1. Adsorption: Concentration of liquid or gaseous molecules over the surface of a solid material is known as adsorption. It is a surface phenomenon. (e.g.) H2 adsorption over nickel.
The solid material is known as adsorbent. (e.g.) Nickel, Silica gel, Charcoal, Zeolite
The gaseous or liquid molecules are adsorbate (e.g.) H2 gas, Bromine, benzene, water vapour
For gases, a special name is given as “occlusion’
2.If the concentration is of bulk phenomenon, it is absorption. (e.g) Absorption of water by anhydrous CaCl2.
3. Adsorption and absorption process are collectively known as ‘sorption’. (e.g) Ammonia sent through Charcoal solution. Here , Charcoal adsorbs ammonia while water absorbs ammonia. [pic]
4. Characteristics of adsorption:
a. It is spontaneous.
b. It is highly selective and specific.
c. It is accompanied by change in enthalpy and entropy.
d. It depends on temp, pressure and concentration.
e. It is a physical phenomenon accompanied by chemical process.
5. Differences between Adsorption and Absorption:
|S.No |Adsorption |Absorption | |1 |Surface Phenomenon |Bulk Phenomenon | |2 |It is the concentration of gas/ liquid on |It is the concentration of gas/liquid throughout the | | |solid surface only. |body of solid. | |3 |Fast process |Slow process | |4 |Equilibrium attainment is easy |Not easy | |5 |Eg. H2 on Nickel |Water vapour absorbed by anhydrous CaCl2 |
TYPES OF ADSORPTION
1. If the adsorbent and adsorbate are bonded by weak Vanderwalls forces, it is known as ‘Physisorption’. (e.g) H2 on charcoal. 2.If the adsorbent and adsorbate are bonded by strong chemical covalent bonds, it is chemisorption. (e.g) H2 on nickel. |No |PHYSISORPTION |CHEMISORPTION | |1 |Adsorption is due to weak Vanderwalls forces. |Due to strong covalent bonding. | |2 |They form multi layer. [pic] |Monolayer[pic] | |3 |Reversible |Irreversible | |4 |Equilibrium is easy |Not easy | |5 |Intermediate surface compound formation does not take place. |Takes place | |6 |Energy of activation (Ea) is low. |High | |7 |Heat of formation and enthalpy (∆H) is low. It is upto 40 Kcal / mole. |High .Upto 400 Kcal / Mole | |8 |Inversely proportional to temperature. |Direct proportion | |9 |Directly proportional to pressure, concentration |Inversely proportion | |10 |Non specific and non selective |Selective and specific | |11 |e.g - H2 on charcoal. |H2 on nickel. |
3.2) - FACTORS AFFECTING ADSORPTION
a) Adsorption of gases on solids: (e.g) H2 on Charcoal (Physisorption).
H2 on nickel. (Chemisorption)
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