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Surface Chemistry 

      Most of the chemical reactions that occur on the surface of solid catalysts. Surface chemistry deals with phenomena that occur at the surface. Surface chemistry finds many applications in analytical work, industry and daily life situations. Some of the important features of surface chemistry are adsorption, catalysis and colloids.
     Production of high vacuum, chromatographic analysis, froth flotation process in the concentration of sulphide ores, separation of noble gases by Dewar's flask, heterogeneous catalysis, Gas masks, control of humidity, removal of colouring matter from the solutions are some of the applications of adsorption.
      The substance that concentrates at the surface of the adsorbent is called 'Adsorbate". The solid on whose surface the accumulation of molecular species is taking place is called "adsorbent". The process of accumulation of molecular species (adsorbate) at the surface of a solid (adsorbent) is known as "adsorption". For example activated charcoal is adsorbent and noble gases are adsorbates in Dewar's method of separation of noble gases.
Where as in absorption, the substance is uniformly distributed through out the bulk of the solid (eg. absorption of water by sponge). If both the absorption and adsorption are taking place simultaneously, it is called 'sorption'. Adsorption of gases is mainly classified into two types. They are physical adsorption and chemical adsorption. Adsorption process is exothermic. During the adsorption of gases ∆S = -Ve, ∆H = -Ve, ∆G = -Ve (as the process is spontaneous)


Physical adsorption: The adsorption of adsorbate molecules on the surface of adsorbent
takes place due to physical forces i.e. weak Vanderwaal's forces. eg. adsorption of noble gases on activated charcoal.

Chemical adsorption: The adsorption of adsorbate molecules on the surface of adsorbent takes place due to chemical forces. eg. adsorption of H2 on Ni metal surface.
Comparision of physical adsorption and chemical adsorption

Physical adsorpiton Chemical adsorption
1. Caused by Vanderwaal's forces 1. Caused by chemical bonding
2. It is not specific 2. It is highly specific.
3. It is reversible 3. It is irreversible
4. Enthalpy of adsorption is low    (20-40 KJ mol-1)

4. Enthalpy of adsorption is high(40-400 KJ mol-1)

5. Low temperature is favoured 5. high temperature is favoured
6. It is multi layered 6. It is Unilayered.
7. It depends on the nature of adsorbate only 7. It depends on the nature adsorbate as well as adsorbent.
8. Desorption is easy 8. Desorption is difficult.
9. It is instantaneous 9. It may be rapid or slow.
10. Decreases with rise in temperature. 10. Increases with rise in temperature.

Adsorption of gases on metals: The adsorption of gases on metals depends upon the surface area of adsorbent, nature of the gas, pressure, temperature and the thickness of adsorbed layer of gas. The change in the amount of gas adsorbed by Unit mass of solid adsorbent  with equilibrium pressure (P) at constant temperature can be expressed by adsorption isotherm.
Freundlich adsorption isotherm: Freundlich gave an empirical relationship between  and pressure at constant temperature by the following relationship     

Where x is the mass of the gas adsorbed on mass m of the adsorbent at a pressure P. K & n are constants which depend on the nature of the adsorbent and the gas at a given temperature. taking logarithm of equation ......... (1)
                                  

    
    These curves indicate that at constant P, extent of physical adsorption decreases with the rise in temperature Freundlich isotherm can be verified by plotting log  on y-axis and log p on x-axis.
         Factor  can have values from 0 to 1. If  = 0, then  = constant i.e. adsorption is independent of pressure.


If   = 1, then  = KP. i.e. adsorption is directly proportional to pressure. This isotherm cannot be explained at high P.
 

Applications of Adsorption:
Some of the important applications of adsorption are
* In the production of high vacuum.
* In making gas masks.
* In controlling humidity.
* In the removal of colouring matter from solutions.
* In the manufacture of NH3, H2SO3 and vanaspathi.
* In the separation of noble gases from their mixture.
* In making drugs to kill germs, to cure diseases.
* In froth flotation process in metallurgy.
* In chromatography analysis.
* In the preparation of adsorption indicators.

 Even by providing suitable conditions like temperature, pressure, change of concentration, the rate of the reaction may be slow. In such conditions it is essential to accelerate the speed of the reaction. Substances, which alter the rate of a chemical reaction and themselves remain chemically, quantitatively unchanged at the end of reaction are called catalysts and the phenomenon is called "Catalysis".
      
      A catalyst does not initiate the reaction. It doesn't effect the position of the equilibrium.
It is not consumed during the reaction. A catalyst may increase or decrease the rate of a reaction.

Change of temperature can change the rate of catalytic reaction. It's action is more in powdered state. A small amount of catalyst is enough to carry the reaction. The catalyst is specific in nature. eg:
                    C2H5OH   CH3CHO + H2 (dehydrogenation)
                    C2H5OH  C2H4 + H2O (dehydration)
    
    A catalyst which increases the rate of reaction is called as positive catalyst, which decreases the rate is called negative catalyst. (e.g.: Glycerol, Acetanilide in the decomposition of H2O2), a substance which increases the activity of a catalyst is called promoter (Mo in the synthesis of NH3 by Haber's process). while which decrease the activity of a catalyst is called "Poisons".
(e.g.: CO is poison in Haber's process of manufacturing NH3).


Types of Catalysis
Auto Catalysis: The Catalysis in which one of the products formed acts as catalyst is called as Auto Catalysis.
2 KMnO4 + 3 H2SO4 + 5 H2C2O4 K2SO4 + 2 MnSO4 + 8 H2O + 10 CO2 (Mn+2 ions: auto catalyst)
       
Homogeneous Catalysis: The catalysis in which the reactants and catalyst are in the same phase (gas or liquid).


Heterogeneous Catalysis: The catalysis in which the reactants and catalyst are in different phases.


Adsorption theory of Heterogeneous Catalysis:
Modern adsorption theory explain the mechanism of heterogeneous catalysis. Modern adsorption theory is the combination of the old adsorption theory and the intermediate compound formation.


The mechanism of catalysis involves 5 steps as follows:
* Diffusion of the reactants to the surface of the catalyst.
* Adsorption of the reactants on the surface of the catalyst.
* Formation of intermediate compound on the suface of the catalyst.
* Desorption of the products from the surface of the catalyst.
* Diffusion of the products from the surface of the catalyst.
Some of the important features of solid catalysts are activity and selectivity. Maximum activity is shown by 7 to 9 group metals. Action of the catalyst is highly specific, i.e., same reactants could give different products with different catalysts.

    When catalysis depends upon shape (structure of the catalyst), then that catalysis is called shape selective catalysis. Zeolites are shape selective catalysts as they have honey comb like (Al - O - Si network) structure. ZSM - 5 catalyst converts alcohols directly into petrol (gasoline).
 

Enzyme Catalysis
Proteins with high molecular mass, which forms colloidal solution in water are called enzymes. The catalysis in which enzymes are involved are called enzyme (or bio chemical) catalysis.

Lacto bacilli converts milk into curd. Pepsin converts proteins into peptides. Trypsin converts proteins into amino acids.
       


Characteristics of enzyme catalysis:
* They are highly efficient and highly specific.
* They are highly active under optimum temperature & pH.
* Inhibitors & Poisons influence the enzyme catalysis.
* Activators & Co - enzymes increase the activity. Just like a key fits into a lock,
   shape of reactant fits into enzyme to form an activated complex (ES*)
E + S  ES*
    
Activated complex is unstable, hence it forms products.
ES* E + P
Some of the catalysts used in the industry are

          Based on the size of the solute particles, solutions are classified into true solutions, colloids and suspensions. True solution is the solution in which particle size of the solute is < 1 mµ. Colloidal solution is the solution in which solute particles having size 1 m µ- 1µ. colloidal solution has minor (dispersed phase) and major (dispersion medium) parts.


Dispersion medium: The medium in which colloidal particles are dispersed is known as dispersion medium.
                          e.g.: Water in gold sol.
Dispersed phase: The particles of colloidal substance distributed in dispersion medium is known as dispersed phase.
                          e.g.: Gold in gold sol.

True solution colloidal solution
1. Homogeneous & very clear 1. Heterogeneous & clear or opaque
2. Diffuses rapidly 2. Diffuses slowly
3. Can not be filtered 3. Can be filtered partially
4. Donot show Tyndal effect 4. Show Tyndal effect
5. High osmotic pressure 5. Low osmotic pressure

Depending upon the affinity of dispersed phase towards dispersion medium, colloids are classified into Lyophilic and Lyophobic colloids.


Lyophilic colloids: Colloidal solution formed due to much affinity between dispersed phase and dispersion medium.
                             e.g.: Gelatin, starch, proteins.
Lyophobic colloids: Colloidal solution formed due to very little affinity (negligible) between dispersed phase and dispersion medium.
                            e.g.: gold sol, silver sol

Classification of colloids (Based on type of particles of the dispersed phase):
Based on the type of particle size of the dispersed phase colloids are classified as
Multimolecular colloids: A large number of atoms or small molecules of the dispersed phase associate together to form multimolecular colloids (colloidal particle diameter > 1 nm).
            e.g.: Gold sol & Suplhur solution.
Macromolecular colloids: Macromolecules having colloidal particle size like starch, cellulose, proteins, enzymes, nylon, polythene, polystyrene, synthetic rubber forms macromolecular colloids.
Micelles: Due to association of 100 or more normal molecules at Kraft temperature and critical micelle concetration, micelles are formed. Colloids can be prepared by chemical methods as follows
As2O3 + 3 H2S  As2S3 (sol.) + 3 H2O
SO2 + 2 H2S  3 S (sol.) + 2 H2O
FeCl3 + 3 H2O

 Fe(OH)3 (sol.) + 3 HCl
2 AuCl3 + 3 H2O + 3 HCHO  2 Au (sol.) + 6 HCl + 3 HCOOH


Bredig's Arc Method: Gold, Silver and Platinum colloids are prepared by this method. It involves dispersion and condensation. Electric arc is given between 2 metal electrodes placed in dispersion medium. Colloidal solution is formed due to heat produced in arc.

Peptization: The conversion of precipitate into colloidal solution by shaking it with dispersion medium in presence of electrolyte.
 
Electrolytes are purified by dialysis (removal of dissolved impurities present in colloidal solution with the help of dialysing membrane) by electrodialysis (dialysis by applying emf) and by ultra filtration (with ultra filter).
Colloids show optical properties (due to large sized particles of disperse phase), electrical properties (due to charge on the colloidal particles) kinetic properties (due to random motion of colloidal particles). Tyndal effect is an optical property. "The phenomenon of scattering of light by colloidal particles in all directions and formation of bright glowing cones is known as Tyndal effect" Due to electron capture by sol particles, or due to preferential adsorption of ions from solution or due to formulation of electrical double layer, charge is present on colloids. British botanist Robert Brown observed that colloidal particles move randomly (zig-zag) in all the directions.

    (The continuous rapid zig - zag movement by colloidal particles in the dispersion medium is known as Brownian movement.)
Colloidal particles always carry an electric charge. First layer of ions is fixed and other layer with opposite charge is mobile (diffused layer). The potential difference between these 2 layers is called "Zeta Potential". Migration of colloidial particles under applied emf is called "electrophoresis".

The movement of colloidal particles and dispersion medium in opposite directions is called "electro osmosis".
Coagulation is taking place when the colloidal particles lose the charge and coming down as precipitate when electrolyte is added to it.
      Al2 O3.xH2O, CrO3. xH2O, Fe2O3 xH2O, TiO2, Haemoglobin, methylene blue are positively charged sols. Where as Cu, Ag, Au, As2S3, Sb2 S3, CdS sols, eosin, congo red sols, starch gum gelatin, clay, charcoal are negatively charged sols. The effectiveness of ion which causes coagulation depends on the charge sign and its magnitude. It was explained by Hardy and Schulze. This law states that "The coagulating ability of oppositely charged ions is directly proportional to the charge of coagulating ions''.
Coagulating ability of positive colloid: Cl- < SO4-2< PO4-3
coagulating ability of negative colloid: K+ < Ba+2< Al+3

     Due to coagulation tendency lyophobic sols are less stable and requires some protective colloid (lyophilic sol). The protective action of a lyophilic sol is expressed in terms of 'gold number', introduced by zigmondy. "The minimum number of milligrams of lyophilic colloid required to prevent the coagulation of a standard gold sol on the addition of 1 ml of 10% NaCl solution" is known as "gold number".

 Gelatin has gold number 0.005 to 0.01, Haemoglobin has 0.03 to 0.07, Albumin has 0.1 to 0.2, potato starch has 25.
        Preparation of lotions, creams, ointments, cosmetics, drugs, separation of oil from water, digestion of fats, froth flotation in concentration of sulphide ores, cleaning action of cloths, separation of butter from cream are some of the applications of emulsions. Emulsion is a colloidal system in which both the dispersion medium and dispersed phase are liquids. Based on the quantity of the components in emulsion, classified into 2 types.


Colloids Around us
* Blue colour of sky is due to scattering of light by dust particles and water vapour present in air.
* Formation of fog, mist, rain.
* Food materials like milk, butter, halwa, ice - creams.
* Clotting of blood by alum and FeCl3.
* Fertility of soil and formation of deltas.


Applications of colloids
Some of the important applications of colloids are:
* Making ink, paint, rubber, lubricants, cement, photographic plates and films, medicines.
* Purification of drinking water by alum.
* Tanning of leather.
* Cleansing action of soaps & detergents.
* Electrical precipitation of smoke by Cottrell smoke precipitator.
             

Posted Date : 31-10-2020

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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