A particular adsorption process has the following characteristics: (i) It arises due to van der Waals forces and (ii) it is reversible. Identify the correct statement that describes the above adsorption process:

We are told that the given adsorption process (i) arises due to van der Waals forces and (ii) is reversible.

First, let us recall the two broad kinds of adsorption usually discussed in surface chemistry:

1. Physisorption (physical adsorption)  —  this is governed only by weak van der Waals interactions between adsorbate and adsorbent.

2. Chemisorption (chemical adsorption)  —  this involves formation of true chemical bonds, generally covalent or ionic in character.

Because the question explicitly mentions van der Waals forces, the process must be physisorption. Physisorption carries several well-documented characteristics, which we list one by one together with the associated energetic facts.

(a) Low enthalpy of adsorption.
For physisorption the intermolecular attraction is weak, so the heat evolved (enthalpy change) is small, generally in the range $$\Delta H_{\text{ads}} \approx -20 \text{ to } -40\ \text{kJ mol}^{-1}.$$ This value is much less than $$100\ \text{kJ mol}^{-1}.$$

(b) Multilayer possibility.
Since there is no specific bonding directionality, more than one layer of adsorbate molecules can pile up one over another. Therefore physisorption is not restricted to a monolayer.

(c) Temperature dependence is inverse.
Adsorption is an exothermic process, so Le-Châtelier’s principle tells us that higher temperature will shift the equilibrium toward desorption. Hence, for physisorption the amount adsorbed decreases as temperature rises.

(d) Low energy of activation.
Only weak forces must be overcome, so the activation barrier is either zero or very small. Consequently the rate of physisorption is fast even at low temperatures.

Now we match these standard properties against the four given statements:

Option A: “Adsorption is monolayer.”  But physisorption can form multilayers, so this is incorrect.

Option B: “Adsorption increases with increase in temperature.”  We have just reasoned that the opposite trend holds, so this is incorrect.

Option C: “Enthalpy of adsorption is greater than $$100\ \text{kJ mol}^{-1}.$$”  Physisorption has a much smaller enthalpy (≈20-40 kJ mol$$^{-1}$$), so this is also incorrect.

Option D: “Energy of activation is low.”  This agrees perfectly with statement (d) above and is therefore correct.

Thus only Option D is consistent with every characteristic of the process described.

Hence, the correct answer is Option 4.