SURFACE CHEMISTRY Adsorption Adsorption is a surface phenomenon .A molecule present in the interior of solid (or liquid) is pulled by other molecules from all sides and the net pull on the molecule is zero, but a molecule at the surface experience a resultant downward pull ie it is left with unsatisfiedor unbalanced forces and therefore it has tendency to attract and retain the molecules of other substances on its surface. Adsorption the phenomenon of attracting and retaining the molecules of a substance on the surface of a solid or liquid resulting into higher concentration of the molecules on the surface is called adsorption . Adsorbent : the solid substance on the surface of which adsorption occurs is known as adsorbent. Adsorbate :the substance which gets adsorbed on solid surface is called adsorbate. Sorption : when both adsorption & absorption take place, the term sorption is used. Occlusion: the adsorption of gases on the surface of metal is called occlusion. Desorption :the process of removal of an adsorbed substance from the surface on which it is adsorbed is called desorption. Difference between adsorption& absorption Adsorption Absorption 1. Adsorption is a surface phenomenon, ieadsorbate 1. Absorption is a bulk phenomenon. are held on the surface . 2. The concentration of adsorbed molecules is more 2. The concentration is same throughout. on the surface. 3. Initially the rate of adsoption is rapid .It decreases 3. Absorption occur with uniform rate. slowly.
Difference between positive & negative adsorption +ve adsorption From solution , if solute is adsorbed on the surface of adsorbent, the concentration of solute is more on the surface of adsorbent compared to bulk, it is called as +ve adsorption.
−Ve adsorption From solution if solvent is adsorbed on adsorbent, the concentration of the solute is less on the surface as compared to bulk It is called –ve adsorption.
Difference between physical adsorption & Chemical adsorption Physical adsorption (physisorption) 1. Molecules of adsorbate and adsorbent are held by weak vanderwaal forces. 2. Enthalpy of adsorption is low (20-40 KJ/mol) in this case. 3. Multimolecularlayer may be formed on
Chemical adsorption (chemisorptions) 1. Molecules of adsorbate and adsorbent are held by chemical bonds. 2. Enthalpy of adsorption is high (80-240 KJ/mol) in this case. 3. Monomolecular layer may be formed
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the surface of adsorbent. 4 It usually takes place at low temperature & decreases with increasing temp. 5.physisorption is non specific in nature. 6. It is reversible in nature
on the surface of adsorbent. 4. It takes place at relatively high tem. 5. Chemisorption is specific in nature 6. It is irreversible.
Adsorption of gases on solids Activated charcoal adsorb Cl2 gas Charcoal also adsorb polluting gases present in air in small amount. Moisture in air is adsorbed by silica gel. Gases like H2,O2, N2, & CO are adsorbed by transition metals like Ni &Co Factors affecting adsorption of gases on solids 1.Nature of adsorbate: physical adsorption is non specific in nature and therefore every gas is adsorbed on the surface of any solid to a lesser or greater extent . However under given conditions of temperature and pressure , the easily liquefiable gases like NH3, HCl, CO2etc are more readily adsorbed. While the chemical adsorbed is highly specific in nature and in this case , a gas gets adsorbed on the solid only if it enters into chemicalcombination with it. 2.Nature of adsorbent; it is observed that same gas is adsorbed to different extent by different solids at the same temperature. 3.Specific area of the adsorbent: is the surface area available for adsorption per gram of the adsorbent. The greater the specific area of the solid , the greater would be its adsorbing capacity. 4.Effect of pressure on adsorption at constant temperature Adsorption isotherm; At constant temperature , the extent of adsorption of a gas (x/ m) on a solid increases with increase of pressure. A graph between x/m and the pressure, p of a gas at constant temperature is called adsorption isotherm.
At low pressure, x/m p, x/m =kp At very high p , x/m x/m = k At moderate p , x/m x/m = kp1/n (1) The above expression (1) is known as Freundlich adsorption isotherm. Where ‘x’ is the mass of gas adsorbed on the ‘m’ mass of adsorbent at pressure p, k & n are constant which depend on the nature of the adsorbent and the gas at a particular temperature Page 2
Taking logarithm of eq(1), Log x/m = logk + 1/n log p The validity of Freundlich isotherm can be verified by plotting log x/m and log p . If it comes out bestraight line , it is valid otherwise not. The slope of the straight line gives the value of 1/n. The intercept on the Yaxis gives the value of log k. The above equation holds good at low p but fails at high p.
Effect of temperature on adsorption at constant pressure The graph between extent of adsorption and temperature at constant pressure is known as adsorption isobar.
Adsorption is exothermic . From graph we see that with increase in temperature x/m decreases in case of physical adsorption while in case of chemical adsorption x/m first increases and then decreases. The initial increase is because reactant molecules need activation energy to get adsorbed and then get desorbed with increase in temperature.
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Adsorption from solutions: see NCERT
1) 2) 3) 4) 5) 6)
Applications of Adsorption Production of high vacuum Gas masks Control of humidity Removal of coloring matter from solution Separation of inert gases Froth floatation process Catalysts A substance that alters the rate of a reaction and themselves remain chemically and quantitatively unchanged after the reaction are known as catalysts andthe phenomenon is known as catalysis. A catalyst speeds up the forward and backward reaction to the same extent and therefore does not affect the equilibrium constant Catalyst lowers the energy of activation by providing a different mechanism for the reaction Promoters: are substances that enhance the activity of a catalysts. eg in Habers process for manufacture of NH3, Molybdenum acts as promoter for Fe, which is used as catalyst. N2 + 3H2 Fe, Mo 2NH3 Poisons:decreases the activity of a catalyst . Types of catalysts 1.Homogeneous catalysts If the reactants & catalysts are present in same phase , it is called homogeneous catalysis Eg (i) Hydrolysis of ester. CH3COOCH3 (l) + H2O (l) HCl(l) CH3COOH(aq) + CH3OH(aq) (ii) Hydrolysis of sugar cane juice in presence of dilute H2SO4. C12H22O11(aq) + H2O (l) H2SO4(l) C6H12O6(aq) + C6H12O6(aq) (iii) Oxidation of SO2 to SO3 SO2 (g) + O2(g) NO(g) SO3(g) 2.Heterogeneous catalysts: When catalysts and reactants are in different phases , it is known asheterogeneous catalysis. Eg (i) Oxidation of SO2 into SO3 in the presence of V2O5 SO2 (g) + O2(g) V2O5(s) SO3(g) (ii) Combination of N2& H2 N2(g) + 3H2(g) Fe(s) 2NH3 (iii) Oxidation of NH3 into HNO3. 4NH3(g) + 5O2(g) Pt(s) 4NO + 6H2O (iv) vegetable oil + H2(g) Ni(s) vegetable ghee. Page 4
Adsorption theory of heterogeneous catalysis. Modern adsorption theory is a combination of old adsorption theory and intermediate compound formation theory . The catalytic activity is localized on the surface ofcatalyst. The mechanism of catalyst involved the following steps. (i) diffusion of reactants to the surface of the catalyst. (ii) adsorption of the reactant molecules on the surface of catalysts. (iii) occurrence of chemical reaction on the catalysts surface through formation of an intermediate (iv) desorption of reaction product from the surface of catalyst. (v) diffusion of reaction product away from the catalyst’s surface.
Important features of solid catalysts: Activity :The ability of a catalyst to increase the rate of reaction is called its activity. For example , a mixture of hydrogen and oxygen does not react at all, however in presence of platinum as catalyst the mixture react with explosive violence to form water. 2H2 + O2 pt 2H2O Selectivity The selectivity of a catalyst is its ability to direct a reaction to yield a particular product .eg CO(g) + 3H2(g) Ni CH4(g) + H2O (g) CO(g) + 3H2(g) Cu/ZnO- Cr2O3 CH3OH(g) CO(g) + 3H2(g) Cu HCHO(g) Thus it can be said that the action of a catalyst is selective Page 5
Shape selective catalysis Zeolites are micro porous alumino-silicate with three dimensional network of silicates in which some silicon atoms are replaced by Al atoms giving Al-O-Si frame work . The reaction taking place in zeolites depend upon the size andshape of reactant and product molecules as well as upon the pores and cavities of the zeolites. Zeolites are used as catalysts in petrochemical industries for cracking of hydrocarbons.ZSM – 5 a synthetic zeolite is used to convert alcohols directly in to gasoline (petrol).
Enzymes: Enzymes are biochemical catalysts.Chemically all enzymes are globular protein. Examples of few enzyme catalysed reactions: 1, Inversion of cane sugar C12H22O11 (aq) + H2O (l) invertase 2. Conversion of glucose intoethyl alcohol C6H12O6 (aq) zymase 2C2H5OH(aq)
C6H12O6 (aq)
+ C6H12O6(aq)
+ 2CO2
3
Conversion of starch into maltose (C6H10O5)(aq) +nH2O (l) diastase n C12H22O11 (aq) 4. Conversion of maltose into glucose . C12H22O11 (aq) + H2O (l) maltase 2C6H12O6 (aq)
6. Decomposition of urea into NH3& CO2 NH2CONH2(aq) + H2O(l) 7
urease
2NH3(g)
+ CO2(g)
Conversion of proteins into amino acids by hydrolysis in intestine. 8 Conversion of milk into curd. Characteristic of Enzyme catalysis. 1 Highly specific in nature : each enzyme is specific for a given reaction. NH2CONH2 CH3NHCONH2
+
H2O
urease
2NH3
+ CO2
+
H2O
urease
no reaction
2.Highly efficient: - Only small amount of enzyme can be highly efficient. Foreg a single molecule ofcarbonic anhydrase can decompose 36 million molecule of carbonic acid into CO2&H2O in one minute. 3.Optimum temperature and PH enzymesare most efficient at PH of around 7.4 & human body temperature of 370C under one atm pressure. Enzyme activators and co-enzymes:- co-enzyme is a small non-protein (vitamin) present along with an enzyme, which enhance the catalytic activity considerably Activators are generally metal ion, such as Na+, Co2+, Mn2+, Cu2+, etc.For eg amylase in presence Page 6
of NaCl i.e Na+ ions are catalytically very active. Influence of inhibitors or poisons: The inhibitors or poisons interact with the active functional groups on enzyme surface & often reduce or completely destroy the catalytic activity of the enzymes. Mechanism of enzyme catalysis There are number of cavities of characteristic shape present on the surface of enzymes. The cavities possess active groups such as –NH2, −COOH, −SH,−OH etc. The molecules of the reactant (substrate) which have complementary shape,fit into theses cavities just like a key fits into a lock .On the account of the presence of actives groups an activated complex is formed which then decompose as to yield the products .
Colloids Colloidal state may be regarded as intermediate state between true solutions and suspensions. The diameter of colloidal particles may range from 1 to 1oonm, A colloidal sol is a heterogeneous system. It consists of two phases of which one is called the dispersed phase while the other is called dispersion medium.The dispersed phase is one which constitute the colloidal particles and the dispersion medium is that in which colloidal particles are dispersed.
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Classification of colloids: - 1. Classification based on physical state of dispersed phase
II.
and dispersion medium. Dispersed phase Dispersion medium
Types of colloids
Example
Solid
Solid
Solid sol
Some coloured glasses
Solid
Liquid
Sol
Paints
Solid
Gas
Aerosol
Smoke, dust
Liquid
Solid
Gel
Cheese, jellies
Liquid
Liquid
Emulsion
Milk, Hair cream
Liquid
Gas
Aerosol
Fog, mist
Gas
Solid
Solid sol
Pumice, Stone
Gas
Liquid
Foam
Froth, Whipped cream
Classification based on nature of interaction between dispersed phase and dispersion medium In it colloidal state sols are divided into two categories :- lyophillic (solvent attractive) ,Lyophobic (solvent repelling) Lyophillic colloids
1. Lyophlicsols are formed when there is attraction
Lyophobic Colloids 1. Lyophobicsols are formed when there n
between dispersed phase and dispersion medium
attraction between dispersedphase
dispersion medium. 2 They are easily formed
2. They are not easily formed.
3. They are very stable
3. We have to add stabilizing agent to make them stable.
4. Lyophilic sols are reversible
4. Lyophobic sols are not reversible.
5. They are highly viscous as compared to the pure
5.their viscosity is almost the same as
solvent.
that of the solvent
6. eg sol of starch, gum,rubber etc
6. Sols of metals, metal oxide &sulphides Page 8
III.
Classification based on type of particle of dispersed phase:- In it, colloid are classified as multimolecular, macro-molecular and associated colloids. Multimolecular colloids
When on dissolution , atoms or small molecules (diameter <1nm)
Macromolecular colloids These are the colloidal dispersion
Asssociated colloids There are some substances
in which the dispersed particles are which at low conc behave
of substance aggregate together to
giant molecules, usually polymers
as normal strong electrolyte,
particles of colloidal size. The particles
For eg , starch, cellulose, protein
but at higher conc(CMC) &
thus formed are called multimolecular
rubber, nylon etc.
colloids For eg Au sol contains particle
kraft temp show colloidal behaviour due to formation of aggregate .The aggregated thus formed are called micelle or associated colloids.
Kraft temperature : The formation of micelle takes place only above a particular temperature called Kraft temperature CMC: The formation of micelle takes place only above a particular concentration called critical micelle concentration(conc of soap or detergent = 10–4 to 10–3 mol/ L) Mechanism of micelle formation In water soap dissociates into RCOO- and Na+ ions . In RCOO- ion, R which is hydrophobic lies on the surface of water and COO− being hydrophilic in water.
But at critical micelle concentration the anions are pulled into the bulk of the solutioand aggregate to form a spherical shape with their hydrocarbon chain pointing towards the center of the sphere with COOpart remaining outward on the surface of the sphere. An aggregate thus formed is known Page 9
as Ionic micelle
Cleansing action of soaps :- the soap molecule form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet & hydrophilic part projects out of grease droplet. The oil droplet is pulled in water and removed from the dirty surface, Preparation of Colloids:a)
Chemical method: - 1.when H2S is passed through a dilute solution of arsenious oxide in water,
a colloidal solution of arsenioussulphide is obtained. As2O3
+
3H2S
As2S3(sol)
+
3H2O
2. Au, Ag, Pt etc are obtained in colloidal form by reduction of very dilute solution of their salts with a suitable reducing agent eg 2AuCl3 + 3HCHO + 3H2O 3
red
2Au(sol) + 3HCOOH + 6HCl
S sol can be obtained when H2S is bubbled through the solution of an oxidizing agent like Bromine water H2S
4
+
Br2
oxi
S (sol)
+ 2HBr
Fe(OH)3 & Al(OH)3 sols are obtained by boiling solutions of their corresponding chlorides. FeCl3
+ 3H2O
Fe(OH)3(sol) + 3HCl
Electrical disintegration or Bredigs arc method This method is commonly employed for the preparation of colloidal solution of metals. Two rods of concerned metalare kept immersed in cold water containing few drops of KOH. An electric arc is struck between them. As a result the metal changes to vapour which then condense to form colloidal particles.
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Peptization: it is`the process of converting a freshly formed precipitate into colloidal particles by adding suitable electrolyte , known as peptizing agent. Eg reddish brown colloidal solution is obtained by adding small amount of FeCl3 solution to freshly precipitated Fe(OH)3. Cause of Peptization As the electrolyte is added to fresh ppt. The particles of ppt preferentially adsorb one particular type of ions of the electrolyte and get dispersed due to electrostatic repulsion. This gives particles of colloidal size. Purification of colloidal sols Dialysis: the process of separating the particles of colloids from those of crystalloids by diffusion of the mixture through a parchment or an animal membrane is known as dialysis. A bag made up of cellophane membrane is filled with the colloidal solution and is then suspended in fresh water. The electrolyte particles pass out leaving behind colloidal solution.
Electrodialysis : movement of ions across the membrane can expedite by applying electric current Through two electrodes. This method is very fast and is known as electrodialysis. Page 11
Ultrafiltration : in this method, colloidal solution are purified by carrying out filtration through specialtype of graded filter paper called ultra – filters . These filter papers allow the electrolytes to pass through them but do not allow colloidal particles. These filter papers are made from ordinary filter paper by impregnating them with collodion and hardened by formaldehyde.
Properties of colloidal sol.
Brownian movement: the rapid Zig-zag movement of colloidal particles is called Brownian movement. Cause :Brownian movement arises because of the impact of the molecules of the dispersion medium with the colloidal particles.
Tyndall effect: when a strong beam of light is passed through a colloidal sol kept in dark , the path of thebeam gets illuminated . This phenomenon is called Tyndall effect. Cause : this phenomenon is due to the scattering of light by the colloidal particles.
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Cause of charge on colloidal particles : 1. Preferentialadsorption of ions is the most accepted reason. The sol particles acquire positive or negative charge by preferential adsorption of +ve or – ve ions. For eg when AgNO3 solution is added to KI solution the ppt AgI adsorb iodide ions from the Dispersion medium and negatively charged colloidal solution results. However , when KI solution is added to AgNO3 solution, positively charged sol results due to adsorption of Ag+ ions from dispersion medium.
+ve charged solution:1.
Haemoglobin
2.
Hydrated Metallic oxides E.g. AS2S3.xH2O –ve charged solution:-
1.
Metals E.g. Cu, Ag, Au
2.
Metallic sulphides
E.g. AS2S3, Sb2S3, CdS solutions
Zeta potential or electrokinetic potential the potential difference between the fixed layer anddiffused layer of opposite charge is called the electrokinetic potential.
Electrophoresis : the colloidal particles are electrically charged and carry same type of charge either positive or negative charge. The movement of colloidal particles under the influence of electric field is known as electrophoresis . Thus the existence of charge is shown by electrophoresis. Page 13
Coagulation the process of settling of colloidal particles is called coagulation or precipitation of the sol. The coagulation of the lyophobic sols can be carried out in the following ways:1. By electrophoresis: - The colloidal particles move towards oppositely changed electrodes get discharged and precipitate. 2. By mixing two oppositely charged sols: - Oppositely charged sols when mixed almost equalproportional neutralize then changed and get partially or completely precipitated. 3. By Boiling: - When a sol is boiled the adsorbed layer is disturbed due to increased collisions with the molecules of the dispersion medium. This reduces the charge on the particles and ultimately led to setting down in the form of a precipitate. 4. By Persistent dialysis: - On prolonged dialysis, traces of the electrolyte present in the sol are removed almost completely collides become unstable and ultimately coagulate. 5. By addition of electrolyte : - When excess of electrolyte is added the colloidal particles precipitated, the reason is that colloids interact with ions carrying change opposite to that present on themselves, this causes neutralisation leading to their coagulation. Hardy –Schulze rule:Greater the valence of flocculating ion added , better is the coagulation. (ii) Coagulating power of an ion: the minimum concentration of an electrolyte in millimoles per litre required to cause precipitated of a sol in two hours is called coagulating value. The smaller the quantity needed , the higher will be the coagulating power of an ion.
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Protective of colloids:- When a lyophilic sol is added to lyophobic sol, the lyophilic particles (colloids) form a layer around lyophobic particles and thus protect the latter from electrolytes. Lyophlic colloids used for this purpose are called protective colloids. Emulsions:-An emulsion is a colloidal in which both the dispersion medium and dispersed phaseare liquids. There are two types of emulsions. 1.
O/W type – water act as a dispersion medium and oil acts as dispersed bphase. Example: - Milk and vanishing cream.
2.
W/O type - oil act as dispersion medium& water acts as dispersed phase Example: - Butter and cream.
Identification / difference of emulsion Dye test some oil soluble dye is added to the emulsion , if the background becomes coloured , the emulsion is water in oil type. Emulsion of oil in water are unstable and sometimes they separate into two layers on standing . For stabilizing of an emulsion , emulsifying agent is usually added . The emulsifying agent form an interfacial film between suspended particles and the medium . The emulsifying agent for O/W
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emulsion are protein , gum , natural &synthetic soaps etc & for W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack etc. Emulsions can be broken into constituent liquids by heating , freezing, centrifuging. Colloids around us: - Most of the substances all we come across in our daily life are colloids, for example meals, clothes, wooden, furniture, houses, newspaper are largelycomposed of colloids. Application of colloids: -colloids are widely used in the industrial sector. Examples- Electro precipitation of smoke: The smoke, before is comes out from the chimney, is led through a chamber containing plateshaving a charged opposite to that carried by smoke particles. The particles on coming in contact with these plates lose their charge get precipitated, the particles thus settle down on the floor of the chamber. The precipitator is called Cottrell precipitate.
Purification drinking water: - Alum is added to water (that contain impurities) to coagulate the suspended impurities make water fit for colloidal in nature.
Medicines : Most of the medicines are colloidal in nature Ex:- Argyrol is a silver sol use as an eye lotion. Tanning :- when a hide (Animal skin) , which has +ve charged particles is soaked in tanning (or chromium salt) which contains – ve charged particles , mutually coagulation take place. This result in the hardening of leather. This process is termed as tanning. Photographic plates and films: - photographic plates and films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films. Rubber industry: - Latex is a colloidal solution of rubber particles which are Page 16
negativecharged Rubber is obtained by coagulation of latex. Industrial products: - Paints inks, synthetic plastics, rubber, cement, graphite lubricants, etc. are all colloidal solution.
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