Characteristics and Features of Adsorption Processes

To determine the correct option(s) related to adsorption processes, let us analyze each statement individually:

1. Option (A): Chemisorption results in a unimolecular layer.

   • Chemisorption involves the formation of strong chemical bonds (covalent or ionic) between the adsorbate molecules and the active sites on the adsorbent surface.
   • Once all the specific surface sites are occupied, further chemical bonding cannot occur directly with the adsorbent. Consequently, chemisorption stops at a single layer, forming a unimolecular (monolayer) surface coverage.
   • Therefore, Option (A) is correct.

2. Option (B): The enthalpy change during physisorption is in the range of $100$ to $140\text{ kJ mol}^{-1}$.

   • Physisorption involves weak van der Waals forces of attraction. Because these interactions are weak, the enthalpy of physisorption is low, typically in the range of $20\text{ to }40\text{ kJ mol}^{-1}$.
   • Higher values of enthalpy of adsorption (typically $80\text{ to }240\text{ kJ mol}^{-1}$) correspond to chemisorption due to chemical bond formation.
   • Therefore, Option (B) is incorrect.

3. Option (C): Chemisorption is an endothermic process.

   • Adsorption involves a decrease in residual attractive forces at the surface, which leads to a release of surface energy. Furthermore, gas molecules lose degrees of freedom upon adsorption, so $\Delta S < 0$.
   • For the process to be spontaneous ($\Delta G = \Delta H - T\Delta S < 0$), the enthalpy change $\Delta H$ must be negative ($\Delta H < 0$). Thus, chemisorption is generally an exothermic process.
   • Therefore, Option (C) is incorrect.

4. Option (D): Lowering the temperature favors physisorption processes.

   • Physisorption is an exothermic process at equilibrium: $\text{Adsorbate (g)} + \text{Adsorbent (s)} \rightleftharpoons \text{Adsorbed State} + \text{Heat} \quad (\Delta H < 0)$
   • According to Le Chatelier's principle, lowering the temperature shifts the equilibrium in the forward (exothermic) direction, thereby increasing the extent of adsorption.
   • Therefore, Option (D) is correct.

Conclusion: The correct options are A and D.