1 00:00:00,000 --> 00:00:00,016 The following content is provided under a Creative 2 00:00:00,016 --> 00:00:00,022 Commons license. 3 00:00:00,022 --> 00:00:00,038 Your support will help MIT OpenCourseWare continue to 4 00:00:00,038 --> 00:00:00,054 offer high quality educational resources for free. 5 00:00:00,054 --> 00:00:00,072 To make a donation or view additional materials from 6 00:00:00,072 --> 00:00:00,088 hundreds of MIT courses, visit MIT OpenCourseWare at 7 00:00:00,088 --> 00:00:00,110 ocw.mit.edu. 8 00:00:00,110 --> 00:00:29,140 PROFESSOR: So, you have now 10 seconds, go ahead and click in 9 00:00:29,140 --> 00:00:43,560 your answer. 10 00:00:43,560 --> 00:00:43,870 OK. 11 00:00:43,870 --> 00:00:47,270 We have a little bit of a mixed response here. 12 00:00:47,270 --> 00:00:49,860 So for those of you coming in late, I'm going to give you a 13 00:00:49,860 --> 00:00:55,260 second chance, and we say now what if the p k a was 4. 14 00:00:55,260 --> 00:00:59,600 Everything else is the same, but the p k a is now 4 -- what 15 00:00:59,600 --> 00:01:17,850 would the answer be? 16 00:01:17,850 --> 00:01:22,600 OK, go ahead and click in your response for a p k a of 4, 17 00:01:22,600 --> 00:01:45,160 everything else being the same. 18 00:01:45,160 --> 00:01:59,920 OK, let's do 10 seconds. 19 00:01:59,920 --> 00:02:01,910 Interesting. 20 00:02:01,910 --> 00:02:04,500 More people got that right. 21 00:02:04,500 --> 00:02:07,730 So it's the same answer for a p k a of 3 as a p k a of 4, 22 00:02:07,730 --> 00:02:11,230 but it gave people the opportunity to think about 23 00:02:11,230 --> 00:02:14,410 that a little more and see what their neighbors had voted 24 00:02:14,410 --> 00:02:16,490 and make a decision based on that. 25 00:02:16,490 --> 00:02:19,220 And so more people came up with the right answer. 26 00:02:19,220 --> 00:02:22,560 So let's consider now for a minute why this is true. 27 00:02:22,560 --> 00:02:25,430 So if you can switch to my lecture notes, I have a couple 28 00:02:25,430 --> 00:02:29,040 of slides on this that are not in your handout but that are 29 00:02:29,040 --> 00:02:31,360 related to things we've talked about. 30 00:02:31,360 --> 00:02:35,920 So just a brief sort of reminder of where we were with 31 00:02:35,920 --> 00:02:37,860 acid based titrations. 32 00:02:37,860 --> 00:02:42,340 So, we last time talked about what's happening at different 33 00:02:42,340 --> 00:02:47,800 points of the p h curve, and introduced this concept of 1/2 34 00:02:47,800 --> 00:02:51,470 equivalence point where the p h equals the p k a, where you 35 00:02:51,470 --> 00:02:54,370 have equal number of moles of molecules that are pronated as 36 00:02:54,370 --> 00:02:54,500 depronated. 37 00:02:54,500 --> 00:02:59,210 And another way to sort of think about this whole thing 38 00:02:59,210 --> 00:03:03,200 is that in the beginning at p h's that are lower than the p 39 00:03:03,200 --> 00:03:06,390 k a, you have more pronated molecules. 40 00:03:06,390 --> 00:03:09,490 And if p h is much above the p k a, you have 41 00:03:09,490 --> 00:03:12,860 more depronated molecules. 42 00:03:12,860 --> 00:03:14,950 So, we can think about things that way. 43 00:03:14,950 --> 00:03:18,020 So let's think about the last question that I asked where 44 00:03:18,020 --> 00:03:20,250 are have a p k a of 4. 45 00:03:20,250 --> 00:03:24,380 So, if you gave someone a sample of molecules where the 46 00:03:24,380 --> 00:03:27,920 p h was equal to the p k a of the molecule, you'd be giving 47 00:03:27,920 --> 00:03:30,980 them a sample where there would be equal amounts of 48 00:03:30,980 --> 00:03:33,280 pronated and depronated. 49 00:03:33,280 --> 00:03:36,670 But instead, if you gave a sample where the p h is much 50 00:03:36,670 --> 00:03:41,340 above the p k a, then you're going to be giving them a 51 00:03:41,340 --> 00:03:43,690 sample that's mostly depronated. 52 00:03:43,690 --> 00:03:48,160 And I emphasize about using, in terms of titrations, using 53 00:03:48,160 --> 00:03:51,070 Henderson Hasselbalch for buffer, but it also can be 54 00:03:51,070 --> 00:03:53,970 applied to thinking about this type of problem. 55 00:03:53,970 --> 00:03:57,030 And so you can be thinking about what sort of ratio are 56 00:03:57,030 --> 00:04:01,290 you going to get of your pronated to depronated when 57 00:04:01,290 --> 00:04:05,360 you have given a particular p k a and a particular p h. 58 00:04:05,360 --> 00:04:11,560 And so, if the p h is really very far above the p k a, then 59 00:04:11,560 --> 00:04:13,920 you're going to be largely depronated, and here's the 60 00:04:13,920 --> 00:04:16,800 math for these two numbers. 61 00:04:16,800 --> 00:04:20,220 If you're at a p h that's really below the p k a, then 62 00:04:20,220 --> 00:04:22,580 your molecules are going to be pronated. 63 00:04:22,580 --> 00:04:24,920 And so, you're going to be thinking about this kind of 64 00:04:24,920 --> 00:04:27,150 thing when you get to organic chemistry, and also 65 00:04:27,150 --> 00:04:28,220 biochemistry. 66 00:04:28,220 --> 00:04:32,250 Biochemists spend a lot of time thinking about the p k 67 00:04:32,250 --> 00:04:35,950 a's of amino acid side chains, and there are often mechanisms 68 00:04:35,950 --> 00:04:38,810 that people propose of how an enzyme works, and they're 69 00:04:38,810 --> 00:04:42,290 proposing that some residue is going to play a role as a 70 00:04:42,290 --> 00:04:45,700 catalytic acid or a base in an enzyme mechanism. 71 00:04:45,700 --> 00:04:49,300 And often people are proposing things that really just don't 72 00:04:49,300 --> 00:04:52,910 make much sense in terms of the p k a of that molecule, 73 00:04:52,910 --> 00:04:55,500 that they're proposing, that it's going to be giving off a 74 00:04:55,500 --> 00:05:00,720 proton, but given the p h of the enzyme in the body and 75 00:05:00,720 --> 00:05:03,050 given the p k a, it wouldn't be pronated. 76 00:05:03,050 --> 00:05:05,000 So how is it going to give off something it's not going to 77 00:05:05,000 --> 00:05:07,990 have. So these are the kinds of questions people talk about 78 00:05:07,990 --> 00:05:11,320 in biochemistry, and if you're in a biochemistry seminar and 79 00:05:11,320 --> 00:05:13,182 someone's talking about something like this, there 80 00:05:13,182 --> 00:05:15,680 will probably be a hand go up in the audience and say "What 81 00:05:15,680 --> 00:05:18,730 do you think the p k a is of that amino acid that you're 82 00:05:18,730 --> 00:05:22,785 proposing that role in this enzyme mechanism?" And there 83 00:05:22,785 --> 00:05:25,930 have been people who are pretty high profile who've 84 00:05:25,930 --> 00:05:28,730 gotten themselves in trouble over p k a issues. 85 00:05:28,730 --> 00:05:31,940 So now, you should all be ready to raise your hand in 86 00:05:31,940 --> 00:05:34,360 those seminars and say "What do you think the p k a is of 87 00:05:34,360 --> 00:05:38,830 that residue?" So, you'll be hearing about p k a's later, 88 00:05:38,830 --> 00:05:42,630 and the thing that makes me very excited is that some 89 00:05:42,630 --> 00:05:45,180 other people who are taking chemistry here, may not be as 90 00:05:45,180 --> 00:05:49,230 aware of the sort of biological role of p k a's. 91 00:05:49,230 --> 00:05:51,690 Now you know something, you've had some of these in your 92 00:05:51,690 --> 00:05:55,440 problem-sets, and so when you go into those advanced classes 93 00:05:55,440 --> 00:05:58,570 and there's a discussion of p k a, you'll raise your hand 94 00:05:58,570 --> 00:06:01,350 and impress my colleagues with your tremendous 95 00:06:01,350 --> 00:06:03,080 knowledge of p k a's. 96 00:06:03,080 --> 00:06:05,150 So I'm very excited about that. 97 00:06:05,150 --> 00:06:08,620 And please send me an email when you get extra points 98 00:06:08,620 --> 00:06:10,440 because of knowledge of p k a's. 99 00:06:10,440 --> 00:06:15,900 I want to collect that information and use it for 100 00:06:15,900 --> 00:06:16,870 evil purposes. 101 00:06:16,870 --> 00:06:18,370 No. 102 00:06:18,370 --> 00:06:21,980 But anyway, you can think about p k a's of molecules 103 00:06:21,980 --> 00:06:24,520 now, which will be very handy to you later on. 104 00:06:24,520 --> 00:06:28,300 So it's not just -- some people tell me, if I promise 105 00:06:28,300 --> 00:06:32,320 never to titrate a weak acid with a strong base, do I have 106 00:06:32,320 --> 00:06:34,120 to take that part of the test? 107 00:06:34,120 --> 00:06:38,190 Well, you know it's not just about acid based titrations. 108 00:06:38,190 --> 00:06:41,320 Some of the things you learn in that are actually relevant 109 00:06:41,320 --> 00:06:43,350 to other things that most of you will see 110 00:06:43,350 --> 00:06:44,560 later in your career. 111 00:06:44,560 --> 00:06:45,960 All right. 112 00:06:45,960 --> 00:06:50,350 So, I love enzymes, enzymes are great, I'm a biochemist. 113 00:06:50,350 --> 00:06:53,750 Acid base is very important in biochemistry. 114 00:06:53,750 --> 00:06:57,180 And the other thing that is very important in biochemistry 115 00:06:57,180 --> 00:06:59,070 is oxidation reduction. 116 00:06:59,070 --> 00:07:01,860 So, in these two units that we're doing now that'll be on 117 00:07:01,860 --> 00:07:04,650 the third exam, you're learning a lot of the basic 118 00:07:04,650 --> 00:07:09,360 principles that apply to how enzymes work. 119 00:07:09,360 --> 00:07:12,930 And so we started last time talking about some rules. 120 00:07:12,930 --> 00:07:15,180 Rules of assigning oxidation number. 121 00:07:15,180 --> 00:07:17,960 So this is sort of the very basic knowledge that you need 122 00:07:17,960 --> 00:07:21,890 to know to go on and do oxidation reduction problems. 123 00:07:21,890 --> 00:07:24,510 And here are those rules, and now let's see look at some 124 00:07:24,510 --> 00:07:35,110 examples of how we're going to apply these rules. 125 00:07:35,110 --> 00:07:39,280 So, first let's look at a compound that has lithium and 126 00:07:39,280 --> 00:07:47,450 oxygen in it. 127 00:07:47,450 --> 00:07:48,640 All right. 128 00:07:48,640 --> 00:07:53,930 What do I know about lithium's oxidation number? 129 00:07:53,930 --> 00:07:55,950 What's it going to be? 130 00:07:55,950 --> 00:07:56,960 Plus 1. 131 00:07:56,960 --> 00:08:00,600 So, things in group 1 are going to be plus 1, 132 00:08:00,600 --> 00:08:02,660 and I have 2 of them. 133 00:08:02,660 --> 00:08:04,600 So we're going to be using one of those rules up 134 00:08:04,600 --> 00:08:06,120 there to assign it. 135 00:08:06,120 --> 00:08:07,300 What about oxygen? 136 00:08:07,300 --> 00:08:12,000 What's oxygen going to be in this molecule? 137 00:08:12,000 --> 00:08:14,620 Minus 2. 138 00:08:14,620 --> 00:08:19,080 So it's not in a peroxide here, so it'll be minus 2. 139 00:08:19,080 --> 00:08:24,290 And then we have plus 2 minus 2 is equal to 0, and that's 140 00:08:24,290 --> 00:08:27,540 good, because there's no charge on this particular 141 00:08:27,540 --> 00:08:30,270 molecule, so all of the oxidation numbers 142 00:08:30,270 --> 00:08:32,140 should add up to 0. 143 00:08:32,140 --> 00:08:33,630 That's also one of our rules. 144 00:08:33,630 --> 00:08:40,990 All right, so let's look at p c l 5. 145 00:08:40,990 --> 00:08:47,010 So, what we know about oxidation number of chloride? 146 00:08:47,010 --> 00:08:48,930 What are you guessing? 147 00:08:48,930 --> 00:08:50,710 Minus 1. 148 00:08:50,710 --> 00:08:55,130 What's the exception to that? 149 00:08:55,130 --> 00:08:59,680 When it's with what? 150 00:08:59,680 --> 00:09:02,100 When it's with oxygen it can be different. 151 00:09:02,100 --> 00:09:04,470 All right, so there's 5 of those. 152 00:09:04,470 --> 00:09:07,670 I haven't told you anything about phosphorous, but what 153 00:09:07,670 --> 00:09:09,470 can you guess it's going to be? 154 00:09:09,470 --> 00:09:11,350 STUDENT: 5. 155 00:09:11,350 --> 00:09:14,100 PROFESSOR: I love that enthusiasm, good. 156 00:09:14,100 --> 00:09:19,700 So, it will be 5, and so overall, we're going 157 00:09:19,700 --> 00:09:22,800 to add up to 0. 158 00:09:22,800 --> 00:09:25,720 So you won't always have a rule about everything in your 159 00:09:25,720 --> 00:09:29,020 molecule, but you'll be given something that you can get a 160 00:09:29,020 --> 00:09:31,220 handle on and then predict what the other 161 00:09:31,220 --> 00:09:34,660 thing is going to be. 162 00:09:34,660 --> 00:09:41,300 So let's do a couple more. 163 00:09:41,300 --> 00:09:47,030 So, what about h n o 3? 164 00:09:47,030 --> 00:09:51,030 So let's start with oxygen -- you told me about oxygen, 165 00:09:51,030 --> 00:09:52,660 what's oxygen going to be here? 166 00:09:52,660 --> 00:09:55,320 STUDENT: Negative 2. 167 00:09:55,320 --> 00:09:57,590 PROFESSOR: Negative 2 and there's 3 of those. 168 00:09:57,590 --> 00:10:01,600 And we need to have this all add up to 0 again. 169 00:10:01,600 --> 00:10:03,280 So, what about hydrogen? 170 00:10:03,280 --> 00:10:05,990 STUDENT: Plus 1. 171 00:10:05,990 --> 00:10:07,330 PROFESSOR: Plus 1. 172 00:10:07,330 --> 00:10:09,170 What's the exception to that rule? 173 00:10:09,170 --> 00:10:14,530 When its with a metal. 174 00:10:14,530 --> 00:10:19,260 So what does that leave for nitrogen? 175 00:10:19,260 --> 00:10:21,400 Plus 5. 176 00:10:21,400 --> 00:10:23,920 And that will all add up. 177 00:10:23,920 --> 00:10:28,020 All right, so you're very good at this, so let's bring in 178 00:10:28,020 --> 00:10:31,110 that clicker competition, and you can tell 179 00:10:31,110 --> 00:10:56,560 me about this molecule. 180 00:10:56,560 --> 00:11:12,640 All right, 10 seconds. 181 00:11:12,640 --> 00:11:15,930 Excellent -- not necessarily for the competition since most 182 00:11:15,930 --> 00:11:17,960 people got it right, but I love seeing 183 00:11:17,960 --> 00:11:19,210 people get it right. 184 00:11:19,210 --> 00:11:31,150 So, 2 times plus 1 minus 2 is 0, so we know our states here. 185 00:11:31,150 --> 00:11:33,390 And I'll just tell you another thing you can do in doing 186 00:11:33,390 --> 00:11:36,330 these problems, if you recognize that something is 187 00:11:36,330 --> 00:11:40,450 often seen has a unit here, you can also think about this 188 00:11:40,450 --> 00:11:47,760 as h plus and n o 3 minus, and you would get the same answers 189 00:11:47,760 --> 00:11:48,610 to the problem. 190 00:11:48,610 --> 00:11:50,950 So, if it's a little complicated and you want to 191 00:11:50,950 --> 00:11:54,050 break it apart, you can do that too, you'll get the same 192 00:11:54,050 --> 00:11:55,620 answers there. 193 00:11:55,620 --> 00:11:59,330 All right, so as I said, in this unit it's a lot of adding 194 00:11:59,330 --> 00:12:04,320 and subtracting, so this part is not too complicated, but 195 00:12:04,320 --> 00:12:07,280 you just have to be paying attention and doing your math, 196 00:12:07,280 --> 00:12:08,650 simple math, correctly. 197 00:12:08,650 --> 00:12:13,060 All right, so we looked at some examples. 198 00:12:13,060 --> 00:12:16,640 So now we're going to give some definitions. 199 00:12:16,640 --> 00:12:21,630 What is oxidation? 200 00:12:21,630 --> 00:12:23,300 What happens when something is oxidized? 201 00:12:23,300 --> 00:12:24,010 Yup. 202 00:12:24,010 --> 00:12:34,700 So, when you oxidize something, electrons are lost. 203 00:12:34,700 --> 00:12:37,270 What happens when you reduce something? 204 00:12:37,270 --> 00:12:39,390 STUDENT: Gain electrons. 205 00:12:39,390 --> 00:12:41,370 PROFESSOR: You gain electrons. 206 00:12:41,370 --> 00:12:43,770 Most people are good with those definitions. 207 00:12:43,770 --> 00:12:46,700 Here are some that give people a little bit more trouble. 208 00:12:46,700 --> 00:12:49,200 Oxidizing agent. 209 00:12:49,200 --> 00:13:01,090 So it is an agent of oxidation, so it accepts -- an 210 00:13:01,090 --> 00:13:03,980 oxidation agent is something that wants to oxidize 211 00:13:03,980 --> 00:13:07,190 something else, so it gets reduced itself. 212 00:13:07,190 --> 00:13:08,640 So it's an agent of oxidation. 213 00:13:08,640 --> 00:13:12,190 It runs around trying to oxidize other things, but it 214 00:13:12,190 --> 00:13:15,190 itself is going to get reduced, and then you can 215 00:13:15,190 --> 00:13:19,370 probably figure out what a reducing agent is, it's an 216 00:13:19,370 --> 00:13:21,890 agent of reduction. 217 00:13:21,890 --> 00:13:24,520 And so it itself will be oxidized. 218 00:13:24,520 --> 00:13:26,170 It'll try to reduce something else. 219 00:13:26,170 --> 00:13:28,910 It'll run around trying to reduce something, trying to 220 00:13:28,910 --> 00:13:33,600 give off its electrons, donate its electrons so that it can 221 00:13:33,600 --> 00:13:36,780 be oxidized. 222 00:13:36,780 --> 00:13:39,840 So, keep these definitions in mind, because you're going to 223 00:13:39,840 --> 00:13:47,580 be using them a lot in this unit. 224 00:13:47,580 --> 00:13:49,580 So now we're going to use them, we're going to look at a 225 00:13:49,580 --> 00:13:52,790 reaction, and we're going to think about what's being 226 00:13:52,790 --> 00:13:57,340 oxidized and what is being reduced. 227 00:13:57,340 --> 00:14:01,870 And this particular type of reaction, a disproportionation 228 00:14:01,870 --> 00:14:10,760 reaction, the same element can be both oxidized and reduced. 229 00:14:10,760 --> 00:14:14,890 So let's break this down into two equations. 230 00:14:14,890 --> 00:14:17,660 In every reaction there's going to be an oxidation and a 231 00:14:17,660 --> 00:14:20,820 reduction, and so you can write those separately. 232 00:14:20,820 --> 00:14:23,940 And poor sodium, sodium is getting kind of a jip. 233 00:14:23,940 --> 00:14:29,140 A lot in these last few units in acid base, and here again, 234 00:14:29,140 --> 00:14:33,300 it's a spectator ion here, so it doesn't even make it into 235 00:14:33,300 --> 00:14:34,670 the equation. 236 00:14:34,670 --> 00:14:38,430 And it was ineffective as a conjugate acid, it hasn't 237 00:14:38,430 --> 00:14:40,160 really been doing very much recently. 238 00:14:40,160 --> 00:14:41,290 But that's OK. 239 00:14:41,290 --> 00:14:44,620 All right, so let's look at what's going on here and try 240 00:14:44,620 --> 00:14:47,540 to figure out what's happening in terms of what's being 241 00:14:47,540 --> 00:14:50,490 reduced and what's being oxidized. 242 00:14:50,490 --> 00:14:54,890 So let's start over here, let's think about what the 243 00:14:54,890 --> 00:14:59,560 oxidation numbers are in this particular molecule. 244 00:14:59,560 --> 00:15:05,130 So what would be true about oxygen here and chloride. 245 00:15:05,130 --> 00:15:08,840 What do we know about this combination of things in terms 246 00:15:08,840 --> 00:15:14,090 of oxidation numbers? 247 00:15:14,090 --> 00:15:25,990 So let's start with chloride, what's that going to be here? 248 00:15:25,990 --> 00:15:28,410 So, we have a minus 2 here. 249 00:15:28,410 --> 00:15:32,010 And the whole thing is going to be minus 1, and so what is 250 00:15:32,010 --> 00:15:32,450 that about chloride? 251 00:15:32,450 --> 00:15:35,900 Plus 1, right. 252 00:15:35,900 --> 00:15:37,950 So this is one of the exceptions. 253 00:15:37,950 --> 00:15:41,650 Chloride is usually minus 1, except when it's with oxygen, 254 00:15:41,650 --> 00:15:44,610 and here we have an overall charge of the molecule of 255 00:15:44,610 --> 00:15:47,980 minus 1, so it all has to add up to minus 1 and it does. 256 00:15:47,980 --> 00:15:51,190 All right, so now let's do that side. 257 00:15:51,190 --> 00:15:54,030 So what's going on here? 258 00:15:54,030 --> 00:15:56,270 Are they going to be similar or different? 259 00:15:56,270 --> 00:16:01,550 Let's start with the oxygen, what's that going to be? 260 00:16:01,550 --> 00:16:04,190 So we have three negative 2's. 261 00:16:04,190 --> 00:16:07,360 It's not a peroxide, so it's negative 2 for oxygen. 262 00:16:07,360 --> 00:16:11,300 The overall has to be equal to minus 1, so 263 00:16:11,300 --> 00:16:12,380 what is chloride here? 264 00:16:12,380 --> 00:16:15,360 Plus 5, right. 265 00:16:15,360 --> 00:16:17,480 So that's unusual, but that's what it is. 266 00:16:17,480 --> 00:16:19,780 So we use our rules. 267 00:16:19,780 --> 00:16:22,750 Chloride is usually minus 1, except when it's with oxygen, 268 00:16:22,750 --> 00:16:24,780 and then it can be something different. 269 00:16:24,780 --> 00:16:28,690 So, here chloride's going from plus 1 to plus 5, so what's 270 00:16:28,690 --> 00:16:29,570 happening to it? 271 00:16:29,570 --> 00:16:37,740 Is it being oxidized or reduced? 272 00:16:37,740 --> 00:16:38,210 Yup. 273 00:16:38,210 --> 00:16:41,450 So it's going from plus 1 to plus 5, so we have an 274 00:16:41,450 --> 00:16:43,200 oxidation going on. 275 00:16:43,200 --> 00:16:46,010 So to tell whether something is an oxidation or not, you 276 00:16:46,010 --> 00:16:49,330 need to figure out what the oxidation numbers are and then 277 00:16:49,330 --> 00:16:52,170 see what's changing in the course of that reaction. 278 00:16:52,170 --> 00:16:57,800 All right, so down here, we've already done this one, so we 279 00:16:57,800 --> 00:16:59,840 can put that down here. 280 00:16:59,840 --> 00:17:03,910 So we have minus 2 for oxygen, overall minus 1, and so 281 00:17:03,910 --> 00:17:05,700 chloride is plus 1 again. 282 00:17:05,700 --> 00:17:08,090 And what's the oxidation number on the 283 00:17:08,090 --> 00:17:11,010 other side for chlorine? 284 00:17:11,010 --> 00:17:12,310 Minus 1. 285 00:17:12,310 --> 00:17:15,300 I just have to look, and so that's minus 1. 286 00:17:15,300 --> 00:17:20,680 So here we're going from plus 1 to minus 1, so what is that? 287 00:17:20,680 --> 00:17:22,320 That's a reduction. 288 00:17:22,320 --> 00:17:25,960 And if we had figured out that they're both oxidations and 289 00:17:25,960 --> 00:17:28,280 something we would have done incorrectly, because in these 290 00:17:28,280 --> 00:17:31,090 reactions you're going to have oxidations and reductions. 291 00:17:31,090 --> 00:17:35,650 So, here, and this is disproportionation. 292 00:17:35,650 --> 00:17:37,280 So, you have chloride in one state, and in another, it's 293 00:17:37,280 --> 00:17:43,030 undergoing a reduction. 294 00:17:43,030 --> 00:17:46,820 OK, so that's how you sort of think about what's happening 295 00:17:46,820 --> 00:17:48,720 in these types of reactions. 296 00:17:48,720 --> 00:17:51,440 So now we need to balance reactions. 297 00:17:51,440 --> 00:17:54,220 And this is very important in getting the correct answer to 298 00:17:54,220 --> 00:17:58,430 the later problems, and so we'll go through an example of 299 00:17:58,430 --> 00:17:59,670 how you're going to balance. 300 00:17:59,670 --> 00:18:01,740 You need to think about whether it's an acidic 301 00:18:01,740 --> 00:18:03,670 solution or basic solution, and we'll talk 302 00:18:03,670 --> 00:18:07,020 about that at the end. 303 00:18:07,020 --> 00:18:11,820 So, first here, we can look at the two 1/2 reactions going 304 00:18:11,820 --> 00:18:14,410 on, we have iron, we have chromium, and so we're going 305 00:18:14,410 --> 00:18:17,590 to look at those separately. 306 00:18:17,590 --> 00:18:22,190 So here is the first one, and let's think about what's 307 00:18:22,190 --> 00:18:23,870 happening over here. 308 00:18:23,870 --> 00:18:38,010 So, what would our oxidation number of the oxygen be here? 309 00:18:38,010 --> 00:18:39,590 What is happening to it? 310 00:18:39,590 --> 00:18:42,800 So, figure out what the oxidation numbers are of the 311 00:18:42,800 --> 00:18:45,480 chromium on one side, you know what it is on the other side, 312 00:18:45,480 --> 00:19:06,270 and then tell me what's happening to it. 313 00:19:06,270 --> 00:19:07,440 I hear some murmuring. 314 00:19:07,440 --> 00:19:09,970 People in one recitation aren't helping out people in 315 00:19:09,970 --> 00:19:16,900 the other recitation, are they? 316 00:19:16,900 --> 00:19:19,460 All right. 317 00:19:19,460 --> 00:19:21,060 Do you need more times? 318 00:19:21,060 --> 00:19:22,130 You good? 319 00:19:22,130 --> 00:19:22,490 You clicked in? 320 00:19:22,490 --> 00:19:39,930 Let's do 10 seconds. 321 00:19:39,930 --> 00:19:40,770 OK. 322 00:19:40,770 --> 00:19:42,170 People did pretty well on that one. 323 00:19:42,170 --> 00:19:44,150 Let's look at the answer to that. 324 00:19:44,150 --> 00:19:50,150 So, let's go back to my Powerpoint. 325 00:19:50,150 --> 00:19:56,640 And so, oxidation number for oxygen here, minus 2. 326 00:19:56,640 --> 00:20:00,890 The overall charge on that is minus 2. 327 00:20:00,890 --> 00:20:04,590 So you have to figure out what the math equals there. 328 00:20:04,590 --> 00:20:08,280 And so, if you run through the math, then you see that you're 329 00:20:08,280 --> 00:20:13,260 going from a plus 6 to a plus 3 state, 330 00:20:13,260 --> 00:20:17,580 so we have a reduction. 331 00:20:17,580 --> 00:20:21,810 OK, 75% did that, got that, good. 332 00:20:21,810 --> 00:20:25,720 So, most of you should be able to get this one now, iron plus 333 00:20:25,720 --> 00:20:26,610 2 to iron plus 3. 334 00:20:26,610 --> 00:20:29,400 Just yell it out, what is that? 335 00:20:29,400 --> 00:20:30,970 Yeah. 336 00:20:30,970 --> 00:20:33,500 So here we have our oxidation. 337 00:20:33,500 --> 00:20:35,500 Now let's balance this. 338 00:20:35,500 --> 00:20:39,180 And I left some nice blanks in your notes, but not so many 339 00:20:39,180 --> 00:20:42,640 that you won't be able to keep up and you should feel free to 340 00:20:42,640 --> 00:20:46,420 yell out the answers and we'll go through balancing this 341 00:20:46,420 --> 00:20:49,430 pretty quickly. 342 00:20:49,430 --> 00:20:51,310 So there's a couple of different rules. 343 00:20:51,310 --> 00:20:53,860 I'll just say some books have things differently. 344 00:20:53,860 --> 00:20:56,330 If you can get the right answer, you can use whatever 345 00:20:56,330 --> 00:20:58,070 procedure you want. 346 00:20:58,070 --> 00:21:01,870 I have found in the past that this particular procedure a 347 00:21:01,870 --> 00:21:06,150 lot of people find to be the easiest. So I'll teach you 348 00:21:06,150 --> 00:21:08,880 this one, but you're free to use whichever ones 349 00:21:08,880 --> 00:21:09,830 work well for you. 350 00:21:09,830 --> 00:21:13,270 All right, so the first thing we want to do is balance all 351 00:21:13,270 --> 00:21:17,060 elements that are not oxygen or hydrogen. 352 00:21:17,060 --> 00:21:19,030 To make it equal on both sides, we're going to do 353 00:21:19,030 --> 00:21:21,020 oxygen and hydrogen later. 354 00:21:21,020 --> 00:21:25,590 But what do we need to do up here to balance our 355 00:21:25,590 --> 00:21:29,440 non-oxygens? 356 00:21:29,440 --> 00:21:31,470 We need to add a what? 357 00:21:31,470 --> 00:21:35,150 A 2. 358 00:21:35,150 --> 00:21:38,250 So we need to add a 2 over here, two chromiums here, two 359 00:21:38,250 --> 00:21:39,390 on the other side. 360 00:21:39,390 --> 00:21:40,590 What about for iron? 361 00:21:40,590 --> 00:21:43,230 Nothing. 362 00:21:43,230 --> 00:21:46,560 All right, so that was pretty simple. 363 00:21:46,560 --> 00:21:52,530 Now, in this procedure you add water to balance the oxygens. 364 00:21:52,530 --> 00:21:56,400 So, what are we going to do up here. 365 00:21:56,400 --> 00:22:01,560 How many waters do we need to add? 366 00:22:01,560 --> 00:22:05,270 So we need to add 7 waters. 367 00:22:05,270 --> 00:22:08,200 And the bottom one, bottom one's pretty easy so far, we 368 00:22:08,200 --> 00:22:11,220 don't have to do anything. 369 00:22:11,220 --> 00:22:15,380 So, go ahead and write in your 7 waters. 370 00:22:15,380 --> 00:22:18,710 Next step is we're going to balance the hydrogens that we 371 00:22:18,710 --> 00:22:24,780 just added, and we can balance here with h plus, and I say 372 00:22:24,780 --> 00:22:26,850 here's one place that books are different. 373 00:22:26,850 --> 00:22:29,400 Some of them balance in the more sort of technically 374 00:22:29,400 --> 00:22:32,310 correct way with hydroium ions, but then your oxygens 375 00:22:32,310 --> 00:22:36,340 get unbalanced again, so it's OK to use the simpler 376 00:22:36,340 --> 00:22:39,520 approach, and just balance with h plus. 377 00:22:39,520 --> 00:22:42,380 So how many h plusses do we need to add to 378 00:22:42,380 --> 00:22:46,720 balance the top part? 379 00:22:46,720 --> 00:22:49,840 Yup, so we need to add 14 over here. 380 00:22:49,840 --> 00:22:53,550 Again, the bottom one, nothing to do. 381 00:22:53,550 --> 00:22:56,800 Pretty simple. 382 00:22:56,800 --> 00:23:00,280 Now we need to balance the charge, so we just added h 383 00:23:00,280 --> 00:23:04,770 plusses, so we just added some charge to this, and now we 384 00:23:04,770 --> 00:23:07,720 have to balance the overall charge, so you want the charge 385 00:23:07,720 --> 00:23:11,350 on one side of the equation to be equal to the charge on the 386 00:23:11,350 --> 00:23:13,440 other side of the equation. 387 00:23:13,440 --> 00:23:17,150 So how many electrons are we going to have to add to this 388 00:23:17,150 --> 00:23:30,860 top equation to balance the charge? 389 00:23:30,860 --> 00:23:31,700 I heard the answer. 390 00:23:31,700 --> 00:23:34,740 6. 391 00:23:34,740 --> 00:23:36,410 So we have to add 6. 392 00:23:36,410 --> 00:23:41,530 I told you, this unit involves adding and subtracting in your 393 00:23:41,530 --> 00:23:45,430 head, and you always want to check your work when you're 394 00:23:45,430 --> 00:23:49,340 doing this on an exam, because it's really sad when you lose 395 00:23:49,340 --> 00:23:52,220 points for something that is adding and subtracting. 396 00:23:52,220 --> 00:23:55,640 So you want to make sure that you don't lose points on these 397 00:23:55,640 --> 00:23:57,560 on an exam. 398 00:23:57,560 --> 00:24:00,120 So what about the bottom -- finally we get to do something 399 00:24:00,120 --> 00:24:03,550 with the bottom one, what do we do? 400 00:24:03,550 --> 00:24:07,010 So, we add how many electrons? 401 00:24:07,010 --> 00:24:08,750 1, right. 402 00:24:08,750 --> 00:24:12,530 OK, so now our charge is balanced. 403 00:24:12,530 --> 00:24:18,350 So now, we want to multiply up one of the 1/2 reactions so 404 00:24:18,350 --> 00:24:23,190 that the electrons are going to cancel, and so what do we 405 00:24:23,190 --> 00:24:27,950 have to multiply by the bottom equation so that the electrons 406 00:24:27,950 --> 00:24:30,320 cancel with the first? 407 00:24:30,320 --> 00:24:32,440 6. 408 00:24:32,440 --> 00:24:40,180 So we have 6, 6, and 6. 409 00:24:40,180 --> 00:24:43,520 And now, we're going to add those together and make the 410 00:24:43,520 --> 00:24:48,750 appropriate cancellations. 411 00:24:48,750 --> 00:24:52,120 So here is our overall equation -- we have the 6 412 00:24:52,120 --> 00:24:56,690 electrons, the 14 protons, we have our chromium oxide 413 00:24:56,690 --> 00:25:00,970 compound, we have the 6 iron 2 plusses, on the other side, 414 00:25:00,970 --> 00:25:05,060 the 2 chromium 3 plusses, the 7 waters, the 6 iron 3 415 00:25:05,060 --> 00:25:07,150 plusses, and the 6 electrons. 416 00:25:07,150 --> 00:25:11,010 So, we should be able to cancel the electrons, so we 417 00:25:11,010 --> 00:25:15,630 cancel those out. 418 00:25:15,630 --> 00:25:17,420 And now we want to double check that, 419 00:25:17,420 --> 00:25:18,710 in fact, it's balanced. 420 00:25:18,710 --> 00:25:21,300 Again, this is very important to do on the test, it's really 421 00:25:21,300 --> 00:25:24,230 easy to make some kind of math mistake, but you should be 422 00:25:24,230 --> 00:25:26,190 able to figure it out at this point. 423 00:25:26,190 --> 00:25:29,000 It won't be balanced if you've made a math mistake. 424 00:25:29,000 --> 00:25:32,830 So there should be 14 hydrogens over here, 14 over 425 00:25:32,830 --> 00:25:36,670 there, 2 chromiums, 2 chromiums, you have 7 oxygens 426 00:25:36,670 --> 00:25:40,590 here, 7 oxygens there, 6 irons, 6 irons. 427 00:25:40,590 --> 00:25:44,680 And the charge also should be balanced on both sides. 428 00:25:44,680 --> 00:25:47,290 So you can double check that and if it's 429 00:25:47,290 --> 00:25:48,910 good, then you're done. 430 00:25:48,910 --> 00:25:52,970 And this was in acidic solution, so we should have 431 00:25:52,970 --> 00:25:57,240 again this number and a charge of plus 24 on each side, 432 00:25:57,240 --> 00:26:01,830 double check, make sure it's correct. 433 00:26:01,830 --> 00:26:05,580 So, that was acidic solution and we ended up with an 434 00:26:05,580 --> 00:26:07,660 equation that had h plusses in it. 435 00:26:07,660 --> 00:26:11,740 You can also be asked to do it in basic solution, and again, 436 00:26:11,740 --> 00:26:14,960 books have different approaches here. 437 00:26:14,960 --> 00:26:19,200 What I like to do is the simplest thing, which is to 438 00:26:19,200 --> 00:26:23,480 use the same steps all the way up to here to get your same 439 00:26:23,480 --> 00:26:27,380 answer that you had for the acidic solution, and then 440 00:26:27,380 --> 00:26:29,590 neutralize it at this point. 441 00:26:29,590 --> 00:26:33,955 So, adjust the p h in quotes by adding hydroxide ions to 442 00:26:33,955 --> 00:26:40,090 both sides, and so, if we have 14 h plusses here, we can add 443 00:26:40,090 --> 00:26:45,120 14 o h's on one side, and 14 o h minus on the other side. 444 00:26:45,120 --> 00:26:49,760 And then we can add those guys together. 445 00:26:49,760 --> 00:26:52,670 So we have now 14 waters over here. 446 00:26:52,670 --> 00:26:56,310 And we have still our 7 waters on this side, and then the 14 447 00:26:56,310 --> 00:26:58,570 hydroxides on this side. 448 00:26:58,570 --> 00:27:00,710 And now, we should be able to cancel out 449 00:27:00,710 --> 00:27:03,810 some of those waters. 450 00:27:03,810 --> 00:27:08,090 So, we had 14 over here, 7 over here, so we can just have 451 00:27:08,090 --> 00:27:09,530 7 on this side. 452 00:27:09,530 --> 00:27:11,480 And now we have an equation that looks like 453 00:27:11,480 --> 00:27:13,040 it's in basic solution. 454 00:27:13,040 --> 00:27:16,090 So instead of having h plus, which you would have in acid, 455 00:27:16,090 --> 00:27:18,990 you have hydroxide for basic solution. 456 00:27:18,990 --> 00:27:20,340 So you can follow the same rules. 457 00:27:20,340 --> 00:27:21,760 Again, some books do it differently. 458 00:27:21,760 --> 00:27:25,030 I think this is the simplest way to do it, the least likely 459 00:27:25,030 --> 00:27:29,510 to make those kind of adding and subtracting errors. 460 00:27:29,510 --> 00:27:31,480 So those are sort of the fundamentals you 461 00:27:31,480 --> 00:27:32,720 need for this unit. 462 00:27:32,720 --> 00:27:36,120 You need to figure out oxidation numbers, looking at 463 00:27:36,120 --> 00:27:40,120 the composition of a molecule, and you need to be able to 464 00:27:40,120 --> 00:27:41,390 balance equations. 465 00:27:41,390 --> 00:27:44,370 When you can do that, you can go on to do other things. 466 00:27:44,370 --> 00:27:49,350 So, at this point -- oh, let me just say, those are the two 467 00:27:49,350 --> 00:27:53,520 answers there, so you can see acid and then in base. 468 00:27:53,520 --> 00:27:58,960 So, at this point then, I want to try to do a little demo to 469 00:27:58,960 --> 00:28:04,390 show you that when you do do oxidation reduction reactions, 470 00:28:04,390 --> 00:28:08,230 cool things can happen. 471 00:28:08,230 --> 00:28:13,070 So, it's more exciting, perhaps, in real life when 472 00:28:13,070 --> 00:28:15,830 oxidation happens than on paper. 473 00:28:15,830 --> 00:28:20,780 So I'm going to turn it over now to Dr. Taylor and Dr. 474 00:28:20,780 --> 00:28:24,310 Patti Christie who's here to help us with this demo. 475 00:28:24,310 --> 00:28:26,520 And they will -- go for it. 476 00:28:26,520 --> 00:28:28,390 PROFESSOR: OK, so this is mostly visual, there's not too 477 00:28:28,390 --> 00:28:29,520 much we'll need to say. 478 00:28:29,520 --> 00:28:30,820 Basically what we're going to do for 479 00:28:30,820 --> 00:28:33,250 you is oxidize magnesium. 480 00:28:33,250 --> 00:28:38,720 So we have a source of oxidation, which is going to 481 00:28:38,720 --> 00:28:42,440 be carbon dioxide, which is dry ice, and what we put 482 00:28:42,440 --> 00:28:45,540 inside the dry ice is solid magnesium. 483 00:28:45,540 --> 00:28:48,830 So we're going to set the magnesium on fire, and then 484 00:28:48,830 --> 00:28:53,430 put the oxidating agent on top of it, and you can see the 485 00:28:53,430 --> 00:28:55,070 rest of what's going to happen there. 486 00:28:55,070 --> 00:28:57,070 And you can actually also determine if this is an 487 00:28:57,070 --> 00:29:34,940 exothermic or an endothermic reaction while you're at it. 488 00:29:34,940 --> 00:29:35,090 [EXPERIMENTING] 489 00:29:35,090 --> 00:29:36,040 [APPLAUSE] 490 00:29:36,040 --> 00:30:36,260 PROFESSOR: Thanks to my helpers. 491 00:30:36,260 --> 00:30:46,160 All right. 492 00:30:46,160 --> 00:30:50,400 So oxidation reduction reactions can be quite 493 00:30:50,400 --> 00:30:56,280 interesting, and don't try that at home. 494 00:30:56,280 --> 00:31:00,890 So, we're going to continue on now, you know the basics and 495 00:31:00,890 --> 00:31:03,110 we're going to talk about electric chemical cells. 496 00:31:03,110 --> 00:31:06,620 We're going to introduce Faraday's law, and a thing I 497 00:31:06,620 --> 00:31:11,320 love to do, is come back to my friend Gibb's free energy. 498 00:31:11,320 --> 00:31:16,540 So, thermodynamics is all over the place in chemistry and you 499 00:31:16,540 --> 00:31:18,420 can't get very far away from it. 500 00:31:18,420 --> 00:31:20,540 So hopefully, by the end of the class today, we'll come 501 00:31:20,540 --> 00:31:23,060 back to free energy. 502 00:31:23,060 --> 00:31:25,790 All right, so what is an electric chemical cell. 503 00:31:25,790 --> 00:31:30,000 It's any device in which electric current, which is a 504 00:31:30,000 --> 00:31:35,630 flow of electrons through a circuit, is either produced by 505 00:31:35,630 --> 00:31:39,470 a spontaneous reaction, or used to bring about a 506 00:31:39,470 --> 00:31:41,780 non-spontaneous reaction. 507 00:31:41,780 --> 00:31:44,530 And so a battery is technically a collection of 508 00:31:44,530 --> 00:31:49,090 cells in a series, so that the voltage that each cell 509 00:31:49,090 --> 00:31:52,905 produces is the sum, the battery has the sum of the 510 00:31:52,905 --> 00:31:55,640 voltages of each cell. 511 00:31:55,640 --> 00:31:59,200 So, let's take a look at what some of these might look like. 512 00:31:59,200 --> 00:32:02,960 Here's a little cartoon of a simple version. 513 00:32:02,960 --> 00:32:04,590 We have a beaker -- 514 00:32:04,590 --> 00:32:07,180 2 beakers with different solutions in them, 2 515 00:32:07,180 --> 00:32:11,270 electrodes, a salt bridge across, and as electrons 516 00:32:11,270 --> 00:32:18,530 transfer through this, you can read a current on an amp meter 517 00:32:18,530 --> 00:32:21,170 here, you can read some kind of voltage coming off. 518 00:32:21,170 --> 00:32:26,590 And so, I like to talk about how good you guys have it at 519 00:32:26,590 --> 00:32:30,750 MIT with the Web and handouts and everything and how 520 00:32:30,750 --> 00:32:33,010 students in the old days at MIT, if they're going to do 521 00:32:33,010 --> 00:32:35,400 their problem-sets at night, first they had to build a 522 00:32:35,400 --> 00:32:39,070 battery to get electricity to be able to see. 523 00:32:39,070 --> 00:32:42,550 So you guys have it easy, you have these electric lights and 524 00:32:42,550 --> 00:32:47,080 all this fancy stuff now here. 525 00:32:47,080 --> 00:32:49,940 So, let's sort of break apart sort of a components. 526 00:32:49,940 --> 00:32:55,010 Here is my beautiful picture that I drew of this system, 527 00:32:55,010 --> 00:32:58,090 that's why I show you a better cartoon first. So this is a 528 00:32:58,090 --> 00:33:01,100 beaker that you -- here's one beaker with one solution on 529 00:33:01,100 --> 00:33:04,030 this side, here is the other beaker. 530 00:33:04,030 --> 00:33:09,520 So we have two electrodes put in, we have a salt bridge, and 531 00:33:09,520 --> 00:33:13,570 then we have a wire across where we can measure voltage. 532 00:33:13,570 --> 00:33:17,000 So, let's think about what is happening on each side. 533 00:33:17,000 --> 00:33:22,260 So in one beaker you're going to have an oxidation 534 00:33:22,260 --> 00:33:24,570 reduction, and in the other beaker you're 535 00:33:24,570 --> 00:33:26,980 going to have a reduction. 536 00:33:26,980 --> 00:33:30,050 So if we think about what's happening over here, you could 537 00:33:30,050 --> 00:33:33,750 have a zinc system, is oxidized from zinc 0 538 00:33:33,750 --> 00:33:36,190 to zinc plus 2. 539 00:33:36,190 --> 00:33:39,270 And so, this is sort of a view of what would be happening at 540 00:33:39,270 --> 00:33:42,720 the electrode, you have zinc solid or zinc 0, and you've 541 00:33:42,720 --> 00:33:45,360 also have zinc plus 2 in solution. 542 00:33:45,360 --> 00:33:50,690 And so, as you take a zinc solid atom, going into zinc 543 00:33:50,690 --> 00:33:53,550 plus 2, you can have an oxidation reaction. 544 00:33:53,550 --> 00:33:56,590 And here would be the equation going down. 545 00:33:56,590 --> 00:33:59,650 So zinc solid to zinc plus 2 with two electrons. 546 00:33:59,650 --> 00:34:03,370 Those electrons can go through to our other beaker on this 547 00:34:03,370 --> 00:34:09,770 other side, and so here we have a copper solid electrode 548 00:34:09,770 --> 00:34:15,150 or a cathode in this case, and we have copper plus 2 in 549 00:34:15,150 --> 00:34:18,940 solution, which is being reduced to copper solid, and 550 00:34:18,940 --> 00:34:22,220 so we're plating on to our electrode on this side. 551 00:34:22,220 --> 00:34:25,040 And so, here we have the reduction reaction, copper 2 552 00:34:25,040 --> 00:34:27,570 with two electrons going to copper solid. 553 00:34:27,570 --> 00:34:29,630 So our oxidation is happening at an 554 00:34:29,630 --> 00:34:31,280 electrode called the anode. 555 00:34:31,280 --> 00:34:33,030 The reduction is happening in an 556 00:34:33,030 --> 00:34:34,480 electrode called the cathode. 557 00:34:34,480 --> 00:34:38,260 And as these reactions occur, you're changing the charge on 558 00:34:38,260 --> 00:34:39,380 either side. 559 00:34:39,380 --> 00:34:43,030 So, you have this salt bridge, and so to neutralize the 560 00:34:43,030 --> 00:34:46,850 change in charge, you would have negative ions come down 561 00:34:46,850 --> 00:34:49,120 here, because we're producing more plus 2. 562 00:34:49,120 --> 00:34:52,790 And on the other side potassium is going in as we're 563 00:34:52,790 --> 00:34:56,470 going from copper plus 2 to copper solid, again, to 564 00:34:56,470 --> 00:34:59,430 balance this change in charge that's occurring. 565 00:34:59,430 --> 00:35:03,800 So those are the basic components of a simple 566 00:35:03,800 --> 00:35:06,980 electrochemical cell. 567 00:35:06,980 --> 00:35:09,510 So, we could talk about the cell -- here's some 568 00:35:09,510 --> 00:35:12,820 nomenclature that represents the cell. 569 00:35:12,820 --> 00:35:18,120 So, the one that I just showed you, you have one electrode, a 570 00:35:18,120 --> 00:35:23,670 zinc solid, and we also have zinc plus 2 in solution. 571 00:35:23,670 --> 00:35:27,650 You have a single line between the solid and the zinc plus 2 572 00:35:27,650 --> 00:35:30,640 ions in solution, that indicates there's a phase 573 00:35:30,640 --> 00:35:34,610 boundary, so you're changing phase from solid to aqueous. 574 00:35:34,610 --> 00:35:38,600 Then if you see two little lines that tells you, okay, 575 00:35:38,600 --> 00:35:41,450 one part of the reaction's in one beaker, the other part is 576 00:35:41,450 --> 00:35:44,240 in the other beaker, that represents the salt bridge 577 00:35:44,240 --> 00:35:47,580 which separates out the two 1/2 reactions. 578 00:35:47,580 --> 00:35:50,360 And then on this side, we're going from copper 2 in 579 00:35:50,360 --> 00:35:54,130 solution, single line which indicates the phase boundary, 580 00:35:54,130 --> 00:35:59,210 to a copper solid over here. 581 00:35:59,210 --> 00:36:08,150 So the amount of charge that goes through the system, it 582 00:36:08,150 --> 00:36:12,850 depends -- and how much of the zinc that is consumed or the 583 00:36:12,850 --> 00:36:15,960 copper that is deposited, is proportional to that charge, 584 00:36:15,960 --> 00:36:18,750 the number of electrons that go through the system, and 585 00:36:18,750 --> 00:36:23,160 this is called Faraday's law. 586 00:36:23,160 --> 00:36:26,280 So let's look at what is happening a little bit here, 587 00:36:26,280 --> 00:36:30,760 so we have a little movie that shows the oxidation reaction. 588 00:36:30,760 --> 00:36:35,800 So in green is the electrode, and then here are the water 589 00:36:35,800 --> 00:36:38,790 molecules, and let's look at what happens when 590 00:36:38,790 --> 00:36:40,660 there's this oxidation. 591 00:36:40,660 --> 00:36:42,610 So here things are floating around, and 592 00:36:42,610 --> 00:36:43,740 there go the electrons. 593 00:36:43,740 --> 00:36:47,860 And this pops off into solutions, so if this is zinc, 594 00:36:47,860 --> 00:36:50,970 so then as this is happening, we have a zinc plus 2 now, 595 00:36:50,970 --> 00:36:51,990 it's aqueous. 596 00:36:51,990 --> 00:36:54,310 There go electrons, we have our oxidation. 597 00:36:54,310 --> 00:36:57,320 Here comes off zinc plus 2 floating around, there's 598 00:36:57,320 --> 00:37:00,280 another zinc plus 2 that just came off. 599 00:37:00,280 --> 00:37:04,600 And so our electrode here is being consumed as this 600 00:37:04,600 --> 00:37:06,290 oxidation occurs. 601 00:37:06,290 --> 00:37:09,430 I think it's really cool, the electrons, you can see them as 602 00:37:09,430 --> 00:37:13,790 this green going along in this movie. 603 00:37:13,790 --> 00:37:18,250 So, this is a little thing to think about what's happening. 604 00:37:18,250 --> 00:37:21,520 So what's happening at the cathode, we have a reduction. 605 00:37:21,520 --> 00:37:24,230 So if we have our copper ions are going to 606 00:37:24,230 --> 00:37:25,990 plate onto our electrode. 607 00:37:25,990 --> 00:37:28,890 So here in blue is the electrode, and again the water 608 00:37:28,890 --> 00:37:32,530 molecules, here's a little copper 2 in solution. 609 00:37:32,530 --> 00:37:37,010 And when it gets electrons and gets reduced, it's going to 610 00:37:37,010 --> 00:37:40,450 start plating, so let's take a look at that. 611 00:37:40,450 --> 00:37:42,990 So, here's -- oh, there comes the electrons. 612 00:37:42,990 --> 00:37:45,770 And so now it becomes part of the electrode there, the 613 00:37:45,770 --> 00:37:49,160 electrons come in, and so you're building up, you're 614 00:37:49,160 --> 00:37:51,970 adding, your plating on to your electrode. 615 00:37:51,970 --> 00:37:55,380 Now it got its electrons, it got reduced, got reduced 616 00:37:55,380 --> 00:38:05,160 again, and it becomes solid, and plates onto the electrode. 617 00:38:05,160 --> 00:38:08,380 And we see the electrons coming in. 618 00:38:08,380 --> 00:38:11,620 So again, the amount of current that flows, the number 619 00:38:11,620 --> 00:38:14,860 of electrons that flow are going to be proportional to 620 00:38:14,860 --> 00:38:18,600 the chemistry that's happening at the two electrodes. 621 00:38:18,600 --> 00:38:22,540 So you can figure out how much zinc would be consumed, how 622 00:38:22,540 --> 00:38:25,440 much of the zinc solid electrode could be consumed, 623 00:38:25,440 --> 00:38:28,600 and how much copper would be deposited on the copper 624 00:38:28,600 --> 00:38:32,380 electrode for a particular amount of current. 625 00:38:32,380 --> 00:38:36,910 So say we have 1 amp of current for 1 hour, what are 626 00:38:36,910 --> 00:38:39,600 we going to do to our system? 627 00:38:39,600 --> 00:38:44,100 So we can calculate this out, so we use this equation to 628 00:38:44,100 --> 00:38:46,420 find out how much charge is going to pass through the 629 00:38:46,420 --> 00:38:50,500 system, we have this term q, the magnitude of charge and 630 00:38:50,500 --> 00:38:54,170 it's in Coulombs, those are our units, and that's equal to 631 00:38:54,170 --> 00:39:00,310 the current in amps, and just for unit conversion, amps are 632 00:39:00,310 --> 00:39:02,910 Coulombs per second, so that's very convenient because we're 633 00:39:02,910 --> 00:39:07,510 going to take the current and times second, so times time. 634 00:39:07,510 --> 00:39:12,520 And so if we work that out, we had 1 amp and we had 1 hour, 635 00:39:12,520 --> 00:39:18,660 so we're going to have 3,600 Coulombs that are our 636 00:39:18,660 --> 00:39:21,110 magnitude of our charge. 637 00:39:21,110 --> 00:39:25,570 So once we know that, we can convert, using Faraday's 638 00:39:25,570 --> 00:39:29,720 constant from charge into moles of electrons that had to 639 00:39:29,720 --> 00:39:33,870 pass through the system to generate that kind of current. 640 00:39:33,870 --> 00:39:37,370 And here is our friend Faraday's constant, and so 641 00:39:37,370 --> 00:39:39,680 that's in Coulombs per mole. 642 00:39:39,680 --> 00:39:43,570 And so, we can we can convert the number of Coulombs into 643 00:39:43,570 --> 00:39:46,970 the number of moles of electrons that were passed 644 00:39:46,970 --> 00:39:48,540 through the system with that amount of 645 00:39:48,540 --> 00:39:51,860 current, 1 amp for 1 hour. 646 00:39:51,860 --> 00:39:56,080 Now, how is this going to relate to the zinc consumed or 647 00:39:56,080 --> 00:39:57,920 the copper deposited? 648 00:39:57,920 --> 00:40:00,430 So we know the total number of moles that 649 00:40:00,430 --> 00:40:02,660 passed through the system. 650 00:40:02,660 --> 00:40:06,810 And so, then we have to think about for every 1 mole of zinc 651 00:40:06,810 --> 00:40:12,020 that was consumed, how many moles of electrons went 652 00:40:12,020 --> 00:40:13,590 through the system? 653 00:40:13,590 --> 00:40:17,560 So why don't you yell out and tell me what 654 00:40:17,560 --> 00:40:18,600 you think that is. 655 00:40:18,600 --> 00:40:20,900 What do you think? 656 00:40:20,900 --> 00:40:26,410 2, right. 657 00:40:26,410 --> 00:40:30,360 So we are going from zinc solid to zinc plus 2. 658 00:40:30,360 --> 00:40:33,130 So, we need to do -- to consume 659 00:40:33,130 --> 00:40:35,590 one, we need two electrons. 660 00:40:35,590 --> 00:40:42,410 So then we can look up the atomic weight of zinc and 661 00:40:42,410 --> 00:40:45,470 calculate the number of grams. 662 00:40:45,470 --> 00:40:49,160 We can do the same with copper. 663 00:40:49,160 --> 00:40:52,440 So for one mole of copper deposited, how many moles of 664 00:40:52,440 --> 00:40:56,800 electrons needed to pass through the system? 665 00:40:56,800 --> 00:41:02,480 two, right. 666 00:41:02,480 --> 00:41:06,630 And then we look up the atomic weight, which is very similar, 667 00:41:06,630 --> 00:41:09,820 and actually with significant figures, it's the same amount, 668 00:41:09,820 --> 00:41:13,180 which won't be true for most things. 669 00:41:13,180 --> 00:41:19,020 All right, so these problems are not too complicated. 670 00:41:19,020 --> 00:41:21,470 So let me just tell you a little bit more about types of 671 00:41:21,470 --> 00:41:24,670 electrodes that can be used. 672 00:41:24,670 --> 00:41:27,900 It's not always true that you have to use electrodes that 673 00:41:27,900 --> 00:41:31,210 are going to be consumed or have species 674 00:41:31,210 --> 00:41:32,700 deposited on them. 675 00:41:32,700 --> 00:41:36,370 You can use an inert electrode, such as a platinum 676 00:41:36,370 --> 00:41:39,640 electorate, and here is an example of that. 677 00:41:39,640 --> 00:41:44,750 So here, this cell has a platinum electrode, and the 678 00:41:44,750 --> 00:41:47,180 reaction that's happening, you have actually two things in 679 00:41:47,180 --> 00:41:50,510 solution instead of going from a solid to a solution. 680 00:41:50,510 --> 00:41:54,470 So this is just another example of a type of cell. 681 00:41:54,470 --> 00:41:57,580 So let's think about, then, what equations we would write 682 00:41:57,580 --> 00:42:00,180 for this chemistry and what's going on. 683 00:42:00,180 --> 00:42:03,810 So over here at the cathode, what happens at a cathode, 684 00:42:03,810 --> 00:42:11,430 oxidation or reduction? 685 00:42:11,430 --> 00:42:13,040 I'm hearing somewhat of a consensus. 686 00:42:13,040 --> 00:42:13,850 That's correct. 687 00:42:13,850 --> 00:42:14,240 Reduction. 688 00:42:14,240 --> 00:42:17,950 That's the thing you're going to need to memorize, it'll be 689 00:42:17,950 --> 00:42:20,390 important in doing problems later on. 690 00:42:20,390 --> 00:42:23,720 So, what is the reduction reaction that you imagine 691 00:42:23,720 --> 00:42:26,000 would be happening if you have copper plus 692 00:42:26,000 --> 00:42:31,590 2 and copper solid? 693 00:42:31,590 --> 00:42:34,940 So, if you're reducing it, you're going to be reducing 694 00:42:34,940 --> 00:42:38,400 copper plus 2 with two electrons to copper solid. 695 00:42:38,400 --> 00:42:41,280 So, same reaction we saw before. 696 00:42:41,280 --> 00:42:42,930 So what about over here. 697 00:42:42,930 --> 00:42:46,940 This is the anode, which has the oxidation reaction going 698 00:42:46,940 --> 00:42:50,130 on, and what oxidation reaction can you imagine 699 00:42:50,130 --> 00:42:53,660 happening with chromium plus 3 and chromium plus 2. 700 00:42:53,660 --> 00:43:03,760 What's going to what? 701 00:43:03,760 --> 00:43:06,350 So, you're going from plus 2 to plus 3 with 702 00:43:06,350 --> 00:43:08,370 one electron here. 703 00:43:08,370 --> 00:43:11,440 So that would be the oxidation reaction. 704 00:43:11,440 --> 00:43:13,490 So let's think about how we would actually 705 00:43:13,490 --> 00:43:15,200 write this down then. 706 00:43:15,200 --> 00:43:18,530 So the notation for this type of cell, we're going to have a 707 00:43:18,530 --> 00:43:22,260 platinum that indicates the electrode, a single line that 708 00:43:22,260 --> 00:43:23,940 indicates a phase boundary. 709 00:43:23,940 --> 00:43:27,550 And in solution, you have the chromium plus 2, chromium plus 710 00:43:27,550 --> 00:43:29,320 3, those are both aqueous, so they have a 711 00:43:29,320 --> 00:43:30,840 comma between them. 712 00:43:30,840 --> 00:43:34,950 So this is on one side in one of the beakers, so that's the 713 00:43:34,950 --> 00:43:37,100 reaction at the anode. 714 00:43:37,100 --> 00:43:40,910 And then we have a bar here that indicates a salt bridge. 715 00:43:40,910 --> 00:43:44,390 And then in the other beaker, we would have the copper plus 716 00:43:44,390 --> 00:43:47,500 2 and the copper solid, separated by a single line, 717 00:43:47,500 --> 00:43:49,370 because there's a phase boundary between the aqueous 718 00:43:49,370 --> 00:43:50,890 and the solid. 719 00:43:50,890 --> 00:43:54,360 And then here are our two equations. 720 00:43:54,360 --> 00:43:57,840 At the anode, we have copper plus 2. 721 00:43:57,840 --> 00:44:00,580 Aqueous going to copper plus 3 aqueous in an 722 00:44:00,580 --> 00:44:02,370 electron at the cathode. 723 00:44:02,370 --> 00:44:05,120 We have copper plus 2 aqueous and two electrons going to 724 00:44:05,120 --> 00:44:06,310 copper solid. 725 00:44:06,310 --> 00:44:08,950 So those are our two equations and how we 726 00:44:08,950 --> 00:44:12,680 would write that down. 727 00:44:12,680 --> 00:44:14,760 Another example of an electrode 728 00:44:14,760 --> 00:44:16,700 is a hydrogen electrode. 729 00:44:16,700 --> 00:44:18,070 And this is very common. 730 00:44:18,070 --> 00:44:21,900 In fact, most potential standard reduction potentials, 731 00:44:21,900 --> 00:44:25,400 or as they're also known, oxidation reduction potentials 732 00:44:25,400 --> 00:44:29,100 are measured against a standard hydrogen electrode, 733 00:44:29,100 --> 00:44:31,970 and it'll be abbreviated s h e. 734 00:44:31,970 --> 00:44:34,985 And so, if you see it was measured against s h e, you 735 00:44:34,985 --> 00:44:36,870 will now know what that means. 736 00:44:36,870 --> 00:44:39,510 So that's a standard hydrogen electrode. 737 00:44:39,510 --> 00:44:41,790 So there's a couple of different variations. 738 00:44:41,790 --> 00:44:45,200 It can be used at the cathode or at the anode. 739 00:44:45,200 --> 00:44:50,280 And so, if you're using it at the cathode, h plus is 740 00:44:50,280 --> 00:44:54,100 reduced, and at the anode, h 2 is oxidized. 741 00:44:54,100 --> 00:44:58,750 And often you have a platinum system there as well, for the 742 00:44:58,750 --> 00:45:02,260 standard hydrogen, so platinum is commonly used in a standard 743 00:45:02,260 --> 00:45:04,000 hydrogen electrode. 744 00:45:04,000 --> 00:45:08,350 So, let's just sort of look at a little picture of this. 745 00:45:08,350 --> 00:45:11,975 So here, we may have like a glass cylinder here, we're 746 00:45:11,975 --> 00:45:13,680 pumping in h 2 gas. 747 00:45:13,680 --> 00:45:16,740 And here you can see it sort of bubbling down. 748 00:45:16,740 --> 00:45:20,050 We may have a solution of hydrochloric acid, which would 749 00:45:20,050 --> 00:45:22,750 have a lot of h plus in it, and so this would be a 750 00:45:22,750 --> 00:45:27,000 hydrogen electrode on one side, the other side may have 751 00:45:27,000 --> 00:45:30,070 something more common or something that we saw before 752 00:45:30,070 --> 00:45:33,290 with the zinc solid and the zinc in solution. 753 00:45:33,290 --> 00:45:37,670 So since on this side we have the cathode, of we can write 754 00:45:37,670 --> 00:45:42,670 about the h plus aqueous, a bar for the phase transition 755 00:45:42,670 --> 00:45:47,300 to h 2 gas, and then we indicate that there's also a 756 00:45:47,300 --> 00:45:49,940 platinum electrode going on here. 757 00:45:49,940 --> 00:45:53,230 And the other side is what we saw before, the zinc solid and 758 00:45:53,230 --> 00:45:54,550 the zinc plus 2. 759 00:45:54,550 --> 00:45:56,920 So you're just introducing different types of electrodes 760 00:45:56,920 --> 00:46:00,360 that you may be seeing in this particular unit. 761 00:46:00,360 --> 00:46:02,550 And again, you want to remember what reaction's going 762 00:46:02,550 --> 00:46:04,590 on at the cathode, and at the anode. 763 00:46:04,590 --> 00:46:14,070 All right, so just very briefly, I will mention cell 764 00:46:14,070 --> 00:46:16,490 potential, which has many different names -- cell 765 00:46:16,490 --> 00:46:19,880 voltage, EMF it's often called. 766 00:46:19,880 --> 00:46:23,020 And so the flow of electrons generates a potential 767 00:46:23,020 --> 00:46:26,900 difference between the electrodes in the current. 768 00:46:26,900 --> 00:46:34,320 And this can be related back to our friend, delta g. 769 00:46:34,320 --> 00:46:39,650 So in this equation you have this potential difference 770 00:46:39,650 --> 00:46:44,760 that's generated by the flow of the electrons, and that 771 00:46:44,760 --> 00:46:49,960 times Faraday's constant times the number moles of electrons, 772 00:46:49,960 --> 00:46:52,590 and if you have a negative sign here, that's equal to the 773 00:46:52,590 --> 00:46:56,040 free energy of the cell. 774 00:46:56,040 --> 00:47:00,850 So we can relate that back. 775 00:47:00,850 --> 00:47:03,340 And a little bit of information. 776 00:47:03,340 --> 00:47:05,390 We have the standard terms as well. 777 00:47:05,390 --> 00:47:09,010 We can have the delta e nought, the cell potential, 778 00:47:09,010 --> 00:47:11,810 cell voltage, you'll see many names for this. 779 00:47:11,810 --> 00:47:14,410 And that's when the products and reactants are in their 780 00:47:14,410 --> 00:47:15,860 standard states. 781 00:47:15,860 --> 00:47:18,740 The units we're talking about here are in volts, sometimes 782 00:47:18,740 --> 00:47:22,690 you'll see things reported in millivolts as well. 783 00:47:22,690 --> 00:47:27,720 And let's end with one final clicker question, and then we 784 00:47:27,720 --> 00:48:11,650 can announce our clicker winner for today. 785 00:48:11,650 --> 00:48:18,980 OK, 10 seconds. 786 00:48:18,980 --> 00:48:21,340 Most people should get this right.