Adsorption


The adsorption is the accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.
Accumulation of Gas particles on Charcoal block


The main difference between adsorption and absorption is that  the substance is concentrated only at the surface and does not penetrate through the surface to the bulk of the adsorbent, while in absorption, the substance is uniformly distributed throughout the bulk of the solid

Types of Adsorption

There are two main types of Adsorption:

1. Physisorption

When gases accumulate on the surface of a solid due to weak van der Waals’ forces, then the Adsorption is termed as Physisorption.


Characteristics of Physisorption:

  • Lack of specificity

The van der Waals’ forces are universal, hence surface of an adsorbent does not show any preference for a particular gas.

  • Nature of adsorbate

The amount of gas adsorbed by a solid depends on the nature of gas. In general, easily liquefiable gases are readily adsorbed as van der Waals’ forces are stronger near the critical temperatures

  • Reversible nature

Physical adsorption of a gas by a solid is generally reversible, ie

  • Surface area of adsorbent

The extent of adsorption increases with the increase of surface area of the adsorbent. Thus, finely divided metals and porous substances having large surface areas are good adsorbents

  • Enthalpy of adsorption


2. Chemisorption

When the gas molecules or atoms are held to the solid surface by chemical bonds, the adsorption is termed chemical adsorption or chemisorption


Characteristics of chemisorption;

  • High specificity

Chemisorption is highly specific and it will only occur if there is some possibility of chemical bonding between adsorbent and adsorbate

  • Irreversibility:

Chemisorption involves compound formation, it is usually irreversible in nature

  • Surface area

Chemisorption also increases with increase of surface area of the adsorbent

Enthalpy of adsorption

  • Enthalpy of chemisorption is high




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