Surface Chemistry

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: Get the Power of Visual Impact on your side Log on to www. topperlearning. com 2 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. . 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.

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.

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. re adsorbed to lesser extent. The chemical adsorption being highly specific, therefore, a gas gets adsorbed Get the Power of Visual Impact on your side Log on to www. topperlearning. com 3 on specific solid only if it enters into chemical combination with it. b. Nature of adsorbent: Activated carbon, metal oxides like aluminum oxide, silica gel and clay are commonly used adsorbents. They have their specific adsorption properties depending upon pores. c. Specific area of the adsorbent: The greater the specific area, more will be the extent of adsorption.

That is why porous or finely divided forms of adsorbents adsorb larger quantities of adsorbate. The pores should be large enough to allow the gas molecules to enter. d. Pressure of the gas: Physical adsorption increases with increase in pressure. 12. Adsorption isotherm: The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve is termed as adsorption isotherm. 13. Freundlich Adsorption isotherm: The relationship between x and m pressure of the gas at constant temperature is called adsorption isotherm and is given by ? k P 1 /n (n ? 1) m Where x- mass of the gas adsorbed on mass m of the adsorbent and the gas at a particular temperature k and n depends upon the nature of gas and the solid x first increases with increase in pressure at low pressure but becomes m independent of pressure at high pressure. Get the Power of Visual Impact on your side Log on to www. topperlearning.