

Chemistry: Batteries



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  • Type: Video Tutorial
  • Length: 11:01
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 118 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: Batteries (2 lessons, $2.97)





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Have you ever wondered how a battery works? Yes, you have. Don't lie to me. And I want to tell you about batteries now, because you want to know. And there's a good reason to know about batteries. Batteries are examples of galvanic cells, essentially - two half-reactions that combine to give us an overall net chemical reaction, but we can take advantage of the transfer of electricity and use that for useful work.
So let's talk first of all about the type of battery you're probably most familiar with, at least if you're an older person, and that's the battery in your car. That's a lead storage battery. And these are the two half-cells that describe it. So let's look quickly at what they are. In fact, I should back up a step and say what you have in your car is actually a series of six batteries all connected together in series. So you end of with a voltage of about 12 volts, even though the individual batteries that make up that 12 volts, each one of those is actually about 2 volts. So I'm going to talk, actually, about only one of the six batteries that you actually have in that battery.
So the two half-reactions would be lead metal reacting with bisulfate. This just comes from sulfuric acid. If you ever look on the top of your batter, it warns you about strong acid and burning you and so forth. Again, this is what you've got to worry about - the sulfuric acid's that in there. This is the bisulfate ion from that. Lead sulfate is the product of this half-reaction. That's lead 2 where we started out with lead zero. So notice that we're doing an oxidation of lead, in this case. And we get H^+ and we get electrons.
Okay, now the other half-cell is lead oxide. The lead oxide is coated on these big plates along with the lead metal. In fact, you see here they alternate between lead and lead oxide plates. And the lead oxide, which is a solid, will react in the presence of acid and the bisulfate. It takes two electrons and it also, interestingly, makes lead sulfate. So interesting example here. We make the same product, but from two different directions. Lead zero gets oxidized to lead 2, and lead 4 gets reduced to lead 2. And we get a little bit of water when that happens. And so when we combine those two half-reactions together, we end up with lead metal and the lead oxide being consumed with acid to give us lead sulfate, which also precipitates on these plates, and water.
Now by the way, it's really important for that lead sulfate to stay on those plates. If it drops down to the bottom, it's no longer in contact electrochemically, and the battery is ruined. So if we knock those things off, that's a problem, so we'll come back to that. Well, we probably won't come back to that, so just don't do that.
So what do we have here? Okay, this is our total potential and that's about 2 volts. Now this works fine. We talked about the problem about this type of thing in the Nernst equation. Now remember, the Nernst equation told us that the cell potential is going to decrease from the standard state cell potential if we change concentrations. Now what concentrations are we worried about? H^+ and HSo[4]. We know that as those concentrations drop, we lose voltage. So as this battery starts to lose its charge, its voltage changes. Okay?
And that's a big problem. I mean, it's okay in a car, where we have an alternator to keep the voltage high and to always keep the battery fully charged. But otherwise, this would be a real pain, because it means as soon as we're down to, let's say, three quarters of the charge that we started with, we've already started to lose some of our voltage. And that's not going to be good, because our lights aren't as bright then. But as long as we have an alternator to keep us highly charged, then we're in good shape.
The other problem about this kind of battery is it's got a very, very low energy to mass ratio, meaning the amount of energy we can store in a battery divided by the weight of this battery - if you've ever lifted up one of these things, this is lead, right? You couldn't have picked a worse thing to make a battery out of from that standpoint. It's just really cheap. But so that's a big problem, because this thing is really heavy. It's fine for a big car, but you certainly don't want to walk around with one of these in your radio. That wouldn't be so convenient. And if you're going off into space and you need a lot of batteries, this is not going to be a good solution either, because it's just too heavy for the amount of energy that you get.
So lots of smart folks have come up with lots of other ideas for batteries. This is probably the battery that you're most familiar with, if its' not that one. And this is the traditional dry cell. This is the kind of thing that you've got in your radios and in your Walkman and in your Gameboys and all of that kind of stuff. And basically, the anode of this reaction is zinc metal, going to zinc 2 plus plus 2 electrons. And the cathode is manganese dioxide, and it reacts in the presence of ammonium to give Mn[2]O[3] plus ammonia, plus water as a liquid.
Now, it's called a dry cell. It's not really dry, because we're making water. But, in fact, it's called dry relative to a storage battery where we had, clearly, a lot of sulfuric acid - a lot of wet. And, in fact, you even had to worry about porting water back in, occasionally, to replace water that you lost. This is dry in that it's a paste. It's a very kind of watery, moist paste that's put in here between the zinc electrode and the MnO[2 ]electrode. And so dry is a relative term here, I guess. But that's what we mean by a dry cell. And the original dry cells were zinc and manganese dioxide.
Now, these things don't sit around on your shelf very well. You can leave them there fore months and they're okay. But if you wait years, eventually, little bits of water get in and this zinc starts to react. And so you lose some of the charge. The battery leaks, in other words. You lose some of the initial charge of the battery. And so, kind of a spin-off of this idea was invented called the alkaline cell. It's very much the same idea except that we're changing now, a little bit, what's going on at the anode. This reaction now is zinc metal in the presence of hydroxide, and that forms zinc hydroxide. Now that gives it an extra little oomph. There's a little bit more voltage here, because the hydroxide stabilizes that zinc 2 plus. So that helps our potential for the half-reaction a little bit. And the cathode is basically the same thing - MnO[2 ]goes to Mn[2]O[3. ]But notice that it generates hydroxide. And so what's nice is, now, when we look at the overall reaction, in contrast to that last battery we saw, everything pretty much is a solid or a liquid. So we don't have any concentrations that are changing.
Remember, we talked about the lead storage battery and the problem with it was that as you started to discharge the battery, concentrations decreased. If concentrations decreased, the Nernst equation tells us that that means the voltage has to decrease. Right? Well, what's wonderful about this is that Q is 1. Well, that's pretty cool. There are no concentrations to change here, because these are all solids or liquids. The amounts of materials change, but their concentrations don't. So just like we could take those out of equilibrium expressions, they don't show up in Q. And so the Nernst equation, essentially, the cell potential stays equal to the standard state potential, regardless of how much we discharge the battery.
So the neat thing about alkaline batteries is their potential is very stable throughout their whole discharge, until you just run out of everything, then they don't work. Then, there is the mercury battery - same basic idea. This is the same idea, but even a little bit more stable, as it turns out - for no obvious reasons to me. But the two reactions here are zinc, again. And zinc is just used in just about everything. And I'll talk about lithium at the end. But zinc is just a wonderful reducing agent, so it gives us a lot of potential.
So zinc with hydroxide, giving us zinc hydroxide - that is half of the reaction. The other half is mercury oxide reacts with water to give mercury liquid plus hydroxide. Once again, the two hydroxides cancel, so the overall reaction involves solids, solid liquid, solid, liquid - nothing that involves a concentration. So once again, these are wonderful batteries for holding a very stable potential of voltage until they're completely used up. Okay? So very valuable, especially in scientific instruments or things where you really depend on your voltage being a constant value. These are just fantastic for that. Now, of course, the big problem with mercury batteries - these were real popular until more recent times. But they're a big problem, because when you're done with them, you've got to throw them away. And they contain mercury and that, of course, is really bad for the environment. Likewise, lead storage batteries are a big problem. So kind of a new thing in batteries, these days, are lithium batteries. So the reductant in the lithium battery is going to be the lithium. That's even better than zinc. So instead of a potential of about 1.5 volts, we have a potential of close to 3 volts now in a lithium battery. And then the other side of it is manganese dioxide, like we saw in the earlier dry cells. So these things are light. They have really high voltage. They have very stable potentials, because they involve mostly solids. And I don't know if they have any liquids, but things that basically hold their potential until the battery is pretty much discharged. And they recharge very nicely too, from an engineering standpoint. So lithium batteries are wonderful.
But anyway, the basic ideas of a battery: It's a galvanic cell, and we choose carefully the engineers, choose carefully their half-reactions, such that all of the components of the net chemical reaction are solids or liquids. And that allows us to not be governed by the Nernst equation, which tells us that as you change concentrations of solutions or of gases that your cell potential will change. So we don't have that problem with this new generation of batteries. The cell potentials stay constant and these batteries work a whole lot better.
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