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Chemistry: Electromotive Force

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About this Lesson

  • Type: Video Tutorial
  • Length: 12:29
  • 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: Electrochemistry (12 lessons, $19.80)
Chemistry: Galvanic Cells (6 lessons, $11.88)

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 http://www.thinkwell.com/student/product/chemistry. 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.

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Thinkwell
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We've been exploring the idea that certain chemical reactions that involve a redox process - an electron transfer from one component to another - that these reactions lend themselves to physical separation of the components and that we could use the transfer of electrons to actually get potential work from that chemical reaction - harness the chemical energy in the form of electricity, in other words. We looked at the example of copper metal reacting with silver and we looked at this first in a beaker - saw that it was a very spontaneous reaction. And the idea was that we could pull away the copper component from the silver component, connect those two by a wire and then use the electricity flowing through that wire to do work. We talked about a galvanic cell, where we have the physical separation of the copper and the silver and on the anode side; we had the copper electrode in one molar solution of copper 2 nitrate. Now this is standard state for both copper and the copper 2 plus. And on the cathode side, we had silver and silver plus - again, that's standard state conditions - and we connected the two electrodes together, we ran it through some load - a light bulb or a motor or what have you. And then, in order to complete the circuit, we added a salt bridge. And the salt bridge, if you recall, was to allow, as this cell operates, the transfer of the counter-ions of the silver to get over to the copper side. Now that allowed us to maintain a balance of charge in the cells as current flowed.
Now the idea of a galvanic cell is the principle behind a fruit clock - something that you might have seen at a science store or toy store. And, again, this is just an example of the cell that we're talking about here. The only difference is that instead of copper and silver, this cell is constructed from copper and zinc. But otherwise, it's exactly the same. In fact, we actually have two of those cells hooked up in series, as it turns out. And the only role of the fruit - I'll point out here - is simply to be the salt bridge. That's all that fruit is doing is providing a way to transfer charge so that we don't build up charge at the electrodes. Other than that, this is exactly the same idea as what we're showing here. And you'll notice that it's running this little clock and it's very cute. That's our load, in this case. That's the work that we're getting out of it.
Now, you can imagine taking that work away - breaking the circuit so that we don't have any current flowing - and then taking a volt meter - and we'll say more about that in a moment - and measuring the voltage across those two electrodes. Now, what is voltage? At this point, we should stop and describe a little bit more about electricity. You've heard that term about a 9-volt battery or a 1.5-volt battery. You know about, in your house, you've got 110 volts. But what exactly is this unit that we're talking about? And we'll come back to this idea in a moment. But before I leave, let's just not lose sight of the idea that we've removed the load so that we've opened the circuit, and now we're measuring the potential across those electrodes - the voltage across those electrodes. So what exactly are we measuring?
Well, a volt is the SI unit for potential difference. It's also sometimes referred to as electrical pressure. Well, what does that mean? Well, imagine that you've got two plates that have opposite charges on them. And then we put a negative charge in between those plates, for instance. You know that that charge will be repelled by the negative plate and attracted to the positive plate. And potential is talking about the potential work that you can get out of that charge by it passing through that potential difference. In other words, the work that you get is whatever the charge is on this - if it's an electron, if it's a much larger body that has a lot more charge built up on it - times that potential difference, or the voltage. Our units for work are going to be energy units - joules - our units of charge - now I'm talking about the charge here. This is not necessarily the fundamental unit of charge. If it's an electron, it is. But in general, this is a charge of any amount. This is the total amount of charge on whatever this object is. And so it's a large Q instead of a small q now. And measured in coulombs, which is our standard unit of charge. And then, the potential difference is what's measured in volts. Okay, and that's a description of what the potential is to get work for moving that charge.
Now, a couple other things about electricity to talk about - because that still hasn't given you a good sense of what voltage is, but at least, it's defined it for is, and we'll say a bit more in a second. But let me also mention that an electron is 1.6 X 10^-19 coulombs, as it turns out. If we took an entire mole of electrons, it might be interesting to know what that amount of charge is. That would be a valuable number - ultimately our conversion - for us to hold onto. We'll see that that's useful a little later.
So if you have 1 mole of electrons, it turns out that that has 96,480 coulombs worth of charge. Okay, so 1 mole of electrons is 96,480 coulombs and that's referred to as one faraday. So a couple of important conversions for us - again, just mostly definitions now about electricity.
Let's come back to this idea, again, of volts and see if we can come to a better understanding of what it means. And maybe the best way to do this is through analogy - an analogy with water and pipes. We're very comfortable with that idea. Imagine taking a tank, filling it with water and putting it up on top of a hill someplace, and then running a pipe down the hill to some location much below that tank. And we'll connect it to a valve and we'll close the valve off so no water is actually flowing through the pipe. And then we'll hook up a pressure gauge and read what the pressure of that water is. Well, I imagine it seems logical to you that the higher that tank is above where we're reading the pressure, the higher the pressure is going to be. Literally, it's the height difference in this column of water here. So it's wherever the high point of that tank is. It won't make any difference how big that tank is. That talks about the capacity - how much water is being held - but it doesn't affect what the pressure is. The only thing that affects the pressure is the difference in height between where you're measuring and the high point of water - wherever the water level is, because that's describing the column height here - how much is actually pushing down to create the pressure.
Now, you can imagine what will happen. If I open that valve, what's going to happen to our reading of the pressure? Well, again, I think intuition tells you that if I open up that valve so the water starts to flow, the pressure gauge is no longer going to read exactly the same value anymore; it's going to drop a little bit. And it's going to drop a little because there's a little resistance in this pipe to that water flowing. Now hold on to that idea. Remember we talked about the difference between the capacity of water and the pressure of the water. And also, we introduced the idea of how much water flows. We can talk about how much water flows per unit times gallons per minute, for instance. Okay?
So, you're comfortable with those ideas. Now, let's talk about a battery and a circuit. Take a battery and let's imagine connecting the top end of the battery to the bottom end of the battery through a switch. Once I connect that, electrons flow through that potential difference in the same way that water can flow through this pipe. But if we want a measure of what the pressure is on those electrons to flow, if you will, like the pressure here, we need to stop the flow so we can get an accurate measurement. We need to open the switch and just measure the voltage between here and here - the potential to have those electrons flow. If we connect the circuit together, then there's going to be a drop in voltage of some kind because there's going to be a resistance to those electrons flowing. It may be inherent in the wire we use. It may the resistance due to a load - like an electrical clock, for instance, or a motor or a light bulb. But the point is, we don't get an accurate measure of the potential to have those electrons flow - of the pressure on the electrons, if you will - unless we have our circuit open so that no current flows. What do I mean by current? Amount per unit of time - like gallons per minute, but in electrical terms, this is going to be amount of charge per unit time. So the current is going to be measured in coulombs per second, and that has a special unit - that's amperes. Okay, so that's just like gallons per minute - coulombs per second - same idea. The current is going to depend on what the potential is. Well, that makes sense. Remember, the amount of water that's going to come out of this pipe is going to depend on how much pressure there is on that water - and that will describe how fast it flows - divided by the resistance. Why that? Well, the more resistance you have to the current flowing, the slower the electrons are going to be able to travel through that wire. And in the same way, the more resistance there is in this pipe, the less water is going to be able to flow for a given pressure. So you see, there's a very strong analogy between flowing water and flowing electrons in a circuit.
Finally, let me talk about what electromotive force is. Now that's this notion that if the circuit is closed - electrons are flowing - we don't get a good measure of what the potential difference is. So I want to know what the maximum potential is. And the only way I can measure that is to open to the circuit where I don't have any current flowing, measure what the potential is to get current flowing, and that we define as the electromotive force - that's just the maximum voltage we can get out of this circuit, and the way we get that is to open it so that we don't have any current flowing. So that's described as electromotive force. You'll also see it described as - when we talk about an electrochemical cell as the cell potential. And sometimes you'll even see it as E not. That all is the same thing. It's talking about the maximum potential to get useful work out of a galvanic cell.
So let's finally return to our galvanic cell again where we're measuring our potential. And I will tell you that if we take copper and silver and we measure the electromotive force - the maximum potential we have across here - it turns out to have a value of .459 volts - so almost 46 volts. That gives us a sense of the driving force on these electrons to move through - more correctly, the pressure on these electrons to transfer through this circuit. Again, I have to remind you that although I'm drawing these arrows here, there's no current flowing when I'm measuring that potential. But it tells me what will happen if I do connect the circuit, what's pushing on those electrons.
Now, our next step, now that we're comfortable with electricity, is to ask, okay, fair enough. I've got a number. That tells me some measure of the ability of this reaction to occur. Maybe that's linked somehow to an equilibrium constant or to a free energy. We're going to go there. But right now, let's just explore what happens if we remove this metal and replace it with another metal. How is that potential going to change and what does that mean? That's where we're going next.
Electrochemistry
Galvanic Cells
Electromotive Force Page [3 of 3]

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