Can oxygen be diffused to a higher pressure?

Question

So I have recently read about Fick’s Law and Henry’s Law. According to them, a certain molecule can be diffused/transferred to a lower-pressure environment, due to the difference in this molecule’s partial pressure. For example, when I inflate a balloon and let it rest still, although the pressure inside the balloon is higher than the air pressure, the oxygen concentration is lower than the outside. Therefore, while the air inside the balloon (which is pretty rich in terms of CO2) tends to go out, while the oxygen outside the baloon get diffused inside. So is this how it really works? Can oxygen be diffused to an environment where the pressure is obviously higher in natural means?

Answer 1

Hi Mehmet K.,

It's a little difficult to "read between the lines" on your question. But, yes, when you are talking diffusion, total pressures don't matter, partial pressure of each individual species does matter. So, although oxygen is diffusing both directions through the rubber balloon membrane, the NET movement of the oxygen is from the higher-partial-pressure-in-oxygen (outside) to the lower-partial-pressure-in-oxygen (inside). And eventually the CO₂ inside will diffuse out, but more slowly (goes by (MW)^-0.5 , where MW is molecular weight of the molecule or atom in question).

That's why, when rubber balloons are inflated with He gas, they rapidly lose bouyancy, even though they don't appear to lose much volume. The He diffuses out, and N₂ diffuses in, with some small loss of net volume.

Now -- since this is contraintuitive -- you expect mass movement from higher total pressure to lower total pressure, don't you? What is going on? The answer is, the scale size of the orifice (the pores in the rubber). For a larger orifice, mass flow dominates, the way you expect. For smaller orifices, in which the movement of individual molecules is more limited by interactions directly with the walls than by interactions with adjoining gas molecules, diffusion dominates. The speeds of the molecules are what is limiting. Kabish?

Now here's an interesting problem for you. A helium-filled balloon (in air) is bouyant, right? But where, exactly, does the net upwards force on the balloon arise from? (Don't just say, bouyancy, I want an argument based on pressures exerted by gas molecules!)

-- Cheers, --Mr. d.