Question

Interphase mass transfer resistance is usually considered to exist in the:

• A. Bulk phases only
• B. Interface only
• C. Thin films on either side of the interface
• D. Solid surface only

Hint:

Logic Tip: Think of crossing a border between two countries. Traveling through the countries is fast (bulk phase), and the actual border line is just a concept (interface). The only thing that slows you down are the customs checkpoints built immediately on either side of the border (the thin films)!

Answer 1

The Correct Option is C

Concept:
When a substance transfers from one phase to another (e.g., from a gas into a liquid), engineers use the "Two-Film Theory" (Whitman's film theory) to model where the resistance to this transfer actually occurs.

Step 1: In industrial equipment, the main body of the gas and the main body of the liquid (the bulk phases) are usually well-mixed or turbulent. Because of this mixing, concentration is uniform, and there is virtually zero resistance to mass transfer in the bulk.

Step 2: The actual, infinitely thin boundary line where the gas touches the liquid (the interface) is assumed to be in perfect, instantaneous thermodynamic equilibrium. Therefore, the interface itself offers zero resistance.

Step 3: Right next to the interface, on both the gas side and the liquid side, there exists a thin, stagnant (or laminar) fluid layer called a "film".

Step 4: Because these thin films are relatively still, convective mixing doesn't happen there. The molecules are forced to rely solely on slow, molecular diffusion to cross these films. Therefore, all the resistance to interphase mass transfer is concentrated entirely within these thin films.