Shaillee Kaushal is teaching live on Unacademy Plus
XII-05 SURFACE CHEMISTRY II
Characterstics of physical adsorption: (i) Lack of specificity: The surface of an adsorbent does not show very strong attraction for a particular gas as the vander waals forces are universal. (ii) Nature of the gas: In general more easily liquefiable gases (i.e higher critical temperatures) are readily adsorbed as the vander waal's forces are greater near critical temperature (iii) Reversible in nature:- Physical adsorption of a gas by a solid is generally reversible.The gas adsorbed can be removed by reversing the conditions of temperature and pressure. More gas is adsorbed at high pressure and low temperature by decreasing the pressure and increasing the temperature.The process of adsorption can be reversed. (iv) Surface area: The extent of adsorption increases with increase in surface area of the adsorbent.Thus finely divided metals and porous substances having large surface area are good adsorbents (v) Heat of adsorption: Heat of adsorption is very low (20 40 kJ mol-1) due to weak vander waals forces of attraction
Characterstics of chemisorption: (i) High specificity: If is highly specific and will only occur if there is some posibility of chemical bonding (ii) Nature of gas: Chemisorption will occur if there is some possibility of chemical action between the gas and solid adsorbent (iii) Irreversability: As chemisorption involves compound formation, it is commonly irreversible in nature. (iv) Surface area: Chemisorption increases with increase in surface area (v) Heat of adsorption: Heat of adsorption is high of the order of 40-400 kJ/mole as it involves and formation.
Physical Adsorption 1. It is caused by intermolecular vander waals forces. 2. It is not specific 3. It is reversible 4. Heat of adsorption is low 5. Low temperature is favourable 6. No appreciable activation energy is required 7. It forms multilayers on adsorbent surface under high pressure Chemical Adsorption It is caused by chemical bond formation It is highly specific It is irreversible. Heat of adsorption is high High temperature is favorable High activation energy is required It forms unimolecular layer
Activation of Adsorption: Adsorption power of an adsorbent can be increased by a number of ways: (i) Metallic adsorbents are activated by rubbing the surface (1) ActIvity of adsorbent can be increased by taking it in powered state as the surface area is maximum in powered state (iii) Adsorbing power of adsorbent can be increased by heating with superheated steam
T constant Adsorption isotherms: The variation of extent of adsorption with pressure at constant temperature gives a curve called adsorption isotherm I Im Freundlich Adsorption isotherm:- kP The empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature was given in 1909 Pressure (P) Ps
k.Pl/n SlopeI mass of the gas adsorbed m mass of the adsorbent log x/mL P pressure K constant log log k + Llog P Intercept log, K This is an equation of straight line. The validity of F isotherm can be verified by plotting log on y reundlich adsorption loge P axis
DrawbackS: (i) Concept of Freundlich adsorption is purely empirical (ii) It does not consider the role of surface area of adsorbent in the process of adsorpton. (iii) It is unable to explain why the degree of adsorption is constant at high pressure. (iv) It is applicable for only physical adsorption because it consider multimolecular layer of adsorption.
Langmiur adsorption isotherm:- He postulated the following theory of adsorption. (i) Gases form unimolecular layer of adsorption on the surface of adsorbate (ii) Gases undergoing adsorption behave ideally. (ii) Adsorption of a gas molecule at a particular site is independent, whether the neighboring sites are adsorbed or not. (iv) Dynamic equilibrium exists on the surface of adsorbent two process are taking place (a) Adsorption or condensation of gas molecules on the surface of adsorbent (b) Desorption or evaporation of gas molecules from the surface of adsorbent
Mathematically: x k1p m 1+KP Case l: when pressure is very high 1 + kp KP n K'P m KP That is at high pressure, degree of adsorption approaches a limiting value constant