Driving force for Osmosis

[Ifat Glassman]

My problem is the following: Why is there only a driving force to compare concentrations of the same substance, but not of different substances?

To clarify the question: suppose you have two tanks, connected by a membrane which is only permeable to water. We have a concentration of 5M NaCl in one side, and 1M NaCl on the other side. Water would flow from the lower concentration to the higher concentration. Fine, no questions here.

Now: we have two tanks, connected by a membrane permeable only to water as before. But now, on one tank, we have 5M Na+, and on the other side we have 1M K+. Water would not do a darn thing, according to the theory I learned. Question is why?

Osmosis: The passage of a liquid from a weak solution to a more concentrated solution across a semiperm- eable membrane that allows passage of the solvent (water) but not the dissolved solids.

[BrassDragon]

According to your definition of osmosis, as well as what I know, osmosis would occur in your second example as well, until the molarity of both sides is equal. What makes you think it woulnd't?

But don't hold me to that like you'd hold me to something philosophical... I'm no chemist.

[Ifat Glassman]

According to your definition of osmosis, as well as what I know, osmosis would occur in your second example as well, until the molarity of both sides is equal. What makes you think it wouldn't?

But don't hold me to that like you'd hold me to something philosophical... I'm no chemist.

Huh, you are right. I just checked Wikipedia:

Osmotic pressure is a colligative property, meaning that the property depends on the concentration of the solute but not on its identity.

Oops... I should have known that.

Okay: so changing the question now: What is the driving force for a substance to move in order to equate concentrations in two sides of a tank?

If we have the same two tanks, 1 side has 10M Na+, the other side has 1M K+, and the membrane is only permeable to K+, the final result (neglecting the electrical forces) should be 1/2M K+ on each tank. Which means, K+ will only "want" to have the same concentration with itself, and it doesn't "care" that Na+ creates a lot of concentration on one side.

So how would you explain this?

[BrassDragon]

So how would you explain this?

Remember, the membrane is permeable to not only the K+, but also the water. Water will flow more quickly towards the side with the higher concentrarion until the concentration on both sides is completely equal. Meanwhile, the K+ ions will randomly disperse throughout the water, just because they randomly move around anyway.

In other words, it's not that the K+ doesn't "care" about the concentration of Na+. It's that it cares about the overall concentration, which the water is equalizing anyway.

To answer your specific underlined queston: The force causing the substance to equalize on both sides of a tank is random motion of all particles (the water and both types of ions). If you're in a crowded room and you're moving randomly, eventually the entire room will have an even density.

I'll have fun extending that metaphor, and perhaps it will be helpful. There is a school dance with 1000 girls in the gymnasium, and 100 guys. 90 of them are 7th graders, and they have to stay in the left half of the gym, but 10 of them are 8th graders, and they can cross to either side. Everybody is really nervous, so they move around randomly to the least crowded parts of the gym. Even though the 7th graders can't cross the middle line, eventually the dispersion of people throughout the gym will be even.

[KendallJ]

Huh, you are right. I just checked Wikipedia:

Oops... I should have known that.

Okay: so changing the question now: What is the driving force for a substance to move in order to equate concentrations in two sides of a tank?

If we have the same two tanks, 1 side has 10M Na+, the other side has 1M K+, and the membrane is only permeable to K+, the final result (neglecting the electrical forces) should be 1/2M K+ on each tank. Which means, K+ will only "want" to have the same concentration with itself, and it doesn't "care" that Na+ creates a lot of concentration on one side.

So how would you explain this?

The answer is "pressure", more specifically vapor pressure. K+ doesn't "want" anything, and it doesn't move anywhere. The mechanism you're using is an epitemological one. Salt solutions change vapor pressure of the liquid. Since the tube is open to the same atmospheric pressure bu thave different vapor pressures, the net effect is that the water is "pushed" to the other side. However, as it does so, the concentration changes in both resorvoirs. The equilibrium state will have a water height difference that directly reflects the difference in vapor pressures of the new resulting solutions.

The reason it is colligative is because the effect of vapor pressure is related to normality, not the particular identity of the solute.

[Ifat Glassman]

In other words, it's not that the K+ doesn't "care" about the concentration of Na+. It's that it cares about the overall concentration, which the water is equalizing anyway.

To answer your specific underlined question: The force causing the substance to equalize on both sides of a tank is random motion of all particles (the water and both types of ions). If you're in a crowded room and you're moving randomly, eventually the entire room will have an even density.

I'll have fun extending that metaphor, and perhaps it will be helpful. There is a school dance with 1000 girls in the gymnasium, and 100 guys. 90 of them are 7th graders, and they have to stay in the left half of the gym, but 10 of them are 8th graders, and they can cross to either side. Everybody is really nervous, so they move around randomly to the least crowded parts of the gym. Even though the 7th graders can't cross the middle line, eventually the dispersion of people throughout the gym will be even.

Here is the problem: in the school dance example the left half of the gym will be almost entirely populated with 7th grader boys. The distribution of girls, for example, will not be equal throughout the room. And according to the theory, the girls should be spread equally all over the gym, whether there is a region of higher density of 7th graders or not.

Your school dance example and the first sentence of yours that I quoted, say opposite things. In the school dance example the girls do mind that the 7th graders occupy the left half of the gym. In the K+ example the K+ doesn't mind that the left half is occupied by Na+.

Your explanation of random movement (increase of entropy) could work. I think maybe the problem is that I am taught that there is a driving force for K+ to have equal concentration of K+ on both sides, when in fact there is no "driving force" like in the case of Osmosis, but just higher probability of K+ to spread equally.

An interesting question though is: why would the most probable result be that both substances are spread equally on both sides? Why can't it be that the same amount of molecules, regardless of kind, are spread in equal densities? Why does the identity "matter"? Dunno, I have a problem with probability, I'm not good at it, I just don't get this subject... (sigh).

Kendall, I don't see the relevancy of the correct things you said (out of everything you said) to my second question, and if it is an answer to the question in the title of the thread, it is just wrong: The vapor pressure is the same (since the system is open), and vapor pressure has nothing to do with why water move from one side to the other. But like I said, I already know the answer to the driving force for Osmosis, and it is not what I am asking here, so I don't wish to discuss it.

[KendallJ]

To answer your specific underlined queston: The force causing the substance to equalize on both sides of a tank is random motion of all particles (the water and both types of ions). If you're in a crowded room and you're moving randomly, eventually the entire room will have an even density.

That's incorrect. Random motion is part of the mechanism, but it is not the driving force. It will not explain why more water will end up on one side than the other, which is what happens.

The driving force is vapor pressure differences, i.e. of an uneven distribution of random motion of particles, due to the changes in vapor pressure caused by the salts.

[KendallJ]

Kendall, I don't see the relevancy of the correct things you said (out of everything you said) to my second question, and if it is an answer to the question in the title of the thread, it is just wrong: The vapor pressure is the same (since the system is open), and vapor pressure has nothing to do with why water move from one side to the other. But like I said, I already know the answer to the driving force for Osmosis, and it is not what I am asking here, so I don't wish to discuss it.

Ifatart, you are incorrect in your terminology. The net equilibrium system pressure is the same on both sides, however the VAPOR PRESSURE (i.e. the partial pressure exerted by the molecules of water vapor a the surface of the solution / air interface) is different. It is this pressure which is the driving force. When the system comes to equilibrium, the difference in height of each of the colums will be exactly equal to the difference in vapor pressures of the resulting equilibrium solutions.

This is exactly the same reason that salt solutions also exhibit boiling point differences. The temperature of the solutions must be higher because the vapor pressures are depressed at a given temperature.

It is also the mechanism behind reverse osmosis water purification where the driving force is a reversal of the pressure mechanism.

THE DRIVING FORCE IS PRESSURE DIFFERENCES. that is why it is called osmotic "pressure"

This is a direct answer to your second question. The driving force to move the water is the added vapor pressure on the side of the weaker salt solution.

Edited October 19, 2006 by KendallJ

[Ifat Glassman]

Ifatart, you are incorrect in your terminology. The net equilibrium system pressure is the same on both sides, however the VAPOR PRESSURE (i.e. the partial pressure exerted by the molecules of water vapor a the surface of the solution / air interface) is different. It is this pressure which is the driving force. When the system comes to equilibrium, the difference in height of each of the columns will be exactly equal to the difference in vapor pressures of the resulting equilibrium solutions.

This is exactly the same reason that salt solutions also exhibit boiling point differences. The temperature of the solutions must be higher because the vapor pressures are depressed at a given temperature.

If we look at the system in more simpler terms, and explore what actually happens there, we see that the driving force is what happens inside the liquid, and not on the surface of the liquid: What happens on the surface of the liquid might be an indication of the state of the system (if it is closed and in equilibrium). The connections of water molecules to the solute is stronger than between water molecules and themselves, or perhaps, the presence of the solute changes the structure of connections between water molecules around it, causing the structure to be more stable. This is why, once a solute is added, water molecules are more reluctant to leave the solution. This is why the vapor pressure decreases and a higher temperature is needed to reach a certain vapor pressure than the one required had we only had a pure solvent. This is why once water randomly passed to the side with the solute, they will remain there, until they balance out the weight of the water column pushing it down to pass to the other side.

Even if you have suction of vapors from the surface of the system, it will not change what happens inside the solution, and it will not stop water from crossing over, and adding more volume to the solution.

THE DRIVING FORCE IS PRESSURE DIFFERENCES. that is why it is called osmotic "pressure"

I can read just fine without capitalized letters, excessive spacing and repetitiveness. I prefer ideas to be presented just once, in a pithy way. This is the best way for me to learn something.

As for the name "Osmotic pressure": liquid creates pressure as well. The pressure is measured in the height of water (you know, 10 meters height on 1 square cm is 1 atmosphere, if you ever took a diving course ).

This is a direct answer to your second question. The driving force to move the water is the added vapor pressure on the side of the weaker salt solution.

It is not an answer to my question. I was asking about movement of a solute, not of water.

[KendallJ]

Ifat,

I owe you an apology. I missed the para where you changed the senario.