1 00:00:00,090 --> 00:00:02,430 The following content is provided under a Creative 2 00:00:02,430 --> 00:00:03,820 Commons license. 3 00:00:03,820 --> 00:00:06,060 Your support will help MIT OpenCourseWare 4 00:00:06,060 --> 00:00:10,150 continue to offer high quality educational resources for free. 5 00:00:10,150 --> 00:00:12,690 To make a donation or to view additional materials 6 00:00:12,690 --> 00:00:16,445 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,445 --> 00:00:17,070 at osw.mit.edu. 8 00:00:26,400 --> 00:00:30,150 CATHERINE DRENNAN: Next hand out-- thermodynamics. 9 00:00:30,150 --> 00:00:31,520 Yes. 10 00:00:31,520 --> 00:00:33,010 Yes. 11 00:00:33,010 --> 00:00:35,520 I love thermodynamics. 12 00:00:35,520 --> 00:00:36,062 All right. 13 00:00:39,540 --> 00:00:42,090 So what is thermodynamics? 14 00:00:44,610 --> 00:00:47,700 So thermodynamics and kinetics I feel go together, 15 00:00:47,700 --> 00:00:51,390 but for kind of weird reasons we do thermodynamics now 16 00:00:51,390 --> 00:00:55,260 and we do kinetics at the very last unit of the semester. 17 00:00:55,260 --> 00:00:59,040 Part of the reason for this is that kinetics is often a unit 18 00:00:59,040 --> 00:01:01,830 that students can pick up really fast, 19 00:01:01,830 --> 00:01:05,220 and so I like doing it at the end when everything-- 20 00:01:05,220 --> 00:01:07,560 your world is sort of crazy and you 21 00:01:07,560 --> 00:01:10,650 have something that you can get a grasp on pretty easily 22 00:01:10,650 --> 00:01:11,784 for the last unit. 23 00:01:11,784 --> 00:01:13,950 So anyway, I'll tell you a little bit about kinetics 24 00:01:13,950 --> 00:01:17,760 now because we won't get to a lot of it until later. 25 00:01:17,760 --> 00:01:22,230 So thermodynamics deals with energy change and spontaneity 26 00:01:22,230 --> 00:01:24,180 of reactions. 27 00:01:24,180 --> 00:01:28,500 And thermodynamics brings you three of my favorite things 28 00:01:28,500 --> 00:01:35,550 in chemistry, which are [MUSIC PLAYING] delta H, 29 00:01:35,550 --> 00:01:45,600 [CYMBAL CRASH] Delta S, and [DRUM ROLL] delta G. 30 00:01:45,600 --> 00:01:47,790 I love these. 31 00:01:47,790 --> 00:01:50,760 I live my life around these things. 32 00:01:50,760 --> 00:01:55,050 I believe entropy should always be increasing. 33 00:01:55,050 --> 00:02:00,150 And I love nothing more than free energy. 34 00:02:00,150 --> 00:02:01,860 That's great stuff. 35 00:02:01,860 --> 00:02:05,310 So today we're going to talk about delta H. Next week we 36 00:02:05,310 --> 00:02:09,479 have delta S and delta G. 37 00:02:09,479 --> 00:02:10,620 What about kinetics? 38 00:02:10,620 --> 00:02:12,030 What does kinetics bring us? 39 00:02:12,030 --> 00:02:17,970 Well, kinetics brings us the rate or speed of a reaction. 40 00:02:17,970 --> 00:02:23,070 It can bring us fast reactions and it 41 00:02:23,070 --> 00:02:31,370 can bring us slow reactions. 42 00:02:31,370 --> 00:02:33,190 I like kinetics too. 43 00:02:33,190 --> 00:02:36,200 I kind of like the fast to be honest with you. 44 00:02:36,200 --> 00:02:36,700 All right. 45 00:02:36,700 --> 00:02:39,190 So thermodynamics and kinetics. 46 00:02:39,190 --> 00:02:41,560 One. thermodynamics tells us whether something 47 00:02:41,560 --> 00:02:44,830 is going to happen spontaneously or not, 48 00:02:44,830 --> 00:02:47,860 but kinetics tells us the rate at which it happens. 49 00:02:47,860 --> 00:02:51,910 So let's just think of an example for a minute. 50 00:02:51,910 --> 00:02:54,220 You may have heard-- this commercial happens 51 00:02:54,220 --> 00:02:56,380 a lot around Valentine's Day. 52 00:02:56,380 --> 00:02:59,600 Diamonds are forever. 53 00:02:59,600 --> 00:03:05,860 So actually, thermodynamically, graphite 54 00:03:05,860 --> 00:03:08,080 is favorable to diamonds. 55 00:03:08,080 --> 00:03:10,210 It's more stable. 56 00:03:10,210 --> 00:03:15,520 So graphite or coal-- thermodynamically, 57 00:03:15,520 --> 00:03:17,680 this is the stuff. 58 00:03:17,680 --> 00:03:22,880 Diamonds-- diamond is not forever. 59 00:03:22,880 --> 00:03:26,800 That's really a kinetic statement. 60 00:03:26,800 --> 00:03:34,060 It's there for a very long time, but you know it isn't inert. 61 00:03:34,060 --> 00:03:36,650 So here's an important question. 62 00:03:36,650 --> 00:03:41,670 What is the best ring for one geek to give another geek? 63 00:03:41,670 --> 00:03:44,690 The thermodynamically stable one or one 64 00:03:44,690 --> 00:03:51,180 that is more kinetically slow to react, more stable, more inert. 65 00:03:51,180 --> 00:03:53,210 Inert is reaction. 66 00:03:53,210 --> 00:03:55,520 What do you-- I should have a clicker question on this, 67 00:03:55,520 --> 00:03:56,020 I know. 68 00:03:56,020 --> 00:03:57,430 I don't know what you think. 69 00:03:57,430 --> 00:04:06,520 But actually, the answer is, in my opinion, neither of these. 70 00:04:06,520 --> 00:04:07,416 Come on. 71 00:04:11,980 --> 00:04:16,810 There's only really one ring that any geek really wants. 72 00:04:16,810 --> 00:04:19,750 Green Lantern's ring has the power 73 00:04:19,750 --> 00:04:23,050 of chemistry at your fingertips. 74 00:04:23,050 --> 00:04:25,780 Who cares if something's inert? 75 00:04:25,780 --> 00:04:28,840 If you're the Green Lantern you can do whatever. 76 00:04:28,840 --> 00:04:31,780 So that's the ring. 77 00:04:31,780 --> 00:04:33,100 Anyway-- 78 00:04:33,100 --> 00:04:34,587 AUDIENCE: What about the one ring? 79 00:04:34,587 --> 00:04:35,920 CATHERINE DRENNAN: Oh, one ring. 80 00:04:35,920 --> 00:04:36,430 Yeah. 81 00:04:36,430 --> 00:04:41,470 I like the Green Lantern ring, but I guess next-- maybe 82 00:04:41,470 --> 00:04:43,910 we should open a blog on this. 83 00:04:43,910 --> 00:04:47,020 It's a really important question. 84 00:04:47,020 --> 00:04:49,930 So let's think about bonding for a minute. 85 00:04:49,930 --> 00:04:52,970 So thermodynamics is telling us about energy change. 86 00:04:52,970 --> 00:04:55,510 It's telling us about spontaneity. 87 00:04:55,510 --> 00:04:58,630 And so we need to think about energy that's going in. 88 00:04:58,630 --> 00:05:00,760 So kinetics is telling us about how fast. 89 00:05:00,760 --> 00:05:03,850 Thermodynamics is really telling us about stability. 90 00:05:03,850 --> 00:05:04,960 How stable is something? 91 00:05:04,960 --> 00:05:07,960 How much does it cost to break it apart? 92 00:05:07,960 --> 00:05:11,870 So we're back to bond association energies. 93 00:05:11,870 --> 00:05:14,750 We have the change in the dissociation 94 00:05:14,750 --> 00:05:16,720 energy E to the little d. 95 00:05:16,720 --> 00:05:18,650 It's the energy to break a bond. 96 00:05:18,650 --> 00:05:20,570 We've seen this plot before. 97 00:05:20,570 --> 00:05:22,130 Now we have methane. 98 00:05:22,130 --> 00:05:24,380 We're breaking off a hydrogen from it, which 99 00:05:24,380 --> 00:05:26,060 is actually very hard to do. 100 00:05:26,060 --> 00:05:29,180 Scientists would love to break apart methane and make 101 00:05:29,180 --> 00:05:32,090 methanol, but it's a hard thing to do. 102 00:05:32,090 --> 00:05:34,460 Up here we have unfavorable reactions. 103 00:05:34,460 --> 00:05:36,590 When the atoms are too close, then you 104 00:05:36,590 --> 00:05:39,140 have a sweet spot where the positions of the atoms 105 00:05:39,140 --> 00:05:40,790 are just right to form a bond. 106 00:05:40,790 --> 00:05:42,380 That's when you get methane. 107 00:05:42,380 --> 00:05:46,440 And then, if you put in energy, you can pull this off, 108 00:05:46,440 --> 00:05:49,450 is they go far apart with the radius this way. 109 00:05:49,450 --> 00:05:51,230 Your bond will dissociate. 110 00:05:51,230 --> 00:05:55,160 You always have to put energy in to get it to dissociate. 111 00:05:55,160 --> 00:05:57,500 So now we can think about it-- we thought about this 112 00:05:57,500 --> 00:05:59,150 in terms of bond dissociation. 113 00:05:59,150 --> 00:06:01,190 We've seen this before, but now we 114 00:06:01,190 --> 00:06:03,500 can think about it in terms of a new term, which 115 00:06:03,500 --> 00:06:06,550 is delta H B, or bond enthalpy. 116 00:06:06,550 --> 00:06:09,770 So bond enthalpy is the change in heat accompanying 117 00:06:09,770 --> 00:06:13,250 the dissociation of a bond and that's measured 118 00:06:13,250 --> 00:06:15,150 at a constant pressure. 119 00:06:15,150 --> 00:06:19,490 In fact, if we relate delta H to Delta E-- delta 120 00:06:19,490 --> 00:06:25,520 H equals delta E plus whatever change in pressure or volume. 121 00:06:25,520 --> 00:06:27,590 And often, this term is pretty small, 122 00:06:27,590 --> 00:06:29,990 so people often really think about reactions 123 00:06:29,990 --> 00:06:33,320 in terms of these being pretty similar to each other. 124 00:06:33,320 --> 00:06:38,240 So for gases, the difference really is 1% to 2%. 125 00:06:38,240 --> 00:06:42,110 And if you're talking about a liquid or a solid 126 00:06:42,110 --> 00:06:44,200 it's really a negligible difference. 127 00:06:44,200 --> 00:06:46,700 So we often really kind of think about these things 128 00:06:46,700 --> 00:06:47,420 in the same way. 129 00:06:47,420 --> 00:06:49,730 We think about the energy going into a system 130 00:06:49,730 --> 00:06:52,850 to break the bond, or we think about the bond enthalpies. 131 00:06:56,440 --> 00:06:59,980 And bond enthalpy, delta H, is often easier to measure, 132 00:06:59,980 --> 00:07:02,870 so it's very convenient. 133 00:07:02,870 --> 00:07:05,900 So again, bond enthalpies You always 134 00:07:05,900 --> 00:07:08,990 have to put energy in if you're going to break a bond. 135 00:07:08,990 --> 00:07:10,850 So it's always going to be positive. 136 00:07:10,850 --> 00:07:12,020 It always takes heat. 137 00:07:12,020 --> 00:07:16,370 It always takes something to break a bond. 138 00:07:16,370 --> 00:07:20,200 And so breaking a bond is endothermic. 139 00:07:20,200 --> 00:07:21,556 Heat must be added. 140 00:07:21,556 --> 00:07:24,560 Whereas bond formation is exothermic. 141 00:07:24,560 --> 00:07:26,270 Heat is being released. 142 00:07:26,270 --> 00:07:28,250 And so we can think about this-- again, 143 00:07:28,250 --> 00:07:33,500 when you form a bond those-- we saw with MO theory-- 144 00:07:33,500 --> 00:07:37,640 there's more electrons in lower energy in the bonding orbitals 145 00:07:37,640 --> 00:07:39,530 than in the antibonding orbitals. 146 00:07:39,530 --> 00:07:40,130 They're happy. 147 00:07:40,130 --> 00:07:42,600 This is a lower energy state. 148 00:07:42,600 --> 00:07:44,900 So if you're going to break that bond, 149 00:07:44,900 --> 00:07:46,270 breaking up is hard to do. 150 00:07:46,270 --> 00:07:48,527 And there's a song that verifies that statement. 151 00:07:48,527 --> 00:07:49,610 Breaking up is hard to do. 152 00:07:49,610 --> 00:07:51,500 You always have to have heat. 153 00:07:51,500 --> 00:07:56,710 But when you go from that stable stay 154 00:07:56,710 --> 00:08:00,470 out-- when you form those bonds, it's 155 00:08:00,470 --> 00:08:03,420 like kind of the married couple, often get a little boring. 156 00:08:03,420 --> 00:08:04,670 They're in a low energy state. 157 00:08:04,670 --> 00:08:07,250 It's hard to get them out of the house 158 00:08:07,250 --> 00:08:10,310 and so they release all of their energy 159 00:08:10,310 --> 00:08:12,980 and they form this nice, little, happy, stable couple. 160 00:08:12,980 --> 00:08:17,197 So when you do bond formation that's an exothermic process. 161 00:08:19,960 --> 00:08:23,720 So we can talk about standard bond enthalpies. 162 00:08:23,720 --> 00:08:26,240 When you see this little circle up there 163 00:08:26,240 --> 00:08:30,530 that means it's a value that's at standard conditions, where 164 00:08:30,530 --> 00:08:33,558 your reactants and products are in their standard states. 165 00:08:36,549 --> 00:08:41,230 And so we can think about what are some delta H or some bond 166 00:08:41,230 --> 00:08:45,900 enthalpies for different kinds of carbon hydrogen bonds. 167 00:08:45,900 --> 00:08:47,607 So here's our friend methane. 168 00:08:47,607 --> 00:08:49,190 If you're going to pull off a hydrogen 169 00:08:49,190 --> 00:08:51,320 you have to put energy in to do that, 170 00:08:51,320 --> 00:08:55,960 and the bond enthalpy for that is 438. 171 00:08:55,960 --> 00:08:58,790 Now we can think about some other kinds of carbon bonds. 172 00:08:58,790 --> 00:09:00,110 We talked about this one. 173 00:09:00,110 --> 00:09:03,480 We have it here in the classroom. 174 00:09:03,480 --> 00:09:08,980 So if we pull off a hydrogen from that, it's plus 410. 175 00:09:08,980 --> 00:09:11,660 So similar but not the same. 176 00:09:11,660 --> 00:09:15,470 If you now substitute three flourines for three 177 00:09:15,470 --> 00:09:17,930 of the hydrogens that changes your value 178 00:09:17,930 --> 00:09:19,700 a little bit, but not much. 179 00:09:19,700 --> 00:09:24,050 If you substitute chlorine it changes it a lot more. 180 00:09:24,050 --> 00:09:25,720 Same with bromine. 181 00:09:25,720 --> 00:09:26,930 So it depends. 182 00:09:26,930 --> 00:09:30,990 The bond enthalpy depends on what else is around that atom 183 00:09:30,990 --> 00:09:32,120 that you're pulling around. 184 00:09:32,120 --> 00:09:34,915 So it's not always the same value. 185 00:09:34,915 --> 00:09:35,540 It's different. 186 00:09:35,540 --> 00:09:37,820 It depends on what else is there. 187 00:09:37,820 --> 00:09:41,630 So often you'll have a table that will report mean bond 188 00:09:41,630 --> 00:09:42,700 enthalpies. 189 00:09:42,700 --> 00:09:44,540 And they take all the bonds enthalpies 190 00:09:44,540 --> 00:09:46,820 and they get the mean value, and they're usually 191 00:09:46,820 --> 00:09:48,820 within 8% of each other. 192 00:09:48,820 --> 00:09:50,960 And for a carbon hydrogen bond it's 193 00:09:50,960 --> 00:09:54,410 around 412 is the mean bond enthalpy. 194 00:09:54,410 --> 00:09:56,500 But when you're using mean bond enthalpies 195 00:09:56,500 --> 00:09:58,880 to calculate something, you have to realize 196 00:09:58,880 --> 00:10:02,870 that there can be some pretty big differences depending 197 00:10:02,870 --> 00:10:08,210 on what's around that bond that you're going to break. 198 00:10:08,210 --> 00:10:10,310 So those are some bond enthalpies. 199 00:10:10,310 --> 00:10:12,450 Why are they important? 200 00:10:12,450 --> 00:10:14,750 Well, they're important because the difference 201 00:10:14,750 --> 00:10:18,260 in bond enthalpies between a product and a reactant 202 00:10:18,260 --> 00:10:21,980 can tell you about the enthalpy of that reaction. 203 00:10:21,980 --> 00:10:24,680 The enthalpy of the reaction of breaking of bonds 204 00:10:24,680 --> 00:10:28,610 and forming new bonds, or the enthalpy of reaction, which 205 00:10:28,610 --> 00:10:34,670 is delta H R-- sub R. And that is in the standard state 206 00:10:34,670 --> 00:10:35,750 in that case. 207 00:10:35,750 --> 00:10:39,530 So let's talk about enthalpies of reactions. 208 00:10:39,530 --> 00:10:42,300 So we have this symbol again. 209 00:10:42,300 --> 00:10:45,690 Standard bond enthalpy for a reaction. 210 00:10:45,690 --> 00:10:47,750 So if it's a little B it's a bond enthalpy. 211 00:10:47,750 --> 00:10:50,780 If it's a little R it's a reaction enthalpy. 212 00:10:50,780 --> 00:10:55,830 If it's negative value it means it's an exothermic reaction, 213 00:10:55,830 --> 00:10:58,490 and if it's a positive value it means that it's 214 00:10:58,490 --> 00:11:00,840 an endothermic reaction. 215 00:11:00,840 --> 00:11:03,050 So we'll use these terms a lot and you'll 216 00:11:03,050 --> 00:11:04,890 get very familiar with them. 217 00:11:04,890 --> 00:11:08,720 So let's look at some examples of reactions. 218 00:11:08,720 --> 00:11:15,801 And here is one of my favorites, and it is yes. 219 00:11:15,801 --> 00:11:19,170 [MUSIC PLAYING] 220 00:11:19,170 --> 00:11:20,220 It has it's own song. 221 00:11:20,220 --> 00:11:24,552 - (SINGING) Photosynthesis Aah. 222 00:11:24,552 --> 00:11:27,426 Photosynthesis. 223 00:11:27,426 --> 00:11:29,342 Aah. 224 00:11:29,342 --> 00:11:31,591 Photosynthesis. 225 00:11:31,591 --> 00:11:32,090 Aah. 226 00:11:39,373 --> 00:11:42,810 Photosynthesis does not involve a camera or a synthesizer, 227 00:11:42,810 --> 00:11:44,283 although that's interesting too. 228 00:11:44,283 --> 00:11:46,901 Photosynthesis is how the plants take in light from the sun 229 00:11:46,901 --> 00:11:48,702 and turn it into energy. 230 00:11:48,702 --> 00:11:51,648 It's actually a thing on which most life depends here 231 00:11:51,648 --> 00:11:53,121 on the planet Earth. 232 00:11:53,121 --> 00:11:55,576 Photosynthesis. 233 00:11:55,576 --> 00:11:56,494 Aah. 234 00:11:56,494 --> 00:11:57,410 CATHERINE DRENNAN: OK. 235 00:11:57,410 --> 00:11:58,640 That gives you an idea. 236 00:11:58,640 --> 00:12:01,520 Unfortunately, every time you will hear the word 237 00:12:01,520 --> 00:12:04,100 photosynthesis you'll go Ah. 238 00:12:04,100 --> 00:12:05,180 It happens. 239 00:12:05,180 --> 00:12:07,340 I'm sorry about that. 240 00:12:07,340 --> 00:12:09,470 So photosynthesis. 241 00:12:09,470 --> 00:12:10,640 Amazing reaction. 242 00:12:10,640 --> 00:12:14,660 People right now are trying to duplicate it in industry 243 00:12:14,660 --> 00:12:16,200 to solve the energy problem. 244 00:12:16,200 --> 00:12:18,650 Good luck with that. 245 00:12:18,650 --> 00:12:20,060 But I know. 246 00:12:20,060 --> 00:12:21,330 I wish them good luck. 247 00:12:21,330 --> 00:12:23,480 That would be awesome. 248 00:12:23,480 --> 00:12:27,680 We use the opposite of that reaction for our energy. 249 00:12:27,680 --> 00:12:31,400 So we take sugar and use oxygen to break it down, 250 00:12:31,400 --> 00:12:34,670 which is an awesome thing because this 251 00:12:34,670 --> 00:12:39,470 has a really negative enthalpy of reaction 252 00:12:39,470 --> 00:12:43,560 minus 2816 kilojoules per mole. 253 00:12:43,560 --> 00:12:44,360 It's huge. 254 00:12:44,360 --> 00:12:47,610 And we store this in something called ATP. 255 00:12:47,610 --> 00:12:50,877 So since this is-- and I'm going to need the help of the TAs 256 00:12:50,877 --> 00:12:53,210 for a minute because we're going to do a very quick demo 257 00:12:53,210 --> 00:12:55,340 at the end of today's class. 258 00:12:55,340 --> 00:13:00,050 This reaction is exothermic big time. 259 00:13:00,050 --> 00:13:03,620 It's a big negative, which raises the question, 260 00:13:03,620 --> 00:13:06,950 if it's that exothermic-- really big value. 261 00:13:06,950 --> 00:13:09,590 We have sugar in air. 262 00:13:09,590 --> 00:13:11,900 Why we should feel heat? 263 00:13:11,900 --> 00:13:13,560 Heat should be released. 264 00:13:13,560 --> 00:13:17,360 So I think we should do this demo now and see 265 00:13:17,360 --> 00:13:18,710 whether that's true. 266 00:13:18,710 --> 00:13:24,620 So I have a bag of sugar and it is sealed under nitrogen 267 00:13:24,620 --> 00:13:27,020 so there's no oxygen in there. 268 00:13:27,020 --> 00:13:31,995 And I forgot my safety glasses but I'll try to-- sorry 269 00:13:31,995 --> 00:13:32,870 about the front room. 270 00:13:32,870 --> 00:13:34,610 I should have had a stellar announcement that you 271 00:13:34,610 --> 00:13:35,840 might want to sit back. 272 00:13:35,840 --> 00:13:40,010 But I'm going to cut this open and let O2 in. 273 00:13:40,010 --> 00:13:44,552 So there should be a lot of heat coming out. 274 00:13:44,552 --> 00:13:47,439 AUDIENCE: [INAUDIBLE] the things inside it individually wrapped? 275 00:13:47,439 --> 00:13:48,980 CATHERINE DRENNAN: Oh, you know what? 276 00:13:48,980 --> 00:13:50,060 They are individually wrapped. 277 00:13:50,060 --> 00:13:51,800 All right, so this is not going to work. 278 00:13:51,800 --> 00:13:55,850 So I need the TAs to come down here, please, 279 00:13:55,850 --> 00:13:58,250 and you've got to help me unwrap them. 280 00:13:58,250 --> 00:14:01,520 Has anyone done the experiment yet? 281 00:14:01,520 --> 00:14:04,410 Do you feel heat coming out? 282 00:14:04,410 --> 00:14:05,369 AUDIENCE: Yeah. 283 00:14:05,369 --> 00:14:06,452 CATHERINE DRENNAN: You do? 284 00:14:06,452 --> 00:14:10,110 [LAUGHTER] 285 00:14:10,110 --> 00:14:10,610 All right. 286 00:14:10,610 --> 00:14:13,610 I better try it up here. 287 00:14:13,610 --> 00:14:14,480 Let's see. 288 00:14:14,480 --> 00:14:17,630 I'm going or unwrap mine. 289 00:14:17,630 --> 00:14:20,730 It's not working very well. 290 00:14:20,730 --> 00:14:27,320 So it turns out that heat should be 291 00:14:27,320 --> 00:14:31,730 released but this is very slow. 292 00:14:31,730 --> 00:14:36,650 So we don't feel the heat when we unwrap our Hershey's Kisses. 293 00:14:36,650 --> 00:14:41,420 I encourage everyone to try this experiment at least once. 294 00:14:41,420 --> 00:14:46,880 But the way that we harness this energy in our bodies 295 00:14:46,880 --> 00:14:49,370 is that we have catalysts, which are enzymes, 296 00:14:49,370 --> 00:14:51,170 that speed up the reaction. 297 00:14:51,170 --> 00:14:55,910 And that's how we get the full force of this reaction out. 298 00:14:55,910 --> 00:15:02,170 So that is actually our introduction to thermodynamics. 299 00:15:02,170 --> 00:15:04,940 And next time we're going to talk 300 00:15:04,940 --> 00:15:11,600 about how we're going to calculate these delta Hr, 301 00:15:11,600 --> 00:15:15,140 these heats of reaction. 302 00:15:15,140 --> 00:15:18,140 So we were talking about delta H, 303 00:15:18,140 --> 00:15:22,360 and so we want to pull out the handouts from last time. 304 00:15:22,360 --> 00:15:26,980 And we were at the bottom of page two 305 00:15:26,980 --> 00:15:31,480 with three different ways to calculate delta H. 306 00:15:31,480 --> 00:15:37,120 So our delta H of reaction, delta Hr, the reaction 307 00:15:37,120 --> 00:15:38,920 enthalpy. 308 00:15:38,920 --> 00:15:41,620 So I introduced you to bond enthalpies, 309 00:15:41,620 --> 00:15:43,480 and today we're going to look at how 310 00:15:43,480 --> 00:15:49,170 you use bond enthalpies to calculate reaction enthalpies. 311 00:15:49,170 --> 00:15:52,030 And remember, bond enthalpies-- sometimes it has nothing. 312 00:15:52,030 --> 00:15:53,740 Just delta H. Sometimes it's delta 313 00:15:53,740 --> 00:15:58,030 H sub B. Capital B for bond. 314 00:15:58,030 --> 00:15:59,800 And we're going to look at that. 315 00:15:59,800 --> 00:16:02,200 Then we're going to look at how you can calculate 316 00:16:02,200 --> 00:16:06,490 delta H for reaction from the standard enthalpies 317 00:16:06,490 --> 00:16:09,770 of formation, and I'll introduce you to what that means. 318 00:16:09,770 --> 00:16:12,130 And then, also tell you about Hess's law 319 00:16:12,130 --> 00:16:15,430 where you can combine known reactions that 320 00:16:15,430 --> 00:16:19,090 have known delta H's to get a new equation 321 00:16:19,090 --> 00:16:22,400 and calculate a new delta H for that reaction. 322 00:16:22,400 --> 00:16:23,590 So three different ways. 323 00:16:23,590 --> 00:16:28,810 So we're going to start with way one, which is bond enthalpies. 324 00:16:28,810 --> 00:16:33,700 So here is the equation for calculating bond enthalpies. 325 00:16:33,700 --> 00:16:38,710 So we have the delta H0 of the reaction 326 00:16:38,710 --> 00:16:43,570 equals the sum of all of the reactants bond 327 00:16:43,570 --> 00:16:49,270 enthalpies minus the sum of all the product bond enthalpies. 328 00:16:49,270 --> 00:16:55,090 And so this is bonds broken minus bonds formed. 329 00:16:55,090 --> 00:16:56,950 And so let's think about this for a minute 330 00:16:56,950 --> 00:16:59,770 and think about what would be true. 331 00:16:59,770 --> 00:17:05,200 If you had stronger bonds in the products than in the reactants, 332 00:17:05,200 --> 00:17:06,040 what would be true? 333 00:17:06,040 --> 00:17:07,354 And this is a clicker question. 334 00:17:15,971 --> 00:17:16,470 All right. 335 00:17:16,470 --> 00:17:18,003 Let's just take 10 more seconds. 336 00:17:27,839 --> 00:17:30,180 OK Yep. 337 00:17:30,180 --> 00:17:33,790 So now let's think about why this is true. 338 00:17:33,790 --> 00:17:35,730 So it's good news that most of you 339 00:17:35,730 --> 00:17:38,850 know that negative means exothermic and positive 340 00:17:38,850 --> 00:17:40,710 means endothermic. 341 00:17:40,710 --> 00:17:42,593 And let's look at why this is true. 342 00:17:46,380 --> 00:17:48,310 So let's look at both of these. 343 00:17:48,310 --> 00:17:51,210 So if we have bonds stronger in the products-- 344 00:17:51,210 --> 00:17:53,650 you can just think about the equation. 345 00:17:53,650 --> 00:17:55,710 So if you are bond stronger in the products 346 00:17:55,710 --> 00:17:57,480 this is a bigger number and that's 347 00:17:57,480 --> 00:17:59,040 the smaller number, which is going 348 00:17:59,040 --> 00:18:01,050 to give you a negative answer. 349 00:18:01,050 --> 00:18:04,390 And a negative value is exothermic. 350 00:18:04,390 --> 00:18:07,620 And you can think about the equation stronger bonds here. 351 00:18:07,620 --> 00:18:09,690 A bigger number minus the smaller number. 352 00:18:09,690 --> 00:18:11,670 Positive or endothermic. 353 00:18:11,670 --> 00:18:14,820 But let's think for a minute about why this is the case 354 00:18:14,820 --> 00:18:17,610 and rationalize it because on an exam 355 00:18:17,610 --> 00:18:20,310 this is one of the equations that you're not given, 356 00:18:20,310 --> 00:18:23,635 so let's help you remember why this would be true. 357 00:18:26,580 --> 00:18:30,930 We can think about this-- if you're going to break bonds-- 358 00:18:30,930 --> 00:18:34,290 and this isn't in your notes but people get confused by this, 359 00:18:34,290 --> 00:18:37,950 so I'm just going to write a little bit on the board. 360 00:18:37,950 --> 00:18:39,970 So if you're going to break bonds 361 00:18:39,970 --> 00:18:44,820 you need to put energy into the system to break bonds. 362 00:18:44,820 --> 00:18:47,070 And we talked about this before. 363 00:18:47,070 --> 00:18:50,460 And since we have exam two coming up 364 00:18:50,460 --> 00:18:53,100 we'll just do a little review of some of the things 365 00:18:53,100 --> 00:18:54,580 that might be on the exam. 366 00:18:54,580 --> 00:18:58,140 So you don't have this in the handout we're doing right now, 367 00:18:58,140 --> 00:19:00,120 but you had this in the lecture nine handout. 368 00:19:00,120 --> 00:19:02,670 And something like this might be on the exam 369 00:19:02,670 --> 00:19:05,100 so we should be thinking about it. 370 00:19:05,100 --> 00:19:07,920 So remember, if there was no energy 371 00:19:07,920 --> 00:19:09,510 that you needed to put in to break 372 00:19:09,510 --> 00:19:12,540 a bond-- if breaking the bond required no energy there would 373 00:19:12,540 --> 00:19:13,230 be no bond. 374 00:19:13,230 --> 00:19:15,990 So when the energy is zero there's no bonds. 375 00:19:15,990 --> 00:19:19,630 These two are not-- these things are not bonded together. 376 00:19:19,630 --> 00:19:23,100 And when you do form a bond, you go down an energy 377 00:19:23,100 --> 00:19:25,440 here so it's at a lower state. 378 00:19:25,440 --> 00:19:26,610 It's more stable. 379 00:19:26,610 --> 00:19:28,470 That's why it forms a bond. 380 00:19:28,470 --> 00:19:30,990 If it was less stable it wouldn't be forming a bond. 381 00:19:30,990 --> 00:19:33,720 But if it's more stable, lower in energy, a larger 382 00:19:33,720 --> 00:19:37,530 negative number, then a bond forms. 383 00:19:37,530 --> 00:19:41,610 So to break this bond you have to put energy into the system. 384 00:19:41,610 --> 00:19:45,360 So breaking bonds always involves energy in. 385 00:19:45,360 --> 00:19:56,220 But forming bonds-- so if we're forming bonds then 386 00:19:56,220 --> 00:19:58,380 we're going to have energy out. 387 00:19:58,380 --> 00:20:06,580 So we're at a lower place here. 388 00:20:06,580 --> 00:20:09,440 So if we want to break bonds we have to put energy in, 389 00:20:09,440 --> 00:20:11,630 but if we're forming bonds then we're 390 00:20:11,630 --> 00:20:14,870 going to have energy that these guys had that is 391 00:20:14,870 --> 00:20:16,460 going to be released somewhere. 392 00:20:16,460 --> 00:20:19,040 So energy goes out of the system. 393 00:20:19,040 --> 00:20:23,090 And the farther down we have the stronger bonds, the more energy 394 00:20:23,090 --> 00:20:24,350 you have to put in to break. 395 00:20:24,350 --> 00:20:29,360 But also, the more energy that comes out when the bonds form. 396 00:20:29,360 --> 00:20:31,910 So energy in to break a bond, but when 397 00:20:31,910 --> 00:20:35,900 a bond is forming it goes to a lower state and that energy 398 00:20:35,900 --> 00:20:37,100 is released. 399 00:20:37,100 --> 00:20:40,160 So now we can think about what happens if you have 400 00:20:40,160 --> 00:20:43,610 a reactant with weak bonds. 401 00:20:43,610 --> 00:20:53,220 So if the reactant then has weak bonds, 402 00:20:53,220 --> 00:20:56,310 how much energy do you have to put in if it has weak bonds 403 00:20:56,310 --> 00:20:58,260 to break them? 404 00:20:58,260 --> 00:20:59,010 Not a lot. 405 00:20:59,010 --> 00:21:02,760 So we have just sort of a little bit of energy in. 406 00:21:02,760 --> 00:21:05,460 Little energy in to break those bonds. 407 00:21:05,460 --> 00:21:16,010 Now in the products, if we have strong bonds, 408 00:21:16,010 --> 00:21:21,040 how much energy goes out if we're forming strong bonds? 409 00:21:21,040 --> 00:21:22,970 A lot. 410 00:21:22,970 --> 00:21:25,850 So energy out. 411 00:21:25,850 --> 00:21:29,630 We have lots of energy out. 412 00:21:29,630 --> 00:21:33,140 So that was the first case that we had. 413 00:21:33,140 --> 00:21:35,420 So we had something where the bonds were stronger 414 00:21:35,420 --> 00:21:39,740 in the product and we said that this was negative. 415 00:21:39,740 --> 00:21:47,990 So net here we have heat or energy out is released, 416 00:21:47,990 --> 00:21:50,390 and so that's an exothermic system. 417 00:21:53,452 --> 00:21:55,016 Oh, the boards work today. 418 00:21:57,980 --> 00:22:02,930 And if we have the other-- if we have, say, strong bonds 419 00:22:02,930 --> 00:22:08,450 in the reactants, then we have to put a lot of energy in. 420 00:22:08,450 --> 00:22:10,190 Big energy in. 421 00:22:10,190 --> 00:22:13,910 And if we have weak bonds that are being formed, 422 00:22:13,910 --> 00:22:16,670 we're not getting much energy back 423 00:22:16,670 --> 00:22:22,595 so the net here is that you have heat in or heat absorbed. 424 00:22:28,160 --> 00:22:32,220 And it's an endothermic reaction. 425 00:22:32,220 --> 00:22:34,010 So this is just one way to think about it. 426 00:22:34,010 --> 00:22:35,966 Remember, whenever you are going to break bond 427 00:22:35,966 --> 00:22:38,090 you always have to put energy in to break the bond. 428 00:22:38,090 --> 00:22:41,120 And when a bond is formed that energy is released. 429 00:22:41,120 --> 00:22:45,080 So we are thinking about the net of the processes, 430 00:22:45,080 --> 00:22:49,250 and that's why this equation works for us. 431 00:22:49,250 --> 00:22:50,360 So keep this in mind. 432 00:22:50,360 --> 00:22:52,280 This is one of the points that people 433 00:22:52,280 --> 00:22:54,440 get confused on the exams. 434 00:22:54,440 --> 00:22:57,449 And sometimes like they say, oh, thermodynamics. 435 00:22:57,449 --> 00:22:59,240 I just don't understand it, and they're not 436 00:22:59,240 --> 00:23:01,460 keeping calm and sciencing on. 437 00:23:01,460 --> 00:23:05,150 They're getting all stressed by thermodynamics, 438 00:23:05,150 --> 00:23:06,860 and it's only this confusion. 439 00:23:06,860 --> 00:23:07,520 That's it. 440 00:23:07,520 --> 00:23:10,040 So if you work this out then thermodynamics 441 00:23:10,040 --> 00:23:12,920 will be your friend and you will love thermodynamics 442 00:23:12,920 --> 00:23:14,042 like I do forever. 443 00:23:17,630 --> 00:23:21,260 Just kind of keep this in mind and those diagrams in mind 444 00:23:21,260 --> 00:23:22,500 and you'll be all good. 445 00:23:22,500 --> 00:23:23,000 All right. 446 00:23:23,000 --> 00:23:25,340 So let's do an example now. 447 00:23:25,340 --> 00:23:30,860 So we can use these bond enthalpies in this equation 448 00:23:30,860 --> 00:23:32,690 where we're summing up all our reactants. 449 00:23:32,690 --> 00:23:35,710 And sometimes you see some of a little i 450 00:23:35,710 --> 00:23:39,430 here for i reactants minus j products. 451 00:23:39,430 --> 00:23:43,090 So the sum of all of the products. 452 00:23:43,090 --> 00:23:45,650 And it really is a lot here because we're talking 453 00:23:45,650 --> 00:23:48,370 about breaking every bond. 454 00:23:48,370 --> 00:23:51,050 We're talking about forming every bond. 455 00:23:51,050 --> 00:23:53,960 So this is not a huge molecule, but let's think 456 00:23:53,960 --> 00:23:55,640 about how many bonds we're actually 457 00:23:55,640 --> 00:23:57,450 going to be breaking here. 458 00:23:57,450 --> 00:24:00,890 So these are all the bonds that are broken. 459 00:24:00,890 --> 00:24:03,410 They're not quite as many being formed here. 460 00:24:03,410 --> 00:24:04,430 So bonds broken. 461 00:24:04,430 --> 00:24:10,190 We have carbon hydrogen bonds, and we have seven of those. 462 00:24:10,190 --> 00:24:14,900 So 1, 2, 3, 4, 5-- let's see if I can count them-- 1, 2, 3, 4, 463 00:24:14,900 --> 00:24:19,370 5, 6, 7. 464 00:24:19,370 --> 00:24:20,240 There it is. 465 00:24:20,240 --> 00:24:22,280 I need my glasses. 466 00:24:22,280 --> 00:24:23,180 OH bonds. 467 00:24:23,180 --> 00:24:25,190 We have these guys up here. 468 00:24:25,190 --> 00:24:27,380 One, two, three, four, five. 469 00:24:27,380 --> 00:24:33,480 We have CO bond over a one double bond over here. 470 00:24:33,480 --> 00:24:36,800 So we also are going to have these ones here. 471 00:24:36,800 --> 00:24:38,660 One, two, three, four, five. 472 00:24:38,660 --> 00:24:41,120 We have the double bond over there. 473 00:24:41,120 --> 00:24:43,820 We have five carbon bonds. 474 00:24:43,820 --> 00:24:45,920 The one single bond here. 475 00:24:45,920 --> 00:24:48,980 And the carbon bond is one, two, three, four, five. 476 00:24:48,980 --> 00:24:50,480 And we have OO bonds. 477 00:24:50,480 --> 00:24:51,506 We have six of those. 478 00:24:51,506 --> 00:24:52,130 Thank goodness. 479 00:24:52,130 --> 00:24:53,421 I didn't have to count anymore. 480 00:24:53,421 --> 00:24:54,530 It's already labeled. 481 00:24:54,530 --> 00:24:56,390 And then the bonds formed. 482 00:24:56,390 --> 00:24:57,730 So we're going to have these. 483 00:24:57,730 --> 00:25:01,310 So it's six of those, so we have 12 altogether 484 00:25:01,310 --> 00:25:03,930 and we have also 12 over here. 485 00:25:03,930 --> 00:25:05,390 So first you have to count. 486 00:25:05,390 --> 00:25:07,430 And counting is not one of my strengths, 487 00:25:07,430 --> 00:25:08,870 so I don't like doing it this way 488 00:25:08,870 --> 00:25:10,536 and I'm going to show you two other ways 489 00:25:10,536 --> 00:25:12,430 to calculate the same thing. 490 00:25:12,430 --> 00:25:15,950 But we can take this and sum these all up. 491 00:25:15,950 --> 00:25:19,280 We can look up the mean bond enthalpies for every single one 492 00:25:19,280 --> 00:25:21,950 of these types of bonds, multiply them 493 00:25:21,950 --> 00:25:24,770 by the appropriate coefficients, and come up 494 00:25:24,770 --> 00:25:29,360 with a sum for all the bonds for i number of bonds 495 00:25:29,360 --> 00:25:31,100 that you have in the reactants. 496 00:25:31,100 --> 00:25:34,790 And you can do the same in the products for j number of bonds 497 00:25:34,790 --> 00:25:39,330 that you have in the products and come up with these numbers. 498 00:25:39,330 --> 00:25:41,750 So if you were told that you have to do it 499 00:25:41,750 --> 00:25:45,080 this way-- use bond enthalpies and you know how to do it-- 500 00:25:45,080 --> 00:25:47,330 or if it's an easier problem and you're only, say, 501 00:25:47,330 --> 00:25:50,690 breaking two things and forming two things this isn't a bad way 502 00:25:50,690 --> 00:25:51,470 to do it. 503 00:25:51,470 --> 00:25:55,180 For big molecules this is definitely a nuisance. 504 00:25:55,180 --> 00:25:59,830 And if we sum all of this up together-- and so 505 00:25:59,830 --> 00:26:03,400 for the total number we have reactants minus products. 506 00:26:03,400 --> 00:26:08,890 And so if we subtract this we get minus 2,740. 507 00:26:08,890 --> 00:26:15,550 And the actual value is minus 2,816. 508 00:26:15,550 --> 00:26:17,560 So it's not even the best agreement 509 00:26:17,560 --> 00:26:18,940 when you do it this way. 510 00:26:18,940 --> 00:26:21,130 And the reason was, if you remember last time, 511 00:26:21,130 --> 00:26:23,320 we were talking about the bonds. 512 00:26:23,320 --> 00:26:27,540 Mean bond enthalpy is about 8% difference. 513 00:26:27,540 --> 00:26:30,370 So if you had, say, CH in a system that 514 00:26:30,370 --> 00:26:33,910 has all the rest of the atoms on carbon or H, 515 00:26:33,910 --> 00:26:35,290 that's a somewhat different value 516 00:26:35,290 --> 00:26:37,750 than if all of those other H atoms 517 00:26:37,750 --> 00:26:41,260 were substituted with bromine or if all those other H atoms were 518 00:26:41,260 --> 00:26:43,110 substituted with carbon. 519 00:26:43,110 --> 00:26:47,650 Then the bond enthalpy for that CH-- it depends on what else 520 00:26:47,650 --> 00:26:51,430 is bonded to the C. And so there's about 8% difference 521 00:26:51,430 --> 00:26:53,750 usually in the values. 522 00:26:53,750 --> 00:26:56,830 And so overall, you're not going to get much better. 523 00:26:56,830 --> 00:26:59,200 You certainly are not going to get better than eight. 524 00:26:59,200 --> 00:27:01,510 So agreement of 3% is pretty good, 525 00:27:01,510 --> 00:27:03,670 but it's not all that precise because we're 526 00:27:03,670 --> 00:27:06,310 using these mean bond enthalpies, which 527 00:27:06,310 --> 00:27:11,530 don't depend on the actual value in that particular system. 528 00:27:11,530 --> 00:27:16,180 So we can do better than this, and it can be also easier. 529 00:27:16,180 --> 00:27:20,710 And it'll be easier if we use standard heats of formation. 530 00:27:20,710 --> 00:27:26,920 So this is delta H sub f, f for formation. 531 00:27:26,920 --> 00:27:33,970 So the delta H sub f not for standard value 532 00:27:33,970 --> 00:27:38,380 is equal to the reaction, delta H, 533 00:27:38,380 --> 00:27:41,500 if you're talking about a reaction that involves 534 00:27:41,500 --> 00:27:46,750 one mole of compound being derived from its pure elements 535 00:27:46,750 --> 00:27:52,000 in their most stable state and in their standards state. 536 00:27:52,000 --> 00:27:57,310 So this is standard state 1 bar and room temperature. 537 00:27:57,310 --> 00:28:01,600 So let's calculate for the same reaction glucose plus oxygen 538 00:28:01,600 --> 00:28:05,860 going to CO2 plus water and see if we 539 00:28:05,860 --> 00:28:09,430 can get a little bit more accurate value that way. 540 00:28:09,430 --> 00:28:12,650 So let's think about what's happening in this reaction. 541 00:28:12,650 --> 00:28:16,720 So every time we oxidize glucose we're forming water. 542 00:28:16,720 --> 00:28:19,840 And so we can think about the heat 543 00:28:19,840 --> 00:28:23,320 of formation for liquid water. 544 00:28:23,320 --> 00:28:28,090 So again, this would be one mole coming from pure elements 545 00:28:28,090 --> 00:28:30,190 in their most standard state. 546 00:28:30,190 --> 00:28:33,070 So we have to think about where the hydrogen is coming from 547 00:28:33,070 --> 00:28:35,350 and where the oxygen is coming from. 548 00:28:35,350 --> 00:28:40,060 So hydrogen in its most stable form is H2 gas, 549 00:28:40,060 --> 00:28:44,800 and oxygen in its most stable form is O2 gas. 550 00:28:44,800 --> 00:28:48,940 So that's then the equation balanced for one mole 551 00:28:48,940 --> 00:28:52,690 of H2O liquid being formed. 552 00:28:52,690 --> 00:28:57,520 And we can look up the delta H for this-- 553 00:28:57,520 --> 00:29:01,180 that delta H of formation-- for this reaction as written 554 00:29:01,180 --> 00:29:06,940 is the delta H of formation, and it's minus 285.83 kilojoules 555 00:29:06,940 --> 00:29:08,260 per mole. 556 00:29:08,260 --> 00:29:12,190 So now let's consider what else that we're forming, water. 557 00:29:12,190 --> 00:29:15,610 And we're also forming CO2. 558 00:29:15,610 --> 00:29:20,890 So CO2 is derived from carbon in its most stable state, which 559 00:29:20,890 --> 00:29:26,080 is graphite as we discussed before, and also O2, oxygen. 560 00:29:26,080 --> 00:29:29,890 And O2 oxygen gas is the most stable state there. 561 00:29:29,890 --> 00:29:32,920 So for this reaction as written that 562 00:29:32,920 --> 00:29:36,160 is the delta H of formation of CO2 gas 563 00:29:36,160 --> 00:29:41,210 and it's minus 393.5 kilojoules per mole. 564 00:29:41,210 --> 00:29:42,740 So those are our products. 565 00:29:42,740 --> 00:29:44,890 We also have two reactants. 566 00:29:44,890 --> 00:29:47,830 One of our reactants is O2. 567 00:29:47,830 --> 00:29:50,740 So it's what's doing the oxidation. 568 00:29:50,740 --> 00:29:55,570 And we're going from O2 gas to O2 gas. 569 00:29:55,570 --> 00:29:57,680 This is the most stable state. 570 00:29:57,680 --> 00:29:59,610 So what do you think the value is here? 571 00:29:59,610 --> 00:30:00,490 AUDIENCE: 0. 572 00:30:00,490 --> 00:30:01,420 CATHERINE DRENNAN: 0. 573 00:30:01,420 --> 00:30:03,490 Yes. 574 00:30:03,490 --> 00:30:06,400 So if you have an element already 575 00:30:06,400 --> 00:30:09,160 in its most stable state, its heat of formation 576 00:30:09,160 --> 00:30:10,450 is going to be 0. 577 00:30:10,450 --> 00:30:12,910 Because it's already the most stable state, 578 00:30:12,910 --> 00:30:14,950 so the heat of formation is 0. 579 00:30:14,950 --> 00:30:17,380 And every year, I think, on an exam someone's 580 00:30:17,380 --> 00:30:19,600 trying to see if they can calculate 581 00:30:19,600 --> 00:30:22,720 a delta H of a reaction and they're looking 582 00:30:22,720 --> 00:30:25,060 and they're like, oh, I want to use heats of formation 583 00:30:25,060 --> 00:30:28,810 because I know that's a lot easier but a value is missing 584 00:30:28,810 --> 00:30:30,010 from my table. 585 00:30:30,010 --> 00:30:32,650 And they're like, the value is missing from the table. 586 00:30:32,650 --> 00:30:35,440 And the TA doesn't know how much information or whatever 587 00:30:35,440 --> 00:30:36,200 to give. 588 00:30:36,200 --> 00:30:38,325 And if you think you should have a value on an exam 589 00:30:38,325 --> 00:30:41,230 and you don't think about, is that element already 590 00:30:41,230 --> 00:30:43,090 in its most standard state? 591 00:30:43,090 --> 00:30:46,720 Perhaps it's zero and that's why it's not listed on the table. 592 00:30:46,720 --> 00:30:48,160 So keep this in mind. 593 00:30:48,160 --> 00:30:51,291 This can be very useful to remember. 594 00:30:51,291 --> 00:30:51,790 All right. 595 00:30:51,790 --> 00:30:55,250 One more thing is involved in the equation. 596 00:30:55,250 --> 00:30:56,860 We have glucose. 597 00:30:56,860 --> 00:30:59,980 So we can think about the reaction that forms glucose 598 00:30:59,980 --> 00:31:02,170 from elements in its most stable state, 599 00:31:02,170 --> 00:31:04,900 and we've actually talked about all these already. 600 00:31:04,900 --> 00:31:05,860 We have O2. 601 00:31:05,860 --> 00:31:07,660 That's in its most stable state. 602 00:31:07,660 --> 00:31:09,160 Carbon graphite. 603 00:31:09,160 --> 00:31:10,690 H2 gas. 604 00:31:10,690 --> 00:31:13,660 And so this reaction as written-- 605 00:31:13,660 --> 00:31:19,250 it has a heat of formation of minus 1,260 kilojoules 606 00:31:19,250 --> 00:31:20,940 per mole. 607 00:31:20,940 --> 00:31:24,140 So now we can calculate the delta H 608 00:31:24,140 --> 00:31:26,800 for the oxidation of glucose. 609 00:31:26,800 --> 00:31:33,020 The delta H of the reaction from these delta H's of formation. 610 00:31:33,020 --> 00:31:35,180 And here's the equation. 611 00:31:35,180 --> 00:31:38,810 Delta H of the reaction is equal to the sum 612 00:31:38,810 --> 00:31:42,020 of all of the delta H's of formation 613 00:31:42,020 --> 00:31:47,180 of the products minus the sum of the delta H's of formation 614 00:31:47,180 --> 00:31:50,250 of the reactants. 615 00:31:50,250 --> 00:31:54,620 So this now is one of the sources of confusion 616 00:31:54,620 --> 00:31:57,380 because if you're using bond enthalpies 617 00:31:57,380 --> 00:31:59,780 it's reactants minus products. 618 00:31:59,780 --> 00:32:02,300 If you're using delta H of formation 619 00:32:02,300 --> 00:32:04,880 it's products minus reactants. 620 00:32:04,880 --> 00:32:06,950 So that's why I spend a little time 621 00:32:06,950 --> 00:32:11,460 over here thinking about what's going on with the bond 622 00:32:11,460 --> 00:32:16,040 enthalpies, so hopefully no one will fall into this delta H 623 00:32:16,040 --> 00:32:18,890 pitfall over here and you'll keep 624 00:32:18,890 --> 00:32:22,520 the reactions-- the equations straight. 625 00:32:22,520 --> 00:32:24,140 So now we can plug it in. 626 00:32:24,140 --> 00:32:27,860 If you remember the equations this is pretty easy. 627 00:32:27,860 --> 00:32:31,880 So we have our delta H of reaction. 628 00:32:31,880 --> 00:32:35,750 We have 6 times the heat of formation 629 00:32:35,750 --> 00:32:37,280 of our products over here. 630 00:32:37,280 --> 00:32:38,420 CO2. 631 00:32:38,420 --> 00:32:41,450 6 times the first product and then 632 00:32:41,450 --> 00:32:44,840 6 times the second product, which is water, 633 00:32:44,840 --> 00:32:49,310 minus the first reactant, which is our glucose, 634 00:32:49,310 --> 00:32:54,150 and we have one of those, and we have 6 oxygens. 635 00:32:54,150 --> 00:32:56,960 So products minus reactants. 636 00:32:56,960 --> 00:32:59,330 Pay attention to the stoichiometry. 637 00:32:59,330 --> 00:33:02,210 You need to multiply the heats of formation 638 00:33:02,210 --> 00:33:06,860 by the number of molecules so then we can put in the values 639 00:33:06,860 --> 00:33:08,180 that we just saw. 640 00:33:08,180 --> 00:33:14,810 C02 minus 392 for our water minus 285 minus-- 641 00:33:14,810 --> 00:33:19,490 and here we have a minus 1,260 for glucose. 642 00:33:19,490 --> 00:33:22,550 And again, 6 times 0 because the oxygen is already 643 00:33:22,550 --> 00:33:24,380 in its most stable state. 644 00:33:24,380 --> 00:33:26,570 And if we do the math correctly, you 645 00:33:26,570 --> 00:33:31,050 get minus 2,816 kilojoules per mole, 646 00:33:31,050 --> 00:33:34,070 and that is exactly the experimental value. 647 00:33:34,070 --> 00:33:36,500 And it's because the heats of formation 648 00:33:36,500 --> 00:33:41,360 are also experimental, so this is a very precise number. 649 00:33:41,360 --> 00:33:43,190 When you use the heats of formation 650 00:33:43,190 --> 00:33:50,180 you're going to get a much closer value to experimental. 651 00:33:50,180 --> 00:33:52,280 And this was a bit easier than thinking 652 00:33:52,280 --> 00:33:54,530 about every bond that would be broken 653 00:33:54,530 --> 00:33:58,310 and every bond that would be formed. 654 00:33:58,310 --> 00:34:01,550 One more way that you can do this. 655 00:34:01,550 --> 00:34:03,800 And this takes advantage of something 656 00:34:03,800 --> 00:34:08,900 known as Hess's Law and the fact that enthalpy 657 00:34:08,900 --> 00:34:14,270 is a state function, which means that it's independent of path. 658 00:34:14,270 --> 00:34:16,429 So if you were climbing a mountain 659 00:34:16,429 --> 00:34:19,159 and you wanted to go from point A to point B, 660 00:34:19,159 --> 00:34:22,130 you could climb all the way up to the top and go back down 661 00:34:22,130 --> 00:34:26,830 or you could just go right from A to B and it wouldn't matter. 662 00:34:26,830 --> 00:34:30,230 Your delta H would be the same in both cases 663 00:34:30,230 --> 00:34:32,540 because it's independent of path. 664 00:34:32,540 --> 00:34:35,960 So it only matters what the values 665 00:34:35,960 --> 00:34:38,510 are for your reactants and your final products. 666 00:34:38,510 --> 00:34:41,030 It doesn't matter how you get from the reactants 667 00:34:41,030 --> 00:34:42,290 to the products. 668 00:34:42,290 --> 00:34:45,050 Delta H is going to be the same. 669 00:34:45,050 --> 00:34:48,800 And because of this, you can take different routes 670 00:34:48,800 --> 00:34:52,969 if their equations for different parts of your reaction 671 00:34:52,969 --> 00:34:55,010 that are already known with values of delta 672 00:34:55,010 --> 00:34:59,360 H-- you can add those equations together and then add together 673 00:34:59,360 --> 00:35:02,450 the delta H's to get a new value. 674 00:35:02,450 --> 00:35:06,380 So Hess's Law-- if there are two or more equations that 675 00:35:06,380 --> 00:35:09,260 are added to give another chemical equations 676 00:35:09,260 --> 00:35:13,520 then you can add up the delta H for the reactions of each 677 00:35:13,520 --> 00:35:16,040 of the individual equations to get 678 00:35:16,040 --> 00:35:19,140 the sum for your new equation. 679 00:35:19,140 --> 00:35:25,170 So let's do this now, again, for glucose and oxygen. 680 00:35:25,170 --> 00:35:27,880 So if we have these three equations 681 00:35:27,880 --> 00:35:30,260 here-- this one is showing glucose 682 00:35:30,260 --> 00:35:33,470 plus oxygen being broken down to the elements that 683 00:35:33,470 --> 00:35:35,090 are in the most stable state. 684 00:35:35,090 --> 00:35:38,900 So graphite, H2 and O2 for glucose. 685 00:35:38,900 --> 00:35:41,730 And then our 6 O2s are there on both sides 686 00:35:41,730 --> 00:35:44,400 because it's already in the most stable state. 687 00:35:44,400 --> 00:35:47,690 We're going to be forming CO2 from the elements in the most 688 00:35:47,690 --> 00:35:50,210 stable state and also water. 689 00:35:50,210 --> 00:35:53,300 So we can add these together, paying attention 690 00:35:53,300 --> 00:35:55,520 to the stoichiometry. 691 00:35:55,520 --> 00:35:59,810 So we need to multiply this equation by 6 and this equation 692 00:35:59,810 --> 00:36:04,000 by 6, and then we should be able to do some canceling 693 00:36:04,000 --> 00:36:07,630 and make sure that we're getting our equation of interest. 694 00:36:07,630 --> 00:36:12,080 So we can cancel these 6 O2s with these, 695 00:36:12,080 --> 00:36:16,130 we can cancel these O2s with these, 696 00:36:16,130 --> 00:36:19,850 and we can cancel this H2 with this. 697 00:36:19,850 --> 00:36:24,770 And that leaves us with glucose plus 6 oxygens going 698 00:36:24,770 --> 00:36:29,360 to 6 CO2s and 6 waters. 699 00:36:29,360 --> 00:36:31,820 So this is going to work now. 700 00:36:31,820 --> 00:36:35,300 And now, since we added this together to get this, 701 00:36:35,300 --> 00:36:38,360 we can add our delta H of reactions 702 00:36:38,360 --> 00:36:42,440 together to get a new delta H of reaction. 703 00:36:42,440 --> 00:36:43,610 Oh, sorry. 704 00:36:43,610 --> 00:36:45,190 I forgot to cancel my graphites. 705 00:36:45,190 --> 00:36:45,870 There we go. 706 00:36:45,870 --> 00:36:47,420 Now we're good. 707 00:36:47,420 --> 00:36:49,310 Didn't notice them there. 708 00:36:49,310 --> 00:36:52,210 So our delta H for reaction. 709 00:36:52,210 --> 00:36:57,710 We saw before that the formation of CO2 from elements 710 00:36:57,710 --> 00:36:59,534 in its most stable state was minus, 711 00:36:59,534 --> 00:37:01,700 so now we've just changed the sign because now we're 712 00:37:01,700 --> 00:37:03,660 going the opposite direction. 713 00:37:03,660 --> 00:37:07,880 So we have a positive value for that delta H of reaction. 714 00:37:07,880 --> 00:37:11,210 Now we have 6 times the heat of formation 715 00:37:11,210 --> 00:37:16,070 of CO2 and 6 times the heat of formation of water 716 00:37:16,070 --> 00:37:17,870 because that's what those equations are. 717 00:37:17,870 --> 00:37:20,170 Those are the heat of formation reactions. 718 00:37:20,170 --> 00:37:25,770 And if we add this all together then we get the number 719 00:37:25,770 --> 00:37:28,170 that we saw before. 720 00:37:28,170 --> 00:37:30,470 So it doesn't matter what path we take. 721 00:37:30,470 --> 00:37:32,570 We're going to get to that same answer. 722 00:37:32,570 --> 00:37:34,740 And this one, since we're using information 723 00:37:34,740 --> 00:37:36,770 that all has to do with heat of formation, 724 00:37:36,770 --> 00:37:39,560 it's not really very different from the one we did before. 725 00:37:39,560 --> 00:37:42,350 But you can use Hess's Law for delta H 726 00:37:42,350 --> 00:37:46,520 of reactions that are not heats of formation. 727 00:37:46,520 --> 00:37:49,940 If equations are available that can be added 728 00:37:49,940 --> 00:37:52,310 or summed to get your net reaction, 729 00:37:52,310 --> 00:37:56,840 then you can add or subtract these values to get a new delta 730 00:37:56,840 --> 00:38:03,980 H. Don't forget kilojoules per mole. 731 00:38:03,980 --> 00:38:07,130 So we have our three different ways-- bond enthalpies, 732 00:38:07,130 --> 00:38:10,450 heat of formation, and Hess's Law.