Surface Chemistry

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  • Topic: Catalysis, Colloid, Adsorption
  • Pages : 10 (2784 words )
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  • Published : August 17, 2012
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Class XII Chemistry Ch 5: Surface Chemistry Chapter Notes Top Concepts / Key learnings: 1. 2. 3. 4. 5. 6. Adsorption: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption. Adsorbate: The substance which is being adsorbed on the surface of another substance is called adsorbate. Adsorbent: The substance present in bulk, on the surface of which adsorption is taking place is called adsorbent. Desorption: The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption. Absorption: It is the phenomenon in which a substance is uniformly distributed all over the surface. Difference between adsorption and absorption: Absorption (i) It is the phenomenon in which a substance is uniformly distributed throughout the bulk of the solid. (ii) It is a bulk phenomenon. (iii) The concentration is uniform throughout the bulk of solid. Adsorption (i) The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption. (ii) It is a surface phenomenon. (iii) The concentration of adsorbate increases only at the surface of the adsorbent.

7. 8.

Sorption: When adsorption and absorption take place simultaneously, it is called sorption. Enthalpy or heat of adsorption: Adsorption generally occurs with release in energy, i.e., it is exothermic in nature. The enthalpy change for the adsorption of one mole of an adsorbate on the surface of adsorbent is called enthalpy or heat of adsorption.

9.

Types of adsorption:

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a. Physical adsorption or physisorption: If the adsorbate is held on a surface of adsorbent by weak van der Waals’ forces, the adsorption is called physical adsorption or physisorption. b. Chemical adsorption or chemisorption: If the forces holding the adsorbate are as strong as in chemical bonds, the adsorption process is known as chemical adsorption of chemisorption. 10. Difference between Physical and chemical adsorption: Chemical adsorption (Chemisorption) (i) It is highly specific (ii) It is irreversible (iii) The amount of gas adsorbed is not related to critical temperature of the gas. Physical adsorption (Physisorption) (i) It is non-specific (ii) It is reversible (iii) The amount of gas depends upon nature of gas, i.e., easily liquefiable gases like NH3, CO2, gas adsorbed to greater extent than H2 and He. Higher the critical temperature of gas, more will be the extent of adsorption. (iv) The extent of adsorption increases with increase in surface area, e.g. porous and finely divided metals are good adsorbents. (v) There are weak van der Waals’ forces of attraction between adsorbate and adsorbent. (vi) It has low enthalpy of adsorption (20 – 40 kJ mol-1) (vii) Low temperature is favourable. (viii) No appreciable activation energy is needed. (ix) It forms multimolecular layers. 11.

(iv) It also increases with increase in surface area. (v) There is strong force of attraction similar to chemical bond. (vi) It has enthalpy heat of adsorption (180 – 240 kJ mol-1) (vii) High temperature is favourable. (viii) High activation energy is sometimes needed. (ix) It forms unimolecuar layers.

Factors affecting adsorption of gases on solids: a. Nature of adsorbate: Physical adsorption is non-specific in nature and therefore every gas gets adsorbed on the surface of any solid to a lesser or greater extent. However, easily liquefiable gases like NH3. HCl, CO2, etc. which have higher critical temperatures are absorbed to greater extent whereas H 2, O2, N2 etc. are adsorbed to lesser extent. The chemical adsorption being highly specific, therefore, a gas gets adsorbed

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on specific solid only if it enters into chemical combination with it. b. Nature of adsorbent: Activated carbon, metal oxides like aluminum...
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