Question

How does chemical adsorption of a gas on a solid vary with temperature?

Answer 1

Chemical adsorption (chemisorption) of a gas on a solid surface involves the formation of strong valence bonds between gas molecules and the adsorbent. Being an exothermic process with high enthalpy of adsorption (80–240 kJ/mol), chemisorption requires activation energy and thus is slow at low temperatures. As the temperature rises, the extent of chemisorption initially increases due to overcoming the activation energy barrier. However, beyond an optimum temperature, further increase causes desorption, leading to a decrease in adsorption. Therefore, the variation of chemisorption with temperature shows a characteristic rise followed by a fall. This contrasts with physical adsorption, which decreases continuously with increasing temperature due to weaker van der Waals forces involved in that process.