Chapter 3(2).pptx

4 increases by increase of pressure 5 surface area of

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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 place readily at low temperature and desorption takes place at higher temperature.
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Chemical Adsorption or Chemisorption
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When atoms or molecules of gases are held by solids on its surface by chemical bonds, the adsorption is called chemisorption.
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Characteristics of Chemisorption 1. 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 )
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Physisorption Chemisorption 1 Occurs due to van der Waals’ force Chemical Bond 2 Reversible Irreversible 3 Not specific Specific 4 Enthalpy of adsorption is low Enthalpy of adsorption is high 5 More liquefiable gases are adsorbed readily Gases which form compounds with adsorbent alone undergo chemisorption 6 Decreases with increase of temperature Increases with increase of temperature 7 Low temperature is favorable. High temperature is favorable 8 High pressure favors physisorption and decrease of pressure causes desorption High pressure is favorable but decreases of pressure does not cause desorption 9 Results in multimolecular layers Only unimolecular layer are formed 10 No activation energy is needed High activation energy is needed 11 It is instantaneous It is a slow process
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Factors Influencing Adsorption
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1. Surface Area of the Adsorbent Same gas is adsorbed to different extent by different solids at identical conditions. Greater the surface area of the adsorbent greater the volume of gas adsorbed.
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2. Temperature Adsorption of a gas generally decreases with rise in temperature. This is because adsorption is exothermic and increases of temperature favors the backward process which is desorption. Heat
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3. Pressure Adsorption of a gas by an adsorbent at constant temperature increases with increase of pressure.
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Mechanisms of ad…
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Adsorbents Adsorbents Silica gel Activated alumina Molecular sieve zeolites PAC EAC GAC
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Characteristics of Adsorbents Adsorbents Pore structure Porosity Nature
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Characteristics of Adsorbents Pore structure Silica gel Activated Carbon zeolite
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Characteristics of Adsorbents Porosity Mesopores Macropores Micropores D = 2-50 nm D > 50 nm D < 2 nm
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Characteristics of Adsorbents Nature Hydrophobic interaction Hydrophilic repulsion
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Adsorption equilibrium If the adsorbent an adsorbate are contacted long enough, an equilibrium will be established Eq
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