1 00:00:00,090 --> 00:00:02,430 The following content is provided under a Creative 2 00:00:02,430 --> 00:00:03,820 Commons license. 3 00:00:03,820 --> 00:00:06,060 Your support will help MIT OpenCourseWare 4 00:00:06,060 --> 00:00:10,150 continue to offer high-quality educational resources for free. 5 00:00:10,150 --> 00:00:12,690 To make a donation or to view additional materials 6 00:00:12,690 --> 00:00:16,445 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,445 --> 00:00:17,070 at ocw.mit.edu. 8 00:00:27,420 --> 00:00:29,760 CATHERINE DRENNAN: Cell potential-- cell 9 00:00:29,760 --> 00:00:34,890 potential delta E cell, and we're 10 00:00:34,890 --> 00:00:39,690 going to relate this back to delta G. 11 00:00:39,690 --> 00:00:44,070 So as we saw with these cells, you have this flow of electrons 12 00:00:44,070 --> 00:00:46,030 through the circuit. 13 00:00:46,030 --> 00:00:49,410 They're generated at the anode from the oxidation reaction, 14 00:00:49,410 --> 00:00:52,080 and they go over to the cathode. 15 00:00:52,080 --> 00:00:53,910 And when your cell is running, it'll 16 00:00:53,910 --> 00:00:57,480 generate a potential difference, this delta E cell, 17 00:00:57,480 --> 00:01:00,600 between those two electrodes in the cell. 18 00:01:00,600 --> 00:01:05,950 So delta E cell has a lot of different names. 19 00:01:05,950 --> 00:01:11,055 It's called cell potential, cell voltage, electron motive force, 20 00:01:11,055 --> 00:01:12,360 EMF. 21 00:01:12,360 --> 00:01:16,350 All of these names are valid, can be used. 22 00:01:16,350 --> 00:01:18,660 I'm going to try to call it cell potential. 23 00:01:18,660 --> 00:01:20,370 Hopefully, I'm not going to switch up. 24 00:01:20,370 --> 00:01:21,870 But if you see any of these, it's 25 00:01:21,870 --> 00:01:23,460 talking about the same thing. 26 00:01:26,410 --> 00:01:31,950 So overall, then, if we know this cell potential, 27 00:01:31,950 --> 00:01:35,130 we can relate this back to delta G. 28 00:01:35,130 --> 00:01:38,280 And so that will then be able to tell us whether the reaction is 29 00:01:38,280 --> 00:01:40,150 spontaneous or not. 30 00:01:40,150 --> 00:01:43,710 So the overall free energy, Gibbs free energy, of the cell 31 00:01:43,710 --> 00:01:46,990 is related to the cell potential by the following equation. 32 00:01:46,990 --> 00:01:49,800 So delta G for the cell minus n, the number of moles 33 00:01:49,800 --> 00:01:53,280 of electrons that pass through the system, Faraday's 34 00:01:53,280 --> 00:01:57,790 constant and that cell potential. 35 00:01:57,790 --> 00:02:01,440 So we can think about this at a particular time. 36 00:02:01,440 --> 00:02:03,810 We can also think about this equation 37 00:02:03,810 --> 00:02:05,760 in the standard states. 38 00:02:05,760 --> 00:02:09,509 So let's just think again about standard states 39 00:02:09,509 --> 00:02:13,390 so we can talk about delta G0 for the cell and delta 40 00:02:13,390 --> 00:02:18,210 E0 for the cell-- so delta E0, the cell potential, cell 41 00:02:18,210 --> 00:02:21,690 voltage, EMF for the cell, in which 42 00:02:21,690 --> 00:02:25,470 the products and the reactants are in their standard state. 43 00:02:25,470 --> 00:02:28,170 And the unit here is volts. 44 00:02:28,170 --> 00:02:30,720 So we have a new unit that we haven't 45 00:02:30,720 --> 00:02:34,260 used before, I think-- volts. 46 00:02:34,260 --> 00:02:38,820 So let's look at an example of how we would then calculate 47 00:02:38,820 --> 00:02:41,017 our delta E0 for a cell. 48 00:02:41,017 --> 00:02:42,600 And we'll do the cell we've been using 49 00:02:42,600 --> 00:02:46,980 this whole time, our cell with zinc at anode and copper 50 00:02:46,980 --> 00:02:48,720 at the cathode. 51 00:02:48,720 --> 00:02:51,660 And so we have our anode reaction. 52 00:02:51,660 --> 00:02:56,370 And again, this is our oxidation, anox. 53 00:02:56,370 --> 00:03:00,730 And our cathode reaction, cathred. 54 00:03:00,730 --> 00:03:03,990 So we can write it out as the reduction reaction happening 55 00:03:03,990 --> 00:03:05,850 at the cathode. 56 00:03:05,850 --> 00:03:10,320 And the equation we're going to use for this is the following. 57 00:03:10,320 --> 00:03:15,610 So delta E0 of the cell, the cell potential, equals E0, 58 00:03:15,610 --> 00:03:20,910 and E here is the standard reduction potential-- 59 00:03:20,910 --> 00:03:23,820 and this is the standard reduction potential 60 00:03:23,820 --> 00:03:27,180 for the couple, the reaction, that's 61 00:03:27,180 --> 00:03:32,760 happening at the cathode-- minus the standard reduction 62 00:03:32,760 --> 00:03:35,880 potential for the reaction that's happening at the anode. 63 00:03:38,460 --> 00:03:45,630 So now we can look those values in the back of the book. 64 00:03:45,630 --> 00:03:48,870 So we have for zinc the standard reduction potential, 65 00:03:48,870 --> 00:03:54,330 minus 0.7629, and the standard reduction 66 00:03:54,330 --> 00:03:56,520 potential for the copper, plus 2 two 67 00:03:56,520 --> 00:04:02,500 electrons to copper solid reaction, plus 0.3402 volts. 68 00:04:02,500 --> 00:04:04,980 So now clicker question, why don't you 69 00:04:04,980 --> 00:04:08,790 calculate for me what the cell potential is 70 00:04:08,790 --> 00:04:10,370 for this type of cell. 71 00:04:24,150 --> 00:04:25,600 So 10 more seconds. 72 00:04:40,880 --> 00:04:43,220 81%, that's right. 73 00:04:43,220 --> 00:04:46,980 So let's take a look at that over here. 74 00:04:46,980 --> 00:04:52,520 So the correct answer involves putting the standard reduction 75 00:04:52,520 --> 00:04:56,510 potential for the reaction at the cathode with the sign, 76 00:04:56,510 --> 00:04:59,750 without changing it as plus over here, 77 00:04:59,750 --> 00:05:04,020 0.3402, and then it's minus. 78 00:05:04,020 --> 00:05:06,680 And then you put the standard reduction potential in there, 79 00:05:06,680 --> 00:05:10,490 and this is a minus value minus, 0.7628. 80 00:05:10,490 --> 00:05:14,210 And minus a minus we get a plus, and so our answer 81 00:05:14,210 --> 00:05:18,110 is 1.103 volts. 82 00:05:18,110 --> 00:05:21,430 And this is very important, and it's 83 00:05:21,430 --> 00:05:24,000 important to think about the reaction 84 00:05:24,000 --> 00:05:26,440 at the cathode, which is the copper reaction. 85 00:05:26,440 --> 00:05:28,900 The reaction at the anode is the zinc reaction. 86 00:05:28,900 --> 00:05:33,070 And a lot of people who try to be too clever with these. 87 00:05:33,070 --> 00:05:38,440 And they're like, OK, but this one's 88 00:05:38,440 --> 00:05:40,780 the one that's being oxidized, so I'll switch the sign 89 00:05:40,780 --> 00:05:44,000 and then put it in the equation and end up getting it wrong. 90 00:05:44,000 --> 00:05:47,620 So if you always remember standard reduction potential 91 00:05:47,620 --> 00:05:50,770 put here-- standard reduction potential put here-- 92 00:05:50,770 --> 00:05:54,410 the equation already takes care of the sign issue for you, 93 00:05:54,410 --> 00:05:56,140 so don't do anything with the sign. 94 00:05:56,140 --> 00:05:59,710 Other people say, I looked through pages and pages 95 00:05:59,710 --> 00:06:03,055 of these potentials in the book, and the reaction 96 00:06:03,055 --> 00:06:04,930 I'm looking for isn't listed, because they're 97 00:06:04,930 --> 00:06:06,940 looking for an oxidation. 98 00:06:06,940 --> 00:06:08,980 They're not going to find the reaction written 99 00:06:08,980 --> 00:06:13,420 as an oxidation in a table of reduction potentials. 100 00:06:13,420 --> 00:06:17,030 So all of these are listed as reduction potentials. 101 00:06:17,030 --> 00:06:18,370 That's what you will find. 102 00:06:18,370 --> 00:06:20,830 The trick is to remember which is happening 103 00:06:20,830 --> 00:06:23,560 at the cathode, which is happening at the anode, 104 00:06:23,560 --> 00:06:26,012 and then you will always get this problem right. 105 00:06:26,012 --> 00:06:27,970 And it's good to get this right, because as you 106 00:06:27,970 --> 00:06:30,100 will see later on, this can be step 107 00:06:30,100 --> 00:06:32,297 one in a multi-part problem. 108 00:06:34,840 --> 00:06:38,200 So when we successfully calculated 109 00:06:38,200 --> 00:06:42,940 delta E of the cell, the cell potential, then we can ask, 110 00:06:42,940 --> 00:06:46,570 is the flow of electrons going to be spontaneous for this? 111 00:06:46,570 --> 00:06:48,520 Will it? 112 00:06:48,520 --> 00:06:49,930 It will be. 113 00:06:49,930 --> 00:06:53,920 And the way we know that is that we can always come back 114 00:06:53,920 --> 00:06:56,800 to delta G. So whenever you're asked if something 115 00:06:56,800 --> 00:06:59,917 is spontaneous or not, you always want to think delta G. 116 00:06:59,917 --> 00:07:01,250 That's what I'm going to go for. 117 00:07:01,250 --> 00:07:03,070 I'm going to think about delta G. 118 00:07:03,070 --> 00:07:06,750 So here delta G equals minus n, number of moles of electrons, 119 00:07:06,750 --> 00:07:10,100 Faraday's constant, times that cell potential. 120 00:07:10,100 --> 00:07:13,960 So if the cell potential is positive, 121 00:07:13,960 --> 00:07:16,510 delta G will be negative. 122 00:07:16,510 --> 00:07:19,998 And is a reaction spontaneous when delta G is negative? 123 00:07:23,740 --> 00:07:24,850 It's easy to answer. 124 00:07:24,850 --> 00:07:26,230 It's on my shirt today. 125 00:07:26,230 --> 00:07:28,450 I'm giving it all the way-- yes. 126 00:07:28,450 --> 00:07:31,150 So when delta G is negative, then you 127 00:07:31,150 --> 00:07:32,620 can be feeling spontaneous. 128 00:07:32,620 --> 00:07:35,180 It's a spontaneous reaction. 129 00:07:35,180 --> 00:07:38,200 So delta E positive means delta G negative. 130 00:07:38,200 --> 00:07:41,350 Reaction is spontaneous. 131 00:07:41,350 --> 00:07:45,640 So now let's have a little reminder, review, 132 00:07:45,640 --> 00:07:49,330 of what types of cells have spontaneous reactions 133 00:07:49,330 --> 00:07:52,890 and what kind involve non-spontaneous reaction. 134 00:08:03,970 --> 00:08:04,860 10 more seconds. 135 00:08:25,180 --> 00:08:28,000 Can anyone tell me what the correct name 136 00:08:28,000 --> 00:08:31,930 is of the cell that involves a non-spontaneous reaction? 137 00:08:31,930 --> 00:08:33,292 Electrolytic cell. 138 00:08:48,970 --> 00:08:54,360 So if we look over here-- so people knew about the galvanic. 139 00:08:54,360 --> 00:08:55,750 That's good. 140 00:08:55,750 --> 00:08:59,470 So a reaction, if it's a spontaneous reaction that 141 00:08:59,470 --> 00:09:01,090 will produce an electric current, 142 00:09:01,090 --> 00:09:03,190 that's called a galvanic cell. 143 00:09:03,190 --> 00:09:05,620 If it's a non-spontaneous reaction that 144 00:09:05,620 --> 00:09:08,440 has to be driven by applying a current, 145 00:09:08,440 --> 00:09:10,690 that's called an electrolytic cell. 146 00:09:10,690 --> 00:09:14,440 So I just threw up another random name 147 00:09:14,440 --> 00:09:17,470 to see if people would go for it. 148 00:09:17,470 --> 00:09:20,750 So it's important to keep these in mind, 149 00:09:20,750 --> 00:09:24,450 because a lot of the problems will say things like, 150 00:09:24,450 --> 00:09:26,470 in this galvanic cell. 151 00:09:26,470 --> 00:09:28,780 And you seem like you don't have enough information 152 00:09:28,780 --> 00:09:31,840 to solve the problem because the problem didn't tell you 153 00:09:31,840 --> 00:09:34,590 which reaction was at the anode or which reaction was 154 00:09:34,590 --> 00:09:35,360 that the cathode. 155 00:09:35,360 --> 00:09:36,790 So how do you do the problem? 156 00:09:36,790 --> 00:09:40,350 Well, the fact they told you it was a galvanic cell, 157 00:09:40,350 --> 00:09:44,350 there's only one way to put the reactions in order 158 00:09:44,350 --> 00:09:45,997 for it to be spontaneous. 159 00:09:45,997 --> 00:09:47,830 So that's part of the problem-- figuring out 160 00:09:47,830 --> 00:09:49,480 which is at the anode and which is 161 00:09:49,480 --> 00:09:52,180 at the cathode based on the fact that it has 162 00:09:52,180 --> 00:09:54,200 to be a spontaneous reaction. 163 00:09:54,200 --> 00:09:56,200 So knowing these terms is really important. 164 00:09:56,200 --> 00:09:57,955 It's hard to do the problems without them. 165 00:10:00,470 --> 00:10:05,780 Summary of this part, then, so a cell operate spontaneously. 166 00:10:05,780 --> 00:10:07,760 Whether it does or not can be determined 167 00:10:07,760 --> 00:10:10,220 by your cell potential. 168 00:10:10,220 --> 00:10:11,960 If it's positive, it's spontaneous. 169 00:10:11,960 --> 00:10:15,800 And that's because if it's positive, delta G is negative. 170 00:10:15,800 --> 00:10:19,730 And delta G is really what tells you about spontaneity. 171 00:10:19,730 --> 00:10:22,550 And you can calculate your cell potential 172 00:10:22,550 --> 00:10:25,520 from your standard reduction potentials 173 00:10:25,520 --> 00:10:28,580 that you will find in your book. 174 00:10:28,580 --> 00:10:30,590 So now let's think about the meaning 175 00:10:30,590 --> 00:10:32,390 of the standard reduction potential. 176 00:10:32,390 --> 00:10:34,410 What's true if it's a large positive value? 177 00:10:34,410 --> 00:10:38,300 What's true if it is a large negative value? 178 00:10:38,300 --> 00:10:40,820 We can look at these values and know something 179 00:10:40,820 --> 00:10:42,710 about the reactions. 180 00:10:42,710 --> 00:10:46,520 So meaning of standard reduction potentials-- what do the values 181 00:10:46,520 --> 00:10:47,750 tell us? 182 00:10:47,750 --> 00:10:51,800 So a large positive standard reduction potential 183 00:10:51,800 --> 00:10:55,460 means that the element is easy to reduce. 184 00:10:55,460 --> 00:10:57,800 So let's look at an example. 185 00:10:57,800 --> 00:11:03,050 So we have fluorine F2 gas plus two electrons 186 00:11:03,050 --> 00:11:07,190 going 2 fluorine minus ions. 187 00:11:07,190 --> 00:11:09,870 And this has a standard reduction potential 188 00:11:09,870 --> 00:11:13,340 of plus 2.87 volts. 189 00:11:13,340 --> 00:11:17,120 So large positive number means that it's 190 00:11:17,120 --> 00:11:19,320 easy to add electrons. 191 00:11:19,320 --> 00:11:21,170 And we can think about this in terms of what 192 00:11:21,170 --> 00:11:23,220 we know about these reactions. 193 00:11:23,220 --> 00:11:26,635 So again, if it's a positive standard reduction potential, 194 00:11:26,635 --> 00:11:29,390 E0, that's going to mean a negative delta 195 00:11:29,390 --> 00:11:31,910 G for this reaction as written, written 196 00:11:31,910 --> 00:11:33,830 as the reduction reaction. 197 00:11:33,830 --> 00:11:37,200 And so it's favorable or spontaneous in that direction. 198 00:11:37,200 --> 00:11:40,250 So the reduction is spontaneous or favorable. 199 00:11:40,250 --> 00:11:42,710 So when you have a large positive value here, 200 00:11:42,710 --> 00:11:46,550 you can think about, yes, fluorine wants those electrons. 201 00:11:46,550 --> 00:11:50,986 It spontaneously will grab those electrons and become F-minus. 202 00:11:50,986 --> 00:11:52,610 And this also makes sense to you if you 203 00:11:52,610 --> 00:11:54,920 think about your periodic table trends. 204 00:11:54,920 --> 00:11:57,770 You have fluorine wants to be F-minus. 205 00:11:57,770 --> 00:11:59,990 It will have its noble gas configuration then. 206 00:11:59,990 --> 00:12:02,280 It likes getting an extra electron. 207 00:12:02,280 --> 00:12:04,700 So this should make sense from other things 208 00:12:04,700 --> 00:12:06,290 that we've talked about. 209 00:12:06,290 --> 00:12:09,050 So now I could ask the question, and I will, and it's a clicker 210 00:12:09,050 --> 00:12:13,240 question, does that make F2 a good oxidizing agent or not? 211 00:12:25,540 --> 00:12:26,340 10 more seconds. 212 00:12:44,200 --> 00:12:46,920 So the answer is yes. 213 00:12:46,920 --> 00:12:50,740 F2 is easy to reduce, which makes it a good oxidizing 214 00:12:50,740 --> 00:12:51,460 agent. 215 00:12:51,460 --> 00:12:54,040 Remember, it's an agent of oxidation. 216 00:12:54,040 --> 00:12:58,210 It wants to go out there and oxidize other things. 217 00:12:58,210 --> 00:13:00,580 It wants to itself be reduced. 218 00:13:00,580 --> 00:13:02,860 So something that's a good oxidizing agent 219 00:13:02,860 --> 00:13:03,940 is easy to reduce. 220 00:13:03,940 --> 00:13:05,140 It wants to be reduced. 221 00:13:05,140 --> 00:13:08,230 It wants to be an agent of oxidation, 222 00:13:08,230 --> 00:13:11,020 bringing oxidation to the world. 223 00:13:11,020 --> 00:13:12,280 So it's easy to reduce. 224 00:13:12,280 --> 00:13:15,380 It's a good oxidizing agent. 225 00:13:15,380 --> 00:13:18,870 So in general, you can think about this the following way. 226 00:13:18,870 --> 00:13:21,870 Let's just like bring the noise down a little bit. 227 00:13:21,870 --> 00:13:24,340 I know it's a clicker competition day. 228 00:13:24,340 --> 00:13:27,970 So we have a large positive value here for a reaction. 229 00:13:27,970 --> 00:13:31,090 For a reduction reaction is written large positive 230 00:13:31,090 --> 00:13:34,000 standard reduction potential. 231 00:13:34,000 --> 00:13:37,500 And the couple here, F2 to F-minus 232 00:13:37,500 --> 00:13:39,580 is the couple we're talking about. 233 00:13:39,580 --> 00:13:42,520 And we'll say that the oxidized species of that couple 234 00:13:42,520 --> 00:13:44,500 is very oxidizing. 235 00:13:44,500 --> 00:13:47,912 And again, F2 here is the oxidized species. 236 00:13:50,950 --> 00:13:54,970 So the oxidized species F2 is a good oxidizing agent. 237 00:13:54,970 --> 00:13:58,900 It's very oxidizing because it has a large positive 238 00:13:58,900 --> 00:14:02,120 standard reduction potential. 239 00:14:02,120 --> 00:14:04,510 So I don't know how well this copied. 240 00:14:04,510 --> 00:14:07,420 But here are some standard reduction potentials. 241 00:14:07,420 --> 00:14:11,560 They're all in your book, so it's OK if they look terrible. 242 00:14:11,560 --> 00:14:13,840 But the important point here is that the top. 243 00:14:13,840 --> 00:14:16,236 We have large positive values. 244 00:14:16,236 --> 00:14:17,860 So if you can see that in your handout, 245 00:14:17,860 --> 00:14:19,820 this is the reaction we just talked about, 246 00:14:19,820 --> 00:14:25,060 or the couple we just talked about-- F2 F-minus and so 247 00:14:25,060 --> 00:14:26,500 large positive. 248 00:14:26,500 --> 00:14:28,360 And then there's a gap here. 249 00:14:28,360 --> 00:14:30,250 There are pages and pages and pages 250 00:14:30,250 --> 00:14:32,410 of these standard reduction potentials. 251 00:14:32,410 --> 00:14:36,010 And at the very bottom, you have a large negative number, 252 00:14:36,010 --> 00:14:39,520 so large positive on the top large, negative on the bottom. 253 00:14:39,520 --> 00:14:44,830 So at the very top, the oxidized species of that couple 254 00:14:44,830 --> 00:14:46,930 is very oxidizing. 255 00:14:46,930 --> 00:14:50,980 And we'll see it in a minute, that at the bottom 256 00:14:50,980 --> 00:14:55,090 the reduced species, when it's a negative number here, 257 00:14:55,090 --> 00:14:59,010 the reduced species is very reducing. 258 00:14:59,010 --> 00:15:00,720 So let's look at the reduced species, 259 00:15:00,720 --> 00:15:03,210 and this is lithium on the bottom. 260 00:15:03,210 --> 00:15:07,320 So a large negative standard reduction potential 261 00:15:07,320 --> 00:15:10,110 means that the element is hard to reduce. 262 00:15:10,110 --> 00:15:13,110 So example, lithium plus 1 plus 1 electron 263 00:15:13,110 --> 00:15:15,570 going to lithium solid. 264 00:15:15,570 --> 00:15:18,180 The standard reduction potential for this reaction 265 00:15:18,180 --> 00:15:22,800 is minus 3.045 volts. 266 00:15:22,800 --> 00:15:27,390 So it's hard to add electrons to lithium-plus. 267 00:15:27,390 --> 00:15:30,730 We have a negative standard reduction potential, 268 00:15:30,730 --> 00:15:33,330 which means a positive delta G for the reaction 269 00:15:33,330 --> 00:15:35,450 is written for the reduction reaction. 270 00:15:35,450 --> 00:15:36,390 So it's not favorable. 271 00:15:36,390 --> 00:15:39,280 Lithium-plus does not want electrons. 272 00:15:39,280 --> 00:15:41,640 It doesn't want to go to lithium solid. 273 00:15:41,640 --> 00:15:44,790 So lithium-plus, if it loses an electron, 274 00:15:44,790 --> 00:15:47,340 it gets its nice noble gas configuration. 275 00:15:47,340 --> 00:15:48,570 It's a group-one element. 276 00:15:48,570 --> 00:15:52,800 It likes to be in the plus-1 oxidation state. 277 00:15:52,800 --> 00:15:55,410 It doesn't want to be reduced. 278 00:15:55,410 --> 00:16:00,960 So is lithium plus 1 a good oxidizing agent? 279 00:16:00,960 --> 00:16:02,730 No, it's not. 280 00:16:02,730 --> 00:16:04,440 It does not want to be reduced. 281 00:16:04,440 --> 00:16:10,920 But lithium solid is a good reducing agent. 282 00:16:10,920 --> 00:16:13,530 It's a good reducing agent because it 283 00:16:13,530 --> 00:16:15,270 wants to reduce other things. 284 00:16:15,270 --> 00:16:19,380 It wants to itself become oxidized. 285 00:16:19,380 --> 00:16:22,680 So lithium solid will reduce other things. 286 00:16:22,680 --> 00:16:25,020 Lithium solid wants to be oxidized. 287 00:16:25,020 --> 00:16:29,140 It wants to be lithium plus 1. 288 00:16:29,140 --> 00:16:33,540 So the rule here for those large, negative standard 289 00:16:33,540 --> 00:16:37,220 reduction potential couples is that 290 00:16:37,220 --> 00:16:39,480 with a large negative standard reduction 291 00:16:39,480 --> 00:16:43,830 potential, the reduced species is very reducing. 292 00:16:43,830 --> 00:16:46,470 The reduced species here is lithium. 293 00:16:46,470 --> 00:16:50,580 So lithium is reduced compared to lithium plus 1. 294 00:16:50,580 --> 00:16:53,480 This, then, is very reducing. 295 00:16:53,480 --> 00:16:56,220 It is a good reducing agent. 296 00:16:56,220 --> 00:17:01,110 Do you think lithium plus 1 would be a good reducing agent? 297 00:17:01,110 --> 00:17:01,860 What do you think? 298 00:17:05,510 --> 00:17:07,894 So if it was a good reducing agent, 299 00:17:07,894 --> 00:17:10,819 it wants to reduce other things and become oxidized. 300 00:17:10,819 --> 00:17:14,970 Do you think lithium plus 2 is a good thing? 301 00:17:14,970 --> 00:17:15,980 No. 302 00:17:15,980 --> 00:17:20,270 So again, in this couple, it's the reduced species 303 00:17:20,270 --> 00:17:21,920 that's very reducing. 304 00:17:21,920 --> 00:17:24,619 And so you have to think about this a little bit. 305 00:17:24,619 --> 00:17:26,599 Make sure that you pick the right one. 306 00:17:26,599 --> 00:17:29,830 Often you'll ask about rank the order 307 00:17:29,830 --> 00:17:32,420 of these as reducing agents. 308 00:17:32,420 --> 00:17:34,970 And people remember this, but don't 309 00:17:34,970 --> 00:17:36,979 remember which species of the couple 310 00:17:36,979 --> 00:17:38,270 is going to be doing the thing. 311 00:17:38,270 --> 00:17:40,250 So think about what's happening. 312 00:17:40,250 --> 00:17:43,790 Lithium solid, does it want to become lithium plus 1? 313 00:17:43,790 --> 00:17:44,420 It sure does. 314 00:17:44,420 --> 00:17:46,940 Does lithium plus 1 want to become lithium plus 2? 315 00:17:46,940 --> 00:17:48,710 No. 316 00:17:48,710 --> 00:17:50,770 So again, back to just our table just 317 00:17:50,770 --> 00:17:54,650 for a second, on the top-- large positive, oxidized species very 318 00:17:54,650 --> 00:18:00,572 oxidizing, negative value, reduced species very reducing. 319 00:18:00,572 --> 00:18:02,030 And now if we look at this, I don't 320 00:18:02,030 --> 00:18:03,821 know how well this came out in your handout 321 00:18:03,821 --> 00:18:06,800 either, but part of the periodic table 322 00:18:06,800 --> 00:18:08,660 for you kind of compressed. 323 00:18:08,660 --> 00:18:13,780 Over here we have fluorine, large positive, standard 324 00:18:13,780 --> 00:18:17,350 reduction potential, easy to reduce over here, 325 00:18:17,350 --> 00:18:19,250 good oxidizing agents. 326 00:18:19,250 --> 00:18:21,470 If they get an electron, if they're reduced, 327 00:18:21,470 --> 00:18:23,750 they get their noble gas configuration. 328 00:18:23,750 --> 00:18:27,500 Over here, we have these big negative numbers, 329 00:18:27,500 --> 00:18:32,930 easy to oxidize the solid or the neutral to their plus 1. 330 00:18:32,930 --> 00:18:35,379 So they would be good reducing agents. 331 00:18:35,379 --> 00:18:37,670 And so this all makes sense when you think about trends 332 00:18:37,670 --> 00:18:40,250 in the periodic table. 333 00:18:40,250 --> 00:18:45,290 So today we're going to have two examples of why all of this 334 00:18:45,290 --> 00:18:48,950 is important across disciplines. 335 00:18:48,950 --> 00:18:52,030 So often these units are really talked-- 336 00:18:52,030 --> 00:18:55,670 you talk about making batteries and things like that. 337 00:18:55,670 --> 00:18:56,870 And that is important. 338 00:18:56,870 --> 00:18:59,600 In fact, with energy initiatives, 339 00:18:59,600 --> 00:19:02,421 making batteries is a really hot area right now. 340 00:19:02,421 --> 00:19:03,920 But all these things you're learning 341 00:19:03,920 --> 00:19:08,220 are also related to biology and to medicine as well. 342 00:19:08,220 --> 00:19:11,770 And so we have one more of the "In Their Own Words" segment. 343 00:19:11,770 --> 00:19:17,510 And John Essigmann studies DNA damage as related to cancer. 344 00:19:17,510 --> 00:19:21,500 And people are like, what is DNA damage and cancer 345 00:19:21,500 --> 00:19:23,990 have to do with oxidation reduction potentials? 346 00:19:23,990 --> 00:19:26,600 But it's not all about oxidation reduction. 347 00:19:26,600 --> 00:19:29,150 That's what a lot of the damage is. 348 00:19:29,150 --> 00:19:32,570 So why don't you hear from John Essigmann. 349 00:19:39,620 --> 00:19:40,860 [VIDEO PLAYBACK] 350 00:19:40,860 --> 00:19:42,200 - My name's John Essigmann. 351 00:19:42,200 --> 00:19:43,770 I'm in the Department of Chemistry 352 00:19:43,770 --> 00:19:46,000 and Biological Engineering MIT. 353 00:19:46,000 --> 00:19:49,800 I'm a toxicologist by training, so I study how cells respond 354 00:19:49,800 --> 00:19:52,790 to toxins, and in particular, the kinds of toxins 355 00:19:52,790 --> 00:19:56,637 that cause nucleic acid damage. 356 00:19:56,637 --> 00:19:58,970 Some of these are chemicals from out in the environment, 357 00:19:58,970 --> 00:20:01,190 organic compounds, that cause DNA damage. 358 00:20:01,190 --> 00:20:05,240 But I'm also interested in how chemicals inside our body, 359 00:20:05,240 --> 00:20:07,075 for example, reactive forms of oxygen, 360 00:20:07,075 --> 00:20:12,870 the molecule that we breathe, also cause nucleic acid damage. 361 00:20:12,870 --> 00:20:14,760 Oxidation is the process by which 362 00:20:14,760 --> 00:20:17,680 we convert our metabolic fuels into energy. 363 00:20:17,680 --> 00:20:21,210 This is a gummy bear being oxidized by potassium chlorate. 364 00:20:21,210 --> 00:20:23,720 This is exactly the kind of chemical reaction 365 00:20:23,720 --> 00:20:25,190 that goes on in your bodies. 366 00:20:25,190 --> 00:20:29,210 But in your body, it goes on at a much lower rate. 367 00:20:29,210 --> 00:20:33,230 Oxygen is a double-edged sword. 368 00:20:33,230 --> 00:20:37,530 There are two aspects to its use that is good for us. 369 00:20:41,370 --> 00:20:44,545 The first one is by acting as an electron acceptor, 370 00:20:44,545 --> 00:20:47,070 sort of like an anode in a battery, 371 00:20:47,070 --> 00:20:48,930 it completes the circuit and allows 372 00:20:48,930 --> 00:20:52,560 us to be able to generate free energy that we can 373 00:20:52,560 --> 00:20:55,280 use to power vital processes. 374 00:20:55,280 --> 00:20:57,060 That's good. 375 00:20:57,060 --> 00:21:01,890 The second good thing is that it's the ammunition 376 00:21:01,890 --> 00:21:03,540 of our innate immune system. 377 00:21:03,540 --> 00:21:06,810 [COUGHING] 378 00:21:06,810 --> 00:21:10,080 Oxygen is used to charge the system 379 00:21:10,080 --> 00:21:14,640 to produce one, two, or three electron-reduced species 380 00:21:14,640 --> 00:21:17,950 that themselves can react with reactive nitrogen species 381 00:21:17,950 --> 00:21:24,260 to create a host of very damaging oxidants. 382 00:21:24,260 --> 00:21:27,120 Our innate immune system uses these to protect us 383 00:21:27,120 --> 00:21:30,330 against invaders and oftentimes, we 384 00:21:30,330 --> 00:21:34,730 believe, developing cancer cells. 385 00:21:34,730 --> 00:21:37,970 The bad thing is that the second system I described 386 00:21:37,970 --> 00:21:41,700 doesn't have a lot of accuracy in how it 387 00:21:41,700 --> 00:21:44,250 focuses that chemical warfare. 388 00:21:44,250 --> 00:21:46,740 So not only are the, let's say, invading 389 00:21:46,740 --> 00:21:49,620 bacterial cells or the cancer cells hit, 390 00:21:49,620 --> 00:21:52,200 but surrounding cells as well. 391 00:21:52,200 --> 00:21:54,710 And that creates oxidative damage 392 00:21:54,710 --> 00:21:58,970 in the genomes of those cells, those innocent bystanders. 393 00:21:58,970 --> 00:22:02,540 If those cells divide, the oxidative damage that's in them 394 00:22:02,540 --> 00:22:07,335 can cause mutations, and that sets the cell along a path. 395 00:22:07,335 --> 00:22:11,349 If you accumulate enough mutations in enough genes, 396 00:22:11,349 --> 00:22:13,015 you're going to convert that normal cell 397 00:22:13,015 --> 00:22:15,980 into a cancer cell that will grow out into a tumor. 398 00:22:20,267 --> 00:22:20,850 [END PLAYBACK] 399 00:22:20,850 --> 00:22:22,900 CATHERINE DRENNAN: So that was one of, I think, 400 00:22:22,900 --> 00:22:26,100 the only video, besides mine, that involved a faculty member. 401 00:22:26,100 --> 00:22:28,150 But I thought it's nice to have a sort 402 00:22:28,150 --> 00:22:30,880 of a collection of different folks talking 403 00:22:30,880 --> 00:22:32,230 about their research. 404 00:22:32,230 --> 00:22:35,410 And this relates to free radical species 405 00:22:35,410 --> 00:22:39,670 as well as to oxidation and reduction. 406 00:22:39,670 --> 00:22:42,820 So now I have another bio example, which hopefully we'll 407 00:22:42,820 --> 00:22:44,120 get to at the end. 408 00:22:44,120 --> 00:22:47,200 But I want to introduce a very important equation 409 00:22:47,200 --> 00:22:49,530 for this unit and oxidation reduction, which 410 00:22:49,530 --> 00:22:51,350 is the Nernst equation. 411 00:22:51,350 --> 00:22:55,240 So here, and this happens, I think, to all of us 412 00:22:55,240 --> 00:22:59,410 all the time-- our exhausted batteries. 413 00:22:59,410 --> 00:23:01,450 So when you have a battery or you 414 00:23:01,450 --> 00:23:05,090 go to turn on your headphones or use your laser pointer 415 00:23:05,090 --> 00:23:08,030 or things like that and the battery doesn't work anymore, 416 00:23:08,030 --> 00:23:11,500 it's because the cell reaction has reached equilibrium. 417 00:23:11,500 --> 00:23:14,380 So equilibrium is a state of calmness. 418 00:23:14,380 --> 00:23:17,950 It's also a state of batteries not working anymore. 419 00:23:17,950 --> 00:23:23,790 And so at this point, when it's at equilibrium, 420 00:23:23,790 --> 00:23:26,170 the cell generates zero potential 421 00:23:26,170 --> 00:23:27,430 across its electrodes. 422 00:23:27,430 --> 00:23:29,500 It's not working anymore. 423 00:23:29,500 --> 00:23:31,390 It is a dead battery. 424 00:23:31,390 --> 00:23:35,200 And we need to think about when it reaches equilibrium, 425 00:23:35,200 --> 00:23:38,110 what state it's in at any given time compared 426 00:23:38,110 --> 00:23:39,717 to its equilibrium state. 427 00:23:39,717 --> 00:23:41,800 So this really brings us back to some of the ideas 428 00:23:41,800 --> 00:23:44,310 that we had in the chemical equilibrium unit, 429 00:23:44,310 --> 00:23:47,800 where we were thinking about composition of a reaction 430 00:23:47,800 --> 00:23:51,860 at any given time compared to the equilibrium state. 431 00:23:51,860 --> 00:23:58,030 So how does cell potential change with cell composition? 432 00:23:58,030 --> 00:24:00,060 So again, we know a lot about this. 433 00:24:00,060 --> 00:24:04,080 And exam three on Friday has equilibrium on it. 434 00:24:04,080 --> 00:24:07,810 So we already know a lot about what happens in relationships 435 00:24:07,810 --> 00:24:10,490 between equilibrium. 436 00:24:10,490 --> 00:24:14,110 And we know that delta G changes as the composition 437 00:24:14,110 --> 00:24:15,700 changes in the cell. 438 00:24:15,700 --> 00:24:18,330 And until equilibrium is reached-- so again, 439 00:24:18,330 --> 00:24:20,920 reactions going forward and back in equilibrium-- 440 00:24:20,920 --> 00:24:23,070 the forward direction equals the reverse direction. 441 00:24:23,070 --> 00:24:27,030 But at some given time that is not always the case. 442 00:24:27,030 --> 00:24:30,940 So the equation that we know from before is 443 00:24:30,940 --> 00:24:36,370 that delta G equals delta G0 plus RT natural log of Q. 444 00:24:36,370 --> 00:24:39,050 And so if you know delta G and Q, 445 00:24:39,050 --> 00:24:41,140 you can talk about what the delta G is going 446 00:24:41,140 --> 00:24:44,580 to be at a particular composition of the cell 447 00:24:44,580 --> 00:24:49,480 with a particular value of Q. So now we 448 00:24:49,480 --> 00:24:53,140 can think, well, OK, here's what we know about Q, that reaction 449 00:24:53,140 --> 00:24:55,210 quotient, and delta G. 450 00:24:55,210 --> 00:25:00,490 What do we know about delta G and E, the cell potential? 451 00:25:00,490 --> 00:25:02,300 And we just talked about that. 452 00:25:02,300 --> 00:25:04,960 So we know that delta G0 equals minus n, 453 00:25:04,960 --> 00:25:06,370 the number of moles of electrons, 454 00:25:06,370 --> 00:25:09,470 Faraday's constant times that cell potential. 455 00:25:09,470 --> 00:25:12,120 So now we can do a rearrangement of terms. 456 00:25:12,120 --> 00:25:16,180 We can combine and come up with the Nernst equation. 457 00:25:16,180 --> 00:25:17,650 So here are the two equations. 458 00:25:17,650 --> 00:25:18,700 One we knew before. 459 00:25:18,700 --> 00:25:21,220 One we just learned about today. 460 00:25:21,220 --> 00:25:23,280 And we can put those together. 461 00:25:23,280 --> 00:25:27,490 We can plug in for delta G here without the 0, 462 00:25:27,490 --> 00:25:30,400 and for delta G0 with the 0 over here. 463 00:25:30,400 --> 00:25:33,120 And then we can divide both sides 464 00:25:33,120 --> 00:25:35,290 by minus the number of moles of electrons 465 00:25:35,290 --> 00:25:38,980 and Faraday's constant, come up with the Nernst equation. 466 00:25:38,980 --> 00:25:43,810 So the Nernst equation compares our cell potential 467 00:25:43,810 --> 00:25:46,620 under standard conditions with our cell potential 468 00:25:46,620 --> 00:25:52,770 at any given time based on that reaction quotient Q. 469 00:25:52,770 --> 00:25:56,970 So let's do an example with the Nernst equation. 470 00:25:56,970 --> 00:26:02,400 So let's calculate the cell potential at a particular time 471 00:26:02,400 --> 00:26:09,510 when the zinc plus 2 ions are 0.1 and copper plus 2 is 0.0010 472 00:26:09,510 --> 00:26:10,470 molar. 473 00:26:10,470 --> 00:26:13,990 And we're same cell as we've done before. 474 00:26:13,990 --> 00:26:19,290 So step one, very easy, we want to calculate the cell potential 475 00:26:19,290 --> 00:26:21,330 under standard conditions. 476 00:26:21,330 --> 00:26:24,210 And this is particularly easy because we already 477 00:26:24,210 --> 00:26:25,470 did this today. 478 00:26:25,470 --> 00:26:30,930 So you already told me it's plus 1.103 volts. 479 00:26:30,930 --> 00:26:33,930 So we could look up our standard reduction potentials 480 00:26:33,930 --> 00:26:37,914 for our copper couple and our zinc couple and calculate this. 481 00:26:37,914 --> 00:26:39,330 So we already did this, and that's 482 00:26:39,330 --> 00:26:41,540 why I said you want to make sure you get this right 483 00:26:41,540 --> 00:26:44,730 because it's often just step one of a multi-part problem. 484 00:26:44,730 --> 00:26:47,140 So we want to get that right. 485 00:26:47,140 --> 00:26:51,748 Step two is calculating Q, and this is a clicker question. 486 00:27:14,250 --> 00:27:15,170 10 more seconds. 487 00:27:31,410 --> 00:27:34,640 I tried to make the math not that hard. 488 00:27:34,640 --> 00:27:40,240 So Q, products over reactants, so one thing you want to do 489 00:27:40,240 --> 00:27:43,070 is make sure you have the reaction written correctly. 490 00:27:43,070 --> 00:27:46,810 So you know what the products are and what the reactants are. 491 00:27:46,810 --> 00:27:50,680 And here we have the zinc ions over copper. 492 00:27:50,680 --> 00:27:53,860 Copper solid and zinc solid don't 493 00:27:53,860 --> 00:27:56,740 appear in the equilibrium expression 494 00:27:56,740 --> 00:27:59,350 or in the Q expression also because they're solids 495 00:27:59,350 --> 00:28:01,810 and their concentration is not changing. 496 00:28:01,810 --> 00:28:03,640 So there is enough information that 497 00:28:03,640 --> 00:28:06,790 was given because we don't include the solids. 498 00:28:06,790 --> 00:28:10,870 So it's just the concentration of zinc ions 499 00:28:10,870 --> 00:28:15,598 over the concentration of copper ions, or 1.0 times 10 to the 2. 500 00:28:17,830 --> 00:28:20,350 Now we need to know n. 501 00:28:20,350 --> 00:28:22,990 So n is the number of moles of electrons that 502 00:28:22,990 --> 00:28:25,060 are involved in this reaction. 503 00:28:25,060 --> 00:28:27,625 This reaction involves how many electrons? 504 00:28:31,650 --> 00:28:33,210 How many? 505 00:28:33,210 --> 00:28:35,330 It involves two, right. 506 00:28:35,330 --> 00:28:41,490 So the zinc to zinc plus 2, copper plus 2 to copper, 507 00:28:41,490 --> 00:28:43,590 so it involves two electrons. 508 00:28:43,590 --> 00:28:47,370 So this is two, and sometimes you can just look at this 509 00:28:47,370 --> 00:28:50,040 and realize it's two electrons involved. 510 00:28:50,040 --> 00:28:52,500 Other times you have to balance the reaction 511 00:28:52,500 --> 00:28:56,710 to figure out how many electrons are involved. 512 00:28:56,710 --> 00:28:59,730 It's not obvious by looking at the reaction. 513 00:28:59,730 --> 00:29:01,980 This is a simple one, but some of them are not. 514 00:29:01,980 --> 00:29:05,170 So that's we're balancing reactions comes in handy. 515 00:29:05,170 --> 00:29:05,670 so 516 00:29:05,670 --> 00:29:07,500 Now we have everything we need to plug it 517 00:29:07,500 --> 00:29:09,360 into the Nernst equation. 518 00:29:09,360 --> 00:29:10,860 So we can put it in. 519 00:29:10,860 --> 00:29:15,320 We calculated this standard cell potential 520 00:29:15,320 --> 00:29:20,130 over here, the plus 1.1 value. 521 00:29:20,130 --> 00:29:24,870 We can look up R. Everything, again, room temperature. 522 00:29:24,870 --> 00:29:27,720 Natural log of Q, we calculated Q. 523 00:29:27,720 --> 00:29:31,230 We have two electrons and Faraday's constant. 524 00:29:31,230 --> 00:29:36,180 And we can multiply this out, get this value. 525 00:29:36,180 --> 00:29:39,880 In terms of significant figures, this also is a lot of fun. 526 00:29:39,880 --> 00:29:43,260 We have log rules of significant figures, multiplication 527 00:29:43,260 --> 00:29:45,270 and division rules of significant figures, 528 00:29:45,270 --> 00:29:49,740 and adding and subtracting rules of significant figures, 529 00:29:49,740 --> 00:29:51,450 and also in terms of units. 530 00:29:51,450 --> 00:29:54,060 All of a sudden, volts appeared. 531 00:29:54,060 --> 00:29:56,310 That was nice because our answer should be in volts. 532 00:29:56,310 --> 00:29:58,320 But where did they come from? 533 00:29:58,320 --> 00:30:02,220 So we had our kelvins are going to cancel out. 534 00:30:02,220 --> 00:30:06,000 But we have joules over here. 535 00:30:06,000 --> 00:30:07,170 And we have coulombs. 536 00:30:07,170 --> 00:30:10,260 But luckily, a joule per coulomb equals a volt. 537 00:30:10,260 --> 00:30:14,740 So we're all set to get our appropriate units. 538 00:30:14,740 --> 00:30:17,190 So just a couple of things that I'll mention about this as 539 00:30:17,190 --> 00:30:20,160 well-- sine almost all these problems are 540 00:30:20,160 --> 00:30:22,650 at room temperature, I try to make your lives 541 00:30:22,650 --> 00:30:28,020 a little bit easier on the exam and multiply this out 542 00:30:28,020 --> 00:30:30,360 for you, because these are all constants, 543 00:30:30,360 --> 00:30:32,100 and they're big constants that involve 544 00:30:32,100 --> 00:30:34,410 plugging a lot of numbers in. 545 00:30:34,410 --> 00:30:39,030 So I will give you this combined value. 546 00:30:39,030 --> 00:30:42,480 And if you use log instead of natural log, 547 00:30:42,480 --> 00:30:45,850 you need to change that value here. 548 00:30:45,850 --> 00:30:48,600 And so what I actually put on equation sheets 549 00:30:48,600 --> 00:30:50,120 are these equations. 550 00:30:50,120 --> 00:30:53,940 I'll also put the full equation there as well but just 551 00:30:53,940 --> 00:30:56,560 to make your lives a little bit easier. 552 00:30:56,560 --> 00:31:00,150 It's horrible to make mistakes up there. 553 00:31:00,150 --> 00:31:06,720 So just briefly, what about at equilibrium, what does Q equal? 554 00:31:06,720 --> 00:31:13,180 Q equals K. Delta G equals 0. 555 00:31:13,180 --> 00:31:17,940 And so we can rewrite this expression again, 556 00:31:17,940 --> 00:31:19,950 which we've done before. 557 00:31:19,950 --> 00:31:25,820 And if we take this and look at this expression now, 558 00:31:25,820 --> 00:31:27,590 we can again combine these-- we have 559 00:31:27,590 --> 00:31:33,700 two expressions for delta G0-- and combine them and solve. 560 00:31:33,700 --> 00:31:36,410 And you can see that you can also 561 00:31:36,410 --> 00:31:41,230 calculate K from standard reduction potentials. 562 00:31:41,230 --> 00:31:45,190 So all the things that we have been learning here, 563 00:31:45,190 --> 00:31:48,560 we can always come back to chemical equilibrium. 564 00:31:48,560 --> 00:31:51,960 So I'm done with everything but my second bio example. 565 00:31:51,960 --> 00:31:54,490 So you'll have to wait until next week 566 00:31:54,490 --> 00:31:58,376 to find out how vitamin B12 gets reduced in the body. 567 00:32:10,370 --> 00:32:13,200 Let's just take 10 more seconds on the clicker question. 568 00:32:36,250 --> 00:32:39,310 Does someone want a nice T-shirt to tell me 569 00:32:39,310 --> 00:32:40,602 how they got the right answer? 570 00:32:48,520 --> 00:32:49,587 Pass this back. 571 00:32:52,330 --> 00:32:54,980 AUDIENCE: So when you're looking at the Pb-- 572 00:32:54,980 --> 00:32:57,000 CATHERINE DRENNAN: Let's all quiet down. 573 00:32:57,000 --> 00:33:01,430 AUDIENCE: [INAUDIBLE] and zinc, zinc is going to reduce-- 574 00:33:01,430 --> 00:33:03,711 CATHERINE DRENNAN: Let's quiet down. 575 00:33:03,711 --> 00:33:07,700 AUDIENCE: --Pb because it goes from a 2 plus charge 576 00:33:07,700 --> 00:33:09,330 to no charge at all. 577 00:33:09,330 --> 00:33:11,600 So that's reducing its charge. 578 00:33:11,600 --> 00:33:13,690 And then in the reverse reaction, 579 00:33:13,690 --> 00:33:17,060 Pb does the same thing to zinc 2 plus 580 00:33:17,060 --> 00:33:22,325 and takes it from 2 plus to a neutral charge. 581 00:33:22,325 --> 00:33:23,575 CATHERINE DRENNAN: Yep, great. 582 00:33:26,180 --> 00:33:28,670 So the trick to this is just look at the equations 583 00:33:28,670 --> 00:33:31,310 and figure out what is being oxidized 584 00:33:31,310 --> 00:33:33,560 and what is being reduced in both directions. 585 00:33:37,260 --> 00:33:41,370 Oxidation reduction, it's not just about batteries. 586 00:33:41,370 --> 00:33:43,730 Batteries, of course, are very important. 587 00:33:43,730 --> 00:33:45,800 There's a lot of active research right now 588 00:33:45,800 --> 00:33:48,750 in trying to make better batteries that are more 589 00:33:48,750 --> 00:33:52,980 environmentally friendly, just solving the energy problem 590 00:33:52,980 --> 00:33:55,770 every single possible way. 591 00:33:55,770 --> 00:33:58,140 But even if you're not interested in solving 592 00:33:58,140 --> 00:34:00,650 the energy problem, oxidation reduction 593 00:34:00,650 --> 00:34:02,570 is really important in medicine. 594 00:34:02,570 --> 00:34:05,900 So I'm just going to give you another bio example for this. 595 00:34:05,900 --> 00:34:09,230 So there's a vitamin in your body, vitamin B12 596 00:34:09,230 --> 00:34:13,230 It has one of the largest negative reduction potentials 597 00:34:13,230 --> 00:34:15,679 of any biological molecule. 598 00:34:15,679 --> 00:34:18,710 So we ask the question, how can something like that 599 00:34:18,710 --> 00:34:20,730 be reduced in the body? 600 00:34:20,730 --> 00:34:22,670 So let me introduce you to vitamin B12. 601 00:34:22,670 --> 00:34:25,610 You're going to see this a couple of times today. 602 00:34:25,610 --> 00:34:29,630 So vitamin B12 needs to be reduced to be active. 603 00:34:29,630 --> 00:34:31,686 But it has a low reduction potential. 604 00:34:34,230 --> 00:34:35,850 This is the vitamin B12, and you'll 605 00:34:35,850 --> 00:34:38,060 see several more views of it, and it's 606 00:34:38,060 --> 00:34:42,620 in pictures in your handout of today's lecture. 607 00:34:42,620 --> 00:34:45,420 There's an enzyme that requires vitamin B12 and another B 608 00:34:45,420 --> 00:34:48,060 vitamin called folic acid. 609 00:34:48,060 --> 00:34:51,440 And if this enzyme is not functioning properly 610 00:34:51,440 --> 00:34:54,030 in your body, it leads to all sorts of things. 611 00:34:54,030 --> 00:34:56,550 It's connected to heart disease. 612 00:34:56,550 --> 00:34:59,420 If women don't have enough folic acid while they're pregnant, 613 00:34:59,420 --> 00:35:02,360 it can lead to birth defects, neural tube disease, 614 00:35:02,360 --> 00:35:05,350 where the brain doesn't form properly. 615 00:35:05,350 --> 00:35:10,370 Also, there is rising evidence connecting a vitamin B12 616 00:35:10,370 --> 00:35:14,480 deficiency with bringing on Alzheimer's disease. 617 00:35:14,480 --> 00:35:18,270 And some people actually buy little B12 squirt bottles 618 00:35:18,270 --> 00:35:21,020 and squirt it into their nose before exams. 619 00:35:21,020 --> 00:35:22,920 I'm not sure that helps. 620 00:35:22,920 --> 00:35:25,610 But as you get older, I would seriously 621 00:35:25,610 --> 00:35:29,330 consider taking extra vitamin B12. 622 00:35:29,330 --> 00:35:32,190 So since we're talking about this, 623 00:35:32,190 --> 00:35:34,920 we'll have a little nutrition quiz. 624 00:35:34,920 --> 00:35:38,790 Where do you get vitamin B12 and folic acid in your diet? 625 00:35:38,790 --> 00:35:40,920 First let's think about vitamin B12. 626 00:35:40,920 --> 00:35:43,670 Does anyone know where you get this, besides vitamin pills, 627 00:35:43,670 --> 00:35:44,630 in food? 628 00:35:44,630 --> 00:35:46,689 Where do you get vitamin B12 in food, yeah? 629 00:35:46,689 --> 00:35:47,480 AUDIENCE: Broccoli? 630 00:35:47,746 --> 00:35:49,079 CATHERINE DRENNAN: Broccoli, no. 631 00:35:53,480 --> 00:35:55,420 We had a hand up there. 632 00:35:55,420 --> 00:35:56,770 Anyone else? 633 00:35:56,770 --> 00:35:58,436 Yeah. 634 00:35:58,436 --> 00:35:59,060 AUDIENCE: Meat? 635 00:35:59,060 --> 00:36:02,270 CATHERINE DRENNAN: Meat, yes-- red meat, actually 636 00:36:02,270 --> 00:36:03,150 any kind of meat. 637 00:36:03,150 --> 00:36:06,110 But hardly ever is red meat the answer 638 00:36:06,110 --> 00:36:07,710 to a nutritional question. 639 00:36:07,710 --> 00:36:11,870 So I thought I'd put it up there in big bold red letters. 640 00:36:11,870 --> 00:36:15,910 Yeah, you get B12 in meat. 641 00:36:15,910 --> 00:36:18,250 Plants don't use vitamin B12. 642 00:36:18,250 --> 00:36:20,440 So you can eat as many plants as you want. 643 00:36:20,440 --> 00:36:23,360 Unless they're covered with bacteria that do use B12, 644 00:36:23,360 --> 00:36:25,640 you will not get any in your diet. 645 00:36:25,640 --> 00:36:28,360 So if you're vegetarian, you should take a vitamin tablet. 646 00:36:28,360 --> 00:36:31,150 You don't need much, so it's not really your problem. 647 00:36:31,150 --> 00:36:32,860 But it does come from red meat. 648 00:36:32,860 --> 00:36:34,480 What about folic acid? 649 00:36:34,480 --> 00:36:38,461 Any guesses where folic acid comes from? 650 00:36:38,461 --> 00:36:38,960 Yeah. 651 00:36:38,960 --> 00:36:40,120 AUDIENCE: Broccoli? 652 00:36:40,120 --> 00:36:41,950 CATHERINE DRENNAN: Yes. 653 00:36:41,950 --> 00:36:43,230 Very good. 654 00:36:43,230 --> 00:36:46,600 So it's the fall in New England, and hopefully some of you 655 00:36:46,600 --> 00:36:49,760 have gone out and seen the foliage. 656 00:36:49,760 --> 00:36:54,300 So folic acid does come from green leafy things. 657 00:36:54,300 --> 00:36:59,820 And if you're Norwegian, you will also say the following. 658 00:36:59,820 --> 00:37:05,140 So Norwegian scientists claim that the great longevity 659 00:37:05,140 --> 00:37:07,120 of Norwegians has to do with the fact 660 00:37:07,120 --> 00:37:09,370 that their beer is rich in folic acid. 661 00:37:09,370 --> 00:37:11,020 And somehow they even got a grant 662 00:37:11,020 --> 00:37:14,200 from the Norwegian government to study the correlation 663 00:37:14,200 --> 00:37:18,640 between beer and health. 664 00:37:18,640 --> 00:37:21,400 Everyone signed up for that study, I'm sure. 665 00:37:21,400 --> 00:37:25,000 So red meat and beer for good health. 666 00:37:25,000 --> 00:37:29,650 But yes, leafy green vegetables, yes, folic acid, 667 00:37:29,650 --> 00:37:32,770 foliage, also orange juice is really high. 668 00:37:32,770 --> 00:37:36,970 And at one point, they had this commercial for orange juice. 669 00:37:36,970 --> 00:37:39,190 It's not just for breakfast anymore. 670 00:37:39,190 --> 00:37:41,210 It's good for your heart. 671 00:37:41,210 --> 00:37:43,300 Well, that was because of the folic acid in there. 672 00:37:43,300 --> 00:37:44,820 And that's actually true. 673 00:37:44,820 --> 00:37:47,980 So there was a commercial that actually provided you 674 00:37:47,980 --> 00:37:50,740 with valuable nutritional advice. 675 00:37:50,740 --> 00:37:52,370 Not so sure about the beer, but I'd 676 00:37:52,370 --> 00:37:56,260 go buy the leafy greens and the orange juice. 677 00:37:56,260 --> 00:37:59,110 So you need both of these to be healthy. 678 00:37:59,110 --> 00:38:01,930 But again, we have this problem that vitamin B12 679 00:38:01,930 --> 00:38:04,640 has this low negative redox potential. 680 00:38:04,640 --> 00:38:06,520 So how is it reduced? 681 00:38:06,520 --> 00:38:09,700 Well, in the body it's reduced by a protein called 682 00:38:09,700 --> 00:38:14,020 flavodoxin, which has a flavon, which is another B vitamin. 683 00:38:14,020 --> 00:38:16,750 So there's lots of B vitamins going on here. 684 00:38:16,750 --> 00:38:20,340 So the standard reduction potential for vitamin B12 685 00:38:20,340 --> 00:38:22,480 is minus 0.526. 686 00:38:22,480 --> 00:38:25,320 That's a really low number for biology. 687 00:38:25,320 --> 00:38:29,680 And for flavodoxin, it's minus 0.23. 688 00:38:29,680 --> 00:38:34,201 So we ask, which is the better reducing agent? 689 00:38:34,201 --> 00:38:35,534 And that's the clicker question. 690 00:38:46,970 --> 00:38:48,405 Let's just do 10 more seconds. 691 00:39:03,480 --> 00:39:05,940 Yep. 692 00:39:05,940 --> 00:39:09,190 So the better reducing agent wants to reduce other things 693 00:39:09,190 --> 00:39:11,940 and get oxidized itself. 694 00:39:11,940 --> 00:39:17,800 So vitamin B12, with its low negative potential, 695 00:39:17,800 --> 00:39:19,780 is very reducing. 696 00:39:19,780 --> 00:39:21,280 It wants to reduce other things. 697 00:39:21,280 --> 00:39:22,910 It wants to be an agent of reduction. 698 00:39:26,020 --> 00:39:28,040 But that's not what's supposed to happen. 699 00:39:28,040 --> 00:39:30,370 It's not supposed to reduce flavodoxin. 700 00:39:30,370 --> 00:39:32,590 It's supposed to happen the other way around. 701 00:39:32,590 --> 00:39:36,190 So we could ask, is the reduction, then, 702 00:39:36,190 --> 00:39:40,390 of vitamin B13 by flavodoxin spontaneous 703 00:39:40,390 --> 00:39:43,440 if this is the case, yes or no? 704 00:39:43,440 --> 00:39:44,830 No. 705 00:39:44,830 --> 00:39:46,360 It shouldn't be spontaneous. 706 00:39:46,360 --> 00:39:49,170 It's not going the way that it should go. 707 00:39:49,170 --> 00:39:54,130 So we can just calculate how exactly non-spontaneous it is. 708 00:39:54,130 --> 00:40:00,340 So we can calculate both what the change in potential 709 00:40:00,340 --> 00:40:05,290 is, and we can also calculate delta G0 from this. 710 00:40:05,290 --> 00:40:08,540 So we saw how to do this before. 711 00:40:08,540 --> 00:40:10,870 But we were talking about different kinds 712 00:40:10,870 --> 00:40:13,360 of galvanic or electrolytic cells. 713 00:40:13,360 --> 00:40:16,510 But now we can use the same equation 714 00:40:16,510 --> 00:40:19,060 that we saw before but talk about it in terms 715 00:40:19,060 --> 00:40:20,890 of a biological system. 716 00:40:20,890 --> 00:40:25,870 So before we saw to calculate E0 of the cell, the cell 717 00:40:25,870 --> 00:40:28,960 potential, we had the standard reduction potential 718 00:40:28,960 --> 00:40:31,600 for the couple at the anode minus the standard reduction 719 00:40:31,600 --> 00:40:33,730 potential for a couple at the cathode. 720 00:40:33,730 --> 00:40:36,100 We can do the same thing here but just 721 00:40:36,100 --> 00:40:40,130 put reduction and oxidation over here. 722 00:40:40,130 --> 00:40:42,760 So we can make this a generic equation. 723 00:40:42,760 --> 00:40:45,250 And we can plug our values in. 724 00:40:45,250 --> 00:40:47,980 So the thing being reduced is vitamin B12. 725 00:40:47,980 --> 00:40:51,550 So we want to put the reduction potential of B12 in there. 726 00:40:51,550 --> 00:40:55,570 And the thing being oxidized is the flavodoxin. 727 00:40:55,570 --> 00:40:59,110 And so minus 0.526six for the B12. 728 00:40:59,110 --> 00:41:00,700 Standard reduction potential would 729 00:41:00,700 --> 00:41:03,340 be 12 minus the standard reduction 730 00:41:03,340 --> 00:41:05,230 potential of flavodoxin. 731 00:41:05,230 --> 00:41:10,810 And we get a negative number, minus 0.0296 volts. 732 00:41:10,810 --> 00:41:13,980 So then we have a negative value, 733 00:41:13,980 --> 00:41:17,730 which should tell us that delta G is going to be positive. 734 00:41:17,730 --> 00:41:20,500 But we can also calculate that value 735 00:41:20,500 --> 00:41:22,570 from the equation you saw before. 736 00:41:22,570 --> 00:41:26,200 So just because we are talking about these equations in terms 737 00:41:26,200 --> 00:41:28,540 of cells before, we can also talk 738 00:41:28,540 --> 00:41:32,170 about it's same exact equation that we use in biology. 739 00:41:32,170 --> 00:41:35,540 So minus n, number of moles of electrons-- 740 00:41:35,540 --> 00:41:37,384 this is a one-electron process. 741 00:41:37,384 --> 00:41:38,800 I would have had to tell you that. 742 00:41:38,800 --> 00:41:41,470 You wouldn't necessarily have known it's one electron. 743 00:41:41,470 --> 00:41:43,300 But that's our one. 744 00:41:43,300 --> 00:41:46,210 What is this term here, this kind of weird-looking F term? 745 00:41:46,210 --> 00:41:48,080 What is that again? 746 00:41:48,080 --> 00:41:49,720 Faraday's constant. 747 00:41:49,720 --> 00:41:52,150 Then we plug in our cell potential 748 00:41:52,150 --> 00:41:54,170 or potential difference. 749 00:41:54,170 --> 00:41:57,850 And we get out a value positive 28.6 kilojoules 750 00:41:57,850 --> 00:42:00,580 per mole-- big positive number. 751 00:42:00,580 --> 00:42:04,150 So is this spontaneous? 752 00:42:04,150 --> 00:42:06,710 Why don't we all have heart disease? 753 00:42:06,710 --> 00:42:09,890 It is not a spontaneous reaction. 754 00:42:09,890 --> 00:42:11,170 So how is it driven forward ? 755 00:42:11,170 --> 00:42:13,040 This happens in our body. 756 00:42:13,040 --> 00:42:14,080 This reaction happens. 757 00:42:14,080 --> 00:42:18,640 So clearly something has to happen to make it spontaneous. 758 00:42:18,640 --> 00:42:21,700 So when we were talking about the different kinds of cells, 759 00:42:21,700 --> 00:42:24,730 we had electrolytic cell, where we could put in a current 760 00:42:24,730 --> 00:42:27,610 to drive a non-spontaneous reaction. 761 00:42:27,610 --> 00:42:30,730 Same thing happens in biology, and the current 762 00:42:30,730 --> 00:42:32,890 in this particular case is a molecule 763 00:42:32,890 --> 00:42:35,170 known as adenosylmethionine. 764 00:42:35,170 --> 00:42:38,710 So it's the methionine amino acid within a denosyl group 765 00:42:38,710 --> 00:42:40,060 attached to it. 766 00:42:40,060 --> 00:42:44,590 And the cleavage of this molecule is very spontaneous. 767 00:42:44,590 --> 00:42:49,120 It has a delta G of minus 30.6 kilojoules, which 768 00:42:49,120 --> 00:42:51,150 is greater than what we need. 769 00:42:51,150 --> 00:42:54,280 And we can even calculate the net delta G 770 00:42:54,280 --> 00:42:56,860 for this reaction, the coupled reaction, 771 00:42:56,860 --> 00:42:59,420 of the cleavage with the reduction. 772 00:42:59,420 --> 00:43:05,470 And if we do that, we have minus 37.6 plus 28.6, so 773 00:43:05,470 --> 00:43:09,350 minus 9.0 kilojoules per mole. 774 00:43:09,350 --> 00:43:13,420 So we can drive this unfavorable reduction reaction 775 00:43:13,420 --> 00:43:16,030 with a very favorable reaction, the cleavage 776 00:43:16,030 --> 00:43:17,731 of adenosylmethionine. 777 00:43:17,731 --> 00:43:18,730 And that's how it works. 778 00:43:18,730 --> 00:43:22,810 So that's why we're all healthy. 779 00:43:22,810 --> 00:43:26,020 So this is the end of our oxidation reduction example. 780 00:43:26,020 --> 00:43:28,980 We'll transition to transition metals. 781 00:43:28,980 --> 00:43:32,500 But we're not moving very far away from oxidation reduction. 782 00:43:32,500 --> 00:43:35,710 Because to do problems that are effective transition metals, 783 00:43:35,710 --> 00:43:38,200 you have to know how to determine oxidation numbers 784 00:43:38,200 --> 00:43:40,410 and all sorts of things.