Surface Chemistry PDF

Surface Chemistry INTRODUCTION : Surface chemistry is that branch of chemistry which deals with study of the phenomena occurring at the surface or interface, i.e. ai the boundary separating two bulk phases. In this chapter our main emphasis will be on three im...

Surface Chemistry INTRODUCTION : Surface chemistry is that branch of chemistry which deals with study of the phenomena occurring at the surface or interface, i.e. ai the boundary separating two bulk phases. In this chapter our main emphasis will be on three important topics related to surface chemistry, viz., adsorption, colloids and emulsions. Section (A) : Adsorption Adsorption : The phenomenon of attracting and retaining the mole.c ules of a substance on the surface of a liquid or a solid resulting into a higher encentralion of the molecules on the surface is called adsorption. As a result of adsorpli9n, there is a decrease of surface energy. The process of rernoval. of an.adsorbed substance from the 5;urface on which it is adsorbec! is called desorption. It Is the reverse of adsorption and can be brought about by heating or by reducing the pressure. Adsorbent and adsorbate : The substance on the surface of which adsorption occurs is known as adsorbent The substances that get adsorbed on the solid surface due to intermolecular attractions are called adsorbate. Charcoal, silk:a, gel, alumina gel are good adsorbents because they have highly porous s.tructures and have large surface area. Colloids on account of their extremely small dimensions possess enoromous surface area per unit mass and are, therefore, also good adsorbents. Examples of adsorption : Adsorption of a gas by charcoal : Finely divided activated charcoal has a tendency to adsorb a number of gases like ammonia, sulphur dioxide, chlorine, phosgene, etc. In this case, charcoal acts as an adsorbent while gas molecules a~ as adsorbate. e.Adsorption of a dye by charcoal : Animal charcoal is used for decolourising a number of organic substances in the form of their solution~. The discharge of the colour is due to the fact that the coloured component (generally an organic dye) gets adsorbed on the surface of the adsorbent (animal charcoal). Sorptlon : When both adsorption and absorption take place simultaneously. Eg : Dyes get adsorbed as well as absorbed in the cotton fibre i.e. sorption takes place. Difference between adsorption and absorption : The Imoortant po nts of distincti on between ad sorDlt on and abs. orptIon Absorption Adsorption It is the phenomenon in which the particles of gas It is the phenomenon of higher concentration of gas or liquid get uniformly distributed throughout the or liquid on the surface than in the bulk of the solid. bod v of the solid. The concentration on fhe surface of the adsorbent is The concentration is the same throughout the different (has higher concentration) from that in the material. bulk. It is a bulk phenomenon. It is a surface phenomenon. Adsorption is rapid in the beginning and its rate Absorption occurs at uniform rate. slowlv decreases It is a slow orocess It is a fast process Thermodynamics of adsorption 1. Adsorption involves attracting molecules of adsorbate on surface of the adsorbent. ou·e to this energy is released and thus heat of adsorption is negative i.e. adsorption is always exothemic 2. The molecules of adsorbate are held on surface of the solid adsorbent due to this entropy decreases i.e. 6 S is negative. 3. AG= AH-TAS, Therefore adsorption will occur only when AG is negative and this is possible only if IAHI > ITASI 4. As adsorption proceeds t.H becomes less and less negative hence t.H becomes equal to T AS and AG becomes zero. This is the state at which equilibrium is attained.  Enthalpy of adsorption AH 111 ptiDn : It is the amount of the heat released when 1 mole of an adsorbate gets adsorbed on a particular adsorbent at adsorption equilibrium. It depends upon the nature of both the adsorbate as well as adsorbent Types of adsorption : The adsorption is classified into two types : (I) Physical adsorption (I.e. physlsorption) : When the particles of the adsorbate are held to the surface of the adsorbent by the physical forces such as van der Waal's forces, the adsorption is called physical adsorption or vanderwaals adsorption. (ii) Chemical adsorption (i.e. chemisorption) : When the molecµles of the adsorbate are held to the surface of the adsorbent b.y the chemical forces, the adsorption is called chemical adsorption. Property, physical Adsorption Chemical Adsorption Nature of adsorption Weak Strong Enthalp.y of adsorption Low High Reversibility of adsorption Reversible and occtJr rapidly Irreversible and occurs slowly Temp. at which adsorption is Low temp. High temp. more pronounced Effect of change in temp. Decreases with rise in temp. Increases with rise in temp. Not specific, generally take Highly specific, take place on Specificity of adsorption olace on all surface soecific surface Multi-layered (at high Mono-layered Nature of adsorbate layer pressure) Enel'.'9y of activation Very low significantly high Easy, since Vander-Waal's Not easy. since chemical forces are Ease of desorption forces are involved involved Graph. Temperature Temperature - Competitive adsorption : When an adsorbent is In contact with more than one species (adsorbate). There will be competition among them to get' adsorbed on to the surface of the adsorbent. The one that is more strongly ad.sorbed gets deposited first in preference to the others. Further a strongly adsorbed substance may displace a weakly adsorbed substance. Ex. NH3can displace 0 2 or N2 from the surface of charcoal. Adsorption of gases on solids : The extent of adsorption of a gas on a solid surface is affected by the following factors: The nature of the gas (i.e. nature of the adsort>ate). The easily liquefiable gases such as HCI, NHJ, Cl2 etc. are adsorbed more than the permanent gases such as H2, N2 and 0 2. extent of adsorption cc critical temperature of gas cc ease of llqulflcatlon Gas H2 CH,. CO2 S02 Tc 33 190K 304K 330K  Nature of adsorb;ent. The extent of adsorption of a gas depends upcm the nature of adsorbent. Activated charcoal (i.e. activated ca.rbon), metal oxides (silica gel and aluminium oxide) and clay can adsorb gases which are easily liquified. Gases such as H2, N2 and 0 2 are generally adsorbed on finely divided transition metals Ni and Co. Extent of adsorption cc surface area of solid. Activation of adsorbent : (a) Metallic adsorbents are activated by mechanical rubbing or by subjecting it to some chemical reactions. (b) To increase the adsorbing power of adsorbents, they are sub-divided into smaller pieces. As a results, the surface area is increased and therefore, the adsorbing power increases. Effect of.t emperature : Mostly the process of adsorption is exothermic and the reverse process or desorption is endothermic. If the above equilibrium is subjected to increase in temperature. then according to Le-Chaterlier's principle, with increase in temperature, the des_orption will be favoured. Physical adsorption decreases continuously with increase in temperature whereas chemisorption increases initially, shows a maximum in the curve and then it decreases continuously. ~\__ Temperature Temperature The initial increase in chemisorption with increase in temperature Is b.ecause of activation energ y required. This is why the chemical adsorption is also known as Activated adsorption". A graph between degree of adsorption (x/m) and temperature 't' at a constant pressure of adsorbate gas is known as adsorption Isobar. Effect of pressure. The extent of adsorption of a gas per unit mass of adsorbent depends upon the pressure of the gas. The variation of extent of xlf adsorption expressed as x/m (where x is the mole of adsorbate and m is the mass of the adsorbent) and the pressure is givern as below. A graph between the amount of adsorption and gas pressure keeping the temperature , constant is called an adso·rpt1on Isotherm. 0 -P P, X -oc: P Agure-1 m Where x = mass of adsorbate M = mass of adsorbent It is clear from the figure-1 that extent of adsorption (x/m) increases with pressure and becomes maximum corresponding to pressure Ps called saturation pressure. Fr.e undlich Adsorption isotherm The variation of extent of adsorption (x/m) with pressure (P) was given mathematically by Freundlich. Where. n can take any whole r11.,1mber value which depends upon the nature of adsorbate and adsorbent. The above relationship is also called.Freundlich's adsorption isotherm. ( ; ) = kp,," Pressure Graph Relath>n X At low pressure straight line - =kP m (x/m) = kp1In At intermediate pressure dependent on power of pressure (O