Physical adsorption versus chemisorption:\nWhich statement correctly contrasts physisorption with chemisorption?

Introduction / Context:
Adsorption processes are central to separations and catalysis. Two limiting mechanisms are physical adsorption (physisorption) and chemical adsorption (chemisorption), which differ in bonding, energetics, and capacity.

Given Data / Assumptions:

• Physisorption arises from van der Waals forces; multilayer adsorption is possible (BET behavior).
• Chemisorption involves specific chemical bonding; typically monolayer-limited (Langmuir behavior).
• Typical heats: physisorption ~ 20–40 kJ/mol; chemisorption often much larger.

Concept / Approach:
Because physisorption is weaker and non-specific, it can build multiple layers, often leading to a higher total uptake per mass at suitable temperatures. Chemisorption is stronger, often activated, and limited to surface sites, reducing capacity to about one monolayer.

Step-by-Step Reasoning:

Verification / Alternative check:
BET isotherms show increasing uptake with relative pressure due to multilayer formation, confirming higher capacities for physisorption at lower temperatures.

Why Other Options Are Wrong:

• Surface-reaction control, high activation energy, very large heat of adsorption, strong bonds: all characterize chemisorption, not physisorption.

Common Pitfalls:
Assuming higher heat means better separation; confusing monolayer (Langmuir) with multilayer (BET) models.

Final Answer:
Quantity adsorbed per unit adsorbent mass is generally higher (multilayer possible)