and lyophobic colloids are given in the TABLE.
Lyophilic colloids | Lyophobic colloids |
1. These are easily formed by direct mixing 2. These are reversible in nature. 3. The particles of colloids are true molecules and are big in size. 4. The particles are not easily visible even under microscope. 5. These are very stable. 6. The addition of small amount of electrolytes cause precipitation(called coagulation) of colloidal solution. 7. The particles do not carry any charge. The particles may migrate in any direction or even under the influence of electric field. 8. The particles of colloids are heavily hydrated due to attraction for solvent. 9. The viscosity and surface tension of the sols are much higher than that of dispersion medium. 10. They do not show Tyndall effect. | 1. These are formed by only under special methods. 2. These are irreversible in nature. 3. The particles are aggregates of many molecules 4. The particles are easily detected under ultramicroscope 5. These are unstable and require traces of stabilizers. 6. The addition of small amount of electrolyte has less effect. Larger quantities of electrolytes are required to cause coagulation. 7. The particles move in specific direction, i.e., either towards anode or cathode depending upon their charge. 8. The particles of colloids are not appreciably hydrated due to hatred for the solvent. 9. The viscosity and surface tension are nearly the same as that of the dispersion medium. 10. They show Tyndall effect |
(d) Based on type of particles of dispersed phase : The colloids have been classified into three types according to the molecular size.
(i) Multimolecular Colloids : In this type, the particles consists of an aggregate of atoms or molecules with molecular size less than 1 nm. For example, in gold sol several atoms of gold are condensed together to form bigger particles of colloidal size. Similarly in sulphur sol, each collodial particle contains more than a thousand individual S8 molecules. In these colloids, the particles are held together by van der Waal’s forces.
(ii) Macromolecular Colloids : In this type, the particles of dispersed phase are sufficiently big in size (macro) to be of colloidal dimensions. In this case, a large number of small molecules are joined together through their primary valencies to form giant molecules. These molecules are called macromolecules and each macromolecule may consist of hundreds or thousands of simple molecules. The solutions of such molecules are called macromolecular solutions. For example, colloidal solutions of starch , proteins, cellulose and enzymes. Examples of man-made macromolecules are polyethene, nylon, polystyrene, synthetic rubber etc.
(iii) Associated colloids (Micelles) : There are some substances which at low cocentrations behave as normal, strong electrolytes but at higher concentrations exhibit colloidal behaviour due to the formation of aggregated particles. The aggregated particles thus formed are called micelles. These are also known as associated colloids. The formation of micelles takes place only above a particular temperature called kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic dtergents belong to this class. For soaps CMC is ~ 10-4 to 10-3 mol L-1. These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules or more.
Mechanism of micelle formation
Let us take the formation of soap solutions. Soap is sodium salt of higher fatty acid and may be represented as RCOO- Na+ e.g., sodium stearate CH3(CH2)16COO-Na+ which is the major component of many bar soaps. When dissolved in water it dissociates into RCOO- and Na+ ions.
Agrregation of RCOO- ions to form an anionic micelle
The RCOO- ions , however, consists of two parts, i.e., long hydrocarbon chain R (also called non-polar ‘tail’ ) which is hydrophobic (water repelling) and the polar group COO- (also called polar–ionic ‘head’) which is hydrophylic (water loving). The RCOO- ions are , therefore present on the surface with their COO- groups in water and the hydrocarbon chains R staying away from it, and remain at the surface, but at higher concentration these are pulled into the bulk of the solution and aggregate in a spherical form with their hydrocarbon chains pointing towards the centre with COO- part remaining outward on the surface. An aggregate thus formed is known as ‘ionic micelle’. These micelles may contain as many as 100 such ions.
Similarly, in case of detergents, e.g., sodium lauryl sulphate viz CH3(CH2)11SO4-Na+, the polar group – SO4- along with the long hydrocarbon chain. Hence the mechanism of micelle formation is same as that of soaps.
PREPARATION OF COLLOIDS
1. Preparation of Lyophilic colloids : The lyophilic colloids have strong affinity between particles of dispersed phase and dispersion medium. Therefore, these colloidal solutions are readily formed by simply mixing the dispersed phase and dispersion medium under ordinary conditions. For example, the substances like gelatin, gum etc. pass rapidly into water to give colloidal solution. These are reversible in nature because, these can be precipiated and directly converted into colloidal state.
2. Preparation of Lyophobic colloids : Lyophobic sols can be prepared by mainly two types of methods.
(i) Condensation method
(ii) Dispersion method
Condensation Methods
In these methods, smaller particles of dispersed phase condensed suitably to be of colloidal size. This is done by the following methods.
(i) Chemical methods
(a) Oxidation : A colloidal sol of sulphur is obtained when hydrogen sulphide is bubbled through oxidising agent such as bromine water, sulphur dioxide , nitric acid etc.
H2S + Br2 ® 2 HBr + S
2 H2S + SO2 ® 2 H2O + 3 S
H2S + [O] ® H2O + S
(b) Reduction : Sols of metals such as silver, gold and platinum are obtained by the reduction of their salts with reducing agents like formaldehyde, phenyl hydrazine, SnCl2 etc.
2AuCl3 + 3 SnCl2 ® 3SnCl4 + 2 Au
( c) Hydrolysis : A colloidal solution of ferric hydroxide is obtained when concentrated solution of ferric chloride is added drop-wise to hot water.
FeCl3 + 3 H2 O ® Fe(OH)3 + 3 HCl
(d) Double decomposition : A colloidal solution of arsenic sulphide is obtained by passing hydrogen sulphide through a dilute solution of arseneous oxide in water.
As2O3 + 3 H2 S ® As2 S3 + 3 H2 O
(ii) Physical methods
a) By excessive cooling : The colloidal solution of ice in some organic solvents such as chloroform or ether can be prepared by freezing a solution of water in organic solvent. The molecules of water which can no longer be held in solution separately combine to form particles of colloidal size.
b) By exchange of solvent : Colloidal solution of certain substances such as sulphur and phosphorus can be prepared by pouring their alcoholic solution in excess of water. For example, alcoholic solution of sulphur on pouring into water gives milky colloidal solution of sulphur.
Dispersion methods
In these methods, large particles of the substance are broken into particles having colloidal size, in presence of dispersion medium.
(i) Mechanical dispersion : The substance whose sol is to be prepared is powdered and mixed with dispersion medium to get a suspension. It is then fed into a colloid mill (Fig),
Mechanical dispersion
which consists of two metallic discs nearly touching each other and rotating in opposite directions at very high speed. The suspension particles are torn off to the colloidal size.
(ii) Electrical disintegration : Bredig’s Arc method
This method is convenient for the preparation of sols of metals like gold, silver, platinum etc. Two electrodes of the metal whose colloidal solution is to be prepared are dipped in the dispersion medium such as water. The dispersion medium is kept cooled by surrounding it with ice. The intense heat of the arc converts the metal into vapours . These vapours are then condensed by ice-cold water to form the sol. The sol is stabilised by trace of sodium hydroxide or potassium carbonate.
Electrical dispersion(iii) Peptisation : The conversion of a freshly prepared precipitate into a colloidal solution by the small amount of a suitable electrolyte is called peptisation. The electrolyte used for this purpose is called peptising agent. For example, by the addition of a few drops of ferric chloride to a freshly formed precipitate of Fe(OH)3 , a reddish brown coloured sol of ferric hydroxide is formed. Peptisation involves the preferential adsorption of suitable ions from electrolyte by the particles of the precipitate. Thus in the above case, the precipitate of Fe(OH)3 adsorbs Fe3+ ions from FeCl3 to form Fe(OH)3 . Fe3+.
Fe(OH)3 + Fe3+ ® Fe(OH)3 . Fe3+
precipitate electrolyte colloidal solution