Surface Chemistry

Adsorption


  • Adsorption: Accumulation of molecules of a substance (a gas, a liquid or a dissolved substance from a solution) on the surface of a liquid (or) solid, is called adsorption.
  • Adsorbent: It is the substance whose surface has been utilized for adsorption.
  • Adsorbate: It is the substance whose molecules accumulate on the surface of adsorbent.
  • Trick: ent eat ate.
  • Positive adsorption: When the concentration of the adsorbate is high on the surface of the adsorbent than in the adjoining bulk, the adsorption is called positive adsorption.
  • Negative adsorption: When the concentration of the adsorbate is less on the surface of the adsorbent than in the adjoining bulk, the adsorption is called negative adsorption.
  • Physical adsorption (or) Physisorption or Vander Waal's adsorption: It is the type of adsorption in which adsorbed molecules are held on the surface of adsorbent by weak Vander waal's forces.
    Eg: adsorption of water vapour on silica gel.
  • Characteristics of physisorption:
    (i) Binding forces between adsorbent and adsorbate are weak Vander Waal's forces.
    (ii) It is general in nature i.e., all substances can undergo physisorption
    (iii) It is a reversible process.
    (iv) Enthalpy of adsorption is 20-40 kJ mole−1, which is quite low.
    (v) It can be reduced by decreasing pressure or by increasing temperature.
    (vi) Multi molecular layers are formed.
  • Chemical adsorption (or) Chemisorption (or) Langmuir adsorption: Adsorption in which adsorbed molecules are held on the surface of adsorbent by strong forces of chemical nature is chemisorption.
  • Characteristics of chemisorption:
    (i) Binding forces between adsorbent and adsorbate are strong chemical bonds.
    (ii) It is specific in nature and occurs only if strong bonds can be formed between adsorbent and adsorbate.
    (iii) It is irreversible in nature.
    (iv) Enthalpy of adsorption is 40 to 400 kJ mole−1
    (v) Chemisorption of H2(g) on glass surface to produce H-atoms is endothermic.
    (vii) Chemisorption is favoured by increase in temperature to a certain limit after which adsorption starts.
    (vii) The layers are generally mono molecular.
  • Absorption: It is the phenomenon in which a substance is uniformly distributed in the bulk of another substance.
    Eg: absorption of water by CaCl2.
  • Sorption (M.C.Bain): It is the phenomenon of simultaneous absorption and adsorption of gases and dyes on solid surface.
    Eg: take up of gases by zeolite.
  • Competing adsorption: From a mixture of gases more strongly adsorbable gas is adsorbed to a greater extent (i.e., it shows higher extent of adsorption) than expected from its partial pressure and the amount present.
  • Factors influencing adsorption of gases on solid surface:
    (a) Nature of adsorbate gas:
    Trick: (i) For physisorption easily liquifiable gases adsorbs more readily.
    (ii) Permanent gases get adsorbed on metals and shows chemisorption.
    (*) Higher the critical temperature more easy the liquification.
       H2O (560 K) > SO2(430 K) > NH3(405.5 K) > HCl (324 K) > CO2(304.2 K) > CH4(190.7 K) > O2(154.4 K) > Ar(151 K) > N2(126 K) > H2(33.5 K) > He(5.3 K)
    (b) Nature of adsorbent: Porous materials like silica gel, charcoal etc., undertake physisorption. Transition metals undertake chemisorption.
    (c) Surface area of the solid:
    (i) Adsorption of gases is directly proportional to the surface area of the adsorbent.
    (ii) Surface of 1 gm substance is called specific surface.
    (iii) Surface of a solid can be activated for higher adsorption by making it rough, cleaning by vacuum cleaners, cleaning by steam etc.,
    (d) Pressure of the gas: Increasing in pressure, increases the extent of adsorption of gases, especially physisorption, but to a certain limit, after which it becomes constant.
    (c) Temperature:
    (i) Adsorption decreases entropy {i.e., ΔS is −ve}
    (ii) Adsorption is an exothermic process {i.e., ΔH is −ve}
    (iii) ΔG = ΔH − TΔS, ΔG will be negative at low temperature.
    (iv) Physical adsorption is favored by lowering of temperatures. Chemical adsorption is related with chemical bond formation, activation energy is to be supplied by increasing temperature to some barrier after which it will decrease.
  • Trick: During chemisorption of gases on metal surfaces, electrons from metals enter into empty molecular orbitals of gases.
    Eg: When O2 is chemisorbed on any metal, electrons from metal enter in π* molecular orbitals of O2.

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