Question

What are the principles of surface sorption by solid?

Answer 1

Sorption includes Adsorption and desorption. Adsorption is the accumulation of a substance giving a higher concentration of molecular species on the surface of another substance. The substance that concentrates on another surface is called Adsorbate and the substance onto which adsorption happens is called adsorbent. Desorption is the process by which adsorbed substance is removed from the Adsorbent. The sorption process can happen between solid/liquid and solid/gas. There are two types of sorption. Physisorption and chemisorption. Physisorption occurs when a gas/liquid gets adsorbed temporarily on to a solid surface. It is guarded by weak van der Waal's force. Chemisorption, on the other hand, is the permanent property where gas/liquid is adsorbed. It is guarded by strong intermolecular forces.

Physisorption is an exothermic process and has low enthalpy values (20– 40 kJ mol-1), due to weak van der Waals forces of attraction. It's a reversible process since its activation energy is very low. Physisorption usually occurs at low temperature and it increases with an increase in pressure.

Chemisorption occurs when chemical force exists between adsorbate and adsorbent. It is also an exothermic process with high enthalpy values (80-240 kJ mol-1). It is irreversible with high activation energy and occurs slowly. the extent of adsorption increases with an increase in temperature up to a certain limit and then it starts decreasing. Chemisorption is affected much by pressure but high pressure can favor chemisorption.