Physical adsorption versus chemisorption: Which statement correctly contrasts physisorption with chemisorption?
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AQuantity adsorbed per unit adsorbent mass is generally higher (multilayer possible)
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BRate is controlled primarily by surface reaction resistance
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CActivation energy is very high
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DHeat of adsorption is very large
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EPhysisorption forms strong chemical bonds with the surface
Answer
Correct Answer: Quantity adsorbed per unit adsorbent mass is generally higher (multilayer possible)
Explanation
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:
Physisorption: weak forces, low activation barrier, possible multilayer → higher overall capacity.Chemisorption: strong bonds, activated process, site-specific → lower capacity (monolayer).Therefore, the statement about higher quantity adsorbed per unit mass fits physisorption.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)