1 00:00:00,090 --> 00:00:02,430 The following content is provided under a Creative 2 00:00:02,430 --> 00:00:03,850 Commons license. 3 00:00:03,850 --> 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 --> 00:00:16,620 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,620 --> 00:00:17,850 at ocw.mit.edu. 8 00:00:27,100 --> 00:00:30,230 CATHERINE DRENNAN: So this is on oxidation--reduction go. 9 00:00:30,230 --> 00:00:34,600 GUEST SPEAKER: OK, so the demo involves a strip of magnesium. 10 00:00:34,600 --> 00:00:36,769 And the magnesium is a pretty reactive metal. 11 00:00:36,769 --> 00:00:38,560 Right now it's coated with magnesium oxide. 12 00:00:38,560 --> 00:00:40,976 And what I'm going to do is I'm going to light it on fire. 13 00:00:40,976 --> 00:00:44,110 When it gets lit, it's going to glow very, very brightly, 14 00:00:44,110 --> 00:00:47,710 and it's going to react with this block of carbon dioxide. 15 00:00:47,710 --> 00:00:50,650 And magnesium oxide is what you have up there. 16 00:00:50,650 --> 00:00:52,360 Both of those magnesium oxide molecules 17 00:00:52,360 --> 00:00:55,060 will react with one molecule of carbon dioxide 18 00:00:55,060 --> 00:00:59,750 to form just graphene, carbon, as well as magnesium dioxide. 19 00:00:59,750 --> 00:01:02,350 So you're going to get a white and black powder 20 00:01:02,350 --> 00:01:03,287 inside of here. 21 00:01:03,287 --> 00:01:05,620 And it's going to be really bright and really dangerous. 22 00:01:05,620 --> 00:01:08,500 And can we lower the lights? 23 00:01:08,500 --> 00:01:10,270 Is that something we can do, because that 24 00:01:10,270 --> 00:01:11,520 makes it a whole lot more fun? 25 00:01:17,910 --> 00:01:19,380 [SIDE CONVERSATION] 26 00:01:55,826 --> 00:01:58,010 CATHERINE DRENNAN: You can try it one more time, OK? 27 00:02:04,560 --> 00:02:05,990 All right. 28 00:02:05,990 --> 00:02:07,485 OK, good try. 29 00:02:09,990 --> 00:02:13,080 So oxidation-reductions are really fun. 30 00:02:13,080 --> 00:02:15,000 And so we're going to continue and tell you 31 00:02:15,000 --> 00:02:16,590 about those on Friday. 32 00:02:16,590 --> 00:02:19,890 So oxidation-reduction, or redox-- so 33 00:02:19,890 --> 00:02:25,320 redox is the shorthand for oxidation-reduction. 34 00:02:25,320 --> 00:02:28,001 Let's bring out your hands, but let's bring the noise level 35 00:02:28,001 --> 00:02:28,500 down. 36 00:02:28,500 --> 00:02:29,916 People have still been complaining 37 00:02:29,916 --> 00:02:31,440 about the noise in class. 38 00:02:31,440 --> 00:02:36,090 All right, so a redox reaction, a chemical reaction 39 00:02:36,090 --> 00:02:38,250 that involves an exchange of electrons. 40 00:02:38,250 --> 00:02:44,640 And so we saw what happens when we mix this with some dry ice. 41 00:02:44,640 --> 00:02:46,870 All right, so let's do some definitions. 42 00:02:46,870 --> 00:02:49,500 We always start units with definitions. 43 00:02:49,500 --> 00:02:52,470 And most of you know some of these definitions already. 44 00:02:52,470 --> 00:02:57,330 Oxidation, electron loss. 45 00:02:57,330 --> 00:03:02,540 Reduction, electrons are gained. 46 00:03:02,540 --> 00:03:06,590 This one you may or may not be familiar with. 47 00:03:06,590 --> 00:03:10,780 Oxidizing agent, it accepts electrons. 48 00:03:10,780 --> 00:03:17,360 So it oxidizes other things, but it itself is reduced. 49 00:03:17,360 --> 00:03:21,530 And reducing agent donates electrons. 50 00:03:21,530 --> 00:03:23,930 It reduces other things. 51 00:03:23,930 --> 00:03:26,570 It itself is oxidized. 52 00:03:26,570 --> 00:03:28,910 And before you turn the page, I just 53 00:03:28,910 --> 00:03:32,190 want to try to get you to remember these two terms. 54 00:03:32,190 --> 00:03:34,640 So one way that I like to think about it 55 00:03:34,640 --> 00:03:37,130 is that a reducing agent might be somebody 56 00:03:37,130 --> 00:03:39,620 who is a little bit shady. 57 00:03:39,620 --> 00:03:42,890 They might walk up to someone and say, in a crowded space, 58 00:03:42,890 --> 00:03:45,272 don't look at me, just listen. 59 00:03:45,272 --> 00:03:48,420 Do you want to buy an electron? 60 00:03:48,420 --> 00:03:51,960 So that would be an agent of reduction going around. 61 00:03:51,960 --> 00:03:54,720 Now, there could be ones that are a little bit 62 00:03:54,720 --> 00:03:58,210 shady, selling electrons to the highest bidder. 63 00:03:58,210 --> 00:04:00,720 But there could also be agents of good. 64 00:04:00,720 --> 00:04:03,220 I told you about free radical species. 65 00:04:03,220 --> 00:04:05,400 There's a lot of free radicals in the universe. 66 00:04:05,400 --> 00:04:06,870 They can be dangerous because they 67 00:04:06,870 --> 00:04:09,510 have a single, unpaired electron, which makes 68 00:04:09,510 --> 00:04:12,660 them really highly reactive. 69 00:04:12,660 --> 00:04:17,970 So there might be an agent, a secret agent of oxidation. 70 00:04:17,970 --> 00:04:22,170 And it might want to shield the universe from the power 71 00:04:22,170 --> 00:04:24,030 of free radicals. 72 00:04:24,030 --> 00:04:28,980 And so it will go along and it would oxidize that, 73 00:04:28,980 --> 00:04:31,365 take that electron away and oxidize 74 00:04:31,365 --> 00:04:35,370 and become reduced itself, take that free radical away. 75 00:04:35,370 --> 00:04:38,321 Because the free radical species might not know how dangerous 76 00:04:38,321 --> 00:04:38,820 it is. 77 00:04:38,820 --> 00:04:42,660 It's for everybody's good to shield you from that power. 78 00:04:42,660 --> 00:04:44,700 You might also have an agent of reduction 79 00:04:44,700 --> 00:04:46,350 that's a secret agent, that might 80 00:04:46,350 --> 00:04:47,920 go up to the free radical species 81 00:04:47,920 --> 00:04:49,170 and give it an extra electron. 82 00:04:49,170 --> 00:04:50,130 And then it has two. 83 00:04:50,130 --> 00:04:52,710 And then it's more stable and less reactive. 84 00:04:52,710 --> 00:04:54,434 So you might have these agents. 85 00:04:54,434 --> 00:04:56,100 So that's how you should think about it. 86 00:04:56,100 --> 00:04:57,720 Think about an agent of oxidation 87 00:04:57,720 --> 00:05:00,870 wants to go out and oxidize for the good of mankind, 88 00:05:00,870 --> 00:05:03,600 or an agent of reduction that wants to go out and reduce 89 00:05:03,600 --> 00:05:05,460 other things. 90 00:05:05,460 --> 00:05:09,030 So that might help you remember those definitions. 91 00:05:09,030 --> 00:05:10,440 We'll hope so. 92 00:05:10,440 --> 00:05:12,750 All right, so let's take a look now-- we always 93 00:05:12,750 --> 00:05:15,240 start every unit with definitions, 94 00:05:15,240 --> 00:05:19,460 and then we have a set of rules and guidelines. 95 00:05:19,460 --> 00:05:23,360 So let's take a look at the guidelines. 96 00:05:23,360 --> 00:05:27,470 Luckily, these guidelines are really pretty straightforward. 97 00:05:27,470 --> 00:05:30,200 And you really shouldn't have a lot of difficulty keeping them 98 00:05:30,200 --> 00:05:31,310 in mind. 99 00:05:31,310 --> 00:05:33,280 Some of them we've really seen already, 100 00:05:33,280 --> 00:05:34,880 we just didn't realize that they were 101 00:05:34,880 --> 00:05:37,440 going to be the guidelines that we would have. 102 00:05:37,440 --> 00:05:40,520 So guidelines for assigning oxidation numbers-- 103 00:05:40,520 --> 00:05:44,990 in a free element, every atom has an oxidation number 104 00:05:44,990 --> 00:05:46,760 of zero. 105 00:05:46,760 --> 00:05:48,980 OK, that seems pretty straightforward. 106 00:05:48,980 --> 00:05:53,190 H2, oxidation number, zero. 107 00:05:53,190 --> 00:05:57,580 Rule two-- ions composed of only one atom, 108 00:05:57,580 --> 00:06:01,600 also called monatomic ions, have an oxidation number 109 00:06:01,600 --> 00:06:03,530 that's equal to their charge. 110 00:06:03,530 --> 00:06:06,130 If this wasn't true, it would be deeply disturbing. 111 00:06:06,130 --> 00:06:11,420 Lithium plus 1 ions has an oxidation number of plus 1. 112 00:06:11,420 --> 00:06:13,280 I think we're good. 113 00:06:13,280 --> 00:06:17,220 Also in number two, in forming compounds, 114 00:06:17,220 --> 00:06:20,150 compounds Group 1 metals have an oxidation number 115 00:06:20,150 --> 00:06:24,890 of plus 1, lithium and all of its friends, sodium, et cetera. 116 00:06:24,890 --> 00:06:29,690 Group 2 metals have an oxidation number of plus 2-- calcium 117 00:06:29,690 --> 00:06:31,130 for example. 118 00:06:31,130 --> 00:06:34,820 And aluminum is plus 3 in its compounds. 119 00:06:34,820 --> 00:06:37,040 This is also something that we've seen before. 120 00:06:37,040 --> 00:06:40,100 Those Group 1 and Group 2 metals are not all that exciting. 121 00:06:40,100 --> 00:06:43,550 They pretty much want to get their noble gas configuration 122 00:06:43,550 --> 00:06:46,340 and they want it to lose either one, if you're in Group 1, 123 00:06:46,340 --> 00:06:48,860 or two electrons if you're in Group 2. 124 00:06:48,860 --> 00:06:50,987 And so that's what they do here. 125 00:06:50,987 --> 00:06:52,820 So this is good to keep in mind because it's 126 00:06:52,820 --> 00:06:55,370 going to help you figure out the oxidation 127 00:06:55,370 --> 00:06:57,710 number of other elements for which there are not 128 00:06:57,710 --> 00:07:00,480 such hard and fast rules. 129 00:07:00,480 --> 00:07:07,440 Number three, oxygen-- oxygen in most compounds is minus 2. 130 00:07:07,440 --> 00:07:09,840 Again, not that surprising that it would want 131 00:07:09,840 --> 00:07:12,610 to gain those two electrons. 132 00:07:12,610 --> 00:07:14,160 However, there's some exceptions. 133 00:07:14,160 --> 00:07:17,580 Oxygen is just an element full of exceptions. 134 00:07:17,580 --> 00:07:23,970 If it is in peroxides-- hydrogen peroxide, O2, 2 minus-- then 135 00:07:23,970 --> 00:07:27,720 it's going to have an oxidation number of minus 1. 136 00:07:27,720 --> 00:07:31,740 So peroxides, and later we'll see superoxides are exceptions. 137 00:07:31,740 --> 00:07:34,520 But if it's not a peroxide or a superoxide, 138 00:07:34,520 --> 00:07:37,260 it's going to be minus 2. 139 00:07:37,260 --> 00:07:41,830 All right, rule number four, hydrogen-- hydrogen 140 00:07:41,830 --> 00:07:45,280 is mostly plus 1. 141 00:07:45,280 --> 00:07:46,910 You probably could guess that. 142 00:07:46,910 --> 00:07:49,675 But there actually are some exceptions for this. 143 00:07:49,675 --> 00:07:52,300 And you're going to see a lot of those exceptions in this unit, 144 00:07:52,300 --> 00:07:56,530 and pretty much after this unit, not so much ever again. 145 00:07:56,530 --> 00:08:00,870 The exceptions are for the Group 1 and the Group 2. 146 00:08:00,870 --> 00:08:03,970 So the Group 1 and Group 2 rules supersede the fact 147 00:08:03,970 --> 00:08:06,160 that hydrogen is usually plus 1. 148 00:08:06,160 --> 00:08:08,530 So if you have hydrogen with lithium, 149 00:08:08,530 --> 00:08:10,390 lithium is going to be plus 1. 150 00:08:10,390 --> 00:08:12,060 So hydrogen will be minus. one. 151 00:08:12,060 --> 00:08:14,560 Sodium is the same here. 152 00:08:14,560 --> 00:08:16,510 And calcium, that's Group 2. 153 00:08:16,510 --> 00:08:18,890 Again, hydrogen would be minus 1. 154 00:08:18,890 --> 00:08:24,660 So Group 1, Group 2 rules supersede hydrogen. 155 00:08:24,660 --> 00:08:27,400 All right, what about our halides? 156 00:08:27,400 --> 00:08:28,780 We have fluorine. 157 00:08:28,780 --> 00:08:30,774 It wants to be minus 1. 158 00:08:30,774 --> 00:08:33,820 Other halogens-- chloride, bromide, 159 00:08:33,820 --> 00:08:38,770 iodide-- are negative when they are in salts, 160 00:08:38,770 --> 00:08:40,809 such as sodium chloride. 161 00:08:40,809 --> 00:08:44,980 But if oxygen is there, the oxygen rules 162 00:08:44,980 --> 00:08:46,730 are the ones that follow. 163 00:08:46,730 --> 00:08:50,950 So here, if you have Cl with oxygen, 164 00:08:50,950 --> 00:08:53,770 then you're going to have a positive oxidation 165 00:08:53,770 --> 00:08:56,380 number for the chlorine, because oxygen 166 00:08:56,380 --> 00:08:59,400 is going to be minus two. 167 00:08:59,400 --> 00:09:03,180 And you'll see a number of these so-called oxyacids 168 00:09:03,180 --> 00:09:05,700 in this unit. 169 00:09:05,700 --> 00:09:10,780 All right, six-- this one, you probably already know. 170 00:09:10,780 --> 00:09:17,080 But in a neutral molecule, the sums of the oxidation numbers 171 00:09:17,080 --> 00:09:19,450 must equal zero. 172 00:09:19,450 --> 00:09:23,230 So the sums must be neutral if the molecule is neutral. 173 00:09:23,230 --> 00:09:28,870 And if you have something that is a polyatomic multiatom ion, 174 00:09:28,870 --> 00:09:31,720 the sum of the individual oxidation numbers 175 00:09:31,720 --> 00:09:36,250 needs to be equal to the overall charge on that ion. 176 00:09:36,250 --> 00:09:39,040 So again, we've seen things like this before. 177 00:09:39,040 --> 00:09:41,740 So we should have no trouble with these words. 178 00:09:41,740 --> 00:09:45,550 OK, so let's just look at an example of this. 179 00:09:45,550 --> 00:09:49,230 Suppose we have our friend NH4 plus. 180 00:09:49,230 --> 00:09:52,710 What is hydrogen's oxidation number going to be 181 00:09:52,710 --> 00:09:54,700 in this molecule? 182 00:09:54,700 --> 00:09:56,680 It will be plus one. 183 00:09:56,680 --> 00:10:00,800 So it's not with a Group 1 or Group 2 element, 184 00:10:00,800 --> 00:10:03,810 so it's going to be plus 1. 185 00:10:03,810 --> 00:10:04,920 What about nitrogen? 186 00:10:04,920 --> 00:10:06,480 There were no rules about nitrogen, 187 00:10:06,480 --> 00:10:09,180 so we have to figure out based on the other rules what 188 00:10:09,180 --> 00:10:11,261 the oxidation number of nitrogen will be. 189 00:10:11,261 --> 00:10:12,427 What is it going to be here? 190 00:10:12,427 --> 00:10:13,327 AUDIENCE: Minus 3. 191 00:10:13,327 --> 00:10:15,160 CATHERINE DRENNAN: It's going to be minus 3. 192 00:10:15,160 --> 00:10:18,090 And the reason for that is because of the rule 193 00:10:18,090 --> 00:10:22,290 we just talked about, that the sum of the oxidation numbers 194 00:10:22,290 --> 00:10:24,690 needs to be equal to the charge on the molecule. 195 00:10:24,690 --> 00:10:26,440 The charge is plus 1. 196 00:10:26,440 --> 00:10:31,230 So we have four hydrogens, 4 plus 1 minus 3 197 00:10:31,230 --> 00:10:32,640 is going to equal plus 1. 198 00:10:32,640 --> 00:10:35,370 So nitrogen would have to be minus 3. 199 00:10:35,370 --> 00:10:37,330 Nitrogen can have a lot of different numbers, 200 00:10:37,330 --> 00:10:40,140 so you want to pay attention to what's around it 201 00:10:40,140 --> 00:10:41,940 and whether there are rules or guidelines 202 00:10:41,940 --> 00:10:45,000 about those other elements. 203 00:10:45,000 --> 00:10:49,320 All right, seven, oxidation numbers 204 00:10:49,320 --> 00:10:51,180 don't have to be integers. 205 00:10:51,180 --> 00:10:55,320 They almost always are, but there is one notable exception, 206 00:10:55,320 --> 00:10:58,160 and it's from oxygen. Perhaps not a surprise, 207 00:10:58,160 --> 00:11:00,820 oxygen is always causing trouble. 208 00:11:00,820 --> 00:11:07,600 So in superoxide, you have O2 minus 1. 209 00:11:07,600 --> 00:11:11,140 So what is the oxidation number on each oxygen 210 00:11:11,140 --> 00:11:14,840 in that O2 minus one molecule? 211 00:11:14,840 --> 00:11:19,040 Yep, so it's minus 1/2. 212 00:11:19,040 --> 00:11:23,390 So oxygen is usually minus 2 except in peroxides 213 00:11:23,390 --> 00:11:26,910 and superoxides. 214 00:11:26,910 --> 00:11:29,680 All right, so let's take a look at some examples. 215 00:11:29,680 --> 00:11:32,100 And I'll write some of these on the board. 216 00:11:32,100 --> 00:11:34,860 And here are all of your rules. 217 00:11:34,860 --> 00:11:37,770 So you can yell out what you think 218 00:11:37,770 --> 00:11:41,710 I should be doing with these as we go along. 219 00:11:41,710 --> 00:11:46,860 OK, so we have lithium 2 oxide. 220 00:11:46,860 --> 00:11:49,350 So here we know a lot about this. 221 00:11:49,350 --> 00:11:54,090 What's the oxygen going to be-- minus 2. 222 00:11:54,090 --> 00:11:57,630 The overall charge on the molecule is zero. 223 00:11:57,630 --> 00:11:59,100 And we have two lithiums. 224 00:11:59,100 --> 00:12:00,810 And what's the charge on each one? 225 00:12:00,810 --> 00:12:01,790 AUDIENCE: Plus 1. 226 00:12:01,790 --> 00:12:03,330 CATHERINE DRENNAN: Plus 1. 227 00:12:03,330 --> 00:12:07,260 And so plus 2 minus 2 is 0. 228 00:12:07,260 --> 00:12:09,130 Great. 229 00:12:09,130 --> 00:12:10,920 Let's do another one. 230 00:12:10,920 --> 00:12:16,150 We'll do PCl5 next. 231 00:12:16,150 --> 00:12:18,100 So what do we know about Cl? 232 00:12:18,100 --> 00:12:20,045 What charge do we expect here? 233 00:12:20,045 --> 00:12:21,160 AUDIENCE: Minus 1. 234 00:12:21,160 --> 00:12:25,030 CATHERINE DRENNAN: Minus 1, and there are five of those. 235 00:12:25,030 --> 00:12:27,660 So chlorine likes to be minus 1. 236 00:12:27,660 --> 00:12:31,960 Exception is with oxygen, but it's not with oxygen here. 237 00:12:31,960 --> 00:12:35,200 The overall charge is going to be zero, because it's 238 00:12:35,200 --> 00:12:36,820 a neutral molecule. 239 00:12:36,820 --> 00:12:40,020 So what does that mean about the charge on our phosphorous? 240 00:12:40,020 --> 00:12:41,520 AUDIENCE: Plus 5. 241 00:12:41,520 --> 00:12:45,600 CATHERINE DRENNAN: Plus 5, right. 242 00:12:45,600 --> 00:12:49,045 OK, so now let's try HNO3. 243 00:12:51,700 --> 00:12:54,340 So let's do oxygen first. 244 00:12:54,340 --> 00:12:56,140 What charge do we expect on oxygen? 245 00:12:56,140 --> 00:12:57,715 AUDIENCE: Minus 2. 246 00:12:57,715 --> 00:12:58,840 CATHERINE DRENNAN: Minus 2. 247 00:12:58,840 --> 00:13:01,590 So we have three of those minus 2. 248 00:13:01,590 --> 00:13:04,750 Overall this is going to be zero. 249 00:13:04,750 --> 00:13:08,260 Nitrogen can be varied, so let's do hydrogen. What 250 00:13:08,260 --> 00:13:10,120 do we expect hydrogen to be? 251 00:13:10,120 --> 00:13:11,050 AUDIENCE: Plus 1. 252 00:13:11,050 --> 00:13:12,425 CATHERINE DRENNAN: Plus 1, right, 253 00:13:12,425 --> 00:13:14,620 because it's not in Group 1 or Group 2. 254 00:13:14,620 --> 00:13:16,990 So what does that leave for nitrogen? 255 00:13:16,990 --> 00:13:17,932 AUDIENCE: Plus 5. 256 00:13:17,932 --> 00:13:20,760 CATHERINE DRENNAN: Plus 5. 257 00:13:20,760 --> 00:13:27,000 So we have plus 6 then minus 6 is 0. 258 00:13:27,000 --> 00:13:29,610 All right, so why don't you try the next one 259 00:13:29,610 --> 00:13:31,490 as a clicker question. 260 00:13:42,243 --> 00:13:44,445 All right, 10 more seconds. 261 00:14:00,890 --> 00:14:02,930 So oxygen is minus 2. 262 00:14:02,930 --> 00:14:06,830 It's not in a peroxide or a superoxide. 263 00:14:06,830 --> 00:14:08,930 The overall charge is zero. 264 00:14:08,930 --> 00:14:13,460 So that means that each nitrogen must be plus 1. 265 00:14:13,460 --> 00:14:17,070 So we have plus 2 minus 2 is 0. 266 00:14:17,070 --> 00:14:17,950 AUDIENCE: Professor? 267 00:14:17,950 --> 00:14:18,950 CATHERINE DRENNAN: Yeah? 268 00:14:18,950 --> 00:14:20,542 AUDIENCE: What exactly is a peroxide? 269 00:14:20,542 --> 00:14:24,980 CATHERINE DRENNAN: A peroxide-- so there really only goes 270 00:14:24,980 --> 00:14:27,230 the hydrogen peroxide example. 271 00:14:27,230 --> 00:14:32,490 So it'll say that it's a peroxide, or a superoxide. 272 00:14:35,000 --> 00:14:36,540 You don't have to worry about that. 273 00:14:36,540 --> 00:14:37,498 It's going to be clear. 274 00:14:40,146 --> 00:14:42,920 OK, good. 275 00:14:42,920 --> 00:14:47,320 So now let's talk about some reactions 276 00:14:47,320 --> 00:14:50,140 that involve oxidations and reductions. 277 00:14:50,140 --> 00:14:52,240 And to talk about these reactions you 278 00:14:52,240 --> 00:14:54,250 need to know how to do this. 279 00:14:54,250 --> 00:14:57,190 And then we're going to talk about balancing reactions. 280 00:14:57,190 --> 00:14:59,230 And you need all of these skills. 281 00:14:59,230 --> 00:15:01,840 All right, so first I just want to tell you about something 282 00:15:01,840 --> 00:15:04,240 called the disproportionation reaction 283 00:15:04,240 --> 00:15:06,340 where you have one element that is going to be 284 00:15:06,340 --> 00:15:09,200 both oxidized and reduced. 285 00:15:09,200 --> 00:15:10,870 And this is a good example to allow 286 00:15:10,870 --> 00:15:12,490 us to think about what's happening 287 00:15:12,490 --> 00:15:14,390 in a different reaction. 288 00:15:14,390 --> 00:15:17,650 So in this reaction, the element that 289 00:15:17,650 --> 00:15:22,600 is going to be both oxidized and reduced is Cl. 290 00:15:22,600 --> 00:15:25,240 Sodium is going to be a spectator ion. 291 00:15:25,240 --> 00:15:30,950 And the oxygen is going to stay in its minus 2 form here. 292 00:15:30,950 --> 00:15:34,060 And so we have chlorine going to chlorine in this compound 293 00:15:34,060 --> 00:15:36,110 and in this compound. 294 00:15:36,110 --> 00:15:38,500 So we can break this down and think 295 00:15:38,500 --> 00:15:42,190 about what's happening with this species going to this, 296 00:15:42,190 --> 00:15:45,640 and then also going to Cl minus. 297 00:15:45,640 --> 00:15:49,420 So let's think about what the oxidation number of chlorine 298 00:15:49,420 --> 00:15:52,300 is in ClO minus. 299 00:15:52,300 --> 00:15:56,680 So first, oxygen, I already told you, is going to be minus 2. 300 00:15:56,680 --> 00:16:00,770 The overall charge on the molecule is minus 1. 301 00:16:00,770 --> 00:16:04,630 So chlorine has to make this equation work. 302 00:16:04,630 --> 00:16:07,030 So what is the oxidation number of Cl? 303 00:16:10,916 --> 00:16:11,820 AUDIENCE: Plus 1. 304 00:16:11,820 --> 00:16:13,690 AUDIENCE: Plus 1, thank you. 305 00:16:13,690 --> 00:16:16,270 So yeah, plus 1 here. 306 00:16:16,270 --> 00:16:18,130 Now let's take a look at this. 307 00:16:18,130 --> 00:16:21,295 So oxygen, we're going to have as minus 2. 308 00:16:21,295 --> 00:16:23,930 There are three of them. 309 00:16:23,930 --> 00:16:27,440 That's going to equal, overall, minus 1. 310 00:16:27,440 --> 00:16:30,284 So what's the oxidation number of chlorine here? 311 00:16:30,284 --> 00:16:31,616 AUDIENCE: Plus 5. 312 00:16:31,616 --> 00:16:33,140 CATHERINE DRENNAN: Plus 5, right. 313 00:16:33,140 --> 00:16:36,350 So we have a case now where we're 314 00:16:36,350 --> 00:16:40,980 going from plus 1 to plus 5. 315 00:16:40,980 --> 00:16:44,700 Is that an oxidation or a reduction? 316 00:16:44,700 --> 00:16:46,000 That's an oxidation. 317 00:16:46,000 --> 00:16:47,700 So we're increasing the number. 318 00:16:47,700 --> 00:16:49,440 It is losing electrons. 319 00:16:49,440 --> 00:16:51,960 It's becoming oxidized. 320 00:16:51,960 --> 00:16:56,100 So down here, we already have done all of our work 321 00:16:56,100 --> 00:16:59,490 and know that this is also going to be plus 1. 322 00:16:59,490 --> 00:17:04,890 So in this case, we're going from plus 1 to minus 1. 323 00:17:04,890 --> 00:17:07,109 Is that an oxidation or a reduction? 324 00:17:07,109 --> 00:17:08,099 AUDIENCE: Reduction. 325 00:17:08,099 --> 00:17:08,796 CATHERINE DRENNAN: That's a reduction, 326 00:17:08,796 --> 00:17:10,720 and that's good because it wouldn't 327 00:17:10,720 --> 00:17:12,359 be a very good disproportionation 328 00:17:12,359 --> 00:17:14,040 reaction otherwise. 329 00:17:14,040 --> 00:17:15,450 Something has to be oxidized. 330 00:17:15,450 --> 00:17:18,190 Same thing has to be oxidized and reduced. 331 00:17:18,190 --> 00:17:22,839 So this is how you think about this type of reaction. 332 00:17:22,839 --> 00:17:24,420 So to consider whether something's 333 00:17:24,420 --> 00:17:26,579 being oxidized or reduced, you need 334 00:17:26,579 --> 00:17:32,100 to be able to dissect out what the oxidation numbers are. 335 00:17:32,100 --> 00:17:35,670 All right, now we're going to balance. 336 00:17:35,670 --> 00:17:37,950 So balancing is going to become very important 337 00:17:37,950 --> 00:17:42,540 for doing problems later on, balancing redox reactions. 338 00:17:42,540 --> 00:17:43,980 You balance differently if you're 339 00:17:43,980 --> 00:17:47,590 in acidic conditions versus basic conditions. 340 00:17:47,590 --> 00:17:50,370 So first we're going to do acidic conditions. 341 00:17:50,370 --> 00:17:55,040 And we're going to run through the steps for doing this. 342 00:17:55,040 --> 00:17:58,070 First step is we're going to write two half 343 00:17:58,070 --> 00:17:59,720 reactions for this. 344 00:17:59,720 --> 00:18:03,397 So we have iron in here and we have chromium in here, 345 00:18:03,397 --> 00:18:04,980 and we're going to separate those out. 346 00:18:04,980 --> 00:18:07,490 So we're going to think about what iron is doing separately 347 00:18:07,490 --> 00:18:09,980 from what the chromium is doing. 348 00:18:09,980 --> 00:18:14,450 So first, let's write out our chromium 349 00:18:14,450 --> 00:18:17,210 and think about what is happening to it here. 350 00:18:17,210 --> 00:18:18,560 And that is a clicker question. 351 00:18:29,670 --> 00:18:31,180 All right, 10 more seconds. 352 00:18:45,520 --> 00:18:50,910 OK, so let's take a look at why two is correct. 353 00:18:50,910 --> 00:18:53,672 For one thing, you just have to remember the definitions. 354 00:18:53,672 --> 00:18:55,380 For another, you have to recognize what's 355 00:18:55,380 --> 00:18:57,480 happening to the chromium. 356 00:18:57,480 --> 00:19:00,390 So let's take a look at the chromium now. 357 00:19:00,390 --> 00:19:03,240 So we have oxygen at minus 2. 358 00:19:03,240 --> 00:19:05,900 There's seven of them. 359 00:19:05,900 --> 00:19:10,700 And the overall charge on the molecule is minus 2. 360 00:19:10,700 --> 00:19:12,920 And so what does that mean for the oxidation 361 00:19:12,920 --> 00:19:14,581 number of this chromium? 362 00:19:14,581 --> 00:19:15,443 AUDIENCE: Plus 6. 363 00:19:15,443 --> 00:19:16,901 CATHERINE DRENNAN: It's going to be 364 00:19:16,901 --> 00:19:20,750 plus 6, because we're going to have 12 minus 14 equals 365 00:19:20,750 --> 00:19:23,090 minus 2. 366 00:19:23,090 --> 00:19:28,600 So we're going from plus 6 over here to plus 3 here. 367 00:19:28,600 --> 00:19:32,060 So is that an oxidation or reduction? 368 00:19:32,060 --> 00:19:35,910 So that's a reduction. 369 00:19:35,910 --> 00:19:39,120 So I kind of like this unit coming after acids and bases 370 00:19:39,120 --> 00:19:41,190 because a lot of those problems, I think, 371 00:19:41,190 --> 00:19:43,140 make people's brains hurt a little bit. 372 00:19:43,140 --> 00:19:46,290 And you get here and it's, like, basic math. 373 00:19:46,290 --> 00:19:48,120 So it's kind of exciting. 374 00:19:48,120 --> 00:19:50,550 All right, so the iron is even simpler. 375 00:19:50,550 --> 00:19:53,910 We're going from plus 2 to plus 3. 376 00:19:53,910 --> 00:19:55,920 So that's an oxidation. 377 00:19:55,920 --> 00:19:58,080 So one thing is being reduced. 378 00:19:58,080 --> 00:19:59,880 The other thing is being oxidized. 379 00:19:59,880 --> 00:20:03,280 That's what makes it a redox reaction. 380 00:20:03,280 --> 00:20:06,280 All right, so we know what's happening 381 00:20:06,280 --> 00:20:10,150 in terms of what's being oxidized, what's being reduced. 382 00:20:10,150 --> 00:20:12,760 And we've broken these into half reactions. 383 00:20:12,760 --> 00:20:15,402 Now we need to balance. 384 00:20:15,402 --> 00:20:17,360 All right, so the first thing we're going to do 385 00:20:17,360 --> 00:20:20,630 is we're going to balance our elements that are not 386 00:20:20,630 --> 00:20:24,710 oxygen and hydrogen. So we're going to balance chromium 387 00:20:24,710 --> 00:20:27,200 and we're going to balance our iron. 388 00:20:27,200 --> 00:20:32,390 What do we need to add to this top to balance chromium? 389 00:20:32,390 --> 00:20:34,680 We need to add a 2. 390 00:20:34,680 --> 00:20:37,450 And we don't need to do anything to our iron. 391 00:20:37,450 --> 00:20:41,350 We have one iron on one side, one iron on the other side. 392 00:20:41,350 --> 00:20:45,160 All right, so all you need to do is insert the 2. 393 00:20:45,160 --> 00:20:51,260 Next step, we're going to add water to balance oxygen. 394 00:20:51,260 --> 00:20:54,940 So how much water do we need to add to the top reaction 395 00:20:54,940 --> 00:20:56,155 to balance the oxygen? 396 00:21:01,740 --> 00:21:07,560 So here we're going to add seven waters to balance the oxygen. 397 00:21:07,560 --> 00:21:12,120 And again, we have nothing to do down there. 398 00:21:12,120 --> 00:21:15,440 All, right, so let's keep going. 399 00:21:15,440 --> 00:21:18,350 Next, we want to balance the hydrogen. 400 00:21:18,350 --> 00:21:20,780 We're just added those waters. 401 00:21:20,780 --> 00:21:22,970 And so now we have to balance hydrogen. 402 00:21:22,970 --> 00:21:26,390 So we're going to balance the hydrogen by adding H plus. 403 00:21:26,390 --> 00:21:28,760 Some books have you balance the hydrogen 404 00:21:28,760 --> 00:21:31,070 by adding a more correct species, which 405 00:21:31,070 --> 00:21:32,660 is hydronium ion. 406 00:21:32,660 --> 00:21:35,810 But when you do that, you screw up your oxygens 407 00:21:35,810 --> 00:21:37,400 that you just balanced. 408 00:21:37,400 --> 00:21:41,810 So that is infinitely harder to balance equations that way. 409 00:21:41,810 --> 00:21:44,210 So I say let's not do that. 410 00:21:44,210 --> 00:21:46,970 So let's just use H plus. 411 00:21:46,970 --> 00:21:49,670 So how many H pluses do we need to add over 412 00:21:49,670 --> 00:21:52,840 here to balance the hydrogen? 413 00:21:52,840 --> 00:21:55,410 14-- you've added 14. 414 00:21:55,410 --> 00:21:57,150 And again, you don't have to do anything 415 00:21:57,150 --> 00:22:01,430 to the bottom expression. 416 00:22:01,430 --> 00:22:05,020 Next, we are going to balance the charge 417 00:22:05,020 --> 00:22:07,090 by inserting electrons. 418 00:22:07,090 --> 00:22:11,350 So over here we have 14 minus 2 plus 12. 419 00:22:11,350 --> 00:22:13,750 And over here we have plus 6. 420 00:22:13,750 --> 00:22:17,814 So how many electrons are we going to need to add? 421 00:22:17,814 --> 00:22:21,190 We're going to need to add six. 422 00:22:21,190 --> 00:22:23,740 So we've added six over here, because they 423 00:22:23,740 --> 00:22:25,450 have a negative charge. 424 00:22:25,450 --> 00:22:28,730 So now we should be balanced on both sides. 425 00:22:28,730 --> 00:22:30,700 And now we finally get to do something 426 00:22:30,700 --> 00:22:32,620 with the bottom expression. 427 00:22:32,620 --> 00:22:35,890 And we can add an electron over here to balance the charge. 428 00:22:35,890 --> 00:22:39,730 So plus 2 here, plus 2 net on the other side. 429 00:22:42,340 --> 00:22:50,900 All right, now we want to multiply so that the electrons 430 00:22:50,900 --> 00:22:53,990 are going to cancel. 431 00:22:53,990 --> 00:22:57,230 Because this is an overall oxidation-reduction reaction, 432 00:22:57,230 --> 00:22:59,510 our electrons should cancel. 433 00:22:59,510 --> 00:23:03,170 So what do we need to multiply the bottom equation by? 434 00:23:03,170 --> 00:23:10,130 6, and there we go-- 6 here, 6 here, and 6 here. 435 00:23:10,130 --> 00:23:14,330 Now this is one of the places that people often, in exams, 436 00:23:14,330 --> 00:23:18,080 make a mistake where they write 6 437 00:23:18,080 --> 00:23:21,020 and then put a parentheses around it indicating 438 00:23:21,020 --> 00:23:23,330 that you're multiplying the whole thing by 6, 439 00:23:23,330 --> 00:23:24,290 which is fine. 440 00:23:24,290 --> 00:23:27,050 But then they forget to actually carry that out 441 00:23:27,050 --> 00:23:28,380 on the next step. 442 00:23:28,380 --> 00:23:32,270 So when you multiply, make sure that you continue 443 00:23:32,270 --> 00:23:35,060 to-- sometimes it's better to just write it all out, 444 00:23:35,060 --> 00:23:38,390 because otherwise it's really easy to forget and not 445 00:23:38,390 --> 00:23:41,210 multiply, and then things won't make sense at the end. 446 00:23:41,210 --> 00:23:44,270 All right, so now we can add the half reactions 447 00:23:44,270 --> 00:23:47,150 and make the appropriate cancellations. 448 00:23:47,150 --> 00:23:52,070 So we've now added all together six electrons, 14 H plus. 449 00:23:52,070 --> 00:23:54,800 We have our chromium oxide compound. 450 00:23:54,800 --> 00:24:00,290 We have now our six iron plus 2s, our two chromium plus 3s, 451 00:24:00,290 --> 00:24:04,940 our seven waters, our six iron plus 3s, and our six electrons. 452 00:24:04,940 --> 00:24:08,900 And if we did everything right, then the electrons 453 00:24:08,900 --> 00:24:10,650 should cancel. 454 00:24:10,650 --> 00:24:11,776 And we can double check. 455 00:24:11,776 --> 00:24:13,400 It's always a good idea to double check 456 00:24:13,400 --> 00:24:14,750 that it's balanced. 457 00:24:14,750 --> 00:24:19,010 So we have 14 H's here, 14 H's there. 458 00:24:19,010 --> 00:24:20,960 Chromium two, two. 459 00:24:20,960 --> 00:24:22,910 Oxygen seven, seven. 460 00:24:22,910 --> 00:24:24,530 Iron, six, six. 461 00:24:24,530 --> 00:24:27,180 This can catch a mistake that you might have made. 462 00:24:27,180 --> 00:24:30,860 And then check the charge-- plus 14, minus 2, 463 00:24:30,860 --> 00:24:33,080 plus 12 on one side. 464 00:24:33,080 --> 00:24:39,790 Plus 6, plus 18, so 24 on both sides. 465 00:24:39,790 --> 00:24:40,895 That's pretty good, right? 466 00:24:40,895 --> 00:24:41,770 That's pretty simple. 467 00:24:41,770 --> 00:24:43,390 So balancing. 468 00:24:43,390 --> 00:24:45,440 That's an acidic solution. 469 00:24:45,440 --> 00:24:48,460 We know it's in acidic because we have 14 H plus. 470 00:24:48,460 --> 00:24:50,290 That feels quite acidic. 471 00:24:50,290 --> 00:24:53,170 What do you do for a basic solution now? 472 00:24:53,170 --> 00:24:56,770 So for a basic solution what I recommend is just 473 00:24:56,770 --> 00:25:00,940 do the exact same thing that you did for your acidic solution. 474 00:25:00,940 --> 00:25:03,730 And here again was the answer that we have. 475 00:25:03,730 --> 00:25:09,910 And then, quote, neutralize the acid and make it basic. 476 00:25:09,910 --> 00:25:15,250 Adjust your pH by adding OH minus or hydroxide iron 477 00:25:15,250 --> 00:25:18,050 concentration to each side. 478 00:25:18,050 --> 00:25:19,930 So if we do that now, we're going 479 00:25:19,930 --> 00:25:26,830 to add 14 OH minus to this side and 14 OH minus to that side. 480 00:25:26,830 --> 00:25:30,910 Now 14 OH minus plus 14 H plus are 481 00:25:30,910 --> 00:25:34,640 going to give us 14 water molecules over here. 482 00:25:34,640 --> 00:25:39,010 And on this side we still have 14 OH. 483 00:25:39,010 --> 00:25:42,520 This is not simplified, because we have 14 waters on one side 484 00:25:42,520 --> 00:25:45,290 and seven on the other side. 485 00:25:45,290 --> 00:25:49,480 So we can take that, cancel out these seven. 486 00:25:49,480 --> 00:25:51,830 And we have seven left over here. 487 00:25:51,830 --> 00:25:54,460 So we have seven waters now on one side 488 00:25:54,460 --> 00:25:57,470 and 14 hydroxides on the other. 489 00:25:57,470 --> 00:26:01,150 So if we look at both of these expressions in acidic solution 490 00:26:01,150 --> 00:26:04,450 and in basic solution here, in acidic solution 491 00:26:04,450 --> 00:26:08,530 we have 14 H plus on one side and seven waters on the other. 492 00:26:08,530 --> 00:26:11,650 In basic we have seven waters on this side and 14 OH 493 00:26:11,650 --> 00:26:12,910 minus on this side. 494 00:26:12,910 --> 00:26:14,780 Everything else should be the same, 495 00:26:14,780 --> 00:26:16,450 and they should all add up. 496 00:26:16,450 --> 00:26:19,480 All right, so there we go. 497 00:26:19,480 --> 00:26:22,300 That is balancing oxidation-reductions. 498 00:26:22,300 --> 00:26:26,000 It seems really straightforward, but it's very important. 499 00:26:26,000 --> 00:26:30,130 We'll see next week that we need to use this information 500 00:26:30,130 --> 00:26:31,860 to be able to do problems. 501 00:26:31,860 --> 00:26:34,030 All right, so end of 14. 502 00:26:34,030 --> 00:26:40,630 Now to electrochemistry-- so today's handout. 503 00:26:40,630 --> 00:26:42,640 So we're talking about oxidation-reduction. 504 00:26:42,640 --> 00:26:45,490 We're continuing talking about oxidation-reduction. 505 00:26:45,490 --> 00:26:47,020 I just want to remind you how really 506 00:26:47,020 --> 00:26:50,770 important oxidation-reduction reactions are. 507 00:26:50,770 --> 00:26:53,230 So we've talked about photosynthesis in this class. 508 00:26:53,230 --> 00:26:55,210 We've talked about fuel cells. 509 00:26:55,210 --> 00:26:58,660 All of these reactions that make energy involve 510 00:26:58,660 --> 00:27:02,140 oxidation-reduction, loss of electrons, gain of electrons. 511 00:27:02,140 --> 00:27:05,230 Basically the human body is a big battery. 512 00:27:05,230 --> 00:27:07,510 We have lots of oxidation-reduction reactions 513 00:27:07,510 --> 00:27:08,890 going on inside of us. 514 00:27:08,890 --> 00:27:10,450 This is how we live. 515 00:27:10,450 --> 00:27:11,900 This is how we make energy. 516 00:27:11,900 --> 00:27:14,410 This is how we survive. 517 00:27:14,410 --> 00:27:17,470 All right, so electrochemistry is the study 518 00:27:17,470 --> 00:27:21,760 of redox reactions, usually thinking about how 519 00:27:21,760 --> 00:27:24,640 they occur at an electrode. 520 00:27:24,640 --> 00:27:27,280 So there are two types of reactions 521 00:27:27,280 --> 00:27:29,860 that you will be considering. 522 00:27:29,860 --> 00:27:33,160 You might be considering a reaction that 523 00:27:33,160 --> 00:27:38,170 generates an electric current, electron flow, or electricity, 524 00:27:38,170 --> 00:27:41,110 from a spontaneous reaction. 525 00:27:41,110 --> 00:27:43,630 And you might consider one where you 526 00:27:43,630 --> 00:27:47,200 need to drive a reaction that is nonspontaneous 527 00:27:47,200 --> 00:27:49,280 using an electric current. 528 00:27:49,280 --> 00:27:51,880 So a spontaneous reaction, delta G 529 00:27:51,880 --> 00:27:54,058 is what, positive or negative? 530 00:27:54,058 --> 00:27:55,372 AUDIENCE: Negative. 531 00:27:55,372 --> 00:27:56,540 CATHERINE DRENNAN: Right. 532 00:27:56,540 --> 00:28:00,580 So a spontaneous reaction, delta G is negative. 533 00:28:00,580 --> 00:28:02,080 It's less than zero. 534 00:28:02,080 --> 00:28:05,530 A nonspontaneous reaction is positive. 535 00:28:05,530 --> 00:28:10,450 Just a little review there for exam three. 536 00:28:10,450 --> 00:28:16,460 All right, so electrochemistry, study of these redox reactions. 537 00:28:16,460 --> 00:28:20,030 Electrochemical cells are devices 538 00:28:20,030 --> 00:28:25,280 where you generate electrons and they flow through the cell. 539 00:28:25,280 --> 00:28:29,000 And so you have the flow of electrons through a circuit. 540 00:28:29,000 --> 00:28:34,140 And again, that can be generated by a spontaneous reaction. 541 00:28:34,140 --> 00:28:36,740 And if it is generated by a spontaneous reaction, 542 00:28:36,740 --> 00:28:39,410 that's called a galvanic cell. 543 00:28:39,410 --> 00:28:41,420 Or you can use an electric current 544 00:28:41,420 --> 00:28:44,360 to force a nonspontaneous reaction to go. 545 00:28:44,360 --> 00:28:46,730 And that's called an electrolytic cell. 546 00:28:46,730 --> 00:28:48,920 And it's important to remember those definitions 547 00:28:48,920 --> 00:28:51,440 because a lot of problems tell you, 548 00:28:51,440 --> 00:28:53,510 oh, in this electrolytic cell. 549 00:28:53,510 --> 00:28:56,420 And that's telling you key information about what's 550 00:28:56,420 --> 00:28:57,980 going on in that problem. 551 00:28:57,980 --> 00:29:01,130 So if you don't have these committed to memories, 552 00:29:01,130 --> 00:29:02,780 you will find yourself in trouble. 553 00:29:02,780 --> 00:29:05,780 But it's not too bad, just two terms. 554 00:29:05,780 --> 00:29:09,860 So a battery then-- so I have a battery here. 555 00:29:09,860 --> 00:29:12,837 A battery is a collection of galvanic cells. 556 00:29:12,837 --> 00:29:14,420 And it's going to give us-- it's going 557 00:29:14,420 --> 00:29:18,800 to use spontaneous reactions to give us electric current. 558 00:29:21,420 --> 00:29:25,600 And again, electrodes, the definition for those, 559 00:29:25,600 --> 00:29:27,330 they're just the conductors through which 560 00:29:27,330 --> 00:29:29,310 the electrons travel. 561 00:29:29,310 --> 00:29:31,180 And there are two types. 562 00:29:31,180 --> 00:29:32,330 Let's quiet down again. 563 00:29:32,330 --> 00:29:33,780 I hear the noise. 564 00:29:33,780 --> 00:29:38,280 Two types of electrodes-- we have anodes and cathodes. 565 00:29:38,280 --> 00:29:42,270 All right, so let's look at an electrochemical cell 566 00:29:42,270 --> 00:29:45,250 and consider, again, the anodes and the cathodes. 567 00:29:45,250 --> 00:29:47,650 So this is my beautiful depiction 568 00:29:47,650 --> 00:29:50,110 of an electrochemical cell. 569 00:29:50,110 --> 00:29:52,630 This might be a better depiction of what it looks like, 570 00:29:52,630 --> 00:29:54,689 but I think mine is not bad. 571 00:29:54,689 --> 00:29:56,230 Just to help you out, that's supposed 572 00:29:56,230 --> 00:29:58,990 to be a beaker that has fluid in it. 573 00:29:58,990 --> 00:30:01,600 This is also a beaker that has fluid in it. 574 00:30:01,600 --> 00:30:03,670 And this is my electrode. 575 00:30:03,670 --> 00:30:05,380 That is an anode in this case. 576 00:30:05,380 --> 00:30:08,979 There is the anode there, the electrode there. 577 00:30:08,979 --> 00:30:09,770 Here's another one. 578 00:30:09,770 --> 00:30:12,100 That's the cathode, another type of electrode. 579 00:30:12,100 --> 00:30:13,450 It's over here. 580 00:30:13,450 --> 00:30:15,250 This is my salt bridge. 581 00:30:15,250 --> 00:30:17,920 The salt bridge here is this plastic tube. 582 00:30:17,920 --> 00:30:20,680 And here I have a wire connecting my electrodes here. 583 00:30:20,680 --> 00:30:22,600 And you can't really see the wire. 584 00:30:22,600 --> 00:30:25,810 But in both cases it goes through some kind of volt meter 585 00:30:25,810 --> 00:30:29,280 that measures the electric current. 586 00:30:29,280 --> 00:30:32,070 All right, so now we get rid of this better picture. 587 00:30:32,070 --> 00:30:34,740 But you can think about this pictures or that picture 588 00:30:34,740 --> 00:30:35,790 if that helps. 589 00:30:35,790 --> 00:30:37,800 And I always like to joke that you 590 00:30:37,800 --> 00:30:40,469 think you have a bad here at MIT, in the old days, 591 00:30:40,469 --> 00:30:42,510 before you could do your problem set at 2 o'clock 592 00:30:42,510 --> 00:30:45,030 in the morning you had to build your own batteries 593 00:30:45,030 --> 00:30:47,040 to be able to do it. 594 00:30:47,040 --> 00:30:54,450 All right, so anode-- electrons are produced from oxidation 595 00:30:54,450 --> 00:30:55,530 at the anode. 596 00:30:55,530 --> 00:31:01,240 So the electrons produced here flow out of the cell. 597 00:31:01,240 --> 00:31:05,640 And in this particular type of electrochemical cell, 598 00:31:05,640 --> 00:31:10,010 zinc solid is oxidized to zinc plus 2. 599 00:31:10,010 --> 00:31:11,720 So if we look at this over here, this 600 00:31:11,720 --> 00:31:14,390 is my little beautiful depiction of the blow 601 00:31:14,390 --> 00:31:16,610 up of the electrode. 602 00:31:16,610 --> 00:31:18,650 We have our zinc solid here. 603 00:31:18,650 --> 00:31:21,860 And we have zinc plus 2 in aqueous solution. 604 00:31:21,860 --> 00:31:26,630 So when the electrons leave the zinc solid, 605 00:31:26,630 --> 00:31:29,990 so that the zinc then leaves the electrode 606 00:31:29,990 --> 00:31:32,780 and goes into solution, forming zinc plus 2, 607 00:31:32,780 --> 00:31:34,820 those electrons go out. 608 00:31:34,820 --> 00:31:38,390 So the electrons will then go into the cathode. 609 00:31:38,390 --> 00:31:40,340 And notice I had a typo in the handout. 610 00:31:40,340 --> 00:31:41,401 I find this pretty funny. 611 00:31:41,401 --> 00:31:43,400 I don't know if it's me or spellcheck right now. 612 00:31:43,400 --> 00:31:47,630 I wrote elections are consumed. 613 00:31:47,630 --> 00:31:51,440 Oh man, it's been a long week, between the national elections 614 00:31:51,440 --> 00:31:53,700 and our T-shirt elections. 615 00:31:53,700 --> 00:31:56,510 Anyway, I didn't mean elections. 616 00:31:56,510 --> 00:31:58,280 I meant electrons. 617 00:31:58,280 --> 00:32:01,550 Yes, of course, electrons are consumed 618 00:32:01,550 --> 00:32:04,950 by a reduction reaction. 619 00:32:04,950 --> 00:32:09,720 Of course I noticed that after I had printed hundreds of copies. 620 00:32:09,720 --> 00:32:13,860 And this particular reaction here, copper plus 2 621 00:32:13,860 --> 00:32:16,110 is reduced to copper solid. 622 00:32:16,110 --> 00:32:17,850 And if we look over here, we have copper 623 00:32:17,850 --> 00:32:20,670 plus 2 in aqueous solution. 624 00:32:20,670 --> 00:32:25,140 When two electrons come in, those two electrons 625 00:32:25,140 --> 00:32:29,010 go to the copper plus 2 and reduce it to copper solid. 626 00:32:29,010 --> 00:32:33,510 And the copper solid actually plates on the electrode. 627 00:32:33,510 --> 00:32:35,880 So we can measure the flow. 628 00:32:35,880 --> 00:32:37,410 We can have a meter that measures 629 00:32:37,410 --> 00:32:40,110 the volts or the flow of electrons, 630 00:32:40,110 --> 00:32:41,790 the electric current. 631 00:32:41,790 --> 00:32:43,890 And also what's that salt bridge doing? 632 00:32:43,890 --> 00:32:47,010 Well, the salt bridge can help maintain 633 00:32:47,010 --> 00:32:48,960 the neutrality in the system. 634 00:32:48,960 --> 00:32:52,540 Because over here, we're generating plus charge. 635 00:32:52,540 --> 00:32:54,990 And over here we're losing plus charge. 636 00:32:54,990 --> 00:32:58,050 And so ions can flow through the salt bridge 637 00:32:58,050 --> 00:33:01,770 and keep the charges more similar in our two 638 00:33:01,770 --> 00:33:03,880 different beakers. 639 00:33:03,880 --> 00:33:06,240 All right, so I know my pictures are gorgeous, 640 00:33:06,240 --> 00:33:11,040 but I have a little animation to show you that will also 641 00:33:11,040 --> 00:33:12,570 show you what I was talking about, 642 00:33:12,570 --> 00:33:16,326 oxidation at the electrode. 643 00:33:16,326 --> 00:33:20,820 Zinc solid is going to lose electrons and form zinc plus 2. 644 00:33:20,820 --> 00:33:24,830 And let me just show you that happening. 645 00:33:24,830 --> 00:33:27,350 So there are the electrons going away. 646 00:33:27,350 --> 00:33:31,210 And here is zinc popping off as zinc plus 2. 647 00:33:31,210 --> 00:33:34,630 And then the zinc plus 2 is going to be in solution. 648 00:33:34,630 --> 00:33:37,000 So there the electrons go again. 649 00:33:37,000 --> 00:33:41,700 And the zinc solid pops off as zinc plus 2. 650 00:33:41,700 --> 00:33:45,490 All right, so now let's consider reduction at the cathode. 651 00:33:45,490 --> 00:33:47,550 So here we have our copper solid. 652 00:33:47,550 --> 00:33:49,920 Here we have a copper plus 2. 653 00:33:49,920 --> 00:33:54,720 And here the electrons are going to come in and join up 654 00:33:54,720 --> 00:33:56,460 with our copper plus 2. 655 00:33:56,460 --> 00:33:59,466 And it's going to plate onto the electrode. 656 00:34:03,200 --> 00:34:05,370 There come the electrons. 657 00:34:05,370 --> 00:34:07,730 There they come again. 658 00:34:07,730 --> 00:34:12,830 And now we have added solid copper to our electrode. 659 00:34:12,830 --> 00:34:15,830 And just one more time-- and I think 660 00:34:15,830 --> 00:34:17,690 it's so cool that we can just watch 661 00:34:17,690 --> 00:34:19,536 those electrons move like that. 662 00:34:23,590 --> 00:34:26,679 All right, so there's our electrochemical cell 663 00:34:26,679 --> 00:34:31,210 where we have an oxidation and reduction reaction. 664 00:34:31,210 --> 00:34:34,690 Instead of drawing this admittedly beautiful picture 665 00:34:34,690 --> 00:34:38,110 every time, we can have a shorthand representation 666 00:34:38,110 --> 00:34:39,100 for this. 667 00:34:39,100 --> 00:34:44,080 So we can say zinc solid. 668 00:34:44,080 --> 00:34:46,540 And then we have a little line here. 669 00:34:46,540 --> 00:34:50,320 That little line indicates we have a phase boundary. 670 00:34:50,320 --> 00:34:53,210 So that means we're going from solid to a different phase. 671 00:34:53,210 --> 00:34:56,659 And in this case we're going to aqueous, so solid zinc 672 00:34:56,659 --> 00:34:58,880 to zinc plus 2. 673 00:34:58,880 --> 00:35:02,360 The two lines here indicate the salt bridge. 674 00:35:02,360 --> 00:35:05,360 So this tells us that the reaction on this side 675 00:35:05,360 --> 00:35:08,060 is in one beaker, and the reaction on this side 676 00:35:08,060 --> 00:35:09,590 is in our other beaker. 677 00:35:09,590 --> 00:35:11,030 The reaction in the other side is 678 00:35:11,030 --> 00:35:14,920 copper plus 2 aqueous, a single line for phase boundary going 679 00:35:14,920 --> 00:35:16,220 to copper solid. 680 00:35:16,220 --> 00:35:19,160 Now if we had, say, two aqueous species, 681 00:35:19,160 --> 00:35:20,870 we wouldn't have a phase boundary, 682 00:35:20,870 --> 00:35:22,380 we'd just have a comma. 683 00:35:22,380 --> 00:35:25,400 But if you have a phase boundary, you get the line. 684 00:35:25,400 --> 00:35:28,100 All right, so it's a clicker competition. 685 00:35:28,100 --> 00:35:30,740 Let's consider another cell. 686 00:35:30,740 --> 00:35:33,680 It still has zinc, but now it has tin. 687 00:35:33,680 --> 00:35:36,890 Why don't you tell me which reaction is at the anode 688 00:35:36,890 --> 00:35:38,450 and which reaction is at the cathode? 689 00:35:42,740 --> 00:35:45,120 And also, which of those is oxidation 690 00:35:45,120 --> 00:35:46,300 and which is reductions? 691 00:35:58,357 --> 00:36:00,065 All right, we'll just do 10 more seconds. 692 00:36:14,220 --> 00:36:15,330 Great. 693 00:36:15,330 --> 00:36:18,900 OK, so some of these had parts of it right, 694 00:36:18,900 --> 00:36:19,980 but there was a mistake. 695 00:36:19,980 --> 00:36:22,920 We have AnOx, so that is wrong. 696 00:36:22,920 --> 00:36:26,040 So you had to pay attention to whether these match, 697 00:36:26,040 --> 00:36:29,190 and then also that the reaction is written consistent 698 00:36:29,190 --> 00:36:29,880 with the word. 699 00:36:29,880 --> 00:36:34,800 So only number two had all the various different parts right. 700 00:36:34,800 --> 00:36:36,510 So we can come back here and you can 701 00:36:36,510 --> 00:36:40,710 enter-- so the reaction at the anode was a zinc reaction. 702 00:36:40,710 --> 00:36:44,020 And that is an oxidation, AnOx. 703 00:36:44,020 --> 00:36:47,870 The reaction at the cathode is the tin reaction. 704 00:36:47,870 --> 00:36:52,380 Tin plus 4, two electrons, tin plus 2 aqueous. 705 00:36:52,380 --> 00:36:54,540 And that's a reduction. 706 00:36:54,540 --> 00:36:57,870 And while you have your clickers out, why don't you 707 00:36:57,870 --> 00:37:02,870 tell me how you would represent that electrochemical cell? 708 00:37:12,100 --> 00:37:13,840 All right, let's just do 10 more seconds. 709 00:37:32,710 --> 00:37:34,150 So right. 710 00:37:34,150 --> 00:37:36,490 So I mentioned this very briefly. 711 00:37:36,490 --> 00:37:38,500 This was my tricky clicker question, 712 00:37:38,500 --> 00:37:40,880 because it's a competition. 713 00:37:40,880 --> 00:37:47,170 So there's no phase transition here, so you have a comma. 714 00:37:47,170 --> 00:37:52,510 And the top one a few people put down, but it's not solid here. 715 00:37:52,510 --> 00:37:56,050 So that one is not correct. 716 00:37:56,050 --> 00:37:58,240 So here you can write down in your notes, 717 00:37:58,240 --> 00:38:00,970 you have this zinc phase transition, 718 00:38:00,970 --> 00:38:02,890 zinc plus 2 aqueous. 719 00:38:02,890 --> 00:38:05,390 And then we have our salt bridge, zinc 720 00:38:05,390 --> 00:38:11,500 plus 4 aqueous comma per zinc, tin plus 4 comma, tine plus 2. 721 00:38:11,500 --> 00:38:15,910 And now we're ready for winners. 722 00:38:15,910 --> 00:38:18,296 We'll see if Sam's also ready to present the winners. 723 00:38:24,620 --> 00:38:31,380 All right, we need a strategy to defeat Dan's recitation, 724 00:38:31,380 --> 00:38:34,970 but we know what T-shirt we're playing for now. 725 00:38:34,970 --> 00:38:37,290 All right, have a great weekend everybody. 726 00:38:48,470 --> 00:38:51,380 All right, let's just take 10 more seconds. 727 00:39:06,550 --> 00:39:09,120 All right, does someone want to tell me 728 00:39:09,120 --> 00:39:10,585 how they got the right answer? 729 00:39:13,458 --> 00:39:13,958 Yep? 730 00:39:19,830 --> 00:39:22,747 AUDIENCE: You know that your hydrogen will have a plus 1 731 00:39:22,747 --> 00:39:24,830 charge, and your oxygen will have a minus 2 charge 732 00:39:24,830 --> 00:39:26,520 because it's not a peroxide. 733 00:39:26,520 --> 00:39:28,830 And you know your sum is going to be neutral. 734 00:39:28,830 --> 00:39:30,226 So you add one. 735 00:39:30,226 --> 00:39:32,700 CATHERINE DRENNAN: Yep, that's right. 736 00:39:32,700 --> 00:39:36,870 So this one, chlorine is usually minus 1. 737 00:39:36,870 --> 00:39:38,940 The exception is when it's with oxygen. 738 00:39:38,940 --> 00:39:41,910 And so oxygen is always minus 2 unless it's 739 00:39:41,910 --> 00:39:44,970 in a peroxide or superoxide. 740 00:39:44,970 --> 00:39:47,220 And hydrogen's always plus 1 except when 741 00:39:47,220 --> 00:39:51,960 it's with a metal like sodium, something Group 1 or Group 2. 742 00:39:51,960 --> 00:39:54,800 OK, so today take out lecture 25 notes. 743 00:39:54,800 --> 00:39:56,610 We're just going to wrap those notes up-- 744 00:39:56,610 --> 00:39:58,410 that shouldn't take too long-- and then 745 00:39:58,410 --> 00:39:59,955 keep going with lecture 26. 746 00:40:02,480 --> 00:40:05,150 So we were talking about this cell. 747 00:40:05,150 --> 00:40:07,520 And we're going to talk about this electrochemical cell 748 00:40:07,520 --> 00:40:09,950 a lot. 749 00:40:09,950 --> 00:40:14,810 So we had zinc on one side, zinc at the anode. 750 00:40:14,810 --> 00:40:19,940 And the zinc was being oxidized, generating electrons, which 751 00:40:19,940 --> 00:40:21,830 come around to the copper. 752 00:40:21,830 --> 00:40:24,590 And then the copper plus 2 ions were 753 00:40:24,590 --> 00:40:26,840 being reduced to copper solid. 754 00:40:26,840 --> 00:40:30,710 So the zinc anode was being consumed. 755 00:40:30,710 --> 00:40:34,370 And the copper cathode had copper solid being 756 00:40:34,370 --> 00:40:36,140 plated onto it. 757 00:40:36,140 --> 00:40:39,530 So Faraday's law says that the amount 758 00:40:39,530 --> 00:40:43,460 in which an anode, like the zinc anode, is consumed 759 00:40:43,460 --> 00:40:47,870 and the amount of copper that gets plated on at the cathode 760 00:40:47,870 --> 00:40:51,020 is proportional to the charge that passes through the system. 761 00:40:51,020 --> 00:40:53,010 So according to Faraday's law, there 762 00:40:53,010 --> 00:40:56,540 is a proportionality between the consumption 763 00:40:56,540 --> 00:41:01,520 and also depositing of metal and the amount of charge 764 00:41:01,520 --> 00:41:03,080 going through the system. 765 00:41:03,080 --> 00:41:07,790 So we can think about how much zinc is consumed 766 00:41:07,790 --> 00:41:15,230 and how much copper is deposited on its cathode or its electrode 767 00:41:15,230 --> 00:41:19,430 if you have a current of 1 amp flowing for 1 hour. 768 00:41:19,430 --> 00:41:22,700 So let's just take a look at how we do a problem like this. 769 00:41:22,700 --> 00:41:25,220 First step is we want to think about, 770 00:41:25,220 --> 00:41:29,100 calculate the charge that's going through the system. 771 00:41:29,100 --> 00:41:33,710 And so Q here, we have another Q-- not the reaction quotient, 772 00:41:33,710 --> 00:41:35,330 but the magnitude of the charge. 773 00:41:35,330 --> 00:41:37,160 And that's in coulombs. 774 00:41:37,160 --> 00:41:40,620 And that's going to be equal to the current times the time. 775 00:41:40,620 --> 00:41:44,210 So we have the current, I. And the current is in amps. 776 00:41:44,210 --> 00:41:45,950 And t is in seconds. 777 00:41:45,950 --> 00:41:49,860 And conveniently, amps equal coulombs per second. 778 00:41:49,860 --> 00:41:51,350 So this equation works out. 779 00:41:51,350 --> 00:41:53,540 Our units will work out. 780 00:41:53,540 --> 00:41:58,580 So Q is going to be equal to-- we have 1.0 amps. 781 00:41:58,580 --> 00:42:02,720 And it was 1 hour, so that's 3,600 seconds, 782 00:42:02,720 --> 00:42:09,620 or 3,600 coulombs, because we have an amp times second. 783 00:42:09,620 --> 00:42:14,180 And then here, we have this number of coulombs. 784 00:42:14,180 --> 00:42:17,690 And now we want to convert to the number of moles 785 00:42:17,690 --> 00:42:21,620 of electrons that that is equivalent to, that that charge 786 00:42:21,620 --> 00:42:23,690 in coulombs is equivalent to. 787 00:42:23,690 --> 00:42:28,280 And here, our Faraday's constant is our conversion factor. 788 00:42:28,280 --> 00:42:35,090 So Faraday's constant, we have 96,485 coulombs per mole 789 00:42:35,090 --> 00:42:36,590 is one faraday. 790 00:42:36,590 --> 00:42:39,720 And its this kind of funny looking F over here. 791 00:42:39,720 --> 00:42:41,390 So we have our coulombs. 792 00:42:41,390 --> 00:42:45,830 So we can use now the Faraday's constant to convert. 793 00:42:45,830 --> 00:42:50,580 And we get 0.00373 moles of electrons. 794 00:42:50,580 --> 00:42:52,550 So that's the number of moles of electrons 795 00:42:52,550 --> 00:42:55,880 that would be going through our system that correspond 796 00:42:55,880 --> 00:42:58,500 to that amount of charge. 797 00:42:58,500 --> 00:43:00,550 All right, so now we need to relate this back 798 00:43:00,550 --> 00:43:02,860 to the zinc and the copper. 799 00:43:02,860 --> 00:43:04,630 So step three, we want to know first 800 00:43:04,630 --> 00:43:07,960 the number of moles of the zinc and copper involved, 801 00:43:07,960 --> 00:43:10,810 and then convert that to grams because the problem asked us 802 00:43:10,810 --> 00:43:12,610 for grams. 803 00:43:12,610 --> 00:43:14,980 So we have the number of moles of electrons that 804 00:43:14,980 --> 00:43:16,730 are going through the system. 805 00:43:16,730 --> 00:43:20,050 And so for every one mole of zinc consumed, 806 00:43:20,050 --> 00:43:23,500 how many electrons go into the system? 807 00:43:23,500 --> 00:43:27,938 What's the ratio, one, two-- how many electrons? 808 00:43:27,938 --> 00:43:29,162 AUDIENCE: [INAUDIBLE] 809 00:43:29,162 --> 00:43:35,094 CATHERINE DRENNAN: So one zinc solid going to one zinc plus. 810 00:43:35,094 --> 00:43:35,822 AUDIENCE: Two 811 00:43:35,822 --> 00:43:36,780 CATHERINE DRENNAN: Two. 812 00:43:36,780 --> 00:43:41,870 So that's going to be two moles of electrons. 813 00:43:41,870 --> 00:43:44,390 And then we need to look up, how are we going 814 00:43:44,390 --> 00:43:49,162 to convert our moles to grams? 815 00:43:49,162 --> 00:43:50,120 What are we looking up? 816 00:43:53,840 --> 00:43:54,770 AUDIENCE: Molar mass. 817 00:43:54,770 --> 00:43:57,680 CATHERINE DRENNAN: Yep, molar mass, molecular weight, 818 00:43:57,680 --> 00:43:59,680 atomic mass. 819 00:43:59,680 --> 00:44:03,590 So in our periodic table, grams per mole. 820 00:44:03,590 --> 00:44:08,290 And so we get 1.2 grams here for this equation. 821 00:44:08,290 --> 00:44:11,440 All right, so for copper, so we have the same number 822 00:44:11,440 --> 00:44:14,440 of moles of electrons that are going through the system. 823 00:44:14,440 --> 00:44:17,860 So for every one mole of copper solid 824 00:44:17,860 --> 00:44:22,510 deposited on its electrode, how many moles of electrons 825 00:44:22,510 --> 00:44:26,100 have to come into the system? 826 00:44:26,100 --> 00:44:26,600 Louder? 827 00:44:26,600 --> 00:44:27,270 AUDIENCE: Two. 828 00:44:27,270 --> 00:44:28,610 CATHERINE DRENNAN: Two, yes. 829 00:44:28,610 --> 00:44:30,980 So that's two as well. 830 00:44:30,980 --> 00:44:35,180 And then we look up again, in the periodic table, 831 00:44:35,180 --> 00:44:38,210 the atomic weight, molecular weight. 832 00:44:38,210 --> 00:44:40,370 And it's almost the same, copper and zinc. 833 00:44:40,370 --> 00:44:42,030 So to the number of significant figures 834 00:44:42,030 --> 00:44:43,280 it's the same amount of grams. 835 00:44:43,280 --> 00:44:45,690 Now this will not always be the case. 836 00:44:45,690 --> 00:44:47,480 They're just very close to each other 837 00:44:47,480 --> 00:44:51,100 in their molecular or atomic weight. 838 00:44:51,100 --> 00:44:54,560 OK, so we have two significant figures. 839 00:44:54,560 --> 00:44:58,230 Also our current in time only had two significant figures. 840 00:44:58,230 --> 00:45:02,820 So we only have two significant figures in this problem. 841 00:45:02,820 --> 00:45:05,880 All right, so in this particular electrochemical cell, 842 00:45:05,880 --> 00:45:09,210 again, our anode is being consumed 843 00:45:09,210 --> 00:45:12,570 and the electrons that were generated 844 00:45:12,570 --> 00:45:14,760 ended up plating the copper on. 845 00:45:14,760 --> 00:45:17,040 Sometimes you're not going to want to do that. 846 00:45:17,040 --> 00:45:20,400 Sometimes you're just going to want to actually plate 847 00:45:20,400 --> 00:45:22,960 something with copper, for example. 848 00:45:22,960 --> 00:45:25,200 And so let's look at some of the uses 849 00:45:25,200 --> 00:45:27,030 for this kind of chemistry. 850 00:45:27,030 --> 00:45:28,920 So here's some electroplating. 851 00:45:31,474 --> 00:45:32,140 [VIDEO PLAYBACK] 852 00:45:32,140 --> 00:45:34,640 - This steel spoon will have a thin coating 853 00:45:34,640 --> 00:45:36,920 of copper deposited on it. 854 00:45:36,920 --> 00:45:39,560 The cell consists of an anode of pure copper, 855 00:45:39,560 --> 00:45:43,850 a solution of copper sulfate, and as a cathode, a spoon. 856 00:45:43,850 --> 00:45:45,500 There is no net reaction. 857 00:45:45,500 --> 00:45:49,066 The copper metal is transferred from the anode to the cathode. 858 00:45:54,000 --> 00:45:56,607 CATHERINE DRENNAN: It's pretty nice. 859 00:45:56,607 --> 00:45:57,190 [END PLAYBACK] 860 00:45:57,190 --> 00:45:58,970 CATHERINE DRENNAN: OK, so is kind 861 00:45:58,970 --> 00:46:01,700 of electroplating is pretty common where 862 00:46:01,700 --> 00:46:04,850 you use these kind of reactions to get your copper, which 863 00:46:04,850 --> 00:46:11,120 is in that solution, to plate as copper solid onto something. 864 00:46:11,120 --> 00:46:12,529 So it could be a spoon. 865 00:46:12,529 --> 00:46:13,570 It could be an electrode. 866 00:46:13,570 --> 00:46:14,069 Yeah? 867 00:46:14,069 --> 00:46:15,680 AUDIENCE: Why is it dangerous? 868 00:46:15,680 --> 00:46:17,420 CATHERINE DRENNAN: Why is it dangerous? 869 00:46:17,420 --> 00:46:19,400 You know, I've seen these demos before, 870 00:46:19,400 --> 00:46:23,220 so I think that's just like security 871 00:46:23,220 --> 00:46:25,970 [INAUDIBLE] to say that. 872 00:46:25,970 --> 00:46:28,100 I don't think it's incredibly dangerous frankly, 873 00:46:28,100 --> 00:46:29,170 but I probably shouldn't say that. 874 00:46:29,170 --> 00:46:30,836 I'll get myself in all sorts of trouble. 875 00:46:30,836 --> 00:46:31,830 Don't try this at home. 876 00:46:31,830 --> 00:46:33,702 I don't know why. 877 00:46:33,702 --> 00:46:35,910 AUDIENCE: Usually when you're doing jewelery plating, 878 00:46:35,910 --> 00:46:38,850 the chemicals that you use are relatively toxic. 879 00:46:38,850 --> 00:46:40,990 CATHERINE DRENNAN: Yeah, but not that much worse 880 00:46:40,990 --> 00:46:42,900 than the lab stuff. 881 00:46:42,900 --> 00:46:45,950 Allena makes jewelry, so she knows all about this. 882 00:46:45,950 --> 00:46:49,550 But she does it very safely-- very safely. 883 00:46:49,550 --> 00:46:51,550 All right, so let's look at a couple other kinds 884 00:46:51,550 --> 00:46:52,570 of electrodes. 885 00:46:52,570 --> 00:46:56,950 It is not always the case that an electrode is consumed 886 00:46:56,950 --> 00:47:00,040 or that an electrode gets stuff deposited on it. 887 00:47:00,040 --> 00:47:02,320 Some electrodes are inert. 888 00:47:02,320 --> 00:47:05,380 An example for this is a platinum electrode. 889 00:47:05,380 --> 00:47:06,490 It is inert. 890 00:47:06,490 --> 00:47:08,080 So it doesn't get consumed and it 891 00:47:08,080 --> 00:47:12,070 doesn't get reacted onto during the reaction. 892 00:47:12,070 --> 00:47:15,530 And so here's an example of a cell that uses this. 893 00:47:15,530 --> 00:47:19,940 So over here now we have still our copper system. 894 00:47:19,940 --> 00:47:22,870 So this is our cathode, again, over here. 895 00:47:22,870 --> 00:47:25,480 And a cathode does what, oxidation or reduction? 896 00:47:25,480 --> 00:47:26,476 AUDIENCE: Reduction. 897 00:47:26,476 --> 00:47:28,100 CATHERINE DRENNAN: There's a reduction. 898 00:47:28,100 --> 00:47:30,680 And so this is our same reaction, copper 899 00:47:30,680 --> 00:47:33,140 plus 2, two electrodes, coated copper solid. 900 00:47:33,140 --> 00:47:34,950 So this is doing the plating again. 901 00:47:34,950 --> 00:47:36,270 But this time it's different. 902 00:47:36,270 --> 00:47:38,390 Before we have the zinc on an electrode 903 00:47:38,390 --> 00:47:39,950 that was being consumed. 904 00:47:39,950 --> 00:47:42,200 But now we have this platinum. 905 00:47:42,200 --> 00:47:44,780 And the reaction that happens at the anode-- it's still 906 00:47:44,780 --> 00:47:47,540 an oxidation, always an oxidation at the anode. 907 00:47:47,540 --> 00:47:50,510 But the oxidation reaction is chromium 908 00:47:50,510 --> 00:47:54,780 plus 2 going to chromium plus 3 and an electron. 909 00:47:54,780 --> 00:47:59,450 So here, the electrode is inert, and the reaction 910 00:47:59,450 --> 00:48:01,520 is all happening in the same phase, 911 00:48:01,520 --> 00:48:04,600 all happening in solution. 912 00:48:04,600 --> 00:48:10,020 So we can also write the notation for this kind of cell. 913 00:48:10,020 --> 00:48:12,100 And so here we would have platinum solid, 914 00:48:12,100 --> 00:48:15,180 the single line, which means the phase difference. 915 00:48:15,180 --> 00:48:19,530 Chromium plus 2 aqueous comma, because now there's 916 00:48:19,530 --> 00:48:24,060 not a phase transition between these, a chromium plus 3 917 00:48:24,060 --> 00:48:25,050 aqueous. 918 00:48:25,050 --> 00:48:26,700 The two lines, that means what? 919 00:48:26,700 --> 00:48:28,773 What do those mean again? 920 00:48:28,773 --> 00:48:29,760 AUDIENCE: Salt bridge. 921 00:48:29,760 --> 00:48:30,650 CATHERINE DRENNAN: Salt bridge. 922 00:48:30,650 --> 00:48:32,480 So this indicates that's one beaker. 923 00:48:32,480 --> 00:48:34,070 This is the other beaker. 924 00:48:34,070 --> 00:48:36,200 And here we have the copper plus 2 aqueous, 925 00:48:36,200 --> 00:48:40,520 single line for phase change and a copper solid. 926 00:48:40,520 --> 00:48:44,335 So again, our anode reaction and our cathode reaction. 927 00:48:44,335 --> 00:48:46,990 All right, so this is another kind. 928 00:48:46,990 --> 00:48:50,440 You can also use platinum electrodes in combination 929 00:48:50,440 --> 00:48:54,100 with what's known as a hydrogen electrode. 930 00:48:54,100 --> 00:48:57,340 So these are often together, the platinum with the hydrogen. 931 00:48:57,340 --> 00:49:01,270 And many reduction potentials-- and when you get problem set 932 00:49:01,270 --> 00:49:03,340 eight, you're going to be looking up in your book 933 00:49:03,340 --> 00:49:06,160 a lot of standard reduction potentials. 934 00:49:06,160 --> 00:49:10,720 And many of them will say, measured against SHE. 935 00:49:10,720 --> 00:49:13,450 And a lot of people say, who is this SHE 936 00:49:13,450 --> 00:49:15,730 that measures all of these potentials 937 00:49:15,730 --> 00:49:16,960 in the back of books? 938 00:49:16,960 --> 00:49:20,710 And that SHE is a standard hydrogen electrode. 939 00:49:20,710 --> 00:49:22,480 All right, so let's think about how 940 00:49:22,480 --> 00:49:25,160 this electrode is being used. 941 00:49:25,160 --> 00:49:32,290 So when it's used as a cathode, we have H plus aqueous phase 942 00:49:32,290 --> 00:49:36,520 transition to H2 gas, and then another phase transition 943 00:49:36,520 --> 00:49:39,580 to our solid platinum inert electrode. 944 00:49:39,580 --> 00:49:41,830 And here when it's used as a cathode, 945 00:49:41,830 --> 00:49:45,040 H plus is being reduced. 946 00:49:45,040 --> 00:49:48,850 But it can also be used as an anode. 947 00:49:48,850 --> 00:49:50,560 And this is a clicker question. 948 00:49:50,560 --> 00:49:53,050 Why didn't you tell me what would be happening 949 00:49:53,050 --> 00:49:55,900 when the hydrogen platinum electrode is 950 00:49:55,900 --> 00:49:58,870 being used as an anode, or reaction's 951 00:49:58,870 --> 00:50:00,628 being used an an anode? 952 00:50:09,560 --> 00:50:10,160 10 Seconds. 953 00:50:17,936 --> 00:50:21,650 Let's see if you can get 90%-- 89, 89%. 954 00:50:25,080 --> 00:50:28,700 Oh, far away. 955 00:50:28,700 --> 00:50:31,040 That is the right answer though. 956 00:50:31,040 --> 00:50:34,400 So some of these, platinum is inert, 957 00:50:34,400 --> 00:50:36,530 so it's not doing anything. 958 00:50:36,530 --> 00:50:37,400 And it's over here. 959 00:50:37,400 --> 00:50:38,710 So it's not doing anything. 960 00:50:38,710 --> 00:50:40,100 So we can rule out all the things 961 00:50:40,100 --> 00:50:42,200 that platinum are doing stuff. 962 00:50:42,200 --> 00:50:46,350 All right, H plus being oxidized-- you ever 963 00:50:46,350 --> 00:50:48,150 heard of H2 plus? 964 00:50:50,810 --> 00:50:53,030 No, so we don't want that. 965 00:50:53,030 --> 00:50:55,250 We know at the anode it should be oxidized, 966 00:50:55,250 --> 00:50:57,960 so we can eliminate all the ones that are reduced. 967 00:50:57,960 --> 00:51:02,300 OK, so we only have really kind of one good option if you 968 00:51:02,300 --> 00:51:05,450 say that the platinum is inert. 969 00:51:05,450 --> 00:51:11,270 So H2 then gets oxidized in this case, forming H plus. 970 00:51:11,270 --> 00:51:15,920 All right, so let's look at a cell like this that 971 00:51:15,920 --> 00:51:21,760 uses the hydrogen electrode. 972 00:51:21,760 --> 00:51:23,620 So here's an example. 973 00:51:23,620 --> 00:51:27,040 Now on this side we have our regular zinc system 974 00:51:27,040 --> 00:51:28,420 for our anode. 975 00:51:28,420 --> 00:51:32,200 And so over here we're using, as the cathode, the hydrogen 976 00:51:32,200 --> 00:51:33,280 electrode. 977 00:51:33,280 --> 00:51:36,370 And so here in this beautiful drawing that I made, 978 00:51:36,370 --> 00:51:39,970 we have a glass tube here where you're pumping 979 00:51:39,970 --> 00:51:42,520 H2 gas in at 1 atmosphere. 980 00:51:42,520 --> 00:51:46,030 In the middle of the tube we have our inert platinum 981 00:51:46,030 --> 00:51:47,350 electrode. 982 00:51:47,350 --> 00:51:51,520 And then these are H2 gas bubbles down here. 983 00:51:51,520 --> 00:51:57,520 And we have strong acid HCl, which is our source of H plus, 984 00:51:57,520 --> 00:51:59,260 in solution. 985 00:51:59,260 --> 00:52:02,000 So over here again, this is our cathode. 986 00:52:02,000 --> 00:52:04,690 So we have a reduction at the cathode. 987 00:52:04,690 --> 00:52:11,080 And we have H plus aqueous, phase transition H2, and gas, 988 00:52:11,080 --> 00:52:12,550 and then another phase transition 989 00:52:12,550 --> 00:52:14,890 to our solid platinum electrode. 990 00:52:14,890 --> 00:52:18,910 And over here is our standard anode, our oxidation, that 991 00:52:18,910 --> 00:52:21,730 involves zinc and zinc plus 2. 992 00:52:21,730 --> 00:52:25,870 So we could also have had the hydrogen electrode over here as 993 00:52:25,870 --> 00:52:26,639 well. 994 00:52:26,639 --> 00:52:28,180 All right, so that gives you a number 995 00:52:28,180 --> 00:52:30,670 of different examples of types of electrodes 996 00:52:30,670 --> 00:52:35,230 that you could have and types of cells that you can have. 997 00:52:35,230 --> 00:52:42,170 And that is the end of this, today's-- or lecture 25 notes. 998 00:52:42,170 --> 00:52:44,240 And as soon as you finish writing that down, 999 00:52:44,240 --> 00:52:46,280 we'll go and look at lecture 26. 1000 00:52:46,280 --> 00:52:49,480 Again, we're continuing on the same theme. 1001 00:52:49,480 --> 00:52:52,090 And now we're going to talk about the potential 1002 00:52:52,090 --> 00:52:53,050 of these cells. 1003 00:52:53,050 --> 00:52:55,000 So we've talked about the reactions happening 1004 00:52:55,000 --> 00:52:57,400 at anodes and cathodes, how to write them. 1005 00:52:57,400 --> 00:53:00,250 And now we're going to think about the change in potential 1006 00:53:00,250 --> 00:53:03,990 that occurs when you run these reactions.