Adsorption of Gases on Solids

Adsorption is a process in which molecules, known as adsorbates, adhere to a surface, referred to as an adsorbent. This process can be classified as physisorption or chemisorption.

Physisorption involves weak intermolecular forces and is nonspecific, while chemisorption forms strong chemical bonds and is highly specific. For example, nitrogen physically adsorbs onto any solid at low temperatures, but its chemisorption depends on the solid type.

Chemisorption forms a monolayer on the surface, whereas physisorption creates multilayers due to intermolecular interactions. However, after chemisorption forms a monolayer on the surface, physisorption can build additional layers on top of it.

Interestingly, certain gases, like hydrogen, usually dissociate upon chemisorption, while others, like ammonia, chemisorb without dissociation.

The adsorption rate is expressed as the product of the adsorption rate constant, the gas concentration, and the number of available adsorption sites.

The surface coverage, represented as θ, is calculated from the ratio of the volume of adsorbate adsorbed to the volume of adsorbate required for complete monolayer coverage.