WEBVTT 00:00:00.090 --> 00:00:02.430 The following content is provided under a Creative 00:00:02.430 --> 00:00:03.820 Commons license. 00:00:03.820 --> 00:00:06.060 Your support will help MIT OpenCourseWare 00:00:06.060 --> 00:00:10.150 continue to offer high-quality educational resources for free. 00:00:10.150 --> 00:00:12.690 To make a donation or to view additional materials 00:00:12.690 --> 00:00:16.445 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.445 --> 00:00:17.070 at ocw.mit.edu. 00:00:27.420 --> 00:00:29.760 CATHERINE DRENNAN: Cell potential-- cell 00:00:29.760 --> 00:00:34.890 potential delta E cell, and we're 00:00:34.890 --> 00:00:39.690 going to relate this back to delta G. 00:00:39.690 --> 00:00:44.070 So as we saw with these cells, you have this flow of electrons 00:00:44.070 --> 00:00:46.030 through the circuit. 00:00:46.030 --> 00:00:49.410 They're generated at the anode from the oxidation reaction, 00:00:49.410 --> 00:00:52.080 and they go over to the cathode. 00:00:52.080 --> 00:00:53.910 And when your cell is running, it'll 00:00:53.910 --> 00:00:57.480 generate a potential difference, this delta E cell, 00:00:57.480 --> 00:01:00.600 between those two electrodes in the cell. 00:01:00.600 --> 00:01:05.950 So delta E cell has a lot of different names. 00:01:05.950 --> 00:01:11.055 It's called cell potential, cell voltage, electron motive force, 00:01:11.055 --> 00:01:12.360 EMF. 00:01:12.360 --> 00:01:16.350 All of these names are valid, can be used. 00:01:16.350 --> 00:01:18.660 I'm going to try to call it cell potential. 00:01:18.660 --> 00:01:20.370 Hopefully, I'm not going to switch up. 00:01:20.370 --> 00:01:21.870 But if you see any of these, it's 00:01:21.870 --> 00:01:23.460 talking about the same thing. 00:01:26.410 --> 00:01:31.950 So overall, then, if we know this cell potential, 00:01:31.950 --> 00:01:35.130 we can relate this back to delta G. 00:01:35.130 --> 00:01:38.280 And so that will then be able to tell us whether the reaction is 00:01:38.280 --> 00:01:40.150 spontaneous or not. 00:01:40.150 --> 00:01:43.710 So the overall free energy, Gibbs free energy, of the cell 00:01:43.710 --> 00:01:46.990 is related to the cell potential by the following equation. 00:01:46.990 --> 00:01:49.800 So delta G for the cell minus n, the number of moles 00:01:49.800 --> 00:01:53.280 of electrons that pass through the system, Faraday's 00:01:53.280 --> 00:01:57.790 constant and that cell potential. 00:01:57.790 --> 00:02:01.440 So we can think about this at a particular time. 00:02:01.440 --> 00:02:03.810 We can also think about this equation 00:02:03.810 --> 00:02:05.760 in the standard states. 00:02:05.760 --> 00:02:09.509 So let's just think again about standard states 00:02:09.509 --> 00:02:13.390 so we can talk about delta G0 for the cell and delta 00:02:13.390 --> 00:02:18.210 E0 for the cell-- so delta E0, the cell potential, cell 00:02:18.210 --> 00:02:21.690 voltage, EMF for the cell, in which 00:02:21.690 --> 00:02:25.470 the products and the reactants are in their standard state. 00:02:25.470 --> 00:02:28.170 And the unit here is volts. 00:02:28.170 --> 00:02:30.720 So we have a new unit that we haven't 00:02:30.720 --> 00:02:34.260 used before, I think-- volts. 00:02:34.260 --> 00:02:38.820 So let's look at an example of how we would then calculate 00:02:38.820 --> 00:02:41.017 our delta E0 for a cell. 00:02:41.017 --> 00:02:42.600 And we'll do the cell we've been using 00:02:42.600 --> 00:02:46.980 this whole time, our cell with zinc at anode and copper 00:02:46.980 --> 00:02:48.720 at the cathode. 00:02:48.720 --> 00:02:51.660 And so we have our anode reaction. 00:02:51.660 --> 00:02:56.370 And again, this is our oxidation, anox. 00:02:56.370 --> 00:03:00.730 And our cathode reaction, cathred. 00:03:00.730 --> 00:03:03.990 So we can write it out as the reduction reaction happening 00:03:03.990 --> 00:03:05.850 at the cathode. 00:03:05.850 --> 00:03:10.320 And the equation we're going to use for this is the following. 00:03:10.320 --> 00:03:15.610 So delta E0 of the cell, the cell potential, equals E0, 00:03:15.610 --> 00:03:20.910 and E here is the standard reduction potential-- 00:03:20.910 --> 00:03:23.820 and this is the standard reduction potential 00:03:23.820 --> 00:03:27.180 for the couple, the reaction, that's 00:03:27.180 --> 00:03:32.760 happening at the cathode-- minus the standard reduction 00:03:32.760 --> 00:03:35.880 potential for the reaction that's happening at the anode. 00:03:38.460 --> 00:03:45.630 So now we can look those values in the back of the book. 00:03:45.630 --> 00:03:48.870 So we have for zinc the standard reduction potential, 00:03:48.870 --> 00:03:54.330 minus 0.7629, and the standard reduction 00:03:54.330 --> 00:03:56.520 potential for the copper, plus 2 two 00:03:56.520 --> 00:04:02.500 electrons to copper solid reaction, plus 0.3402 volts. 00:04:02.500 --> 00:04:04.980 So now clicker question, why don't you 00:04:04.980 --> 00:04:08.790 calculate for me what the cell potential is 00:04:08.790 --> 00:04:10.370 for this type of cell. 00:04:24.150 --> 00:04:25.600 So 10 more seconds. 00:04:40.880 --> 00:04:43.220 81%, that's right. 00:04:43.220 --> 00:04:46.980 So let's take a look at that over here. 00:04:46.980 --> 00:04:52.520 So the correct answer involves putting the standard reduction 00:04:52.520 --> 00:04:56.510 potential for the reaction at the cathode with the sign, 00:04:56.510 --> 00:04:59.750 without changing it as plus over here, 00:04:59.750 --> 00:05:04.020 0.3402, and then it's minus. 00:05:04.020 --> 00:05:06.680 And then you put the standard reduction potential in there, 00:05:06.680 --> 00:05:10.490 and this is a minus value minus, 0.7628. 00:05:10.490 --> 00:05:14.210 And minus a minus we get a plus, and so our answer 00:05:14.210 --> 00:05:18.110 is 1.103 volts. 00:05:18.110 --> 00:05:21.430 And this is very important, and it's 00:05:21.430 --> 00:05:24.000 important to think about the reaction 00:05:24.000 --> 00:05:26.440 at the cathode, which is the copper reaction. 00:05:26.440 --> 00:05:28.900 The reaction at the anode is the zinc reaction. 00:05:28.900 --> 00:05:33.070 And a lot of people who try to be too clever with these. 00:05:33.070 --> 00:05:38.440 And they're like, OK, but this one's 00:05:38.440 --> 00:05:40.780 the one that's being oxidized, so I'll switch the sign 00:05:40.780 --> 00:05:44.000 and then put it in the equation and end up getting it wrong. 00:05:44.000 --> 00:05:47.620 So if you always remember standard reduction potential 00:05:47.620 --> 00:05:50.770 put here-- standard reduction potential put here-- 00:05:50.770 --> 00:05:54.410 the equation already takes care of the sign issue for you, 00:05:54.410 --> 00:05:56.140 so don't do anything with the sign. 00:05:56.140 --> 00:05:59.710 Other people say, I looked through pages and pages 00:05:59.710 --> 00:06:03.055 of these potentials in the book, and the reaction 00:06:03.055 --> 00:06:04.930 I'm looking for isn't listed, because they're 00:06:04.930 --> 00:06:06.940 looking for an oxidation. 00:06:06.940 --> 00:06:08.980 They're not going to find the reaction written 00:06:08.980 --> 00:06:13.420 as an oxidation in a table of reduction potentials. 00:06:13.420 --> 00:06:17.030 So all of these are listed as reduction potentials. 00:06:17.030 --> 00:06:18.370 That's what you will find. 00:06:18.370 --> 00:06:20.830 The trick is to remember which is happening 00:06:20.830 --> 00:06:23.560 at the cathode, which is happening at the anode, 00:06:23.560 --> 00:06:26.012 and then you will always get this problem right. 00:06:26.012 --> 00:06:27.970 And it's good to get this right, because as you 00:06:27.970 --> 00:06:30.100 will see later on, this can be step 00:06:30.100 --> 00:06:32.297 one in a multi-part problem. 00:06:34.840 --> 00:06:38.200 So when we successfully calculated 00:06:38.200 --> 00:06:42.940 delta E of the cell, the cell potential, then we can ask, 00:06:42.940 --> 00:06:46.570 is the flow of electrons going to be spontaneous for this? 00:06:46.570 --> 00:06:48.520 Will it? 00:06:48.520 --> 00:06:49.930 It will be. 00:06:49.930 --> 00:06:53.920 And the way we know that is that we can always come back 00:06:53.920 --> 00:06:56.800 to delta G. So whenever you're asked if something 00:06:56.800 --> 00:06:59.917 is spontaneous or not, you always want to think delta G. 00:06:59.917 --> 00:07:01.250 That's what I'm going to go for. 00:07:01.250 --> 00:07:03.070 I'm going to think about delta G. 00:07:03.070 --> 00:07:06.750 So here delta G equals minus n, number of moles of electrons, 00:07:06.750 --> 00:07:10.100 Faraday's constant, times that cell potential. 00:07:10.100 --> 00:07:13.960 So if the cell potential is positive, 00:07:13.960 --> 00:07:16.510 delta G will be negative. 00:07:16.510 --> 00:07:19.998 And is a reaction spontaneous when delta G is negative? 00:07:23.740 --> 00:07:24.850 It's easy to answer. 00:07:24.850 --> 00:07:26.230 It's on my shirt today. 00:07:26.230 --> 00:07:28.450 I'm giving it all the way-- yes. 00:07:28.450 --> 00:07:31.150 So when delta G is negative, then you 00:07:31.150 --> 00:07:32.620 can be feeling spontaneous. 00:07:32.620 --> 00:07:35.180 It's a spontaneous reaction. 00:07:35.180 --> 00:07:38.200 So delta E positive means delta G negative. 00:07:38.200 --> 00:07:41.350 Reaction is spontaneous. 00:07:41.350 --> 00:07:45.640 So now let's have a little reminder, review, 00:07:45.640 --> 00:07:49.330 of what types of cells have spontaneous reactions 00:07:49.330 --> 00:07:52.890 and what kind involve non-spontaneous reaction. 00:08:03.970 --> 00:08:04.860 10 more seconds. 00:08:25.180 --> 00:08:28.000 Can anyone tell me what the correct name 00:08:28.000 --> 00:08:31.930 is of the cell that involves a non-spontaneous reaction? 00:08:31.930 --> 00:08:33.292 Electrolytic cell. 00:08:48.970 --> 00:08:54.360 So if we look over here-- so people knew about the galvanic. 00:08:54.360 --> 00:08:55.750 That's good. 00:08:55.750 --> 00:08:59.470 So a reaction, if it's a spontaneous reaction that 00:08:59.470 --> 00:09:01.090 will produce an electric current, 00:09:01.090 --> 00:09:03.190 that's called a galvanic cell. 00:09:03.190 --> 00:09:05.620 If it's a non-spontaneous reaction that 00:09:05.620 --> 00:09:08.440 has to be driven by applying a current, 00:09:08.440 --> 00:09:10.690 that's called an electrolytic cell. 00:09:10.690 --> 00:09:14.440 So I just threw up another random name 00:09:14.440 --> 00:09:17.470 to see if people would go for it. 00:09:17.470 --> 00:09:20.750 So it's important to keep these in mind, 00:09:20.750 --> 00:09:24.450 because a lot of the problems will say things like, 00:09:24.450 --> 00:09:26.470 in this galvanic cell. 00:09:26.470 --> 00:09:28.780 And you seem like you don't have enough information 00:09:28.780 --> 00:09:31.840 to solve the problem because the problem didn't tell you 00:09:31.840 --> 00:09:34.590 which reaction was at the anode or which reaction was 00:09:34.590 --> 00:09:35.360 that the cathode. 00:09:35.360 --> 00:09:36.790 So how do you do the problem? 00:09:36.790 --> 00:09:40.350 Well, the fact they told you it was a galvanic cell, 00:09:40.350 --> 00:09:44.350 there's only one way to put the reactions in order 00:09:44.350 --> 00:09:45.997 for it to be spontaneous. 00:09:45.997 --> 00:09:47.830 So that's part of the problem-- figuring out 00:09:47.830 --> 00:09:49.480 which is at the anode and which is 00:09:49.480 --> 00:09:52.180 at the cathode based on the fact that it has 00:09:52.180 --> 00:09:54.200 to be a spontaneous reaction. 00:09:54.200 --> 00:09:56.200 So knowing these terms is really important. 00:09:56.200 --> 00:09:57.955 It's hard to do the problems without them. 00:10:00.470 --> 00:10:05.780 Summary of this part, then, so a cell operate spontaneously. 00:10:05.780 --> 00:10:07.760 Whether it does or not can be determined 00:10:07.760 --> 00:10:10.220 by your cell potential. 00:10:10.220 --> 00:10:11.960 If it's positive, it's spontaneous. 00:10:11.960 --> 00:10:15.800 And that's because if it's positive, delta G is negative. 00:10:15.800 --> 00:10:19.730 And delta G is really what tells you about spontaneity. 00:10:19.730 --> 00:10:22.550 And you can calculate your cell potential 00:10:22.550 --> 00:10:25.520 from your standard reduction potentials 00:10:25.520 --> 00:10:28.580 that you will find in your book. 00:10:28.580 --> 00:10:30.590 So now let's think about the meaning 00:10:30.590 --> 00:10:32.390 of the standard reduction potential. 00:10:32.390 --> 00:10:34.410 What's true if it's a large positive value? 00:10:34.410 --> 00:10:38.300 What's true if it is a large negative value? 00:10:38.300 --> 00:10:40.820 We can look at these values and know something 00:10:40.820 --> 00:10:42.710 about the reactions. 00:10:42.710 --> 00:10:46.520 So meaning of standard reduction potentials-- what do the values 00:10:46.520 --> 00:10:47.750 tell us? 00:10:47.750 --> 00:10:51.800 So a large positive standard reduction potential 00:10:51.800 --> 00:10:55.460 means that the element is easy to reduce. 00:10:55.460 --> 00:10:57.800 So let's look at an example. 00:10:57.800 --> 00:11:03.050 So we have fluorine F2 gas plus two electrons 00:11:03.050 --> 00:11:07.190 going 2 fluorine minus ions. 00:11:07.190 --> 00:11:09.870 And this has a standard reduction potential 00:11:09.870 --> 00:11:13.340 of plus 2.87 volts. 00:11:13.340 --> 00:11:17.120 So large positive number means that it's 00:11:17.120 --> 00:11:19.320 easy to add electrons. 00:11:19.320 --> 00:11:21.170 And we can think about this in terms of what 00:11:21.170 --> 00:11:23.220 we know about these reactions. 00:11:23.220 --> 00:11:26.635 So again, if it's a positive standard reduction potential, 00:11:26.635 --> 00:11:29.390 E0, that's going to mean a negative delta 00:11:29.390 --> 00:11:31.910 G for this reaction as written, written 00:11:31.910 --> 00:11:33.830 as the reduction reaction. 00:11:33.830 --> 00:11:37.200 And so it's favorable or spontaneous in that direction. 00:11:37.200 --> 00:11:40.250 So the reduction is spontaneous or favorable. 00:11:40.250 --> 00:11:42.710 So when you have a large positive value here, 00:11:42.710 --> 00:11:46.550 you can think about, yes, fluorine wants those electrons. 00:11:46.550 --> 00:11:50.986 It spontaneously will grab those electrons and become F-minus. 00:11:50.986 --> 00:11:52.610 And this also makes sense to you if you 00:11:52.610 --> 00:11:54.920 think about your periodic table trends. 00:11:54.920 --> 00:11:57.770 You have fluorine wants to be F-minus. 00:11:57.770 --> 00:11:59.990 It will have its noble gas configuration then. 00:11:59.990 --> 00:12:02.280 It likes getting an extra electron. 00:12:02.280 --> 00:12:04.700 So this should make sense from other things 00:12:04.700 --> 00:12:06.290 that we've talked about. 00:12:06.290 --> 00:12:09.050 So now I could ask the question, and I will, and it's a clicker 00:12:09.050 --> 00:12:13.240 question, does that make F2 a good oxidizing agent or not? 00:12:25.540 --> 00:12:26.340 10 more seconds. 00:12:44.200 --> 00:12:46.920 So the answer is yes. 00:12:46.920 --> 00:12:50.740 F2 is easy to reduce, which makes it a good oxidizing 00:12:50.740 --> 00:12:51.460 agent. 00:12:51.460 --> 00:12:54.040 Remember, it's an agent of oxidation. 00:12:54.040 --> 00:12:58.210 It wants to go out there and oxidize other things. 00:12:58.210 --> 00:13:00.580 It wants to itself be reduced. 00:13:00.580 --> 00:13:02.860 So something that's a good oxidizing agent 00:13:02.860 --> 00:13:03.940 is easy to reduce. 00:13:03.940 --> 00:13:05.140 It wants to be reduced. 00:13:05.140 --> 00:13:08.230 It wants to be an agent of oxidation, 00:13:08.230 --> 00:13:11.020 bringing oxidation to the world. 00:13:11.020 --> 00:13:12.280 So it's easy to reduce. 00:13:12.280 --> 00:13:15.380 It's a good oxidizing agent. 00:13:15.380 --> 00:13:18.870 So in general, you can think about this the following way. 00:13:18.870 --> 00:13:21.870 Let's just like bring the noise down a little bit. 00:13:21.870 --> 00:13:24.340 I know it's a clicker competition day. 00:13:24.340 --> 00:13:27.970 So we have a large positive value here for a reaction. 00:13:27.970 --> 00:13:31.090 For a reduction reaction is written large positive 00:13:31.090 --> 00:13:34.000 standard reduction potential. 00:13:34.000 --> 00:13:37.500 And the couple here, F2 to F-minus 00:13:37.500 --> 00:13:39.580 is the couple we're talking about. 00:13:39.580 --> 00:13:42.520 And we'll say that the oxidized species of that couple 00:13:42.520 --> 00:13:44.500 is very oxidizing. 00:13:44.500 --> 00:13:47.912 And again, F2 here is the oxidized species. 00:13:50.950 --> 00:13:54.970 So the oxidized species F2 is a good oxidizing agent. 00:13:54.970 --> 00:13:58.900 It's very oxidizing because it has a large positive 00:13:58.900 --> 00:14:02.120 standard reduction potential. 00:14:02.120 --> 00:14:04.510 So I don't know how well this copied. 00:14:04.510 --> 00:14:07.420 But here are some standard reduction potentials. 00:14:07.420 --> 00:14:11.560 They're all in your book, so it's OK if they look terrible. 00:14:11.560 --> 00:14:13.840 But the important point here is that the top. 00:14:13.840 --> 00:14:16.236 We have large positive values. 00:14:16.236 --> 00:14:17.860 So if you can see that in your handout, 00:14:17.860 --> 00:14:19.820 this is the reaction we just talked about, 00:14:19.820 --> 00:14:25.060 or the couple we just talked about-- F2 F-minus and so 00:14:25.060 --> 00:14:26.500 large positive. 00:14:26.500 --> 00:14:28.360 And then there's a gap here. 00:14:28.360 --> 00:14:30.250 There are pages and pages and pages 00:14:30.250 --> 00:14:32.410 of these standard reduction potentials. 00:14:32.410 --> 00:14:36.010 And at the very bottom, you have a large negative number, 00:14:36.010 --> 00:14:39.520 so large positive on the top large, negative on the bottom. 00:14:39.520 --> 00:14:44.830 So at the very top, the oxidized species of that couple 00:14:44.830 --> 00:14:46.930 is very oxidizing. 00:14:46.930 --> 00:14:50.980 And we'll see it in a minute, that at the bottom 00:14:50.980 --> 00:14:55.090 the reduced species, when it's a negative number here, 00:14:55.090 --> 00:14:59.010 the reduced species is very reducing. 00:14:59.010 --> 00:15:00.720 So let's look at the reduced species, 00:15:00.720 --> 00:15:03.210 and this is lithium on the bottom. 00:15:03.210 --> 00:15:07.320 So a large negative standard reduction potential 00:15:07.320 --> 00:15:10.110 means that the element is hard to reduce. 00:15:10.110 --> 00:15:13.110 So example, lithium plus 1 plus 1 electron 00:15:13.110 --> 00:15:15.570 going to lithium solid. 00:15:15.570 --> 00:15:18.180 The standard reduction potential for this reaction 00:15:18.180 --> 00:15:22.800 is minus 3.045 volts. 00:15:22.800 --> 00:15:27.390 So it's hard to add electrons to lithium-plus. 00:15:27.390 --> 00:15:30.730 We have a negative standard reduction potential, 00:15:30.730 --> 00:15:33.330 which means a positive delta G for the reaction 00:15:33.330 --> 00:15:35.450 is written for the reduction reaction. 00:15:35.450 --> 00:15:36.390 So it's not favorable. 00:15:36.390 --> 00:15:39.280 Lithium-plus does not want electrons. 00:15:39.280 --> 00:15:41.640 It doesn't want to go to lithium solid. 00:15:41.640 --> 00:15:44.790 So lithium-plus, if it loses an electron, 00:15:44.790 --> 00:15:47.340 it gets its nice noble gas configuration. 00:15:47.340 --> 00:15:48.570 It's a group-one element. 00:15:48.570 --> 00:15:52.800 It likes to be in the plus-1 oxidation state. 00:15:52.800 --> 00:15:55.410 It doesn't want to be reduced. 00:15:55.410 --> 00:16:00.960 So is lithium plus 1 a good oxidizing agent? 00:16:00.960 --> 00:16:02.730 No, it's not. 00:16:02.730 --> 00:16:04.440 It does not want to be reduced. 00:16:04.440 --> 00:16:10.920 But lithium solid is a good reducing agent. 00:16:10.920 --> 00:16:13.530 It's a good reducing agent because it 00:16:13.530 --> 00:16:15.270 wants to reduce other things. 00:16:15.270 --> 00:16:19.380 It wants to itself become oxidized. 00:16:19.380 --> 00:16:22.680 So lithium solid will reduce other things. 00:16:22.680 --> 00:16:25.020 Lithium solid wants to be oxidized. 00:16:25.020 --> 00:16:29.140 It wants to be lithium plus 1. 00:16:29.140 --> 00:16:33.540 So the rule here for those large, negative standard 00:16:33.540 --> 00:16:37.220 reduction potential couples is that 00:16:37.220 --> 00:16:39.480 with a large negative standard reduction 00:16:39.480 --> 00:16:43.830 potential, the reduced species is very reducing. 00:16:43.830 --> 00:16:46.470 The reduced species here is lithium. 00:16:46.470 --> 00:16:50.580 So lithium is reduced compared to lithium plus 1. 00:16:50.580 --> 00:16:53.480 This, then, is very reducing. 00:16:53.480 --> 00:16:56.220 It is a good reducing agent. 00:16:56.220 --> 00:17:01.110 Do you think lithium plus 1 would be a good reducing agent? 00:17:01.110 --> 00:17:01.860 What do you think? 00:17:05.510 --> 00:17:07.894 So if it was a good reducing agent, 00:17:07.894 --> 00:17:10.819 it wants to reduce other things and become oxidized. 00:17:10.819 --> 00:17:14.970 Do you think lithium plus 2 is a good thing? 00:17:14.970 --> 00:17:15.980 No. 00:17:15.980 --> 00:17:20.270 So again, in this couple, it's the reduced species 00:17:20.270 --> 00:17:21.920 that's very reducing. 00:17:21.920 --> 00:17:24.619 And so you have to think about this a little bit. 00:17:24.619 --> 00:17:26.599 Make sure that you pick the right one. 00:17:26.599 --> 00:17:29.830 Often you'll ask about rank the order 00:17:29.830 --> 00:17:32.420 of these as reducing agents. 00:17:32.420 --> 00:17:34.970 And people remember this, but don't 00:17:34.970 --> 00:17:36.979 remember which species of the couple 00:17:36.979 --> 00:17:38.270 is going to be doing the thing. 00:17:38.270 --> 00:17:40.250 So think about what's happening. 00:17:40.250 --> 00:17:43.790 Lithium solid, does it want to become lithium plus 1? 00:17:43.790 --> 00:17:44.420 It sure does. 00:17:44.420 --> 00:17:46.940 Does lithium plus 1 want to become lithium plus 2? 00:17:46.940 --> 00:17:48.710 No. 00:17:48.710 --> 00:17:50.770 So again, back to just our table just 00:17:50.770 --> 00:17:54.650 for a second, on the top-- large positive, oxidized species very 00:17:54.650 --> 00:18:00.572 oxidizing, negative value, reduced species very reducing. 00:18:00.572 --> 00:18:02.030 And now if we look at this, I don't 00:18:02.030 --> 00:18:03.821 know how well this came out in your handout 00:18:03.821 --> 00:18:06.800 either, but part of the periodic table 00:18:06.800 --> 00:18:08.660 for you kind of compressed. 00:18:08.660 --> 00:18:13.780 Over here we have fluorine, large positive, standard 00:18:13.780 --> 00:18:17.350 reduction potential, easy to reduce over here, 00:18:17.350 --> 00:18:19.250 good oxidizing agents. 00:18:19.250 --> 00:18:21.470 If they get an electron, if they're reduced, 00:18:21.470 --> 00:18:23.750 they get their noble gas configuration. 00:18:23.750 --> 00:18:27.500 Over here, we have these big negative numbers, 00:18:27.500 --> 00:18:32.930 easy to oxidize the solid or the neutral to their plus 1. 00:18:32.930 --> 00:18:35.379 So they would be good reducing agents. 00:18:35.379 --> 00:18:37.670 And so this all makes sense when you think about trends 00:18:37.670 --> 00:18:40.250 in the periodic table. 00:18:40.250 --> 00:18:45.290 So today we're going to have two examples of why all of this 00:18:45.290 --> 00:18:48.950 is important across disciplines. 00:18:48.950 --> 00:18:52.030 So often these units are really talked-- 00:18:52.030 --> 00:18:55.670 you talk about making batteries and things like that. 00:18:55.670 --> 00:18:56.870 And that is important. 00:18:56.870 --> 00:18:59.600 In fact, with energy initiatives, 00:18:59.600 --> 00:19:02.421 making batteries is a really hot area right now. 00:19:02.421 --> 00:19:03.920 But all these things you're learning 00:19:03.920 --> 00:19:08.220 are also related to biology and to medicine as well. 00:19:08.220 --> 00:19:11.770 And so we have one more of the "In Their Own Words" segment. 00:19:11.770 --> 00:19:17.510 And John Essigmann studies DNA damage as related to cancer. 00:19:17.510 --> 00:19:21.500 And people are like, what is DNA damage and cancer 00:19:21.500 --> 00:19:23.990 have to do with oxidation reduction potentials? 00:19:23.990 --> 00:19:26.600 But it's not all about oxidation reduction. 00:19:26.600 --> 00:19:29.150 That's what a lot of the damage is. 00:19:29.150 --> 00:19:32.570 So why don't you hear from John Essigmann. 00:19:39.620 --> 00:19:40.860 [VIDEO PLAYBACK] 00:19:40.860 --> 00:19:42.200 - My name's John Essigmann. 00:19:42.200 --> 00:19:43.770 I'm in the Department of Chemistry 00:19:43.770 --> 00:19:46.000 and Biological Engineering MIT. 00:19:46.000 --> 00:19:49.800 I'm a toxicologist by training, so I study how cells respond 00:19:49.800 --> 00:19:52.790 to toxins, and in particular, the kinds of toxins 00:19:52.790 --> 00:19:56.637 that cause nucleic acid damage. 00:19:56.637 --> 00:19:58.970 Some of these are chemicals from out in the environment, 00:19:58.970 --> 00:20:01.190 organic compounds, that cause DNA damage. 00:20:01.190 --> 00:20:05.240 But I'm also interested in how chemicals inside our body, 00:20:05.240 --> 00:20:07.075 for example, reactive forms of oxygen, 00:20:07.075 --> 00:20:12.870 the molecule that we breathe, also cause nucleic acid damage. 00:20:12.870 --> 00:20:14.760 Oxidation is the process by which 00:20:14.760 --> 00:20:17.680 we convert our metabolic fuels into energy. 00:20:17.680 --> 00:20:21.210 This is a gummy bear being oxidized by potassium chlorate. 00:20:21.210 --> 00:20:23.720 This is exactly the kind of chemical reaction 00:20:23.720 --> 00:20:25.190 that goes on in your bodies. 00:20:25.190 --> 00:20:29.210 But in your body, it goes on at a much lower rate. 00:20:29.210 --> 00:20:33.230 Oxygen is a double-edged sword. 00:20:33.230 --> 00:20:37.530 There are two aspects to its use that is good for us. 00:20:41.370 --> 00:20:44.545 The first one is by acting as an electron acceptor, 00:20:44.545 --> 00:20:47.070 sort of like an anode in a battery, 00:20:47.070 --> 00:20:48.930 it completes the circuit and allows 00:20:48.930 --> 00:20:52.560 us to be able to generate free energy that we can 00:20:52.560 --> 00:20:55.280 use to power vital processes. 00:20:55.280 --> 00:20:57.060 That's good. 00:20:57.060 --> 00:21:01.890 The second good thing is that it's the ammunition 00:21:01.890 --> 00:21:03.540 of our innate immune system. 00:21:03.540 --> 00:21:06.810 [COUGHING] 00:21:06.810 --> 00:21:10.080 Oxygen is used to charge the system 00:21:10.080 --> 00:21:14.640 to produce one, two, or three electron-reduced species 00:21:14.640 --> 00:21:17.950 that themselves can react with reactive nitrogen species 00:21:17.950 --> 00:21:24.260 to create a host of very damaging oxidants. 00:21:24.260 --> 00:21:27.120 Our innate immune system uses these to protect us 00:21:27.120 --> 00:21:30.330 against invaders and oftentimes, we 00:21:30.330 --> 00:21:34.730 believe, developing cancer cells. 00:21:34.730 --> 00:21:37.970 The bad thing is that the second system I described 00:21:37.970 --> 00:21:41.700 doesn't have a lot of accuracy in how it 00:21:41.700 --> 00:21:44.250 focuses that chemical warfare. 00:21:44.250 --> 00:21:46.740 So not only are the, let's say, invading 00:21:46.740 --> 00:21:49.620 bacterial cells or the cancer cells hit, 00:21:49.620 --> 00:21:52.200 but surrounding cells as well. 00:21:52.200 --> 00:21:54.710 And that creates oxidative damage 00:21:54.710 --> 00:21:58.970 in the genomes of those cells, those innocent bystanders. 00:21:58.970 --> 00:22:02.540 If those cells divide, the oxidative damage that's in them 00:22:02.540 --> 00:22:07.335 can cause mutations, and that sets the cell along a path. 00:22:07.335 --> 00:22:11.349 If you accumulate enough mutations in enough genes, 00:22:11.349 --> 00:22:13.015 you're going to convert that normal cell 00:22:13.015 --> 00:22:15.980 into a cancer cell that will grow out into a tumor. 00:22:20.267 --> 00:22:20.850 [END PLAYBACK] 00:22:20.850 --> 00:22:22.900 CATHERINE DRENNAN: So that was one of, I think, 00:22:22.900 --> 00:22:26.100 the only video, besides mine, that involved a faculty member. 00:22:26.100 --> 00:22:28.150 But I thought it's nice to have a sort 00:22:28.150 --> 00:22:30.880 of a collection of different folks talking 00:22:30.880 --> 00:22:32.230 about their research. 00:22:32.230 --> 00:22:35.410 And this relates to free radical species 00:22:35.410 --> 00:22:39.670 as well as to oxidation and reduction. 00:22:39.670 --> 00:22:42.820 So now I have another bio example, which hopefully we'll 00:22:42.820 --> 00:22:44.120 get to at the end. 00:22:44.120 --> 00:22:47.200 But I want to introduce a very important equation 00:22:47.200 --> 00:22:49.530 for this unit and oxidation reduction, which 00:22:49.530 --> 00:22:51.350 is the Nernst equation. 00:22:51.350 --> 00:22:55.240 So here, and this happens, I think, to all of us 00:22:55.240 --> 00:22:59.410 all the time-- our exhausted batteries. 00:22:59.410 --> 00:23:01.450 So when you have a battery or you 00:23:01.450 --> 00:23:05.090 go to turn on your headphones or use your laser pointer 00:23:05.090 --> 00:23:08.030 or things like that and the battery doesn't work anymore, 00:23:08.030 --> 00:23:11.500 it's because the cell reaction has reached equilibrium. 00:23:11.500 --> 00:23:14.380 So equilibrium is a state of calmness. 00:23:14.380 --> 00:23:17.950 It's also a state of batteries not working anymore. 00:23:17.950 --> 00:23:23.790 And so at this point, when it's at equilibrium, 00:23:23.790 --> 00:23:26.170 the cell generates zero potential 00:23:26.170 --> 00:23:27.430 across its electrodes. 00:23:27.430 --> 00:23:29.500 It's not working anymore. 00:23:29.500 --> 00:23:31.390 It is a dead battery. 00:23:31.390 --> 00:23:35.200 And we need to think about when it reaches equilibrium, 00:23:35.200 --> 00:23:38.110 what state it's in at any given time compared 00:23:38.110 --> 00:23:39.717 to its equilibrium state. 00:23:39.717 --> 00:23:41.800 So this really brings us back to some of the ideas 00:23:41.800 --> 00:23:44.310 that we had in the chemical equilibrium unit, 00:23:44.310 --> 00:23:47.800 where we were thinking about composition of a reaction 00:23:47.800 --> 00:23:51.860 at any given time compared to the equilibrium state. 00:23:51.860 --> 00:23:58.030 So how does cell potential change with cell composition? 00:23:58.030 --> 00:24:00.060 So again, we know a lot about this. 00:24:00.060 --> 00:24:04.080 And exam three on Friday has equilibrium on it. 00:24:04.080 --> 00:24:07.810 So we already know a lot about what happens in relationships 00:24:07.810 --> 00:24:10.490 between equilibrium. 00:24:10.490 --> 00:24:14.110 And we know that delta G changes as the composition 00:24:14.110 --> 00:24:15.700 changes in the cell. 00:24:15.700 --> 00:24:18.330 And until equilibrium is reached-- so again, 00:24:18.330 --> 00:24:20.920 reactions going forward and back in equilibrium-- 00:24:20.920 --> 00:24:23.070 the forward direction equals the reverse direction. 00:24:23.070 --> 00:24:27.030 But at some given time that is not always the case. 00:24:27.030 --> 00:24:30.940 So the equation that we know from before is 00:24:30.940 --> 00:24:36.370 that delta G equals delta G0 plus RT natural log of Q. 00:24:36.370 --> 00:24:39.050 And so if you know delta G and Q, 00:24:39.050 --> 00:24:41.140 you can talk about what the delta G is going 00:24:41.140 --> 00:24:44.580 to be at a particular composition of the cell 00:24:44.580 --> 00:24:49.480 with a particular value of Q. So now we 00:24:49.480 --> 00:24:53.140 can think, well, OK, here's what we know about Q, that reaction 00:24:53.140 --> 00:24:55.210 quotient, and delta G. 00:24:55.210 --> 00:25:00.490 What do we know about delta G and E, the cell potential? 00:25:00.490 --> 00:25:02.300 And we just talked about that. 00:25:02.300 --> 00:25:04.960 So we know that delta G0 equals minus n, 00:25:04.960 --> 00:25:06.370 the number of moles of electrons, 00:25:06.370 --> 00:25:09.470 Faraday's constant times that cell potential. 00:25:09.470 --> 00:25:12.120 So now we can do a rearrangement of terms. 00:25:12.120 --> 00:25:16.180 We can combine and come up with the Nernst equation. 00:25:16.180 --> 00:25:17.650 So here are the two equations. 00:25:17.650 --> 00:25:18.700 One we knew before. 00:25:18.700 --> 00:25:21.220 One we just learned about today. 00:25:21.220 --> 00:25:23.280 And we can put those together. 00:25:23.280 --> 00:25:27.490 We can plug in for delta G here without the 0, 00:25:27.490 --> 00:25:30.400 and for delta G0 with the 0 over here. 00:25:30.400 --> 00:25:33.120 And then we can divide both sides 00:25:33.120 --> 00:25:35.290 by minus the number of moles of electrons 00:25:35.290 --> 00:25:38.980 and Faraday's constant, come up with the Nernst equation. 00:25:38.980 --> 00:25:43.810 So the Nernst equation compares our cell potential 00:25:43.810 --> 00:25:46.620 under standard conditions with our cell potential 00:25:46.620 --> 00:25:52.770 at any given time based on that reaction quotient Q. 00:25:52.770 --> 00:25:56.970 So let's do an example with the Nernst equation. 00:25:56.970 --> 00:26:02.400 So let's calculate the cell potential at a particular time 00:26:02.400 --> 00:26:09.510 when the zinc plus 2 ions are 0.1 and copper plus 2 is 0.0010 00:26:09.510 --> 00:26:10.470 molar. 00:26:10.470 --> 00:26:13.990 And we're same cell as we've done before. 00:26:13.990 --> 00:26:19.290 So step one, very easy, we want to calculate the cell potential 00:26:19.290 --> 00:26:21.330 under standard conditions. 00:26:21.330 --> 00:26:24.210 And this is particularly easy because we already 00:26:24.210 --> 00:26:25.470 did this today. 00:26:25.470 --> 00:26:30.930 So you already told me it's plus 1.103 volts. 00:26:30.930 --> 00:26:33.930 So we could look up our standard reduction potentials 00:26:33.930 --> 00:26:37.914 for our copper couple and our zinc couple and calculate this. 00:26:37.914 --> 00:26:39.330 So we already did this, and that's 00:26:39.330 --> 00:26:41.540 why I said you want to make sure you get this right 00:26:41.540 --> 00:26:44.730 because it's often just step one of a multi-part problem. 00:26:44.730 --> 00:26:47.140 So we want to get that right. 00:26:47.140 --> 00:26:51.748 Step two is calculating Q, and this is a clicker question. 00:27:14.250 --> 00:27:15.170 10 more seconds. 00:27:31.410 --> 00:27:34.640 I tried to make the math not that hard. 00:27:34.640 --> 00:27:40.240 So Q, products over reactants, so one thing you want to do 00:27:40.240 --> 00:27:43.070 is make sure you have the reaction written correctly. 00:27:43.070 --> 00:27:46.810 So you know what the products are and what the reactants are. 00:27:46.810 --> 00:27:50.680 And here we have the zinc ions over copper. 00:27:50.680 --> 00:27:53.860 Copper solid and zinc solid don't 00:27:53.860 --> 00:27:56.740 appear in the equilibrium expression 00:27:56.740 --> 00:27:59.350 or in the Q expression also because they're solids 00:27:59.350 --> 00:28:01.810 and their concentration is not changing. 00:28:01.810 --> 00:28:03.640 So there is enough information that 00:28:03.640 --> 00:28:06.790 was given because we don't include the solids. 00:28:06.790 --> 00:28:10.870 So it's just the concentration of zinc ions 00:28:10.870 --> 00:28:15.598 over the concentration of copper ions, or 1.0 times 10 to the 2. 00:28:17.830 --> 00:28:20.350 Now we need to know n. 00:28:20.350 --> 00:28:22.990 So n is the number of moles of electrons that 00:28:22.990 --> 00:28:25.060 are involved in this reaction. 00:28:25.060 --> 00:28:27.625 This reaction involves how many electrons? 00:28:31.650 --> 00:28:33.210 How many? 00:28:33.210 --> 00:28:35.330 It involves two, right. 00:28:35.330 --> 00:28:41.490 So the zinc to zinc plus 2, copper plus 2 to copper, 00:28:41.490 --> 00:28:43.590 so it involves two electrons. 00:28:43.590 --> 00:28:47.370 So this is two, and sometimes you can just look at this 00:28:47.370 --> 00:28:50.040 and realize it's two electrons involved. 00:28:50.040 --> 00:28:52.500 Other times you have to balance the reaction 00:28:52.500 --> 00:28:56.710 to figure out how many electrons are involved. 00:28:56.710 --> 00:28:59.730 It's not obvious by looking at the reaction. 00:28:59.730 --> 00:29:01.980 This is a simple one, but some of them are not. 00:29:01.980 --> 00:29:05.170 So that's we're balancing reactions comes in handy. 00:29:05.170 --> 00:29:05.670 so 00:29:05.670 --> 00:29:07.500 Now we have everything we need to plug it 00:29:07.500 --> 00:29:09.360 into the Nernst equation. 00:29:09.360 --> 00:29:10.860 So we can put it in. 00:29:10.860 --> 00:29:15.320 We calculated this standard cell potential 00:29:15.320 --> 00:29:20.130 over here, the plus 1.1 value. 00:29:20.130 --> 00:29:24.870 We can look up R. Everything, again, room temperature. 00:29:24.870 --> 00:29:27.720 Natural log of Q, we calculated Q. 00:29:27.720 --> 00:29:31.230 We have two electrons and Faraday's constant. 00:29:31.230 --> 00:29:36.180 And we can multiply this out, get this value. 00:29:36.180 --> 00:29:39.880 In terms of significant figures, this also is a lot of fun. 00:29:39.880 --> 00:29:43.260 We have log rules of significant figures, multiplication 00:29:43.260 --> 00:29:45.270 and division rules of significant figures, 00:29:45.270 --> 00:29:49.740 and adding and subtracting rules of significant figures, 00:29:49.740 --> 00:29:51.450 and also in terms of units. 00:29:51.450 --> 00:29:54.060 All of a sudden, volts appeared. 00:29:54.060 --> 00:29:56.310 That was nice because our answer should be in volts. 00:29:56.310 --> 00:29:58.320 But where did they come from? 00:29:58.320 --> 00:30:02.220 So we had our kelvins are going to cancel out. 00:30:02.220 --> 00:30:06.000 But we have joules over here. 00:30:06.000 --> 00:30:07.170 And we have coulombs. 00:30:07.170 --> 00:30:10.260 But luckily, a joule per coulomb equals a volt. 00:30:10.260 --> 00:30:14.740 So we're all set to get our appropriate units. 00:30:14.740 --> 00:30:17.190 So just a couple of things that I'll mention about this as 00:30:17.190 --> 00:30:20.160 well-- sine almost all these problems are 00:30:20.160 --> 00:30:22.650 at room temperature, I try to make your lives 00:30:22.650 --> 00:30:28.020 a little bit easier on the exam and multiply this out 00:30:28.020 --> 00:30:30.360 for you, because these are all constants, 00:30:30.360 --> 00:30:32.100 and they're big constants that involve 00:30:32.100 --> 00:30:34.410 plugging a lot of numbers in. 00:30:34.410 --> 00:30:39.030 So I will give you this combined value. 00:30:39.030 --> 00:30:42.480 And if you use log instead of natural log, 00:30:42.480 --> 00:30:45.850 you need to change that value here. 00:30:45.850 --> 00:30:48.600 And so what I actually put on equation sheets 00:30:48.600 --> 00:30:50.120 are these equations. 00:30:50.120 --> 00:30:53.940 I'll also put the full equation there as well but just 00:30:53.940 --> 00:30:56.560 to make your lives a little bit easier. 00:30:56.560 --> 00:31:00.150 It's horrible to make mistakes up there. 00:31:00.150 --> 00:31:06.720 So just briefly, what about at equilibrium, what does Q equal? 00:31:06.720 --> 00:31:13.180 Q equals K. Delta G equals 0. 00:31:13.180 --> 00:31:17.940 And so we can rewrite this expression again, 00:31:17.940 --> 00:31:19.950 which we've done before. 00:31:19.950 --> 00:31:25.820 And if we take this and look at this expression now, 00:31:25.820 --> 00:31:27.590 we can again combine these-- we have 00:31:27.590 --> 00:31:33.700 two expressions for delta G0-- and combine them and solve. 00:31:33.700 --> 00:31:36.410 And you can see that you can also 00:31:36.410 --> 00:31:41.230 calculate K from standard reduction potentials. 00:31:41.230 --> 00:31:45.190 So all the things that we have been learning here, 00:31:45.190 --> 00:31:48.560 we can always come back to chemical equilibrium. 00:31:48.560 --> 00:31:51.960 So I'm done with everything but my second bio example. 00:31:51.960 --> 00:31:54.490 So you'll have to wait until next week 00:31:54.490 --> 00:31:58.376 to find out how vitamin B12 gets reduced in the body. 00:32:10.370 --> 00:32:13.200 Let's just take 10 more seconds on the clicker question. 00:32:36.250 --> 00:32:39.310 Does someone want a nice T-shirt to tell me 00:32:39.310 --> 00:32:40.602 how they got the right answer? 00:32:48.520 --> 00:32:49.587 Pass this back. 00:32:52.330 --> 00:32:54.980 AUDIENCE: So when you're looking at the Pb-- 00:32:54.980 --> 00:32:57.000 CATHERINE DRENNAN: Let's all quiet down. 00:32:57.000 --> 00:33:01.430 AUDIENCE: [INAUDIBLE] and zinc, zinc is going to reduce-- 00:33:01.430 --> 00:33:03.711 CATHERINE DRENNAN: Let's quiet down. 00:33:03.711 --> 00:33:07.700 AUDIENCE: --Pb because it goes from a 2 plus charge 00:33:07.700 --> 00:33:09.330 to no charge at all. 00:33:09.330 --> 00:33:11.600 So that's reducing its charge. 00:33:11.600 --> 00:33:13.690 And then in the reverse reaction, 00:33:13.690 --> 00:33:17.060 Pb does the same thing to zinc 2 plus 00:33:17.060 --> 00:33:22.325 and takes it from 2 plus to a neutral charge. 00:33:22.325 --> 00:33:23.575 CATHERINE DRENNAN: Yep, great. 00:33:26.180 --> 00:33:28.670 So the trick to this is just look at the equations 00:33:28.670 --> 00:33:31.310 and figure out what is being oxidized 00:33:31.310 --> 00:33:33.560 and what is being reduced in both directions. 00:33:37.260 --> 00:33:41.370 Oxidation reduction, it's not just about batteries. 00:33:41.370 --> 00:33:43.730 Batteries, of course, are very important. 00:33:43.730 --> 00:33:45.800 There's a lot of active research right now 00:33:45.800 --> 00:33:48.750 in trying to make better batteries that are more 00:33:48.750 --> 00:33:52.980 environmentally friendly, just solving the energy problem 00:33:52.980 --> 00:33:55.770 every single possible way. 00:33:55.770 --> 00:33:58.140 But even if you're not interested in solving 00:33:58.140 --> 00:34:00.650 the energy problem, oxidation reduction 00:34:00.650 --> 00:34:02.570 is really important in medicine. 00:34:02.570 --> 00:34:05.900 So I'm just going to give you another bio example for this. 00:34:05.900 --> 00:34:09.230 So there's a vitamin in your body, vitamin B12 00:34:09.230 --> 00:34:13.230 It has one of the largest negative reduction potentials 00:34:13.230 --> 00:34:15.679 of any biological molecule. 00:34:15.679 --> 00:34:18.710 So we ask the question, how can something like that 00:34:18.710 --> 00:34:20.730 be reduced in the body? 00:34:20.730 --> 00:34:22.670 So let me introduce you to vitamin B12. 00:34:22.670 --> 00:34:25.610 You're going to see this a couple of times today. 00:34:25.610 --> 00:34:29.630 So vitamin B12 needs to be reduced to be active. 00:34:29.630 --> 00:34:31.686 But it has a low reduction potential. 00:34:34.230 --> 00:34:35.850 This is the vitamin B12, and you'll 00:34:35.850 --> 00:34:38.060 see several more views of it, and it's 00:34:38.060 --> 00:34:42.620 in pictures in your handout of today's lecture. 00:34:42.620 --> 00:34:45.420 There's an enzyme that requires vitamin B12 and another B 00:34:45.420 --> 00:34:48.060 vitamin called folic acid. 00:34:48.060 --> 00:34:51.440 And if this enzyme is not functioning properly 00:34:51.440 --> 00:34:54.030 in your body, it leads to all sorts of things. 00:34:54.030 --> 00:34:56.550 It's connected to heart disease. 00:34:56.550 --> 00:34:59.420 If women don't have enough folic acid while they're pregnant, 00:34:59.420 --> 00:35:02.360 it can lead to birth defects, neural tube disease, 00:35:02.360 --> 00:35:05.350 where the brain doesn't form properly. 00:35:05.350 --> 00:35:10.370 Also, there is rising evidence connecting a vitamin B12 00:35:10.370 --> 00:35:14.480 deficiency with bringing on Alzheimer's disease. 00:35:14.480 --> 00:35:18.270 And some people actually buy little B12 squirt bottles 00:35:18.270 --> 00:35:21.020 and squirt it into their nose before exams. 00:35:21.020 --> 00:35:22.920 I'm not sure that helps. 00:35:22.920 --> 00:35:25.610 But as you get older, I would seriously 00:35:25.610 --> 00:35:29.330 consider taking extra vitamin B12. 00:35:29.330 --> 00:35:32.190 So since we're talking about this, 00:35:32.190 --> 00:35:34.920 we'll have a little nutrition quiz. 00:35:34.920 --> 00:35:38.790 Where do you get vitamin B12 and folic acid in your diet? 00:35:38.790 --> 00:35:40.920 First let's think about vitamin B12. 00:35:40.920 --> 00:35:43.670 Does anyone know where you get this, besides vitamin pills, 00:35:43.670 --> 00:35:44.630 in food? 00:35:44.630 --> 00:35:46.689 Where do you get vitamin B12 in food, yeah? 00:35:46.689 --> 00:35:47.480 AUDIENCE: Broccoli? 00:35:47.746 --> 00:35:49.079 CATHERINE DRENNAN: Broccoli, no. 00:35:53.480 --> 00:35:55.420 We had a hand up there. 00:35:55.420 --> 00:35:56.770 Anyone else? 00:35:56.770 --> 00:35:58.436 Yeah. 00:35:58.436 --> 00:35:59.060 AUDIENCE: Meat? 00:35:59.060 --> 00:36:02.270 CATHERINE DRENNAN: Meat, yes-- red meat, actually 00:36:02.270 --> 00:36:03.150 any kind of meat. 00:36:03.150 --> 00:36:06.110 But hardly ever is red meat the answer 00:36:06.110 --> 00:36:07.710 to a nutritional question. 00:36:07.710 --> 00:36:11.870 So I thought I'd put it up there in big bold red letters. 00:36:11.870 --> 00:36:15.910 Yeah, you get B12 in meat. 00:36:15.910 --> 00:36:18.250 Plants don't use vitamin B12. 00:36:18.250 --> 00:36:20.440 So you can eat as many plants as you want. 00:36:20.440 --> 00:36:23.360 Unless they're covered with bacteria that do use B12, 00:36:23.360 --> 00:36:25.640 you will not get any in your diet. 00:36:25.640 --> 00:36:28.360 So if you're vegetarian, you should take a vitamin tablet. 00:36:28.360 --> 00:36:31.150 You don't need much, so it's not really your problem. 00:36:31.150 --> 00:36:32.860 But it does come from red meat. 00:36:32.860 --> 00:36:34.480 What about folic acid? 00:36:34.480 --> 00:36:38.461 Any guesses where folic acid comes from? 00:36:38.461 --> 00:36:38.960 Yeah. 00:36:38.960 --> 00:36:40.120 AUDIENCE: Broccoli? 00:36:40.120 --> 00:36:41.950 CATHERINE DRENNAN: Yes. 00:36:41.950 --> 00:36:43.230 Very good. 00:36:43.230 --> 00:36:46.600 So it's the fall in New England, and hopefully some of you 00:36:46.600 --> 00:36:49.760 have gone out and seen the foliage. 00:36:49.760 --> 00:36:54.300 So folic acid does come from green leafy things. 00:36:54.300 --> 00:36:59.820 And if you're Norwegian, you will also say the following. 00:36:59.820 --> 00:37:05.140 So Norwegian scientists claim that the great longevity 00:37:05.140 --> 00:37:07.120 of Norwegians has to do with the fact 00:37:07.120 --> 00:37:09.370 that their beer is rich in folic acid. 00:37:09.370 --> 00:37:11.020 And somehow they even got a grant 00:37:11.020 --> 00:37:14.200 from the Norwegian government to study the correlation 00:37:14.200 --> 00:37:18.640 between beer and health. 00:37:18.640 --> 00:37:21.400 Everyone signed up for that study, I'm sure. 00:37:21.400 --> 00:37:25.000 So red meat and beer for good health. 00:37:25.000 --> 00:37:29.650 But yes, leafy green vegetables, yes, folic acid, 00:37:29.650 --> 00:37:32.770 foliage, also orange juice is really high. 00:37:32.770 --> 00:37:36.970 And at one point, they had this commercial for orange juice. 00:37:36.970 --> 00:37:39.190 It's not just for breakfast anymore. 00:37:39.190 --> 00:37:41.210 It's good for your heart. 00:37:41.210 --> 00:37:43.300 Well, that was because of the folic acid in there. 00:37:43.300 --> 00:37:44.820 And that's actually true. 00:37:44.820 --> 00:37:47.980 So there was a commercial that actually provided you 00:37:47.980 --> 00:37:50.740 with valuable nutritional advice. 00:37:50.740 --> 00:37:52.370 Not so sure about the beer, but I'd 00:37:52.370 --> 00:37:56.260 go buy the leafy greens and the orange juice. 00:37:56.260 --> 00:37:59.110 So you need both of these to be healthy. 00:37:59.110 --> 00:38:01.930 But again, we have this problem that vitamin B12 00:38:01.930 --> 00:38:04.640 has this low negative redox potential. 00:38:04.640 --> 00:38:06.520 So how is it reduced? 00:38:06.520 --> 00:38:09.700 Well, in the body it's reduced by a protein called 00:38:09.700 --> 00:38:14.020 flavodoxin, which has a flavon, which is another B vitamin. 00:38:14.020 --> 00:38:16.750 So there's lots of B vitamins going on here. 00:38:16.750 --> 00:38:20.340 So the standard reduction potential for vitamin B12 00:38:20.340 --> 00:38:22.480 is minus 0.526. 00:38:22.480 --> 00:38:25.320 That's a really low number for biology. 00:38:25.320 --> 00:38:29.680 And for flavodoxin, it's minus 0.23. 00:38:29.680 --> 00:38:34.201 So we ask, which is the better reducing agent? 00:38:34.201 --> 00:38:35.534 And that's the clicker question. 00:38:46.970 --> 00:38:48.405 Let's just do 10 more seconds. 00:39:03.480 --> 00:39:05.940 Yep. 00:39:05.940 --> 00:39:09.190 So the better reducing agent wants to reduce other things 00:39:09.190 --> 00:39:11.940 and get oxidized itself. 00:39:11.940 --> 00:39:17.800 So vitamin B12, with its low negative potential, 00:39:17.800 --> 00:39:19.780 is very reducing. 00:39:19.780 --> 00:39:21.280 It wants to reduce other things. 00:39:21.280 --> 00:39:22.910 It wants to be an agent of reduction. 00:39:26.020 --> 00:39:28.040 But that's not what's supposed to happen. 00:39:28.040 --> 00:39:30.370 It's not supposed to reduce flavodoxin. 00:39:30.370 --> 00:39:32.590 It's supposed to happen the other way around. 00:39:32.590 --> 00:39:36.190 So we could ask, is the reduction, then, 00:39:36.190 --> 00:39:40.390 of vitamin B13 by flavodoxin spontaneous 00:39:40.390 --> 00:39:43.440 if this is the case, yes or no? 00:39:43.440 --> 00:39:44.830 No. 00:39:44.830 --> 00:39:46.360 It shouldn't be spontaneous. 00:39:46.360 --> 00:39:49.170 It's not going the way that it should go. 00:39:49.170 --> 00:39:54.130 So we can just calculate how exactly non-spontaneous it is. 00:39:54.130 --> 00:40:00.340 So we can calculate both what the change in potential 00:40:00.340 --> 00:40:05.290 is, and we can also calculate delta G0 from this. 00:40:05.290 --> 00:40:08.540 So we saw how to do this before. 00:40:08.540 --> 00:40:10.870 But we were talking about different kinds 00:40:10.870 --> 00:40:13.360 of galvanic or electrolytic cells. 00:40:13.360 --> 00:40:16.510 But now we can use the same equation 00:40:16.510 --> 00:40:19.060 that we saw before but talk about it in terms 00:40:19.060 --> 00:40:20.890 of a biological system. 00:40:20.890 --> 00:40:25.870 So before we saw to calculate E0 of the cell, the cell 00:40:25.870 --> 00:40:28.960 potential, we had the standard reduction potential 00:40:28.960 --> 00:40:31.600 for the couple at the anode minus the standard reduction 00:40:31.600 --> 00:40:33.730 potential for a couple at the cathode. 00:40:33.730 --> 00:40:36.100 We can do the same thing here but just 00:40:36.100 --> 00:40:40.130 put reduction and oxidation over here. 00:40:40.130 --> 00:40:42.760 So we can make this a generic equation. 00:40:42.760 --> 00:40:45.250 And we can plug our values in. 00:40:45.250 --> 00:40:47.980 So the thing being reduced is vitamin B12. 00:40:47.980 --> 00:40:51.550 So we want to put the reduction potential of B12 in there. 00:40:51.550 --> 00:40:55.570 And the thing being oxidized is the flavodoxin. 00:40:55.570 --> 00:40:59.110 And so minus 0.526six for the B12. 00:40:59.110 --> 00:41:00.700 Standard reduction potential would 00:41:00.700 --> 00:41:03.340 be 12 minus the standard reduction 00:41:03.340 --> 00:41:05.230 potential of flavodoxin. 00:41:05.230 --> 00:41:10.810 And we get a negative number, minus 0.0296 volts. 00:41:10.810 --> 00:41:13.980 So then we have a negative value, 00:41:13.980 --> 00:41:17.730 which should tell us that delta G is going to be positive. 00:41:17.730 --> 00:41:20.500 But we can also calculate that value 00:41:20.500 --> 00:41:22.570 from the equation you saw before. 00:41:22.570 --> 00:41:26.200 So just because we are talking about these equations in terms 00:41:26.200 --> 00:41:28.540 of cells before, we can also talk 00:41:28.540 --> 00:41:32.170 about it's same exact equation that we use in biology. 00:41:32.170 --> 00:41:35.540 So minus n, number of moles of electrons-- 00:41:35.540 --> 00:41:37.384 this is a one-electron process. 00:41:37.384 --> 00:41:38.800 I would have had to tell you that. 00:41:38.800 --> 00:41:41.470 You wouldn't necessarily have known it's one electron. 00:41:41.470 --> 00:41:43.300 But that's our one. 00:41:43.300 --> 00:41:46.210 What is this term here, this kind of weird-looking F term? 00:41:46.210 --> 00:41:48.080 What is that again? 00:41:48.080 --> 00:41:49.720 Faraday's constant. 00:41:49.720 --> 00:41:52.150 Then we plug in our cell potential 00:41:52.150 --> 00:41:54.170 or potential difference. 00:41:54.170 --> 00:41:57.850 And we get out a value positive 28.6 kilojoules 00:41:57.850 --> 00:42:00.580 per mole-- big positive number. 00:42:00.580 --> 00:42:04.150 So is this spontaneous? 00:42:04.150 --> 00:42:06.710 Why don't we all have heart disease? 00:42:06.710 --> 00:42:09.890 It is not a spontaneous reaction. 00:42:09.890 --> 00:42:11.170 So how is it driven forward ? 00:42:11.170 --> 00:42:13.040 This happens in our body. 00:42:13.040 --> 00:42:14.080 This reaction happens. 00:42:14.080 --> 00:42:18.640 So clearly something has to happen to make it spontaneous. 00:42:18.640 --> 00:42:21.700 So when we were talking about the different kinds of cells, 00:42:21.700 --> 00:42:24.730 we had electrolytic cell, where we could put in a current 00:42:24.730 --> 00:42:27.610 to drive a non-spontaneous reaction. 00:42:27.610 --> 00:42:30.730 Same thing happens in biology, and the current 00:42:30.730 --> 00:42:32.890 in this particular case is a molecule 00:42:32.890 --> 00:42:35.170 known as adenosylmethionine. 00:42:35.170 --> 00:42:38.710 So it's the methionine amino acid within a denosyl group 00:42:38.710 --> 00:42:40.060 attached to it. 00:42:40.060 --> 00:42:44.590 And the cleavage of this molecule is very spontaneous. 00:42:44.590 --> 00:42:49.120 It has a delta G of minus 30.6 kilojoules, which 00:42:49.120 --> 00:42:51.150 is greater than what we need. 00:42:51.150 --> 00:42:54.280 And we can even calculate the net delta G 00:42:54.280 --> 00:42:56.860 for this reaction, the coupled reaction, 00:42:56.860 --> 00:42:59.420 of the cleavage with the reduction. 00:42:59.420 --> 00:43:05.470 And if we do that, we have minus 37.6 plus 28.6, so 00:43:05.470 --> 00:43:09.350 minus 9.0 kilojoules per mole. 00:43:09.350 --> 00:43:13.420 So we can drive this unfavorable reduction reaction 00:43:13.420 --> 00:43:16.030 with a very favorable reaction, the cleavage 00:43:16.030 --> 00:43:17.731 of adenosylmethionine. 00:43:17.731 --> 00:43:18.730 And that's how it works. 00:43:18.730 --> 00:43:22.810 So that's why we're all healthy. 00:43:22.810 --> 00:43:26.020 So this is the end of our oxidation reduction example. 00:43:26.020 --> 00:43:28.980 We'll transition to transition metals. 00:43:28.980 --> 00:43:32.500 But we're not moving very far away from oxidation reduction. 00:43:32.500 --> 00:43:35.710 Because to do problems that are effective transition metals, 00:43:35.710 --> 00:43:38.200 you have to know how to determine oxidation numbers 00:43:38.200 --> 00:43:40.410 and all sorts of things.