Adsorption and Colloids — Chemistry STD 11 Science — Question

When the gas molecules accumulate at the surface of a solid or adsorbate by means of chemical bonds (covalent or ionic), the adsorption is termed as chemical adsorption or chemisorption.
Features of chemical adsorption:

• Chemisorption is specific in nature.
• Chemisorption involving the gas-solid as the adsorbate and adsorbent is usually exothermic i.e., heat is released during this process (Exception: The adsorption of hydrogen on glass is endothermic).
• The heat evolved in chemisorption per mole of adsorbate is nearly the same order of magnitude as that accompanying chemical bonding.
• Chemisorption involves a large energy of activation and hence, it is also referred as activated adsorption.
• Chemisorption increases with increase in temperature in the beginning, as a greater number of molecules can have activation energy. But after certain temperature chemisorption decreases with increase in temperature as the chemical bonds break.
• Sometimes at low’ temperature, physisorption occurs which passes into chemisorption as the temperature is raised.
• Chemisorption is dependent on surface area of the adsorbent.