Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: S_Ch_S on March 21, 2024, 07:15:00 AM
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Can anyone recommend how to show an inverse diffusion process? Is it possible? Any hint or link much appreciated. Thank you.
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What do you mean by reverse diffusion? Diffusion is process in which species tend to move along a concentration gradient from high concentration to low concentration. It is driven primarily by entropy. You cannot "reverse" this process short of reversing the second law of thermodynamics. That would be a neat trick.
But, there are examples in which molecules do not diffuse because entropy is not the only consideration at play - there is also enthalpy. When the energy cost of molecules moving to a new location exceeds the entropy gain, molecules will not diffuse to areas of lower concentration. Indeed, if you start in a mixed state, they will appear to separate, which may look like "reverse" diffusion.
Take oil and vinegar. The water will not diffuse into the vinegar. In fact, if you shake the mixture up, it will, over time, spontaneously separate. Water will go from a state of low concentration to locally high concentration. This I guess might be taken as "reverse diffusion", although I don't know that I've ever heard it called that. The reason the mixture separates is because there's an energy cost to water mixing with fat. In this case, the energy cost of having fat molecules next to water molecules exceeds the entropy gain of having water molecules (and fat molecules) spread out. As you might surmise, you can change the energy balance equation by adding a small amount of new molecules that reduce the energy cost of mixing. Add a little egg yolk that contains lecithin to your oil and vinegar mixture, and the energy cost goes down considerably because the lecithin molecules make it more energetically favorable for fat molecules to be near water molecules, and now the entropy factor becomes more competitive. The mixture will persist for much longer. This is the magic of a surfactant.
So, you want an example of "reverse diffusion"? Make a vinaigrette!
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Hi! Many thanks for the great explanation and the entertaining response:) Would it be possible to create a model of "reverse diffusion" by applying current? For example to pull one liquid/element apart from another by applying current as in electrolysis and when releasing going back to a diffusion state? Would something like this be possible (and visible)? Any hint, link, suggestion highly appreciated. Many thanks.
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Sure, in principle. Suppose the following thought experiment: you take a sodium chloride solution and sandwich it between two metal plates connected by a wire. Apply a voltage across the wire using a battery or something. You basically now have a capacitor. On one plate you build up a negative charge and on the other you build up a positive charge, creating an electrical field across the solution. Without the electrical bias, the charge distribution in the solution is uniform. With the electrical bias, the positively charged sodium atoms should move toward the negatively charged plate and the negatively charged chloride atoms should move toward the positively charged plate. So you've basically increased the concentration gradient for both sodium and chloride ions - sort of "reverse diffusion" if you want to call it that. This was enabled by the fact that you've applied energy to the system, when compensates for the decrease in entropy that accompanies the creation of a concentration gradient. Remove the electrical bias, and the electric field disappears, and the ions will diffuse back to the normal equilibrium position (zero concentration gradient). This is both because of the tendency to maximize the system's entropy but also to minimize charge interaction energies. (So, even in this direction it's not a pure Fickian diffusion.)
There are some practical reasons this experiment wouldn't work that well due to the nature of capacitors, but hopefully you get the principle I am trying to illustrate - to "reverse" diffusion you need to apply energy to the system.
Incidentally, biology figured this out a long time ago. Every cell in your body uses chemical energy from cellular respiration to propel ions across a membrane against a concentration gradient. These ions then flow back across the membrane through ion channels to create ATP, which can then be used to drive reactions elsewhere in the cell. Signal transduction in neurons is also facilitated by the creation of ion gradients across cell membranes using similar "reverse diffusion" processes driven by the consumption of cellular energy provided by ATP.
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Hi! Thanks for another amazing answer! I tried out the setup that you described: water, sodium chloride, 12V. See Attachment. So when I apply voltage I have a visible movement of particles. But phenomenologically speaking, there is a paradox here if I am not wrong: When I apply the voltage the water gets diffused because of the particle movement (but this is the moment when I have the "inverse diffusion", right?). When I turn the voltage off, the heavier sodium chloride resides at the bottom of my water tank so my water (tank) gets clear and does not look diffused anymore. It seems turning on and off the voltage will make the effect more intense and visible. Would applying more voltage (from 12V to 24V) create a more intense effect? And could theoretically speaking this system run "perpetually"? Thank you for your amazing explanations.
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It also seems that when I add vinegar to the water the effect is more intense...
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Sorry, was out of town for a while.
Honestly, I'm not sure I understand exactly what you have done here. A more detailed explanation would be helpful.
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Hi! I just put 12V, two wires +,- in the water with salt. When I apply the voltage the water around the cathode gets diffused. Is this the moment that theoretically speaking I have an inverse diffusion? It seems after a while the effect is not so intense? Could theoretically speaking this system run "perpetually"? Thank you for your amazing explanations.
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... Could theoretically speaking this system run "perpetually"? ...
Sounds too good to be true
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First, as water is the solvent here, the concentration is high everywhere. It is also electrically neutral so any electromotive force created by the electric field between the two wires is unlikely to create any mass movement of water - although since water molecules are polar, you might expect a realignment of the water molecules along the field lines.
As noted above, I would expect that the field between the two wires will cause some separation of the sodium and chloride ions dissolved in the water. But this isn't something you'd be likely to see - sodium and chloride being transparent in the wavelengths your eyes can pick up.
I'm not really sure what you mean when you say that "water gets diffused". If you mean that you see some disturbance in the water (ripples or whatever): were I to hazard a guess, it would be that when you initially apply the voltage, you have some electrolysis of water at the electrodes, liberating some gas. As in any cell of this nature, the effect will subside as (for instance) sodium/chloride ions adhere to the electrode surfaces, reducing their efficiency over time.
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Sorry for the late response and many thanks for your explanation! Please see below:
https://streamable.com/7e4pme (https://streamable.com/7e4pme)
The setup is sodium chloride in water with 12V.
What exactly is happening on the wire of the cathode on this VERY visible effect?
Is there any kind of diffusion happening here?
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To go back to the initial question: would it be possible to have diffusion and then (even partially) reverse it?
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Rather than reproducing the wheel here, I can direct you to one of many videos on the internet that describe what happens during elecrtolysis of brine:
E.g., https://www.youtube.com/watch?v=9X-ht85YYsI
Notice around the time 1:25, the "reverse diffusion" you have been asking about.
The gas you seen in your video liberated will be the hydrolysis products of brine, for example hydrogen and chlorine gas. (Do note, chlorine gas is toxic and hydrogen is explosive, so you should do this in a well-ventilated space without ignition sources.
The reason your visual reaction slows down is two-fold: first, the pH raises because one of the biproducts of brine electrolysis is NaOH; also side reactions can cause rapid buildup (say, of oxides) on your electrode surfaces, which decreases their efficiency. This is why electrochemical cells utilize things like salt-bridges prevent this kind of thing. As you can see, it happens pretty quickly.
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Thank you, now I understand!! This video is very helpful! May I ask a last question: is there a way to prolong the process by adding something to the mix? Thank you again!
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Well the first thing I'd do is probably equip a stir bar to mix the ingredients, and increasing the volume would probably help sustain the reaction as well. In the end, though, like any reaction the only way to keep it going is to remove products as they are formed, which would mean counteracting the pH change. Unfortunately the reaction is inherently self-limiting.
At this point it becomes a chemical engineering problem. I could envision using a pump and circulator to continually cycle fresh brine across the electrodes. In effect, make the solution volume infinite (as long as your reservoir doesn't run out).
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Thank you so much for your amazing answers! Probably the best responses I have gotten in any forum ever!
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Thank you so much for your amazing answers! Probably the best responses I have gotten in any forum ever!
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Incidentally, biology figured this out a long time ago. Every cell in your body uses chemical energy from cellular respiration to propel ions across a membrane against a concentration gradient. These ions then flow back across the membrane through ion channels to create ATP, which can then be used to drive reactions elsewhere in the cell. Signal transduction in neurons is also facilitated by the creation of ion gradients across cell membranes using similar "reverse diffusion" processes driven by the consumption of cellular energy provided by ATP.
Just to piggyback on this concept. Bacteria such as E. coli move lactose (milk sugar) against its concentration gradient, which requires energy. Evidently they obtain more energy by catabolizing this disaccharide than is takes to drive the transport. This might be tangential to your question.
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I don't think it's tangential at all. Given that life itself is made possible only by utilizing energy to (locally) defy the thermodynamic tendency to maximize entropy, it should not be surprising that there are many microscopic and macroscopic examples of local increases in concentration due to life processes. We can even think of examples on the large scale: imagine people milling about outside in a large field, rain clouds come, people will concentrate themselves inside due to the energetic payoff of staying dry. Grazing animals concentrate themselves into herds when predators are present due to the payoff of increased change of survival. We may even look to sociological principles: societies self-segregating themselves into neighborhoods based on wealth, race, religion, etc. It may seem silly to think of these things as examples of thermodynamic principles in action, but looked at analytically, they typify how living systems can run counter to the entropic tendency toward maximum mixing and maximum decrease of concentration for some energetic benefit. The processes appear to happen naturally without little conscious thought on the part of the participating organisms. Less abstract examples also occur in our technology: every house that runs an air conditioner or refrigerator is using energy to run a "reverse diffusion machine". In this case the diffusing "substance" is heat rather than matter, but the principles are the same all the way down to the equations used to describe it.
(As a matter of fact, versions of the diffusion equation and related thermodynamic/entropy principles have been used to successfully model everything from mass transfer, heat transfer, currency flow, information exchange, sociological change, and so on. Thermodynamics truly is everywhere. Seriously, next time you're on a train or bus or theater, watch how strangers naturally spread out to fill the available space, whereas family members sit together. Have you ever considered the thermodynamics of why they do that? In my view, it's remarkable how large ensembles of particles behave in ways that might be predicted by the laws of statistical mechanics, whether those particles are atoms or people. It makes me question the authenticity of choice. Is free will real or just an illusion deriving from the fact that when you are part of the system, it's hard to see the system at large. Your choice may be the product of conscious thought, but does it matter if the behavior of the system of which you are part is predetermined? Even if you consciously chose the seat you chose, is the choice meaningful if the aggregate of all choices by all people is easily predicted by a statistical bell curve modeled by a basic law of thermodynamics? Put another way, if you had not chosen the seat you chose, someone else would have, and the system would be unchanged. Hmmm. Well, I guess that's a philosophical question for you all ;).)
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Thank you both for the very interesting exchange of ideas!
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@Corribus Blown away by your last post. Any good read/link/youtube on these ideas?
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Well, entropy as a concept is foundational to some disciplines like information theory (https://en.wikipedia.org/wiki/Entropy_(information_theory)). In others like sociology (https://en.wikipedia.org/wiki/Social_entropy) and economics (https://en.wikipedia.org/wiki/Thermoeconomics), it's a little more controversial, but I'm not sure why anyone would be surprised that thermodynamic principles guide human constructs. A quick search indicates there are youtube videos available for these, particularly Information Theory (you might try looking into Shannon Entropy), but I haven't vetted them.
As for the philosophical aspects, well those are my own thoughts, although I can't say for sure whether others have put them forward first. So no youtube video for that (yet). But I see that the idea isn't too far fetched, as others have ruminated on the link between thermodynamics guiding human decision-making, e.g., here (https://en.wikipedia.org/wiki/Entropy_(information_theory)). (I haven't read that article, just found it as the first link when searching for "thermodynamics and human behavior".) Googling "free will and thermodynamics" also turns up a number of hits, mostly to other forums where random internet peoples have given voice to the topic.
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Thank you so much for sharing your amazing ideas!!
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Another interesting link:
https://www.sciencedirect.com/science/article/abs/pii/S0378437120309973
Quote from the intro:
"It is shown that there is a deep analogy between the parameters of thermodynamic and economic systems (markets)"
Your view of the world that you described above shifts the perspective of looking at things completely. And it seems to be touching a much deeper level...than what is usually observed.. It is very interesting to think for example volatility in markets as a non-equilibrium state...it literally is. Thanks again!
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May I ask something: I did the sodium chloride infused water electrolysis in a self-built thin and long water tank for about 3 hours and after I turned the voltage (12V) off, my glass water tank broke. I didn't see how this happened but I found it broken 5 minutes after I turned the voltage off. Any explanation on this? Was the hydrogen product trapped in the water tank but which other factor contributed to this? Thanks!
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If the tank was hermetically sealed and it is not pressure rated, then, sure, gas generation at the electrode could certainly cause failure.
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Thank you for the response, the tank was absolutely not hermetically closed, and the 12V electrolysis of sodium chloride infused water was happening for 3hrs. After I turned it off it broke. Any logical explanation? It was a thin and long tank but its top side was completely open.
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Three factors that could contribute to the failure: hydrogen, chlorine, and pH going up. Hydrogen seems to be the least problematic, but both chlorine and high pH can easily speed up corrosion/decomposition of many substances/glues.
Doesn't mean it wasn't just a completely random thing.
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Thank you again for the response, it seems very weird that this happened especially after turning the voltage off.
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The failure after the battery being turned off could just be a matter of kinetics. Under Borek's theory of corrosion, corrosive substances generated during the cell operation could take some time to cause enough damage to reach the point of failure.
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Many thanks for your response! I will look into what you are describing. My tank was held vertically by two clamps, maybe the glass broke from the tension of the clamps and the fact that it happened after I turned the voltage off is just a coincidence?