Adsorption energetics: For physical adsorption (physisorption), the typical heat of adsorption is of what order on a kcal per kg·mole basis?

Introduction / Context:
The heat released when molecules adsorb on surfaces reflects the strength of adsorbate–surface interactions. Distinguishing physisorption from chemisorption by their heats of adsorption is fundamental in surface science and catalysis.

Given Data / Assumptions:

• Physical adsorption (van der Waals forces), not involving new chemical bonds.
• Energy scale requested in kcal per kg·mole (i.e., per kilomole).

Concept / Approach:
Physisorption heats are modest, typically a few kcal per mole (e.g., 1–5 kcal/mol). On a per kmol (kg·mole) basis, multiply by 1000, giving on the order of 10^3 kcal/kg·mol. Chemisorption, by contrast, can be tens to over a hundred kcal/mol, i.e., 10^4–10^5 kcal/kg·mol.

Step-by-Step Solution:
Use rule of thumb: physisorption ≈ 1–5 kcal/mol.Convert to per kmol: 1–5 kcal/mol × 1000 mol/kmol → 1000–5000 kcal/kg·mol.Hence, the correct order is 10^3 kcal/kg·mole.

Verification / Alternative check:
Textbook tables list heats of physisorption orders of magnitude lower than typical bond-forming chemisorption, confirming the 10^3 kcal/kg·mol scale for physisorption.

Why Other Options Are Wrong:

• 100 or 10 kcal/kg·mol: too small; would imply fractions of a kcal per mole.
• 10000 or 100000 kcal/kg·mol: these correspond to 10–100 kcal/mol, typical of chemisorption or bond energies, not physisorption.

Common Pitfalls:
Mixing units per mole vs. per kmol; always check the requested basis.

Final Answer:
1000