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5 SURFACE CHEMISTRY Word Document 2 .Pdf STD XII CHEMISTRY CHAPTER 5 Surface Chemistry “The branch of physical chemistry, which deals the Examples :(i) Water vapours Examples : (i) Water vapours nature of surfaces and also with the chemical and physical adsorbed by silica gel. absorbed by anhydrous CaCl2 processes which takes place on the surfaces, is called surface (ii) NH3is adsorbed by charcoal. (ii) NH3is absorbed in water chemistry”. forming NH4OH In surface chemistry, we study the phenomenon of (4) Surface forces :Only the surface atoms of an adsorption, catalysis and colloidal properties. adsorbent play an active role in adsorption. These atoms posses Adsorption unbalanced forces of various types such as, Vander Waal’s forces (1) Definition : The phenomenon of attracting and and chemical bond forces. retaining the molecules of a substance on the surface of a liquid Thus, the residual force-field on a free surface or solid resulting in to higher concentration of the molecules on which is responsible for adsorption is produced. For the surface is called adsorption. example, when a solid substance is broken into two pieces, two (2) Causes of adsorption:Unbalanced forces of new surfaces are formed and therefore, the number of attraction or free valencies which is present at the solid or liquid unbalanced forces becomes more. As a result the tendency for surface, have the property to attract and retain the molecules of adsorption become large. a gas or a dissolved substance on to their surfaces with which (5) Reversible and Irreversible adsorption :The they come in contact. adsorption is reversible, if the adsorbate can be easily removed Example:Ammonia gas placed in contact with charcoal from the surface of the adsorbent by physical methods.If the gets adsorbed on the charcoal whereas ammonia gas placed in adsorbatecan not be easily removed from the surface of the contact with water gets absorbed into water, adsorbent is called irreversible adsorption. Table : 14.1 Some basic terms used in adsorption Example for reversible adsorption: A gas adsorbed on a Interface :Any surface is a Adsorbate and Adsorbent solid surface can be completely removed in vacuum. plane which separates any two :The substance which gets Example for irreversible adsorption: Adsorption of O2 on phases in contact with each adsorbed on any surface is called tungusten adsorbent. other. The plane which separates adsorbate for example, if a gas any two phase is generally called gets adsorbed on to the surface (6) Characteristics of adsorption an interface between the two of a solid, then the gas is termed (i) Adsorption refers to the existence of a higher phases. as the adsorbate. concentration of any particular component at the surface of a The substance on the surface of liquid or a solid phase. which adsorption takes place is called adsorbent. (ii) Adsorption is accompanied by decrease in the G (free energy change) of the system when G 0 , adsorption Desorption :The removal of Absorption :When the equilibrium is said to be established. the adsorbed substance from a molecules of a substance are (iii) Adsorption is invariably accompanied by evolution of surface is called desorption. uniformly distributed heat, i.e. it is an exothermic process. In other words, Hof throughout the body of a solid or adsorption is always negative. liquid. This phenomenon is called absorption. (iv) When a gas is adsorbed, the freedom of movement of its molecules becomes restricted. On account of it decrease in Sorption :The phenomenon in Occlusion :When adsorption of which adsorption and absorption gases occur on the surface of the entropy of the gas after adsorption, i.e. Sis negative. occur simultaneously is called metals this is called occlusion. (v) For a process to be spontaneous, the thermodynamic sorption. requirement is that G must be negative, i.e. there is Mc. Bain introduced a general decrease in free energy. On the basis of Gibb’s Helmholtz term sorption describeing both the processes, however equation, G H TS , G can be negative if H has adsorption is instantaneous i.e. a sufficiently high negative value and TS has positive value. fast process while absorption is a Classification of adsorption slow process. Adsorption can be classified into two categories as (3) Difference between adsorption and absorption described below, Adsorption Absorption (1) Depending upon the concentration : In It is a surface phenomenon. It concerns with the whole mass adsorption the concentration of one substance is different at the of the absorbent. surface of the other substance as compared to adjoining bulk or In it, the substance is only It implies that a substance is interior phase. retained on the surface and does uniformly distributed, through (i) Positive adsorption: If the concentration of not go into the bulk or interior of the body of the solid or liquid. adsorbate is more on the surface as compared to its the solid or liquid. concentration in the bulk phase then it is called positive In it the concentration of the In it the concentration is low. adsorbed molecules is always adsorption. greater at the free phase. Example :When a concentrated solution of KClis shaken It is rapid in the beginning and It occurs at the uniform rate. with blood charcoal, it shows positive adsorption. slows down near the equilibrium. STD XII CHEMISTRY CHAPTER 5 Surface Chemistry (ii)Negative adsorption: If the concentration of the (i) An increase in the pressure of the adsorbate gas adsorbate is less than its concentration in the bulk then it is increases the extent of Adsorptiontends to called negative adsorption. adsorption. reach limiting value Example :When a dilute solution of KClis shaken with (ii) At low temperature, blood charcoal, it shows negative adsorption. the extent of adsorption (2) Depending upon the nature of force existing increases rapidly with pressure. Extent of between adsorbate molecule and adsorbent (iii) Small range of adsorption Adsorption increases pressure, the extent of rapidly at the beginning (i) Physical adsorption: If the forces of attraction adsorption is found to be directly Pressure existing between adsorbate and adsorbent are Vander Waal’s proportional to the pressure. forces, the adsorption is called physical adsorption. This type of adsorption is also known as physisorption or Vander (iv) At high pressure (closer to the saturation vapour Waal’s adsorption. It can be easily reversed by heating or pressure of the gas), the adsorption tends to achieve a limiting decreasing the pressure. value. (ii) Chemical adsorption :If the forces of attraction (4) Effect of temperature existing between adsorbate particles and adsorbent are almost (i) As adsorption is accompanied by evolution of heat, so of the same strength as chemical bonds, the adsorption is called according to the Le-Chatelier’s principle, the magnitude of chemical adsorption. This type of adsorption is also called as adsorption should decrease with rise in temperature. chemisorption or Langmuir adsorption. This type of adsorption cannot be easily reversed. P - Constant P - Constant Comparison between physisorption and chemisorption Physisorption Chemisorption x /m x/m (Vander Waal's adsorption) (Langmuir adsorption) Low heat of adsorption usually High heat of adsorption in the in range of 20-40 kJ/mol range of 50-400 kJ/mol Temperature Temperature Force of attraction areVander Forces of attraction are Physical adsorption Chemical adsorption Waal's forces. chemical bond forces. (ii) The relationship between the extent of adsorption and It is reversible It is irreversible temperature at any constant pressure is called adsorption It is usually takes place at low It takes place at high isobar. temperature and decreases temperature. (iii) A physical adsorption isobar shows a decrease in x/m with increasing temperature. (where ‘m’ is the mass of the adsorbent and ‘x’ that of adsorbate) It is related to the case of It is not related. as the temperature rises. liquefication of the gas. (iv) The isobar of chemisorption show an increase in the It forms multimolecular It forms monomolecular layers. layers. beginning and then decrease as the temperature rises. It does not require any activation It requires high activation Adsorption isotherms energy. energy. A mathematical equation, which describes the High pressure is favourable. High pressure is favourable. Decrease of pressure causes Decrease of pressure does not relationship between pressure (p) of the gaseous adsorbate and desorption cause desorption. the extent of adsorption at any fixed temperature, is called It is not very specific. It is highly specific. adsorption isotherms. The extent of adsorption is expressed as mass of the Factors which affect the extent of adsorption :The adsorbate adsorbed on one unit mass of the adsorbent. following are the factors which affect the adsorption, Thus, if x g of an adsorbate is adsorbed on m gof the (1) Nature of the adsorbate(gas) and adsorbent adsorbent, then (solid) x (i) In general, easily liquefiable gases e.g., CO2, NH3, Extent of adsorption m Cl2and SO2etc. are adsorbed to a greater extent than the Various adsorption isotherms are commonly employed in elemental gases e.g. H2, O2, N2, He etc. (while chemisorption is specific in nature.) describing the adsorption data. (ii) Porous and finely powdered solid e.g. charcoal, fullers (1) Freundlich adsorption isotherm earth, adsorb more as compared to the hard non-porous (i) Freundlich adsorption isotherm is obeyed by the materials. Due to this property powdered charcoal is used in gas adsorptions where the adsorbate forms a monomolecular masks. layer on the surface of the adsorbent. 1 (2) Surface area of the solid adsorbent x kp n (Freundlich adsorption isotherm) or (i) The extent of adsorption depends directly upon the m surface area of the adsorbent, i.e. larger the surface area of the x 1 adsorbent, greater is the extent of adsorption. log log k log p m n (ii) Surface area of a powdered solid adsorbent depends where x is the weight of the gas adsorbed by m gm of the upon its particle size.
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