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

Step 1: Understanding the Concept:
When mass is transferred from one phase (e.g., a gas) to another (e.g., a liquid), it must cross the boundary between them. This is explained by the "Two-Film Theory."
Step 2: Key Formula or Approach:
The total resistance ($1/K_L$) is the sum of the individual phase resistances: \[ \frac{1}{K_L} = \frac{1}{k_L} + \frac{1}{H k_G} \]
Step 3: Detailed Explanation:
According to the two-film theory, there is a stagnant thin film of fluid on both sides of the interface. While the bulk phases are well-mixed (low resistance), the mass must pass through these stagnant films by molecular diffusion, which is a slow process. Therefore, the primary resistance to mass transfer is concentrated in these two films. The interface itself is usually assumed to offer zero resistance.
Step 4: Final Answer:
Resistance is considered to exist in the thin films on either side of the interface.