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Chemistry: Spontaneous Processes


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About this Lesson

  • Type: Video Tutorial
  • Length: 12:26
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 134 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Thermodynamics (8 lessons, $14.85)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

About this Author

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Recent Reviews

Ok but redudant with unclear examples
~ Jayreimer

This movie defines spntaneous carefully, and uses several examples of spontaneous "but not chemical" responses, like diffusion of ink in water, BUT the distinction (why diffusion is not a chemical reaction) is not explained. This could be shortened to about two minutes without appreciable loss of meaning.

Ok but redudant with unclear examples
~ Jayreimer

This movie defines spntaneous carefully, and uses several examples of spontaneous "but not chemical" responses, like diffusion of ink in water, BUT the distinction (why diffusion is not a chemical reaction) is not explained. This could be shortened to about two minutes without appreciable loss of meaning.

The word spontaneous has a lot of different meanings to us in English. It may describe somebody's personality - their willingness to go out and try new things or spur of the moment kind of thing and can talk about something happening or not happening. Chemists have a very specific definition of what they are talking about when they use that word "spontaneous." So let's go ahead and see from a chemist's perspective what we're talking about.
The term "spontaneous process", to a chemist, is talking about a change that occurs by itself without any intervention. Now, let's do a quick example of that and then we'll try to highlight that a little bit more. If I simply take a ball and I drop it into this container, it spontaneously falls to the bottom of the container. It does not spontaneously come back out of the container though.
Now, the fact that it happens, without me intervening - other than just the fact that I let go of the ball to start the process - makes it spontaneous. But how quickly it happens has nothing to do whatsoever with it being spontaneous. So as another example, if I take this honey and I hold it upside-down, eventually, the honey is going to drip out all over the place. But the fact that it's spontaneous does not promise anything about how quickly it will happen - just that eventually it will happen. If it's sap running out of the top part of a tree, it may take forty years before it finally gets to the bottom of the tree. It could take a hundred years for a rock to manage to make its way down a mountain through windstorms and rainstorms and whatever. The point is that regardless of how long it takes something to happen, if it wants to happen on its own, without outside intervention, that process is considered to be spontaneous.
Now, what causes something to be spontaneous? Well, in the case of the ball or the honey, we certainly would describe that, again, by a force of gravity acting on the ball. We could talk about the potential energy of the ball going from a higher potential energy to a lower potential energy and we're really comfortable with that idea that nature likes to go from high energy to low energy and that that causes something to be spontaneous. Good enough.
We can talk about things in terms of a chemical energy difference, as well. Let's go ahead and take this candle here and light it. We can talk about the chemical process going on as the wax is converted to carbon dioxide and water. We can take butane - which is the propellant of this hairspray - and do a chemical reaction with butane and that's a spontaneous reaction. We can understand why it would be spontaneous, because in this case, butane and oxygen - our interpretation is that must be a higher level of energy than the products - the carbon dioxide and the water. And when we talked about thermochemistry, we were comfortable with the idea that as we go from high energy to low energy, that's a process that's going to occur. Again, it's a spontaneous process.
I remind you that it again has nothing to do with how fast it occurs. This balloon here is filled with hydrogen and oxygen and they'll just continue to sit in here for 100 years with nothing happening, even though we know that it's a very, very exothermic reaction if the two things combine with each other to form water. Yet the rate is immeasurably slow for these guys unless we initiate that reaction with a spark. So the idea, "spontaneous", has to do with whether or not the process will happen given an infinite amount of time, not how fast it occurs. So that's a little different than our English definition, at least, of spontaneity.
Now, let's talk about the reverse idea. Let's go back to this chemical reaction for a moment of butane and oxygen forming carbon dioxide and water. We know that this is energetically downhill, and so let's think about the reaction happening in reverse. How spontaneous is it for carbon dioxide and water to come up to here? Well, we know that if it's spontaneous in this direction, it's not going to happen the other way. Our intuition kind of tells us that on the same level that our intuition tells us that the ball at the bottom here is not going to be all of a sudden bouncing back out again. If energy was released by going from high to low energy, we have a pretty good sense that it's not just going to spontaneously pop back out, that the reverse process won't be spontaneous if the forward process is. Let's say that again. We know that if a forward process is spontaneous, the reverse process won't be spontaneous. Here we have, again, a pretty good intuitive sense for that.
Let me point out that everything we've been talking about here - none of it violates the first law of thermodynamics. That just says energy is conserved. In other words, coming back to this ball example, there's nothing in the first law of thermodynamics that says that that ball can't just jump back up to my hand. So I'll just kind of wait for it to do it and you know that I could wait a long time and it's not going to happen. But it wouldn't violate the first law of thermodynamics if it did happen. And just like the chemical reaction, CO[2] plus water goes to butane and oxygen, that reaction also would not violate the first law of thermodynamics, even though it's not spontaneous. There's something else at play here, which we're getting to.
Now, everything we've talked about so far involves energy - chemical energy differences - but there are lots of things that are spontaneous in our world that don't involve big energy changes. For instance, if I have this container of water here and I put a drop of food coloring in it and we create a drop at the top, you can watch what happens over time and you'll notice that the main thing that you're seeing here is that the food coloring is spreading out. And if I come back tomorrow, it will be throughout the entire volume of the water here. And common sense - intuition - tells me that this is not in fact a reversible process, that this is not going to just suddenly form a drop of food coloring again if I wait or if I turn my back. We have a pretty good sense that the reverse process is not spontaneous if this process is spontaneous. And yet, I'm telling you, this has nothing to do with energy - nothing to do with differences in chemical energy - why this should want to happen. In the same sense that something that you're a little more familiar with thinking about - or at least a kind of everyday life example - if I have a collection of marbles here that I'm holding together, if I release what's holding these guys together and I just kind of shake this tray around a little bit, those marbles are going to go every which way. And no matter how much I try to shake this thing and round them up again, I can't get all those marbles to meet in the center the way they were before. So even though this has nothing to do with energy - these marbles aren't under some mystical field being pushed away from each other or anything like that - the chances of getting those marbles all to spontaneously roll back into a neat little pile are very, very small. The chances are even smaller if it's a pile of sand. I mean, think about that - a pile of sand that gets blown by wind. Think of the chances that it's all going to collect back up into a little tiny pile again.
So clearly, there are some things that happen that don't involve energy, per se, or changes in potential energy, or chemical energy, yet they're very spontaneous and they don't reverse. The reverse process, in other words, is not spontaneous.
I wanted to say one other thing about this balloon, and that is we have gas inside this balloon. If I pop the balloon, since the pressure inside is higher than outside, we know that the gas rushes out - this is a common, everyday experience to us - but the chances of that gas rushing back in and assembling in the shape of that balloon, we know that's silly. Why is anybody even talking about that? We know that that is a non-spontaneous process. But why? It has nothing to do with energy. Why would that process be irreversible? Why doesn't air just - if you take a balloon and hold it open and say, "Come on, air; Jump in," and have all the air rush in to the balloon and blow it up? But it doesn't happen. And we know it doesn't happen, but again, that's a case where there's not a good explanation in terms of energy - in terms of chemical energy or potential energy even.
Now, for one more example - a particularly funny example - let's go ahead and look what happens to a tangerine that has been cooled down to the temperature of liquid nitrogen. So we've taken a tangerine; we've dropped the temperature down to 80 Kelvins and now we're going to find out what happens to that tangerine when it comes in contact with a hammer. Okay, so I guess you haven't seen a tangerine do that before, but again, you're not used to tangerines at 80 Kelvins.
Now, even stranger, though, would be this process. Let's watch again. So what you saw certainly seems rather silly and, in fact, it seems silly because you know it can't happen and, once again, your common sense tells you that if it's spontaneous for the tangerine to burst apart when it's in contact with the hammer, it is very much non-spontaneous for all the pieces to reassemble themselves into the tangerine again.
So again we have this notion - maybe we've pounded it into the ground - but this notion that a process that is spontaneous in one direction is therefore not spontaneous in the other. In other words, all spontaneous processes have directionality. They're spontaneous only in one direction. So, again, just a cartoon. You're not going to see a ball spontaneously popping up a wall, for instance, going from low energy to high energy.
And finally, let's make one other point and that is that there are some processes that, indeed, cost energy yet are still spontaneous. The best example I know is putting a glass of water out and coming back a week later. That water is going to evaporate and that is a process that we know costs energy, yet it spontaneously happens. Why is that? Why is it that something that actually would cost energy happens all by itself? Well, clearly, something is at work here other than just chemical energy or potential energy, and that's this notion of disorder - that nature strives to maximize its disorder.
So, in order to discuss this notion a little bit further, let's go ahead now and formally define what we mean exactly by "entropy".
An Introduction to Thermodynamics
Spontaneous Processes Page [2 of 2]

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