Recall the key features of physisorption and chemisorption

• Physisorption:

  • due to weak van der Waals forces
  • generally exothermic
  • low enthalpy of adsorption, usually about $20$ to $40\,\mathrm{kJ\,mol^{-1}}$ (sometimes up to roughly $80\,\mathrm{kJ\,mol^{-1}}$)
  • favored by low temperature
  • can form multilayers

• Chemisorption:

  • due to formation of chemical bonds
  • usually exothermic overall, though activation energy may be required
  • high enthalpy of adsorption, typically about $80$ to $240\,\mathrm{kJ\,mol^{-1}}$
  • generally forms a monolayer (unimolecular layer)

Evaluate each option

Option A: Chemisorption results in a unimolecular layer.

Chemisorption occurs by formation of chemical bonds at the surface, so once the surface sites are occupied, further adsorption does not continue in layers.

Therefore, chemisorption gives a monolayer / unimolecular layer.

$\Rightarrow$ A is correct.

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

This is too high for physisorption. Such large values are characteristic of chemisorption, not physisorption.

Physisorption has much smaller enthalpy change.

$\Rightarrow$ B is incorrect.

Option C: Chemisorption is an endothermic process.

Chemisorption involves bond formation, so the adsorption itself is generally exothermic, not endothermic.

It may require activation energy initially, but that does not make the overall process endothermic.

$\Rightarrow$ C is incorrect.

Option D: Lowering the temperature favors physisorption processes.

Since physisorption is an exothermic process, according to Le Chatelier’s principle, lowering temperature favors adsorption.

$\Rightarrow$ D is correct.

Final answer

The correct options are: $\boxed{A,\ D}$

Comparison with stored answer

Stored correct answer: A, D

My derived answer matches the stored answer.