Dr. SURENDRAN PARAMBADATH (M.Sc, M.Phil, M.Tech) Formerly: Post Doctoral Research Associate,Nano-Information Materials Research Laboratory, Pusan National University, Busan-South Korea Currently: Assistant Professor Govt. Polytechnic College, Perinthalmanna
Adsorption: The accumulationof molecular species at thesurface of a solid or liquidrather than in the bulk is calledadsorption.
Adsorbate: The molecularspecies that accumulate at thesurface is termed adsorbate. Adsorbate
Adsorbent: The material on thesurface of which adsorptiontakes place is calledadsorbent. Adsorben t
Desorption: The removal ofadsorbate from the surface iscalled desorption.
Occlusion: The adsorption ofgases on the surface of metalsis called occlusion.
Examples for adsorption…………1. If a gas like H2, O2, Cl2 etc is taken in a vessel containing powdered charcoal, pressure of the gas slowly decreases as the gas is adsorbed on the surface of charcoal.2. Air becomes dry in presence of silica gel because adsorption of water takes place on the surface of the gel.3. Aqueous solution of raw sugar becomes colourless when passed over a bed of animal charcoal. The coloring matter is adsorbed by the charcoal.4. Litmus solution or a solution of a dye like methylene blue when shaken with animal charcoal turns colourless due to adsorption of coloring material.
Distinction between adsorption & Absorption Adsorption AbsorptionIt is a surface It is a bulk phenomenonphenomenonAdsorbed species is It is uniformly distributedaccumulated in the throughout the bulksurfaceIt is a fast process It is a slow processesRate of adsorption Absorption takes place atdecreases gradually steady rate
If accumulation of gas molecules onthe surface of solids occurs due toweak van der Waals’ forces ofattraction, the adsorption is calledphysisorption.
When atoms or molecules of gasesare held by solids on its surface bychemical bonds, the adsorption iscalled chemisorption.
Characteristics of Physisorption1. Non-specific nature: An adsorbent does not show any preference for a gas as the van der Waals’ forces are universal.2. Easily liquefiable gases like CO2, SO2, NH3 etc, are readily adsorbed.3. Reversible nature: Physisortion of a gas by a solid is reversible.4. Increases by increase of pressure.5. Surface area of adsorbant: When the surface area of the adsorbent increases, more gas is adsorbed, ie extent of adsorption increases.6. Enthalpy of adsorption: Enthalpy of adsorption of physisorption is very low (20-40 KJ mol-1)Since adsorption is exothermic, physisorption takes placereadily at low temperature and desorption takes place at highertemperature.
Characteristics of Chemisorption1. High specificity: It is highly specific and will occur only if chemical bond formation takes between adsorbate and adsorbent.2. Irreversibility: Chemisorption is irreversible because the chemical bond formed is difficult to break.3. Chemisorption increases with temperature.4. Increases by increase of pressure.5. Surface area of adsorbent: When the surface area of the adsorbent increases, more gas is adsorbed, ie extent of adsorption increases.6. Enthalpy of adsorption: Enthalpy of adsorption of chemisorption is high (80-240 KJ mol-1)
Physisorption Chemisorption1 Occurs due to van der Waals’ force Chemical Bond2 Reversible Irreversible3 Not specific Specific4 Enthalpy of adsorption is low Enthalpy of adsorption is high5 More liquefiable gases are adsorbed Gases which form compounds with readily adsorbent alone undergo chemisorption6 Decreases with increase of temperature Increases with increase of temperature7 Low temperature is favorable. High temperature is favorable8 High pressure favors physisorption and High pressure is favorable but decreases of decrease of pressure causes desorption pressure does not cause desorption9 Results in multimolecular layers Only unimolecular layer are formed10 No activation energy is needed High activation energy is needed11 It is instantaneous It is a slow process
2. Surface Area of the AdsorbentSame gas is adsorbed to different extent bydifferent solids at identical conditions. Greaterthe surface area of the adsorbent greater thevolume of gas adsorbed.
3. TemperatureAdsorption of a gas generally decreases withrise in temperature. This is because adsorptionis exothermic and increases of temperaturefavors the backward process which isdesorption. Heat
4. PressureAdsorption of a gas by an adsorbent at constanttemperature increases with increase ofpressure.