The correct option(s) related to adsorption processes is (are)

The correct options are:
1. Chemisorption results in a unimolecular layer.
2. Lowering the temperature favors physisorption processes.

Let us evaluate each of the given options step-by-step to understand the principles governing physical and chemical adsorption processes:

1. Chemisorption results in a unimolecular layer:
Chemical adsorption (chemisorption) involves the formation of true chemical bonds (covalent or ionic) between the adsorbate molecules and the adsorbent surface. Since chemical bonding occurs directly with the specific active sites on the surface, once these active sites are occupied by a single layer of molecules, no further chemical bonding can take place. Therefore, chemisorption results in a unimolecular (monolayer) coverage. Thus, this statement is correct.

2. The enthalpy change during physisorption is in the range of 100 to 140 kJ mol^–1:
Physical adsorption (physisorption) is driven by weak van der Waals forces of attraction. Because these interactions are relatively weak, the enthalpy of physisorption is typically very low, usually in the range of 20 to 40 kJ mol^–1. On the other hand, an enthalpy change in the range of 80 to 240 kJ mol^–1 (or higher) is characteristic of chemisorption. Hence, the statement claiming it is 100 to 140 kJ mol^–1 for physisorption is incorrect.

3. Chemisorption is an endothermic process:
Adsorption is generally an exothermic process because bond formation or attractive interaction releases energy ($\Delta H<0$), and the restriction of movement of adsorbate molecules leads to a decrease in entropy ($\Delta S<0$). For the process to be spontaneous ($\Delta G<0$), $\Delta H$ must be negative. Thus, chemisorption is naturally an exothermic process, making this statement incorrect.

4. Lowering the temperature favors physisorption processes:
Physisorption is an exothermic process ($\Delta H<0$). According to Le Chatelier's principle, lowering the temperature shifts the equilibrium in the direction that releases heat, which is the forward direction (adsorption). Consequently, physisorption increases with a decrease in temperature. Thus, this statement is correct.