2.If the concentration is of bulk phenomenon, it is absorption. (e.g) Absorption of Ink on the surface of a chalk.

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1 EGIEERIG CHEMISTRY UIT 3 SURFACE CHEMISTRY TERMS AD DEFIITIOS: 1. Adsorpton: Concentraton of lqud or gaseous molecules over the surface of a sold materal s known as adsorpton. It s a surface phenomenon. (e.g.) H 2 adsorpton over nckel. The sold materal s known as adsorbent. (e.g.) ckel The gaseous or lqud molecules are adsorbate (e.g.) H 2 gas 2.If the concentraton s of bulk phenomenon, t s absorpton. (e.g) Absorpton of Ink on the surface of a chalk. TYPES OF ADSORPTIO 1. If the adsorbent and adsorbate are bonded by weak Vanderwalls forces, t s known as Physsorpton. (e.g) H 2 on charcoal. 2.If they are bonded by strong chemcal covalent bonds, t s chemsorpton. (e.g) H 2 on nckel. o PHYSISORPTIO CHEMISORPTIO 1 Adsorpton s due to weak Vanderwalls forces. Due to strong covalent bondng. 2 They form mult layer. Monolayer 3 Reversble Irreversble 4 Equlbrum s easy ot easy 5 Intermedate surface compound formaton does Takes place not take place. 6 Energy of actvaton (E a ) s low. Hgh 7 Heat of formaton and enthalpy ( H) s low Hgh 8 Inversely proportonal to temperature Drect proporton 9 Drectly proportonal to pressure, concentraton Inversely proporton 10 on specfc and non selectve Selectve and specfc 11 e.g - H 2 on charcoal. H 2 on nckel. FACTORS AFFECTIG ADSORPTIO a) Adsorpton of gases on solds: (e.g) H 2 on Charcoal (Physsorpton). H 2 on nckel. (Chemsorpton). ature of gases. ature of sold. Actvaton v. Reversble nature v. Thermodynamc quanttes. ( H, E a, T, P, C)

2 1. ature of gases: There are two types of gases. Permanent (e.g. - 2, H 2, O 2 ) and easly lquefable (e.g.- HCl, H 3, SO 2 ) gases. Easly lquefable gases are more adsorbed than the permanent gases. Because they are havng very hgh crtcal temperature (T c ) and great Vander walls forces. E.g H 2 -- T c = 33K Adsorpton = 4.5ml / 1g charcoal SO 2 -- T c = 430K, Adsorpton = 380ml / 1g charcoal 2. ature of solds: For greater adsorpton, the sold adsorbent must have ) Hgh porosty ) More surface area (e.g) powdered alumna and charcoals are havng greater adsorpton capacty. 3. Actvaton: For adsorbents, porosty s ncreased by rubbng, scratchng and passng super heated steam. Ther surface area s ncreased by subdvdng and powderng. The entre process s known as actvaton of adsorbents. (e.g) Charcoal steam / Actvated charcoal 4. Reversble nature: Gas + Adsorbent sold Gas Sold For Physsorpton, reversblty s possble. For Chemsorpton, reversblty s very least possble. 5. a) Enthalpy, Heat of absorpton, Energy of actvatons are low for Physsorpton and hgh for chemsorpton. b) Multlayer s favoured by Physsorpton and monolayer s favoured by chemsorpton. c) Pressure s drectly proportonal to Physsorpton and nversely proportonal to chemsorpton. d) Temperature s nversely proportonal to Physsorpton. In chemsorpton, t s ncreasng wth temperature, attans a maxmum value and agan t s comng down. Physsorpton Chemsorpton Absorpton Absorpton Temperature Temperature b)factors affectng adsorpton of solutes from soluton: 1. ature of solutes 2. ature of adsorbent 1. ature of solute: Hgh molecular weght solutes are easly adsorbed. 2. ature of adsorbent: Adsorbent should have hgh porosty and surface area.

3 ADSORPTIO ISOTHERMS AD 5 MODELS The relaton between adsorpton and pressure at constant temperature s known as adsorpton sotherm. It may be mathematcal relatonshp or of graphcal. (x / m) = k. P 1/n (at constant T) Where, x = amount of Adsorbate m = amount of adsorbent x/m= extent of adsorpton P= pressure n = whole number k = adsorpton coeffcent x / m Pressure To convert the equaton as y = mx + C form, takng log on both sdes, log (x / m) = log k + (1/n) log P (at constant T) Where y = log (x / m) : m = (1/n) : C = log k so, the plot wll be a lne wth slope 1/n and ntercept log k. 5 models of adsorpton sotherms: ( P s = Saturaton pressure) x/m x/m x/m P P s P P s P P s Type I Type II Type -III x/m x/m P P s P P s Type IV Type - V Model Adsorbate Adsorbent Temp Characterstcs I trogen Charcoal C Chemsorpton (monolayer) II trogen Iron / ckel C Physsorpton (Multlayer) III Bromne Slca 80 0 C Physsorpton (Multlayer) IV Benzene Slca gel 50 0 C Capllary Condensed state Physsorpton V Water vapour Carbon C Capllary Condensed state Physsorpton

4 FREUDLICH ADSORPTIO ISOTHERM: The relaton between adsorpton and pressure at constant temperarute s known as adsorpton sotherm. It may be mathematcal relatonshp or of graphcal. (x / m) = k. P 1/n (at constant T) To convert the equaton as y = mx + C form, takng log on both sdes, log (x / m) = log k + (1/n) log P (at constant T) where y = log (x / m) m = (1/n) C = log k so, the plot wll be a lne wth slope 1/n and ntercept log k. From the above graph, there may be three cases: Case 1- At low pressure, adsorpton s proportonal to pressure. (x/m) P (or) (x/m) = k. P 1 Case 2 - At hgh pressure, adsorpton s almost constant. (x/m) = k (or) (x/m) = k. P 0 Case 3 - At Intermedate pressure. (x/m) = k. P (or) (x/m) = k. P 1/n, where n s an nteger. Ths s known as Freundlch adsorpton sotherm. Lmtatons of Freundlch sotherm: 1. It s only emprcal formula and no theoretcal evdences. 2. It s devated at hgh pressure. 3. It s not good at hgh concentraton. 4. n and k are temperature dependent.

5 LAGMUIR ADSORPTIO ISOTHERM Postulates: 1. In adsorbent, surface valences are not fulflled. 2. Adsorbates are of mono layer thckness. 3. Adsorbates are of unform dstrbuton. 4. o nteracton between adjacent gas molecules. 5. The gas molecules do not move around on the surface. Dervaton: 1. As per dynamc equlbrum, k a Gas + Sold G S where, k a = Adsorpton rate constant k d k d = Desorpton rate constant 2. Let us consder, Total area of adsorbent = 1 Gas covered area =θ Then, uncovered area =(1-θ) 3. Rate of adsorpton, R a = K a (1-θ) P Rate of desorpton, R d = k d. θ 4. At equlbrum, R d = R a k d. θ = Ka (1-θ) P k d. θ = K a P - K a θ P k d. θ + K a θ P = K a P θ ( k d + k a P ) = k a P θ = k a P (dvdng by k d ) ( k d + k a P ) θ = ( k a / k d ).P (k d / k d ) + (k a / k d ).P AS, ( k a / k d ) = K, another constant, known as Adsorpton co-effcent, θ = K. P 1 + K.P 5.But, amount of gas adsorbed (x) s proportonal to θ.

6 x θ and x = K 1 θ x = K 1 K P 1 + KP 1 + KP = K 1 K P x 1 + KP = P K.K 1 x P = 1 + K.P x KK 1 KK 1, whch s n the form of y = C + m. x, where, ntercept C = 1 / KK 1 and slope m = K / KK 1 P = 1 + K.P x KK 1 KK 1, Case 1 -, At low pressure, P s neglgble. So, 1 >> K.P KK 1 KK 1 Hence, P = 1 (or) x = KK 1.P = K P 1 x KK 1 Case 2 - At hgh pressure, P s very bg. So, 1 << K.P KK 1 KK 1 Hence, P = K.P (or) x = constant (or) x=k P 0 x KK 1, Case 3, At normal pressure, x = K P 0 1 (or) x = K P n, where n= 0 1 Ths proves that at normal pressure, Langmur adsorpton resembles Freundlch sotherm. But stll, Langmur adsorpton holds good at low pressure but fals at hgh pressure.

7 ROLE OF ADSORBETS I CATALYSIS A substance used to alter the rate of a reacton s called catalyst. But t should not nvolve n the reacton. The process s known as Catalyss. If t ncreases the rate of reacton, t s postve catalyst. If t decreases the rate of a reacton, t s negatve catalyst. If reactants and catalyst are n same phase t s homogeneous catalyss. If they are n dfferent phases, t s heterogeneous catalyss. E.g H 2 Fe 2 H 3. Fe s Postve / Heterogeneous catalyst. H 2 O 2 Dl. H 2 SO 4 H 2 O + ½ O 2,. Dl. H 2 SO 4 s negatve / Homogenous catalyst. Adsorpton s manly used n Heterogeneous catalyss. Steps nvolved n Heterogeneous Catalyss Contact theory. 1. Adsorpton - Here the reactant molecules (A, B) are adsorbed on the sold adsorbent(c).by strong chemcal bond or weak Vander walls bond. 2. Actvated complex formaton - The adjacent adsorbate molecules are formng weak bond (A-B) and the actvated complex s formed. 3. Decomposton The bond between A and B s gettng strengthen whle the bond between A-B and C s gettng weaken. Ths s called decomposton. 4. Desorpton From the decomposed actvated complex, the fnal product s released. Ths s known as desorpton. A B A B A B A B C C C C C C C C Step I Step-II step III Step -IV Adsorpton Actvated complex Decomposton Desorpton E.g + CH 2 = CH 2 + H 2 ckel CH 3 - CH 3 (Ethylene to Ehtene) Where A = Ethylene B = Hydrogen C= ckel and A-B = Ethene Factors affectng heterogeneous catalyss: 1. Fnely dvded catalyst 2. Rough surface 3. Promoters 1. In fnely dvded catalysts, actve centre ncreases. So, adsorpton ncreases.

8 E.g - Fnely dvded 6 Free valences n combned state = 10 Dvded state valences = (6 x 4)=24 2. Rough surfaces may have cracks, peaks and corners. So, number of actve centre ncreases and adsorpton ncreases. 3. Promoters are the substances ncreasng the actvty of catalyst. (e.g) Mo ncreases actvty n Haber s process. They ncrease the peaks and cracks and also the spaces between the catalyst, so adsorbed molecules bonds are further weaken and cleaved. ADSORPTIO AD IO EXCHAGE PROCESS Ion exchange method ( Demneralsaton) Ion exchange process depends on the concept of adsorpton of Ca and Mg ons on resns. Workng: Here all the catons and anons are completely removed. It uses two cylnders of caton exchange cylnder and anon exchange cylnder flled wth resns.resns are lnear, nsoluble, cross lnked, organc polymers. There are 2 types. ) Caton exchange resns RH 2 (e.g) Sulphonated coals, RSO 3 H ) Anon exchange resns. R (OH) 2 (e.g)ureaformaldehyde,amnes R-H 2 The water s fed nto cylnder I where all the catons are replaced by RH 2 Resns. The caton free water s fed to cylnder II, where all the anons are replaced. So, the resultant water s free from all types of ons. RH 2 + CaCl 2 R Ca + 2 HCl R (OH) HCl R Cl 2 + 2H 2 O Regeneraton: On prolonged use, as all the resns are exhausted, there wll be no H + oroh ons to exchange the unwanted ons. So, they have to be regenerated. Caton resns are regenerated by HCl and anon resns by aoh. R Ca + 2 HCl RH 2 + CaCl 2 R Cl 2 + 2aOH R (OH) 2 + 2aCl APPLICATIO I POLLUTIO ABATEMET: 1.When wood or coconut shell s heated n absence of ar, charcoal s obtaned. Ths s called carbonzaton. The charcoal s actvated by passng steam and heated wth Phosphorc acd as dehydratng agent at C to make t more porous. 2. There are two types of actvated charcoal. GAC(Granular actvated Charcoal) and PAC( Powdered Actvated charcoal.

9 o GAC PAC 1 For long term use Short term use 2 Da s greater than 0.1mm Less than 0.1mm 3 Used for gases and lquds Manly for lquds 4 Regeneraton s possble ot possble WATER POLLUTIO TREATMET ( USIG GAC) 1. Down flow contactors: The polluted water s fed at the top of the columns contanng GAC. The mpurtes are adsorbed on GAC. If hgher degree of purty s needed, then numbers of columns are ncreased. The cloggng n GAC s removed by backwashng. There are two types of down flow contactors. A) Parallel type b) Seres type o Seres model Parallel model 1 Hgh degree of purty Low degree of purty 2 Low volume only purfed Hgh volume can be purfed 3 Backwash s frequently needed ot frequently needed. 4 Low lfe tme of GAC Hgh lfe tme 5 Out put of one column wll be n put for another column. Sngle source and sngle termnus.