1 00:00:00,000 --> 00:00:00,016 The following content is provided under a Creative 2 00:00:00,016 --> 00:00:00,022 Commons license. 3 00:00:00,022 --> 00:00:00,038 Your support will help MIT OpenCourseWare continue to 4 00:00:00,038 --> 00:00:00,054 offer high quality educational resources for free. 5 00:00:00,054 --> 00:00:00,072 To make a donation or view additional materials from 6 00:00:00,072 --> 00:00:00,088 hundreds of MIT courses, visit MIT OpenCourseWare at 7 00:00:00,088 --> 00:00:00,110 ocw.mit.edu. 8 00:00:00,110 --> 00:00:23,690 PROFESSOR: So again, this is one more question on 9 00:00:23,690 --> 00:00:24,440 hybridization. 10 00:00:24,440 --> 00:00:27,850 This is the last question you'll get on hybridization 11 00:00:27,850 --> 00:00:29,630 before you see the exam. 12 00:00:29,630 --> 00:00:32,810 So now we're talking about hybridization of two different 13 00:00:32,810 --> 00:00:34,500 atoms in a molecule. 14 00:00:34,500 --> 00:00:35,940 So we're looking at this carbon atom 15 00:00:35,940 --> 00:00:38,710 here and this nitrogen. 16 00:00:38,710 --> 00:00:42,300 So using the rules that we've learned in terms of doing this 17 00:00:42,300 --> 00:00:45,400 quickly, let's see if you can get these answers in quick. 18 00:00:45,400 --> 00:00:48,190 So let's go ahead and just take 10 more seconds on this 19 00:00:48,190 --> 00:00:51,200 question here. 20 00:00:51,200 --> 00:01:01,530 Your parents are free to help if they're here with you. 21 00:01:01,530 --> 00:01:02,100 OK. 22 00:01:02,100 --> 00:01:05,970 Excellent. excellent job, 98% of you. 23 00:01:05,970 --> 00:01:09,860 You're a lot louder with the parents here, so let's make 24 00:01:09,860 --> 00:01:10,740 sure we're still listening. 25 00:01:10,740 --> 00:01:14,460 OK, so let's go over what we just we just established here. 26 00:01:14,460 --> 00:01:17,310 Carbon a is going to be s p 3 hybridized. 27 00:01:17,310 --> 00:01:21,930 What is the geometry, what's the vsper geometry there? 28 00:01:21,930 --> 00:01:23,830 Good, it's tetrahedral. 29 00:01:23,830 --> 00:01:27,900 And this nitrogen b here, that's s p 3 hybrid as well. 30 00:01:27,900 --> 00:01:31,800 What's the geometry around that nitrogen? 31 00:01:31,800 --> 00:01:32,660 Trigonal pyramidal, right. 32 00:01:32,660 --> 00:01:35,960 We have to take into account the fact that there's that 33 00:01:35,960 --> 00:01:39,380 lone pair there. 34 00:01:39,380 --> 00:01:41,850 All right, so let's go ahead and switch over to the lecture 35 00:01:41,850 --> 00:01:45,260 notes and see what we just did here. 36 00:01:45,260 --> 00:01:48,010 And essentially actually what we just did was I had you 37 00:01:48,010 --> 00:01:51,820 identify two of four components that make up what's 38 00:01:51,820 --> 00:01:53,790 called the morphine rule. 39 00:01:53,790 --> 00:01:56,980 And the morphine rule is a set of structural elements that 40 00:01:56,980 --> 00:01:59,950 are responsible for the biological activity of 41 00:01:59,950 --> 00:02:03,680 morphine and other morphine-like drugs that have 42 00:02:03,680 --> 00:02:07,690 a similar pharmacological activity in terms of how 43 00:02:07,690 --> 00:02:08,330 they're active. 44 00:02:08,330 --> 00:02:11,430 So specifically, the morphine rule is a set of four 45 00:02:11,430 --> 00:02:14,520 components, which start with the phenyl ring here, so, an 46 00:02:14,520 --> 00:02:15,830 aromatic ring. 47 00:02:15,830 --> 00:02:19,260 The second thing you identified, which is an s p 3 48 00:02:19,260 --> 00:02:20,130 hybrid carbon. 49 00:02:20,130 --> 00:02:24,540 And then following that we have a c h 2 c h 2 group here. 50 00:02:24,540 --> 00:02:26,720 And the last part of the rule is this s p 51 00:02:26,720 --> 00:02:28,880 3 hybridized nitrogen. 52 00:02:28,880 --> 00:02:31,840 So, in terms of thinking about what this means, I said this 53 00:02:31,840 --> 00:02:32,810 is responsible for the 54 00:02:32,810 --> 00:02:34,700 biological activity of morphine. 55 00:02:34,700 --> 00:02:39,970 If we take a look at the morphine molecule here, what 56 00:02:39,970 --> 00:02:42,670 you can see is that it's a little bit more complicated 57 00:02:42,670 --> 00:02:44,920 than this structure that we see right here. 58 00:02:44,920 --> 00:02:47,790 But if I go ahead and highlight the residues in 59 00:02:47,790 --> 00:02:50,300 purple, you can see that it, in fact, does follow the 60 00:02:50,300 --> 00:02:51,490 morphine rule. 61 00:02:51,490 --> 00:02:54,780 And what this means is this is what allows morphine to 62 00:02:54,780 --> 00:02:58,310 actually bind into its receptor, which is a pain 63 00:02:58,310 --> 00:03:00,720 receptor and block the feeling of pain. 64 00:03:00,720 --> 00:03:03,470 So that's a very important thing in terms of thinking 65 00:03:03,470 --> 00:03:05,510 about hybridization in geometry, because we've 66 00:03:05,510 --> 00:03:09,150 actually established a very important structure and shape, 67 00:03:09,150 --> 00:03:12,070 which is this morphine rule here, that if it's found in a 68 00:03:12,070 --> 00:03:14,130 molecule, as long as there aren't other parts of the 69 00:03:14,130 --> 00:03:16,850 molecule that are messing up that interaction, we can 70 00:03:16,850 --> 00:03:18,510 actually form a very tight binder 71 00:03:18,510 --> 00:03:20,310 into these pain receptors. 72 00:03:20,310 --> 00:03:23,910 So morphine, as you know, is a very potent pain killer. 73 00:03:23,910 --> 00:03:26,470 But you also, I'm sure, know that it's also very addictive, 74 00:03:26,470 --> 00:03:30,850 so it's only used sparingly in terms of treating pain in a 75 00:03:30,850 --> 00:03:33,870 general sense, and it's always used in very closely-monitored 76 00:03:33,870 --> 00:03:36,590 situations in terms of hospital care. 77 00:03:36,590 --> 00:03:40,250 But what's interesting to take note of is that the bioaction 78 00:03:40,250 --> 00:03:44,110 of morphine is very similar to endorphins that we naturally 79 00:03:44,110 --> 00:03:45,490 biosynthesize. 80 00:03:45,490 --> 00:03:49,350 Endorphins have a structure that's very similar to this 81 00:03:49,350 --> 00:03:53,080 structure up here -- endorphins are small peptide 82 00:03:53,080 --> 00:03:56,120 molecules that we biosynthesize and have in very 83 00:03:56,120 --> 00:03:58,110 low concentrations in our brain. 84 00:03:58,110 --> 00:03:59,920 They bind to pain receptors and 85 00:03:59,920 --> 00:04:01,600 block those pain receptors. 86 00:04:01,600 --> 00:04:04,510 So, when someone talks about an endorphin high, which you 87 00:04:04,510 --> 00:04:07,770 sometimes get -- one example would be the runners' high, if 88 00:04:07,770 --> 00:04:10,750 you run for a long way running fast, eventually you'll hit 89 00:04:10,750 --> 00:04:13,550 that runner's high where you get this burst of endorphins. 90 00:04:13,550 --> 00:04:15,890 All of a sudden your feet don't hurt, your shins don't 91 00:04:15,890 --> 00:04:17,460 hurt, the pain goes away and you got 92 00:04:17,460 --> 00:04:18,830 this feeling of eurphoria. 93 00:04:18,830 --> 00:04:21,470 It's the exact same interaction and it's because 94 00:04:21,470 --> 00:04:22,870 of that structure there. 95 00:04:22,870 --> 00:04:25,590 It's not exactly the morphine rule in endorphins but it's 96 00:04:25,590 --> 00:04:27,400 very similar. 97 00:04:27,400 --> 00:04:30,310 So, if we look at other derivatives of morphine, such 98 00:04:30,310 --> 00:04:32,480 as codeine and diacetylmorphine, these also 99 00:04:32,480 --> 00:04:36,840 you can see have this morphin rule within them, this 100 00:04:36,840 --> 00:04:39,630 structure that you've identified using your vsper 101 00:04:39,630 --> 00:04:41,360 rules and thinking about hybridization. 102 00:04:41,360 --> 00:04:46,610 Codeine, as you may know, is less of a potent pain killer 103 00:04:46,610 --> 00:04:48,660 compared to morphine, but it's also less addictive. 104 00:04:48,660 --> 00:04:51,470 So for that reason it's prescribed with 105 00:04:51,470 --> 00:04:53,530 slightly less oversight. 106 00:04:53,530 --> 00:04:55,620 So if you get your wisdom teeth removed or something 107 00:04:55,620 --> 00:04:58,640 like that, some of you might have had taken codeine before, 108 00:04:58,640 --> 00:05:01,570 and now you can think about by looking at the structure, a 109 00:05:01,570 --> 00:05:03,760 little bit more about how that worked in terms of 110 00:05:03,760 --> 00:05:05,460 blocking your pain. 111 00:05:05,460 --> 00:05:07,400 One that I just want to mention because it's so 112 00:05:07,400 --> 00:05:09,370 interesting is this derivative, which is 113 00:05:09,370 --> 00:05:09,480 diacetylmorphine. 114 00:05:09,480 --> 00:05:13,130 The only difference between morphine and diacetylmorphine 115 00:05:13,130 --> 00:05:18,330 is the changing of an alcohol group, or two actually, two 116 00:05:18,330 --> 00:05:20,210 acetyl groups here. 117 00:05:20,210 --> 00:05:23,410 This was synthesized by the company Bayer, and you 118 00:05:23,410 --> 00:05:26,100 probably know Bayer from Bayer's aspirin. 119 00:05:26,100 --> 00:05:28,730 Bayer had a huge success with aspirin where they took 120 00:05:28,730 --> 00:05:33,280 salicylic acid and replaced an o h group with an acetyl group 121 00:05:33,280 --> 00:05:34,760 here, and made acetylsalicylic acid. 122 00:05:34,760 --> 00:05:39,130 Does anyone know the common name for that? 123 00:05:39,130 --> 00:05:39,470 Yup. 124 00:05:39,470 --> 00:05:41,370 So that's aspirin, and we often 125 00:05:41,370 --> 00:05:42,620 think of Bayer's aspirin. 126 00:05:42,620 --> 00:05:44,560 We associate it really closely. 127 00:05:44,560 --> 00:05:47,010 They worked really hard in marketing to make that really 128 00:05:47,010 --> 00:05:49,630 close association and with copywriting, so that when we 129 00:05:49,630 --> 00:05:51,730 think about aspirin we think about Bayers. 130 00:05:51,730 --> 00:05:55,160 Bayers also made this change here, and they did find that 131 00:05:55,160 --> 00:05:59,810 diacetylmorphine is much, much more potent than morphine is. 132 00:05:59,810 --> 00:06:02,870 They could use just a tiny, tiny amount of this compound 133 00:06:02,870 --> 00:06:05,190 and get the same pain-killing properties that 134 00:06:05,190 --> 00:06:07,160 they saw with morphine. 135 00:06:07,160 --> 00:06:09,700 The problem is that this masked some of the side 136 00:06:09,700 --> 00:06:12,510 effects that they didn't realize initially, which was 137 00:06:12,510 --> 00:06:16,830 the extreme, extreme ability for you to become addicted to 138 00:06:16,830 --> 00:06:18,940 diacetylmorphine. 139 00:06:18,940 --> 00:06:21,800 And this was first thought of as a hero drug because it was 140 00:06:21,800 --> 00:06:24,090 such a strong pain killer, and this was 141 00:06:24,090 --> 00:06:27,070 called a Bayer's heroin. 142 00:06:27,070 --> 00:06:30,940 We don't usually associate the name Bayers with heroin, like 143 00:06:30,940 --> 00:06:34,090 we did with aspirin -- that's because Bayers did not work so 144 00:06:34,090 --> 00:06:37,420 hard to keep that association there. 145 00:06:37,420 --> 00:06:42,070 But it's an interesting story from the history of drugs. 146 00:06:42,070 --> 00:06:45,030 One more example I want to show you is demerol, which is 147 00:06:45,030 --> 00:06:46,880 used clinically quite often. 148 00:06:46,880 --> 00:06:49,430 Demerol, if we look at the structure here, actually looks 149 00:06:49,430 --> 00:06:50,960 nothing like morphine at all. 150 00:06:50,960 --> 00:06:53,690 So to an untrained eye, you might not see the 151 00:06:53,690 --> 00:06:54,650 relationship there. 152 00:06:54,650 --> 00:06:57,200 But because you all know some of these basic principles of 153 00:06:57,200 --> 00:07:00,390 chemistry, you should be able to pick out that morphine rule 154 00:07:00,390 --> 00:07:01,440 right in the middle of demerol. 155 00:07:01,440 --> 00:07:04,170 And, in fact, demerol is a pain killer, it's an 156 00:07:04,170 --> 00:07:06,120 alternative to using morphine. 157 00:07:06,120 --> 00:07:08,990 Again, not quite as strong as morphine in terms of pain 158 00:07:08,990 --> 00:07:11,690 killing abilities, but also not as addictive either, and 159 00:07:11,690 --> 00:07:14,280 there are some other side effects, such as nausea that 160 00:07:14,280 --> 00:07:16,910 are limited with demerol compared to morphine. 161 00:07:16,910 --> 00:07:20,510 Demerol has its own set of problems, so it's not the 162 00:07:20,510 --> 00:07:23,440 perfect solution to a different alternative to 163 00:07:23,440 --> 00:07:25,870 morphine, but you do see it used in certain cases. 164 00:07:25,870 --> 00:07:29,540 All right, so that's thinking about vsper theory, and that's 165 00:07:29,540 --> 00:07:30,430 thinking about hybridization. 166 00:07:30,430 --> 00:07:33,840 We're now going to shift gears to talking about some new 167 00:07:33,840 --> 00:07:35,270 topics today. 168 00:07:35,270 --> 00:07:39,750 So, we ended in Wednesday's lecture with giving that set 169 00:07:39,750 --> 00:07:42,930 of rules to very quickly determine hybridization. 170 00:07:42,930 --> 00:07:45,820 And then after we'd established that, we moved on 171 00:07:45,820 --> 00:07:48,830 to starting to talk about a little bit of thermochemistry. 172 00:07:48,830 --> 00:07:51,180 Specifically what we were talking about is the 173 00:07:51,180 --> 00:07:54,120 enthalpies of chemical reactions, so either the 174 00:07:54,120 --> 00:07:56,440 amount of heat that's released, or the amount of 175 00:07:56,440 --> 00:07:58,740 heat that's required in order to have a 176 00:07:58,740 --> 00:08:00,940 chemical reaction go. 177 00:08:00,940 --> 00:08:05,250 And we'd established so far two ways to think about how we 178 00:08:05,250 --> 00:08:08,510 can measure what the overall enthalpy of a reaction is. 179 00:08:08,510 --> 00:08:11,250 And I just wanted to introduced one more technique 180 00:08:11,250 --> 00:08:13,920 that we can use to do that. 181 00:08:13,920 --> 00:08:16,240 And this relies on the fact that when we talk about 182 00:08:16,240 --> 00:08:19,470 enthalpy or we talk about the heat change of a reaction, 183 00:08:19,470 --> 00:08:22,540 this is a state function, and any time we're talking about 184 00:08:22,540 --> 00:08:25,410 state function, it's independent of path. 185 00:08:25,410 --> 00:08:28,850 So essentially, all that we're worried about is the state 186 00:08:28,850 --> 00:08:29,770 that we started in. 187 00:08:29,770 --> 00:08:32,790 So, for example, when we were talking about the oxidation of 188 00:08:32,790 --> 00:08:36,680 glucose, the state that we start in are the reactants, so 189 00:08:36,680 --> 00:08:38,650 it's glucose plus oxygen. 190 00:08:38,650 --> 00:08:41,540 And what we ended up with were six moles each of carbon 191 00:08:41,540 --> 00:08:43,320 dioxide and water. 192 00:08:43,320 --> 00:08:46,180 So, we don't care how we get there in terms of making this 193 00:08:46,180 --> 00:08:48,660 calculation, all we're interested in is the 194 00:08:48,660 --> 00:08:51,930 difference between those two states, to think about the 195 00:08:51,930 --> 00:08:55,660 overall change in enthalpy. 196 00:08:55,660 --> 00:08:58,200 So, for example, when we talked about how we could 197 00:08:58,200 --> 00:09:01,180 calculate this, one way that we can do it, and we'll look 198 00:09:01,180 --> 00:09:04,800 at these numbers even more specifically in a second, is 199 00:09:04,800 --> 00:09:07,630 thinking about instead of going straight down here, 200 00:09:07,630 --> 00:09:10,180 which we might not have the numbers available to us to 201 00:09:10,180 --> 00:09:13,030 calculate directly, we can think about well, what happens 202 00:09:13,030 --> 00:09:16,760 if instead we break apart this glucose molecule and decompose 203 00:09:16,760 --> 00:09:19,500 it into its elements, because using tables in the back of 204 00:09:19,500 --> 00:09:22,670 our book, for example, we can figure out what the change in 205 00:09:22,670 --> 00:09:26,070 enthalpy is of this reaction here. 206 00:09:26,070 --> 00:09:28,470 That doesn't quite get us to where we need to go though, so 207 00:09:28,470 --> 00:09:31,920 then we can think about the enthalpy of formation of six 208 00:09:31,920 --> 00:09:34,180 carbon dioxide molecules. 209 00:09:34,180 --> 00:09:37,410 And then we can go one step further and think about the 210 00:09:37,410 --> 00:09:40,590 enthalpy of formation for six water molecules. 211 00:09:40,590 --> 00:09:43,250 So this is just a round about way of going 212 00:09:43,250 --> 00:09:44,570 from here to here. 213 00:09:44,570 --> 00:09:47,680 Instead we went up and then down and then down, but 214 00:09:47,680 --> 00:09:50,130 because it's a state function, we're absolutely 215 00:09:50,130 --> 00:09:52,290 allowed to do this. 216 00:09:52,290 --> 00:09:54,680 And to formalize this, we can talk about what's called 217 00:09:54,680 --> 00:09:56,960 Hess's law. 218 00:09:56,960 --> 00:10:01,230 And Hess's law just tell us that if we have two or three 219 00:10:01,230 --> 00:10:04,560 or any number of chemical reactions that we can add 220 00:10:04,560 --> 00:10:07,330 together in order to get the chemical equation that we're 221 00:10:07,330 --> 00:10:10,950 actually interested in, we can figure out the change in 222 00:10:10,950 --> 00:10:14,230 enthalpy of that reaction that we're interested in, just by 223 00:10:14,230 --> 00:10:18,250 adding together all of the other enthalpies of reaction 224 00:10:18,250 --> 00:10:20,780 for every other reaction that we had to add together in 225 00:10:20,780 --> 00:10:21,730 order to get there. 226 00:10:21,730 --> 00:10:26,460 So, let's take a look at what we're talking about here. 227 00:10:26,460 --> 00:10:29,260 So when I just showed you that example graphically, I'm just 228 00:10:29,260 --> 00:10:31,470 writing it out more like an equation here, because the 229 00:10:31,470 --> 00:10:34,260 nice thing about Hess's law, is it allows you to think 230 00:10:34,260 --> 00:10:37,520 about chemical reactions kind of as if they're algebraic 231 00:10:37,520 --> 00:10:38,530 expressions. 232 00:10:38,530 --> 00:10:41,560 So we can just add all of these different expressions 233 00:10:41,560 --> 00:10:43,690 together, and if we're thinking about algebra, we can 234 00:10:43,690 --> 00:10:45,690 just think about crossing things out that are in the 235 00:10:45,690 --> 00:10:48,220 reactants versus in the products. 236 00:10:48,220 --> 00:10:51,220 So if we do this, what we end up getting is the reaction 237 00:10:51,220 --> 00:10:54,560 that we're interested in, the oxidation of glucose to form 238 00:10:54,560 --> 00:10:56,940 carbon dioxide in water. 239 00:10:56,940 --> 00:10:59,930 So let's look pretty carefully at exactly the fact that these 240 00:10:59,930 --> 00:11:02,930 all cancel out to give us the reaction I say we get. 241 00:11:02,930 --> 00:11:05,940 So, for example, we can think about the oxygen molecules -- 242 00:11:05,940 --> 00:11:10,110 we can cancel out 6 here and 6 here, as long as we multiply 243 00:11:10,110 --> 00:11:12,540 this whole reaction by 6. 244 00:11:12,540 --> 00:11:16,790 Again, we can get rid of those last 3 oxygen molecules there. 245 00:11:16,790 --> 00:11:19,650 Similarly, we can cross out our 6 hydrogens from the 246 00:11:19,650 --> 00:11:23,590 products with 6 molecular hydrogens from the reactants. 247 00:11:23,590 --> 00:11:27,860 And then the last thing we can cancel out are our carbon 248 00:11:27,860 --> 00:11:30,550 atoms -- we have 6 in the products, and we have 6 in the 249 00:11:30,550 --> 00:11:32,030 reactants here. 250 00:11:32,030 --> 00:11:34,570 So you'll see that the only things that are not canceled 251 00:11:34,570 --> 00:11:37,770 out is what's left in our bottom reaction. 252 00:11:37,770 --> 00:11:41,150 So we can go ahead and just add up all of these different 253 00:11:41,150 --> 00:11:42,710 reaction enthalpies. 254 00:11:42,710 --> 00:11:45,380 In the first case what we're going to have is an enthalpy 255 00:11:45,380 --> 00:11:47,240 of 1260, and that's in 256 00:11:47,240 --> 00:11:53,390 kilojoules per mole of glucose. 257 00:11:53,390 --> 00:11:56,290 And the reason that this is positive is because it's 258 00:11:56,290 --> 00:12:01,560 essentially the reverse reaction of the change in 259 00:12:01,560 --> 00:12:02,730 enthalpy of formation. 260 00:12:02,730 --> 00:12:04,740 So it's going to be a positive number there. 261 00:12:04,740 --> 00:12:08,470 We add to that the enthalpy for this reaction here, which 262 00:12:08,470 --> 00:12:10,110 is negative 393 . 263 00:12:10,110 --> 00:12:13,500 5, but we need to remember to multiply that by 6, because 264 00:12:13,500 --> 00:12:15,460 we're actually adding together six of 265 00:12:15,460 --> 00:12:17,710 those individual reactions. 266 00:12:17,710 --> 00:12:21,060 And lastly, we also add together six of this final 267 00:12:21,060 --> 00:12:23,810 reaction here, which has an enthalpy change of 268 00:12:23,810 --> 00:12:25,620 negative 285 . 269 00:12:25,620 --> 00:12:26,980 8. 270 00:12:26,980 --> 00:12:30,000 So if we go ahead and just add up all of our individual 271 00:12:30,000 --> 00:12:33,430 enthalpy changes, what we're going to end up with for our 272 00:12:33,430 --> 00:12:37,260 entire reaction here, is a change in enthalpy of negative 273 00:12:37,260 --> 00:12:41,290 2816, and that's kilojoules per mole of glucose. 274 00:12:41,290 --> 00:12:44,090 Does anyone remember from Wednesday's class, does this 275 00:12:44,090 --> 00:12:45,640 match what we had seen experimentally? 276 00:12:45,640 --> 00:12:47,440 STUDENT: [INAUDIBLE] 277 00:12:47,440 --> 00:12:49,600 PROFESSOR: Yeah, so this is the exact number that we 278 00:12:49,600 --> 00:12:53,190 calculated when we used heats of formation, and it's also 279 00:12:53,190 --> 00:12:57,570 the exact number that's seen in terms of experimental. 280 00:12:57,570 --> 00:12:57,980 All right. 281 00:12:57,980 --> 00:13:02,850 So there's actually three ways we now have to calculate heats 282 00:13:02,850 --> 00:13:06,850 of the change in enthalpy of an overall reaction. 283 00:13:06,850 --> 00:13:10,710 So, we have these in our notes from last time and from 284 00:13:10,710 --> 00:13:14,170 finishing the end of this unit right now, but I'm just going 285 00:13:14,170 --> 00:13:16,450 to summarize them for you on the board, because there are a 286 00:13:16,450 --> 00:13:19,450 couple things that can get confusing that I want to make 287 00:13:19,450 --> 00:13:21,620 sure no one is confusing, particularly in 288 00:13:21,620 --> 00:13:23,190 the upcoming exam. 289 00:13:23,190 --> 00:13:25,990 So what was the first way that we learned to think about 290 00:13:25,990 --> 00:13:28,050 calculating enthalpies of reaction? 291 00:13:28,050 --> 00:13:30,120 STUDENT: [INAUDIBLE] 292 00:13:30,120 --> 00:13:30,390 PROFESSOR: Um-hmm. 293 00:13:30,390 --> 00:13:37,340 So, it's talking about bond enthalpies. 294 00:13:37,340 --> 00:13:40,000 And when we talk about bond enthalpy, the symbol that we 295 00:13:40,000 --> 00:13:43,310 see for a bond enthalpy is usually delta h. 296 00:13:43,310 --> 00:13:45,190 And this is where all the confusion starts, because 297 00:13:45,190 --> 00:13:48,020 everything's delta h, it just depends on the subscript here, 298 00:13:48,020 --> 00:13:50,310 right, what kind of delta h we're talking about. 299 00:13:50,310 --> 00:13:52,680 In the case of bond enthalpy often you'll see no 300 00:13:52,680 --> 00:13:54,520 subscript at all. 301 00:13:54,520 --> 00:13:57,250 But sometimes you do see a subscript, which would then 302 00:13:57,250 --> 00:14:01,050 just be delta h sub b here. 303 00:14:01,050 --> 00:14:03,940 So if we're trying to do this calculation based on bond 304 00:14:03,940 --> 00:14:09,700 enthalpies, for the delta h for an overall reaction, what 305 00:14:09,700 --> 00:14:13,770 we want to do is take the sum of all of those bond 306 00:14:13,770 --> 00:14:20,990 enthalpies of our bonds that are broken, and what we want 307 00:14:20,990 --> 00:14:26,050 to do is subtract from that the sum of all our bond 308 00:14:26,050 --> 00:14:29,870 enthalpies of our bonds that are formed. 309 00:14:29,870 --> 00:14:37,190 All right, so let's think a little bit more about what 310 00:14:37,190 --> 00:14:38,240 this means. 311 00:14:38,240 --> 00:14:41,050 So, if we're talking about bonds broken, are we talking 312 00:14:41,050 --> 00:14:42,510 about reactants or products? 313 00:14:42,510 --> 00:14:44,190 STUDENT: [INAUDIBLE] 314 00:14:44,190 --> 00:14:44,370 PROFESSOR: . 315 00:14:44,370 --> 00:14:44,620 Reactants. 316 00:14:44,620 --> 00:14:47,560 So, essentially we're summing up the bond enthalpy of 317 00:14:47,560 --> 00:14:51,210 reactants and subtracting it from the products. 318 00:14:51,210 --> 00:14:54,620 All right, great. 319 00:14:54,620 --> 00:14:57,040 So that's our first strategy there. 320 00:14:57,040 --> 00:15:01,450 The second strategy that we learned is thinking about bond 321 00:15:01,450 --> 00:15:06,400 enthalpies of reaction by using the enthalpy change in 322 00:15:06,400 --> 00:15:08,200 terms of the enthalpy of formation. 323 00:15:08,200 --> 00:15:12,450 So this one is sometimes a little bit more intuitive, 324 00:15:12,450 --> 00:15:15,160 because we're talking about the enthalpy change it takes 325 00:15:15,160 --> 00:15:17,770 in order to form a given molecule. 326 00:15:17,770 --> 00:15:20,250 So, if we're trying to do a calculation using enthalpies 327 00:15:20,250 --> 00:15:24,860 of formation, what we find is that delta h of the reaction 328 00:15:24,860 --> 00:15:30,750 is equal to the sum of the delta h of formation of 329 00:15:30,750 --> 00:15:31,640 products or reactants? 330 00:15:31,640 --> 00:15:32,930 STUDENT: Products. 331 00:15:32,930 --> 00:15:36,580 PROFESSOR: Products, good. 332 00:15:36,580 --> 00:15:42,440 Of products minus the delta h of formation, and this is the 333 00:15:42,440 --> 00:15:45,310 sum again of reactants. 334 00:15:45,310 --> 00:15:51,900 All right, and our third strategy is what we just 335 00:15:51,900 --> 00:15:55,120 talked about, which is Hess's law, and in fact, both one and 336 00:15:55,120 --> 00:15:59,080 two are applications of Hess's law -- specific applications, 337 00:15:59,080 --> 00:16:01,830 but any application of Hess's law you can use any way of 338 00:16:01,830 --> 00:16:04,760 adding up different equations to get the final enthalpy of 339 00:16:04,760 --> 00:16:06,450 the reaction. 340 00:16:06,450 --> 00:16:09,270 The reason I wanted to put these two strategies, however, 341 00:16:09,270 --> 00:16:11,880 on the board right next to each other, is so we can just 342 00:16:11,880 --> 00:16:14,310 confront a point of confusion that happens a lot with 343 00:16:14,310 --> 00:16:17,230 students, which is why in the first case do we have 344 00:16:17,230 --> 00:16:20,510 reactants minus product, and in the second case we have 345 00:16:20,510 --> 00:16:22,690 products minus reactants. 346 00:16:22,690 --> 00:16:25,190 So, you need to keep these two straight when you're doing 347 00:16:25,190 --> 00:16:28,490 your calculations in terms of enthalpies, and the reason is 348 00:16:28,490 --> 00:16:31,500 because the definition of a bond enthalpy when we're 349 00:16:31,500 --> 00:16:35,240 talking about bonds, is the enthalpy that it takes in 350 00:16:35,240 --> 00:16:38,230 order to break the bond, the amount of heat that goes in to 351 00:16:38,230 --> 00:16:39,560 breaking a bond. 352 00:16:39,560 --> 00:16:42,240 So it's typically a positive number, because if you have a 353 00:16:42,240 --> 00:16:44,500 stable bond you have to put heat into it in 354 00:16:44,500 --> 00:16:45,660 order to break it. 355 00:16:45,660 --> 00:16:48,920 So you end up having a positive bond enthalpy here. 356 00:16:48,920 --> 00:16:51,390 In contrast, when we're talking about enthalpies of 357 00:16:51,390 --> 00:16:54,720 formation, now we're talking about is the change of 358 00:16:54,720 --> 00:16:57,360 enthalpy in order to form a molecule. 359 00:16:57,360 --> 00:17:00,710 So, for example, if we have a nice stable molecule with 360 00:17:00,710 --> 00:17:04,240 strong bonds in it, what we're going to find is that the 361 00:17:04,240 --> 00:17:08,990 delta h of formation, would that be positive or negative? 362 00:17:08,990 --> 00:17:10,190 So it's negative. 363 00:17:10,190 --> 00:17:13,770 So you'd end up with a negative delta h of formation. 364 00:17:13,770 --> 00:17:16,410 So because you're talking about a term that's negative 365 00:17:16,410 --> 00:17:19,650 here where it would be positive in the first case, 366 00:17:19,650 --> 00:17:22,050 you end up flipping signs to have it work out. 367 00:17:22,050 --> 00:17:24,600 So, if this doesn't make sense as I'm saying it right now, 368 00:17:24,600 --> 00:17:27,650 just go back in your notes and look through and make sure 369 00:17:27,650 --> 00:17:29,570 that you aren't going to get mixed up when you're using 370 00:17:29,570 --> 00:17:32,510 bond enthalpies versus enthalpies of formation here. 371 00:17:32,510 --> 00:17:36,750 All right, so that's pretty much all we're 372 00:17:36,750 --> 00:17:39,170 going to say on enthalpy. 373 00:17:39,170 --> 00:17:41,470 So I really just want to stress, this is going to be 374 00:17:41,470 --> 00:17:44,050 the end of exam 2 material here, I said I'd be clear 375 00:17:44,050 --> 00:17:45,550 about it in class today. 376 00:17:45,550 --> 00:17:46,360 This is it. 377 00:17:46,360 --> 00:17:47,940 It's also written in your notes when 378 00:17:47,940 --> 00:17:49,140 you go ahead to study. 379 00:17:49,140 --> 00:17:52,560 So remember, in terms of exam 2, which I'll remind you is on 380 00:17:52,560 --> 00:17:55,640 Wednesday, it's going to be all the way from the material 381 00:17:55,640 --> 00:17:58,580 on lecture 10 up to this point here. 382 00:17:58,580 --> 00:18:01,960 So only through enthalpy, and it's also going to be on 383 00:18:01,960 --> 00:18:05,070 problem-sets four and five, and I'll mention that this 384 00:18:05,070 --> 00:18:08,020 afternoon, as I said, I'll post a bunch of extra practice 385 00:18:08,020 --> 00:18:12,000 problems, which are more review of these same concepts, 386 00:18:12,000 --> 00:18:15,430 so you can get even more practice this weekend and 387 00:18:15,430 --> 00:18:17,760 early next week to prepare. 388 00:18:17,760 --> 00:18:20,080 All right, so that's enthalpy, that's the 389 00:18:20,080 --> 00:18:22,390 end of exam 2 material. 390 00:18:22,390 --> 00:18:25,680 But there's actually another really important concept that 391 00:18:25,680 --> 00:18:27,810 we need to talk about -- we really don't want to stop with 392 00:18:27,810 --> 00:18:30,450 enthalpy, luckily it's exam 2 and not the end of the class, 393 00:18:30,450 --> 00:18:33,090 because a really important concept to think about is 394 00:18:33,090 --> 00:18:34,560 spontaneous change. 395 00:18:34,560 --> 00:18:37,560 So thinking about whether a reaction is spontaneous or 396 00:18:37,560 --> 00:18:39,680 non-spontaneous. 397 00:18:39,680 --> 00:18:42,180 And when we think about a spontaneous reaction, I think 398 00:18:42,180 --> 00:18:45,080 this is a term many of you are familiar with, spontaneous 399 00:18:45,080 --> 00:18:47,310 just means that the reactions going to proceed in the 400 00:18:47,310 --> 00:18:50,890 forward direction without any kind of outside intervention. 401 00:18:50,890 --> 00:18:53,960 So thinking of a spontaneous process, can talk about a 402 00:18:53,960 --> 00:18:55,970 chemical reaction, but you could just picture, for 403 00:18:55,970 --> 00:19:00,320 example, putting a round rock on the top of a hill -- it 404 00:19:00,320 --> 00:19:03,200 will spontaneously just roll right down that hill. 405 00:19:03,200 --> 00:19:05,940 So a spontaneous process also has direction, right, because 406 00:19:05,940 --> 00:19:08,910 the rock won't spontaneously roll back up the hill without 407 00:19:08,910 --> 00:19:10,070 putting in work. 408 00:19:10,070 --> 00:19:12,920 So essentially we're talking about a spontaneous process 409 00:19:12,920 --> 00:19:15,130 when something's going to happen without actually having 410 00:19:15,130 --> 00:19:20,040 to do anything else to force it to happen. 411 00:19:20,040 --> 00:19:23,180 So let's think about in terms of chemistry what we're 412 00:19:23,180 --> 00:19:25,840 talking about is a spontaneous reaction -- that's the 413 00:19:25,840 --> 00:19:27,900 specific type of spontaneous process that 414 00:19:27,900 --> 00:19:29,230 we're interested in. 415 00:19:29,230 --> 00:19:32,260 So let's think about a few different types of spontaneous 416 00:19:32,260 --> 00:19:35,950 reactions, and see if we can come up with some idea of 417 00:19:35,950 --> 00:19:38,350 what's going to cause them to be spontaneous. 418 00:19:38,350 --> 00:19:42,010 So one spontaneous reaction is written here, so this is just 419 00:19:42,010 --> 00:19:46,130 the oxidation of iron, or the formation of rust. This turns 420 00:19:46,130 --> 00:19:49,950 out to also be an exothermic reaction, it has a negative 421 00:19:49,950 --> 00:19:55,020 delta h of 824 kilojoules per mole. 422 00:19:55,020 --> 00:19:57,670 Another spontaneous reaction is written here, the 423 00:19:57,670 --> 00:19:59,660 combination of an acid and a base, which 424 00:19:59,660 --> 00:20:00,950 neutralizes each other. 425 00:20:00,950 --> 00:20:05,540 So we have a hydronium ion and a hydroxide ion interacting to 426 00:20:05,540 --> 00:20:06,810 form water. 427 00:20:06,810 --> 00:20:08,840 This is spontaneous, and again, it's exothermic. 428 00:20:08,840 --> 00:20:12,520 So, its delta h is negative 55 . 429 00:20:12,520 --> 00:20:13,970 9 kilojoules per mole. 430 00:20:13,970 --> 00:20:17,850 Let's think about some really relevant 431 00:20:17,850 --> 00:20:19,040 reactions in our bodies. 432 00:20:19,040 --> 00:20:21,800 So one incredibly important reaction, of course, is ATP 433 00:20:21,800 --> 00:20:25,350 hydrolysis where we have adenosine here, a 434 00:20:25,350 --> 00:20:29,550 triphosphate, so we have three phosphate groups here. 435 00:20:29,550 --> 00:20:30,470 That's called ATP. 436 00:20:30,470 --> 00:20:33,540 It has a total charge of minus four. 437 00:20:33,540 --> 00:20:36,810 So if we hydrolize this, one of the phosphate bonds here 438 00:20:36,810 --> 00:20:39,460 and lose a phosphate, we end up with adenosine diphosphate 439 00:20:39,460 --> 00:20:42,870 or ADP and that has a charge of minus three. 440 00:20:42,870 --> 00:20:43,160 Yup? 441 00:20:43,160 --> 00:20:50,010 STUDENT: On our notes, this equation [INAUDIBLE]. 442 00:20:50,010 --> 00:20:50,520 PROFESSOR: Oh, no. 443 00:20:50,520 --> 00:20:51,120 OK, thank you. 444 00:20:51,120 --> 00:20:52,500 Which equation are we talking about? 445 00:20:52,500 --> 00:21:03,670 STUDENT: [INAUDIBLE] 446 00:21:03,670 --> 00:21:05,730 PROFESSOR: I'm sorry, what page are you on? 447 00:21:05,730 --> 00:21:09,140 Page two. 448 00:21:09,140 --> 00:21:12,760 So the one with h 3 -- 449 00:21:12,760 --> 00:21:14,660 OK. 450 00:21:14,660 --> 00:21:19,350 All right, let's go back to that. 451 00:21:19,350 --> 00:21:24,790 OK, so if you can fix this reaction in your notes here. 452 00:21:24,790 --> 00:21:26,990 I'll also fix it on the website, so if you don't want 453 00:21:26,990 --> 00:21:29,610 to fix it now I'll just re-post the notes. 454 00:21:29,610 --> 00:21:35,740 So it should be four irons plus three oxygens is two f e 455 00:21:35,740 --> 00:21:38,400 2 o 3 solid. 456 00:21:38,400 --> 00:21:39,360 Thanks for pointing that out. 457 00:21:39,360 --> 00:21:41,520 All right, does everyone have that down? 458 00:21:41,520 --> 00:21:46,680 I know you had to flip a page there. 459 00:21:46,680 --> 00:21:48,230 All right, so I'll post it in the notes if you 460 00:21:48,230 --> 00:21:49,230 didn't get it there. 461 00:21:49,230 --> 00:21:52,310 Let's go back to ATP hydrolysis here. 462 00:21:52,310 --> 00:21:58,050 So we're going from ATP to ADP, and that is a spontaneous 463 00:21:58,050 --> 00:22:02,560 process, and it's an exothermic reaction as well. 464 00:22:02,560 --> 00:22:05,790 So we find that it's negative 24 kilojoules per mole in 465 00:22:05,790 --> 00:22:08,700 terms of the change in enthalpy there. 466 00:22:08,700 --> 00:22:10,610 All right, so at this point I just showed you three 467 00:22:10,610 --> 00:22:13,070 reactions where we have something that's spontaneous 468 00:22:13,070 --> 00:22:14,170 and it's also exothermic. 469 00:22:14,170 --> 00:22:16,880 And I could show you a countless number of other 470 00:22:16,880 --> 00:22:18,750 reactions where it's the same thing. 471 00:22:18,750 --> 00:22:21,140 It's actually quite common if you have an exothermic 472 00:22:21,140 --> 00:22:23,800 reaction that it's also spontaneous at room 473 00:22:23,800 --> 00:22:24,770 temperature. 474 00:22:24,770 --> 00:22:27,600 So we might start to draw the conclusion that, in fact, it's 475 00:22:27,600 --> 00:22:30,630 enthalpy that's responsible for whether or not a reaction 476 00:22:30,630 --> 00:22:33,090 is spontaneous or non-spontaneous. 477 00:22:33,090 --> 00:22:35,420 So it's very easy to get that impression, but let me show 478 00:22:35,420 --> 00:22:37,590 you a few more reactions before we draw any 479 00:22:37,590 --> 00:22:38,830 conclusions. 480 00:22:38,830 --> 00:22:41,430 Let me show you some more spontaneous reactions. 481 00:22:41,430 --> 00:22:44,840 So, for example, the conversion of solid h 2 o to 482 00:22:44,840 --> 00:22:47,990 liquid h 2 o at room temperature, I think we all 483 00:22:47,990 --> 00:22:50,530 know this is spontaneous. 484 00:22:50,530 --> 00:22:53,790 Ice melts at room temperature, but it turns out that the 485 00:22:53,790 --> 00:22:58,490 enthalpy change is positive, it's 7 kilojoules per mole. 486 00:22:58,490 --> 00:23:01,410 Similarly, if we look at this reaction here, which is 487 00:23:01,410 --> 00:23:03,060 ammonium nitrate. 488 00:23:03,060 --> 00:23:05,800 This is a very commonly used fertilizer, a very commonly 489 00:23:05,800 --> 00:23:08,860 used nitrogen source in agriculture. 490 00:23:08,860 --> 00:23:12,590 If we think about solvating this and forming the two ions, 491 00:23:12,590 --> 00:23:14,770 ammonium ion and nitrate ion. 492 00:23:14,770 --> 00:23:17,770 Again, this is a spontaneous reaction, but what we find 493 00:23:17,770 --> 00:23:20,790 here is that delta h again is positive, it's also 494 00:23:20,790 --> 00:23:21,000 endothermic. 495 00:23:21,000 --> 00:23:25,180 So thinking about all of this, would you say that enthalpy is 496 00:23:25,180 --> 00:23:26,540 the key to spontaneity? 497 00:23:26,540 --> 00:23:27,930 STUDENT: No. 498 00:23:27,930 --> 00:23:30,180 PROFESSOR: No, it's definitely not the key to spontaneity. 499 00:23:30,180 --> 00:23:33,770 It tends to correlate in many cases at room temperature, but 500 00:23:33,770 --> 00:23:36,530 it is not the key to spontaneity. 501 00:23:36,530 --> 00:23:39,580 And the key to spontaneity is instead something called Gibbs 502 00:23:39,580 --> 00:23:43,760 free energy or delta g. 503 00:23:43,760 --> 00:23:46,720 And if we think about what delta g is, we can relate it 504 00:23:46,720 --> 00:23:48,980 to enthalpy, and this will sort of show us why there's 505 00:23:48,980 --> 00:23:52,220 often this correlation, because delta g is equal delta 506 00:23:52,220 --> 00:23:57,250 h minus this term here, which is temperature, and that's 507 00:23:57,250 --> 00:23:59,810 temperature times a change in entropy. 508 00:23:59,810 --> 00:24:03,920 And we'll talk very in-depth in just a few minutes about 509 00:24:03,920 --> 00:24:07,570 what entropy actually is, but for now I just want you to 510 00:24:07,570 --> 00:24:09,780 think about the fact that in addition to thinking about 511 00:24:09,780 --> 00:24:11,590 enthalpy there's this other term here 512 00:24:11,590 --> 00:24:13,090 that comes into play. 513 00:24:13,090 --> 00:24:16,820 So when we're talking about what the sign about delta g is 514 00:24:16,820 --> 00:24:19,960 or free energy is, some of you might already be familiar with 515 00:24:19,960 --> 00:24:22,730 this, when we're talking about a negative delta g, is this 516 00:24:22,730 --> 00:24:27,570 spontaneous or non-spontaneous. 517 00:24:27,570 --> 00:24:30,740 So, maybe not so familiar, which is totally fine. 518 00:24:30,740 --> 00:24:33,570 So in terms of thinking about a negative delta g, that's 519 00:24:33,570 --> 00:24:35,840 going to be a spontaneous process. 520 00:24:35,840 --> 00:24:38,940 Any time you see a negative free energy, which just like 521 00:24:38,940 --> 00:24:40,800 when we talk about entropy, it's a similar idea, it means 522 00:24:40,800 --> 00:24:44,730 that free energy is released as the reaction progresses. 523 00:24:44,730 --> 00:24:47,360 That's going to be a spontaneous process. 524 00:24:47,360 --> 00:24:50,140 So that means that if delta g is greater than zero, then 525 00:24:50,140 --> 00:24:53,560 what we're going to see a non-spontaneous process. 526 00:24:53,560 --> 00:24:56,900 And finally in the case where delta g is equal to zero, at 527 00:24:56,900 --> 00:24:59,050 this point we're at equilibrium, which basically 528 00:24:59,050 --> 00:25:01,870 means that there's no net change in either the forward 529 00:25:01,870 --> 00:25:05,500 or the reverse reaction in terms of thinking about 530 00:25:05,500 --> 00:25:07,300 whether something's spontaneous or not 531 00:25:07,300 --> 00:25:09,430 spontaneous, it's just at equilibrium 532 00:25:09,430 --> 00:25:11,470 we see no net change. 533 00:25:11,470 --> 00:25:14,020 And something I want to point out is that this reaction is 534 00:25:14,020 --> 00:25:16,290 valid only when where at constant temperature and 535 00:25:16,290 --> 00:25:20,020 pressure, which we will be throughout the discussions in 536 00:25:20,020 --> 00:25:22,650 this class in terms of using this equation. 537 00:25:22,650 --> 00:25:25,510 And also, it turns out that when you do most chemical 538 00:25:25,510 --> 00:25:28,940 equations, you are, in fact, at a pretty constant 539 00:25:28,940 --> 00:25:31,860 temperature, and a constant pressure in terms of most 540 00:25:31,860 --> 00:25:34,180 things are done in the open atmosphere, so that's really 541 00:25:34,180 --> 00:25:35,150 not going to change. 542 00:25:35,150 --> 00:25:39,320 All right, so let's just talk very briefly about why it is 543 00:25:39,320 --> 00:25:42,480 that free energy is what determines spontaneity, and 544 00:25:42,480 --> 00:25:44,350 not this enthalpy here, not what the 545 00:25:44,350 --> 00:25:45,920 change in enthalpy is. 546 00:25:45,920 --> 00:25:48,600 And it really comes in terms remembering it in terms of the 547 00:25:48,600 --> 00:25:49,200 definition. 548 00:25:49,200 --> 00:25:52,310 When we think about delta g, what that is is that it's 549 00:25:52,310 --> 00:25:55,840 energy that's released or used in the reaction, but if it is 550 00:25:55,840 --> 00:25:58,430 released, that it can directly be used to do work. 551 00:25:58,430 --> 00:26:00,130 It's what we call free energy -- it's 552 00:26:00,130 --> 00:26:01,920 free to do other things. 553 00:26:01,920 --> 00:26:04,680 Whereas when we were talking about enthalpy here, well, 554 00:26:04,680 --> 00:26:07,300 this is the amount of heat that's released when we break 555 00:26:07,300 --> 00:26:09,390 and form the bonds in the reaction. 556 00:26:09,390 --> 00:26:12,840 But some of it actually gets stuck in the molecules, and 557 00:26:12,840 --> 00:26:15,240 this is just a way to think about it, this term here is 558 00:26:15,240 --> 00:26:17,580 one way we can think about it, it's just some energy that's 559 00:26:17,580 --> 00:26:18,660 getting stuck. 560 00:26:18,660 --> 00:26:21,370 So, for example, molecules, which we don't really go into 561 00:26:21,370 --> 00:26:23,990 in this class, have vibrational movements, they 562 00:26:23,990 --> 00:26:26,740 have rotational movements, there's energy associated with 563 00:26:26,740 --> 00:26:30,600 that that can sort of get stuck, as we could say, some 564 00:26:30,600 --> 00:26:33,490 of the enthalpy that's released in terms of forming 565 00:26:33,490 --> 00:26:35,510 and breaking the bonds in a reaction. 566 00:26:35,510 --> 00:26:38,170 So that's our case where we could see that we have a 567 00:26:38,170 --> 00:26:40,820 negative delta h, but we still end up with a 568 00:26:40,820 --> 00:26:42,350 positive delta g. 569 00:26:42,350 --> 00:26:45,150 And again, I haven't really gone into what entropy is here 570 00:26:45,150 --> 00:26:49,350 yet, and we will just in a few minutes. 571 00:26:49,350 --> 00:26:52,180 But first, I want to take a look at one of the examples 572 00:26:52,180 --> 00:26:55,050 that we talked about, which is the conversion of ammonium 573 00:26:55,050 --> 00:26:58,900 nitrate when its solvated, breaking up into its ions. 574 00:26:58,900 --> 00:27:02,410 So what I had told you is that, in fact, this is a 575 00:27:02,410 --> 00:27:06,340 positive delta h, but still that the reaction is 576 00:27:06,340 --> 00:27:07,080 spontaneous. 577 00:27:07,080 --> 00:27:10,610 So let's just do this calculation and see if we can 578 00:27:10,610 --> 00:27:13,410 confirm that in terms of delta g. 579 00:27:13,410 --> 00:27:16,660 So the reaction again that we're going to use is delta g 580 00:27:16,660 --> 00:27:19,980 equals delta h minus t delta s. 581 00:27:19,980 --> 00:27:25,570 So if we're talking about room temperature here, so we're 582 00:27:25,570 --> 00:27:29,970 going to say that delta g in this case is going to be equal 583 00:27:29,970 --> 00:27:37,230 to 28 kilojoules per mole, minus -- we're at room 584 00:27:37,230 --> 00:27:40,900 temperature or 298 kelvin -- in this reaction, you always 585 00:27:40,900 --> 00:27:42,940 put temperature into kelvin. 586 00:27:42,940 --> 00:27:46,120 And then we need to multiply it by the entropy, which is 587 00:27:46,120 --> 00:27:48,760 109 joules per kelvin mole. 588 00:27:48,760 --> 00:27:51,040 I want to point out that entropy tends to be a much 589 00:27:51,040 --> 00:27:54,360 smaller value than enthalpy, so it's reported in joules 590 00:27:54,360 --> 00:27:55,100 instead of kilojoules. 591 00:27:55,100 --> 00:27:58,220 When you solve these problems, it's really important to 592 00:27:58,220 --> 00:28:01,300 convert it into kilojoules so that your units work out and 593 00:28:01,300 --> 00:28:04,150 you don't get a completely crazy answer. 594 00:28:04,150 --> 00:28:06,080 So we want to convert this to 0 . 595 00:28:06,080 --> 00:28:12,860 109 kilojoules per kelvin mole. 596 00:28:12,860 --> 00:28:15,770 So we can just re-write this, so we have 28 kilojoules per 597 00:28:15,770 --> 00:28:19,850 mole, then this term here will be 32 . 598 00:28:19,850 --> 00:28:21,920 5 kilojoules per mole. 599 00:28:21,920 --> 00:28:26,460 So what we end up with our delta g for this reaction is 600 00:28:26,460 --> 00:28:30,570 going to be negative 4 kilojoules per mole. 601 00:28:30,570 --> 00:28:38,930 All right, so talking about this as a negative delta g, 602 00:28:38,930 --> 00:28:40,760 would you say this is a spontaneous or 603 00:28:40,760 --> 00:28:43,410 non-spontaneous reaction. 604 00:28:43,410 --> 00:28:44,650 Yeah, it's spontaneous. 605 00:28:44,650 --> 00:28:48,390 So what we're going to see is delta g is negative 4, so this 606 00:28:48,390 --> 00:28:50,720 reaction is spontaneous even though our 607 00:28:50,720 --> 00:28:54,320 enthalpy change was positive. 608 00:28:54,320 --> 00:28:56,900 Let's take a look at one more reaction since we spent so 609 00:28:56,900 --> 00:29:00,040 much time discussing the oxidation of glucose. 610 00:29:00,040 --> 00:29:03,050 So again, what we know about this reaction, we've already 611 00:29:03,050 --> 00:29:05,470 calculated the delta h is negative 2816 612 00:29:05,470 --> 00:29:07,660 kilojoules per mole. 613 00:29:07,660 --> 00:29:09,820 And you could look up and we'll figure out how to 614 00:29:09,820 --> 00:29:13,650 calculate it soon, the change in entropy, which is plus 233 615 00:29:13,650 --> 00:29:16,210 joules per k mole. 616 00:29:16,210 --> 00:29:18,790 So before we actually do this calculation, let's go to a 617 00:29:18,790 --> 00:29:21,490 clicker question and I want you to think about what's 618 00:29:21,490 --> 00:29:23,800 happening here. 619 00:29:23,800 --> 00:29:27,040 So if you're thinking about the oxidation of glucose and 620 00:29:27,040 --> 00:29:29,890 what the sign is of the change in enthalpy and the change in 621 00:29:29,890 --> 00:29:32,700 entropy, which of these statements is true. 622 00:29:32,700 --> 00:29:35,660 Is it going to be spontaneous at all temperatures, 623 00:29:35,660 --> 00:29:38,390 non-spontaneous at all temperatures, or will it 624 00:29:38,390 --> 00:29:40,750 depend on the temperature whether this reaction is 625 00:29:40,750 --> 00:29:42,720 spontaneous or not spontaneous? 626 00:29:42,720 --> 00:29:45,590 So, remember this is our relationship here. 627 00:29:45,590 --> 00:30:02,380 So let's go ahead and just take 10 seconds on this one. 628 00:30:02,380 --> 00:30:06,220 OK, so we have a pretty mixed response here. 629 00:30:06,220 --> 00:30:09,520 So this could have made or broke the chances of your team 630 00:30:09,520 --> 00:30:11,810 on the competition today, but they'll be a few more left to 631 00:30:11,810 --> 00:30:14,700 redeem yourselves if you got it incorrect. 632 00:30:14,700 --> 00:30:17,490 So what we're going to find is that it's spontaneous at all 633 00:30:17,490 --> 00:30:20,440 temperatures, and we can actually just look at this 634 00:30:20,440 --> 00:30:23,560 equation here to figure out why that is. 635 00:30:23,560 --> 00:30:26,350 If our reaction is spontaneous, that means that 636 00:30:26,350 --> 00:30:28,500 delta g is negative. 637 00:30:28,500 --> 00:30:31,640 So in terms of delta h being negative, well, that's going 638 00:30:31,640 --> 00:30:34,010 to always contribute to delta g being negative no matter 639 00:30:34,010 --> 00:30:35,430 what the temperature is. 640 00:30:35,430 --> 00:30:39,980 And if delta s is positive, since it's minus t delta s, 641 00:30:39,980 --> 00:30:42,840 and our temperature is always positive because we're on the 642 00:30:42,840 --> 00:30:46,900 kelvin scale, so it starts at zero there, then this 643 00:30:46,900 --> 00:30:49,930 component here is always going to be negative as well. 644 00:30:49,930 --> 00:30:52,780 So we're always going to have two negative numbers so that 645 00:30:52,780 --> 00:30:56,300 our combination, our delta g's always going to be negative, 646 00:30:56,300 --> 00:30:58,130 it's always going to be spontaneous. 647 00:30:58,130 --> 00:31:01,200 All right, so let's make sure that's what we do see when we 648 00:31:01,200 --> 00:31:07,730 calculate this for the oxidation of glucose here. 649 00:31:07,730 --> 00:31:10,470 So if we do this and we plug in our numbers, we see that 650 00:31:10,470 --> 00:31:15,450 delta g is going to be equal to negative 2816 times the 651 00:31:15,450 --> 00:31:18,050 temperature, room temperature, times again, 652 00:31:18,050 --> 00:31:19,240 remember to put 0. 653 00:31:19,240 --> 00:31:22,080 233, because we need to convert from joules to 654 00:31:22,080 --> 00:31:24,720 kilojoules for our entropy term. 655 00:31:24,720 --> 00:31:28,310 So what we find out is that this reaction has a delta g of 656 00:31:28,310 --> 00:31:31,770 negative 2885 kilojoules per mole. 657 00:31:31,770 --> 00:31:33,700 This is a spontaneous reaction. 658 00:31:33,700 --> 00:31:37,860 All right, so let's get back to that entropy term. 659 00:31:37,860 --> 00:31:40,490 I introduced it without explaining at all 660 00:31:40,490 --> 00:31:41,960 what it really is. 661 00:31:41,960 --> 00:31:44,420 So let's take a few moments and think about entropy. 662 00:31:44,420 --> 00:31:48,500 Entropy is actually a very easy concept to think about, 663 00:31:48,500 --> 00:31:50,180 it's a measure of disorder. 664 00:31:50,180 --> 00:31:53,280 I think most of us have a very good concept of how things 665 00:31:53,280 --> 00:31:56,090 tend to go to disorder, so it's something we can 666 00:31:56,090 --> 00:31:59,320 conceptualize just in an infinite number of ways. 667 00:31:59,320 --> 00:32:01,260 And entropy is simply a measure of the 668 00:32:01,260 --> 00:32:02,850 disorder of a system. 669 00:32:02,850 --> 00:32:05,120 And when we're talking about chemical reactions, what we're 670 00:32:05,120 --> 00:32:07,620 talking about is a change in entropy. 671 00:32:07,620 --> 00:32:11,810 So whether, as a reaction goes forward, are we becoming more 672 00:32:11,810 --> 00:32:16,110 ordered or are we becoming more disordered? 673 00:32:16,110 --> 00:32:20,020 So just like we saw for enthalpy, entropy also is a 674 00:32:20,020 --> 00:32:23,270 state function, so it doesn't matter in terms of path how we 675 00:32:23,270 --> 00:32:25,260 got from point a to point b. 676 00:32:25,260 --> 00:32:27,680 All we need to worry about is the current state of our 677 00:32:27,680 --> 00:32:31,000 system, what the current entropy is and take the 678 00:32:31,000 --> 00:32:32,350 difference. 679 00:32:32,350 --> 00:32:35,880 So, an example that I like to use in terms of conceptually 680 00:32:35,880 --> 00:32:38,940 thinking about what's going on in terms of disorder, and 681 00:32:38,940 --> 00:32:41,470 which is especially relevant in New England is thinking 682 00:32:41,470 --> 00:32:43,510 about stone wall. 683 00:32:43,510 --> 00:32:46,880 So you can think about a stone wall in terms of being very, 684 00:32:46,880 --> 00:32:47,940 very ordered, right. 685 00:32:47,940 --> 00:32:51,400 If you just built your stone wall, every stone is in place, 686 00:32:51,400 --> 00:32:53,530 it has a high degree of order. 687 00:32:53,530 --> 00:32:56,710 So in other words, the disorder is very low. 688 00:32:56,710 --> 00:33:01,140 So it has a low entropy, a low level of disorder. 689 00:33:01,140 --> 00:33:03,950 But if we're looking at a New England stone wall, which are 690 00:33:03,950 --> 00:33:06,370 typically found if you're hiking in the woods you see 691 00:33:06,370 --> 00:33:10,100 ones that are just ancient and are completely crumbling down, 692 00:33:10,100 --> 00:33:12,620 you'll find that the stone wall is no longer quite so 693 00:33:12,620 --> 00:33:14,410 ordered as it was. 694 00:33:14,410 --> 00:33:17,940 In fact, let's say there's a few stones that have fallen 695 00:33:17,940 --> 00:33:21,540 off, our disorder has increased, so we end up 696 00:33:21,540 --> 00:33:23,890 increasing the entropy. 697 00:33:23,890 --> 00:33:26,365 And I do just want to point out with this analogy that it 698 00:33:26,365 --> 00:33:27,690 is a state function. 699 00:33:27,690 --> 00:33:31,300 So for thinking about the change in entropy, which is 700 00:33:31,300 --> 00:33:34,776 this distance right here, we can either calculate it 701 00:33:34,776 --> 00:33:37,930 directly from here to here. but it also doesn't matter, 702 00:33:37,930 --> 00:33:40,420 let's say the wall crumbled down completely and we ended 703 00:33:40,420 --> 00:33:44,110 up having a very high degree of disorder, if then someone 704 00:33:44,110 --> 00:33:47,620 put most of the stones back in place and we went back down to 705 00:33:47,620 --> 00:33:50,290 this degree of disorder here, it doesn't matter, it's a 706 00:33:50,290 --> 00:33:54,020 state function, all we care about is the actual difference 707 00:33:54,020 --> 00:33:58,100 between our starting point and our ending point. 708 00:33:58,100 --> 00:34:00,820 And I just can't resist putting in a little quote, 709 00:34:00,820 --> 00:34:03,845 "something there is that doesn't love a wall." Does 710 00:34:03,845 --> 00:34:06,800 anybody know where this quote is from -- very famous poem, 711 00:34:06,800 --> 00:34:07,640 very famous poet. 712 00:34:07,640 --> 00:34:13,640 Good guesses, parents can help with this. 713 00:34:13,640 --> 00:34:15,140 Yes, excellent. 714 00:34:15,140 --> 00:34:18,340 I've never heard the correct answer this question before. 715 00:34:18,340 --> 00:34:18,930 Excellent. 716 00:34:18,930 --> 00:34:21,520 So this is Robert Frost in The Mending Wall. 717 00:34:21,520 --> 00:34:25,950 He talks about, it sounds like a lot of you did know this, 718 00:34:25,950 --> 00:34:29,880 this makes me full of joy. "Something there is that 719 00:34:29,880 --> 00:34:33,050 doesn't love a wall," there's many interpretations for this 720 00:34:33,050 --> 00:34:35,050 poem, but maybe it's about entropy. 721 00:34:35,050 --> 00:34:38,980 Entropy doesn't love a wall, we're going from order to 722 00:34:38,980 --> 00:34:41,290 disorder, that wall wants to come down. 723 00:34:41,290 --> 00:34:44,520 All right. 724 00:34:44,520 --> 00:34:47,240 So let's formalize these thoughts on entropy and in 725 00:34:47,240 --> 00:34:49,460 terms of what we're talking about. 726 00:34:49,460 --> 00:34:53,170 Again, what I said on that's chart there is as we increase 727 00:34:53,170 --> 00:34:55,770 entropy, we're increasing disorder. 728 00:34:55,770 --> 00:34:58,810 So if you saw a positive change in entropy in a 729 00:34:58,810 --> 00:35:02,590 reaction, would you think about that as being more or 730 00:35:02,590 --> 00:35:03,530 less ordered? 731 00:35:03,530 --> 00:35:06,660 STUDENT: [INAUDIBLE] 732 00:35:06,660 --> 00:35:08,190 PROFESSOR: Yeah, so it's actually going to be less 733 00:35:08,190 --> 00:35:10,800 ordered, and, in fact, I should stop saying order, I 734 00:35:10,800 --> 00:35:13,310 should be just saying disorder because disorder's actually 735 00:35:13,310 --> 00:35:15,930 what we're measuring, and there's an increase in 736 00:35:15,930 --> 00:35:19,840 disorder here when we have a positive change in enthalpy. 737 00:35:19,840 --> 00:35:22,010 So if we have a negative change in enthalpy, we're 738 00:35:22,010 --> 00:35:26,980 going to see a decrease in disorder. 739 00:35:26,980 --> 00:35:30,620 So in terms of considering different types of states that 740 00:35:30,620 --> 00:35:33,780 we can be in, whether we're in a gas state or a liquid state 741 00:35:33,780 --> 00:35:37,020 or a solid state for any given molecule or any given 742 00:35:37,020 --> 00:35:41,070 compound, we can think about how much order or disorder 743 00:35:41,070 --> 00:35:44,710 these states have. So which of these three states would you 744 00:35:44,710 --> 00:35:46,540 call the most disordered? 745 00:35:46,540 --> 00:35:48,060 STUDENT: Gas. 746 00:35:48,060 --> 00:35:48,910 PROFESSOR: Yeah, it's the gas state. 747 00:35:48,910 --> 00:35:51,940 So it turns out that the disorder gas is greater than 748 00:35:51,940 --> 00:35:54,240 liquid., which is greater than solid. 749 00:35:54,240 --> 00:35:56,960 This makes sense and the gas molecules are free to bounce 750 00:35:56,960 --> 00:35:58,610 around, go wherever they want. 751 00:35:58,610 --> 00:36:01,010 Once you're in a liquid, they have a little bit more 752 00:36:01,010 --> 00:36:02,990 limitation, they can't go anywhere. 753 00:36:02,990 --> 00:36:05,060 But the molecules can still slide all around 754 00:36:05,060 --> 00:36:06,210 and pass each other. 755 00:36:06,210 --> 00:36:09,190 Once you're in a solid state, this molecule is not going to 756 00:36:09,190 --> 00:36:12,380 flip places with the other molecules there. 757 00:36:12,380 --> 00:36:16,660 It's going to be kind of stuck in its place within the solid. 758 00:36:16,660 --> 00:36:20,770 So, just understanding this explanation of entropy that 759 00:36:20,770 --> 00:36:24,180 we're going to have an increase in entropy if we have 760 00:36:24,180 --> 00:36:27,370 an increase in disorder, allows us to make predictions 761 00:36:27,370 --> 00:36:30,690 about reactions even without doing any calculations, and we 762 00:36:30,690 --> 00:36:33,180 always like when that happens, because then once we do 763 00:36:33,180 --> 00:36:35,710 calculations, we can check our calculations and make sure 764 00:36:35,710 --> 00:36:38,440 they make sense with what we would be predicting. 765 00:36:38,440 --> 00:36:41,270 So before we talk about how to actually do a calculation, 766 00:36:41,270 --> 00:36:44,550 let's go to a clicker question here and make sure everyone is 767 00:36:44,550 --> 00:36:48,110 on the same page in terms of thinking about changes in 768 00:36:48,110 --> 00:36:52,060 entropy or changes or delta s for the reaction. 769 00:36:52,060 --> 00:36:56,190 So let's say we're talking about the decomposition of 770 00:36:56,190 --> 00:36:59,500 hydrogen peroxide into water and oxygen. 771 00:36:59,500 --> 00:37:01,600 I want you to tell me if you think it's going to have a 772 00:37:01,600 --> 00:37:06,470 positive delta s, a negative, or zero, or maybe this will 773 00:37:06,470 --> 00:37:08,120 change with temperature as well. 774 00:37:08,120 --> 00:37:12,660 All right, so let's go ahead and take 10 775 00:37:12,660 --> 00:37:25,580 seconds on this one. 776 00:37:25,580 --> 00:37:26,640 OK, excellent. 777 00:37:26,640 --> 00:37:28,740 Great job with the questions today. 778 00:37:28,740 --> 00:37:33,090 So we are actually going to see a positive delta s. this 779 00:37:33,090 --> 00:37:36,230 should be very clear because we're going from two moles of 780 00:37:36,230 --> 00:37:40,726 liquid, to now having two moles of liquid, plus a mole 781 00:37:40,726 --> 00:37:43,600 of gas, so we're going to be increasing the disorder of our 782 00:37:43,600 --> 00:37:44,480 system here. 783 00:37:44,480 --> 00:37:50,580 All right, so let's think about how we actually can 784 00:37:50,580 --> 00:37:51,960 calculate, though, delta s. 785 00:37:51,960 --> 00:37:54,420 So far I've just been giving you the delta s's for the 786 00:37:54,420 --> 00:37:57,270 reactions, but, as I'm sure you can guess, you won't be 787 00:37:57,270 --> 00:37:59,540 given that information, for example, on upcoming 788 00:37:59,540 --> 00:38:02,650 problem-sets, you'll actually need to calculate the entropy. 789 00:38:02,650 --> 00:38:06,230 So what we can do is we can actually calculate the entropy 790 00:38:06,230 --> 00:38:10,930 of a reaction from absolute entropies of individual 791 00:38:10,930 --> 00:38:13,530 molecules that we're discussing. 792 00:38:13,530 --> 00:38:17,060 So this is very similar to how we calculated the enthalpy of 793 00:38:17,060 --> 00:38:20,420 a reaction by taking the change of enthalpy of 794 00:38:20,420 --> 00:38:22,650 formation, except we don't even have to worry about a 795 00:38:22,650 --> 00:38:26,710 change in entropy because we can talk about entropy as an 796 00:38:26,710 --> 00:38:27,950 absolute value. 797 00:38:27,950 --> 00:38:31,860 There's an absolute zero in terms of entropy where there 798 00:38:31,860 --> 00:38:33,490 is no disorder at all. 799 00:38:33,490 --> 00:38:38,220 And this is described for any molecule as being its perfect 800 00:38:38,220 --> 00:38:41,170 crystal at absolute zero. 801 00:38:41,170 --> 00:38:43,720 So this is what we're going to say there's absolute complete 802 00:38:43,720 --> 00:38:46,690 order, there's no disorder at all in this system. 803 00:38:46,690 --> 00:38:49,220 So what we can take to calculate the entropy in a 804 00:38:49,220 --> 00:38:52,800 reaction, is just to take the sum of the entropy of all the 805 00:38:52,800 --> 00:38:55,970 products, and subtract from that the entropy from all of 806 00:38:55,970 --> 00:38:59,000 the reactants. 807 00:38:59,000 --> 00:39:02,110 So let's try this for the example that you just did on 808 00:39:02,110 --> 00:39:05,000 the clicker question, and make sure that our calculation 809 00:39:05,000 --> 00:39:07,720 matches up with our prediction here. 810 00:39:07,720 --> 00:39:11,600 So again, we're just going to take the sum of the entropy of 811 00:39:11,600 --> 00:39:15,100 the products minus out of the reactants. 812 00:39:15,100 --> 00:39:17,340 So if we talk about this, we're starting with our 813 00:39:17,340 --> 00:39:20,710 product, so our first product is water. 814 00:39:20,710 --> 00:39:22,830 And what I want to point out is you need to make sure you 815 00:39:22,830 --> 00:39:26,120 multiply this by 2, because, in fact, we have two moles of 816 00:39:26,120 --> 00:39:27,550 water that are formed here. 817 00:39:27,550 --> 00:39:31,980 STUDENT: [INAUDIBLE] 818 00:39:31,980 --> 00:39:33,580 PROFESSOR: OK, I'm so sorry. 819 00:39:33,580 --> 00:39:37,230 Yeah, so yikes, this should be water. 820 00:39:37,230 --> 00:39:43,630 You know what, let's just write this on the board. 821 00:39:43,630 --> 00:39:49,270 So we're going to talk about delta s of the reaction. 822 00:39:49,270 --> 00:39:53,630 So that's to be sum of the entropy of the products. 823 00:39:53,630 --> 00:39:57,500 So we're going to be talking about 2 times entropy of 824 00:39:57,500 --> 00:40:07,190 water, and we'll add to that our other product, which is 825 00:40:07,190 --> 00:40:12,180 molecular oxygen here, and we don't need to put a number out 826 00:40:12,180 --> 00:40:14,480 here because it's just one mole. 827 00:40:14,480 --> 00:40:19,190 So that's of o 2, and we'll subtract from that, the 828 00:40:19,190 --> 00:40:24,790 entropy of hydrogen peroxide here. 829 00:40:24,790 --> 00:40:27,010 And do we need to put anything out here for this? 830 00:40:27,010 --> 00:40:28,260 STUDENT: 2. 831 00:40:28,260 --> 00:40:30,590 PROFESSOR: So, yeah, because there's two moles there. 832 00:40:30,590 --> 00:40:35,610 So water plus oxygen minus hydrogen peroxide. 833 00:40:35,610 --> 00:40:46,170 Let's hope I at least got the values right. 834 00:40:46,170 --> 00:40:48,300 Yeah, and actually I think this is right, I believe this 835 00:40:48,300 --> 00:40:49,460 is the entropy of water. 836 00:40:49,460 --> 00:40:52,920 So OK, so the actual numbers are right here for what is 837 00:40:52,920 --> 00:40:56,700 written on the board and hopefully in your notes. 838 00:40:56,700 --> 00:41:00,610 So what we find out is that the entropy of this reaction 839 00:41:00,610 --> 00:41:04,090 here, the change in entropy, the delta s, is 125 joules per 840 00:41:04,090 --> 00:41:05,450 k per mole. 841 00:41:05,450 --> 00:41:11,340 All right, so we can also -- well, first, we already 842 00:41:11,340 --> 00:41:13,870 thought about why delta s is positive. 843 00:41:13,870 --> 00:41:17,390 Delta s is positive because we're going from a liquid to a 844 00:41:17,390 --> 00:41:20,270 liquid and a gas -- we're increasing our disorder. 845 00:41:20,270 --> 00:41:24,590 So let's think about whether this reaction is spontaneous 846 00:41:24,590 --> 00:41:27,280 or not, and to do that, we actually need to go ahead and 847 00:41:27,280 --> 00:41:29,720 completely calculate delta g. 848 00:41:29,720 --> 00:41:33,900 So to do this, we can take our values here, and delta g is 849 00:41:33,900 --> 00:41:37,980 equal to delta h minus t delta s. 850 00:41:37,980 --> 00:41:42,190 So if we plug all of this in, we have our delta h is 851 00:41:42,190 --> 00:41:46,250 negative 196 kilojoules per mole, minus our temperature, 852 00:41:46,250 --> 00:41:48,430 and I wanted you to write this again, because I want to make 853 00:41:48,430 --> 00:41:51,350 sure you get in the habit of converting our joules to 854 00:41:51,350 --> 00:41:51,570 kilojoules. 855 00:41:51,570 --> 00:41:52,930 It's 0 . 856 00:41:52,930 --> 00:41:57,120 125 kilojoules per kelvin mole. 857 00:41:57,120 --> 00:42:00,570 So we end up with a delta g of negative 233 858 00:42:00,570 --> 00:42:02,440 kilojoules per mole. 859 00:42:02,440 --> 00:42:03,640 Is that spontaneous or non-spontaneous? 860 00:42:03,640 --> 00:42:05,980 STUDENT: [INAUDIBLE] 861 00:42:05,980 --> 00:42:06,860 PROFESSOR: Great. 862 00:42:06,860 --> 00:42:09,240 So the reaction here is spontaneous. 863 00:42:09,240 --> 00:42:11,200 So actually, this is probably a reaction that you're 864 00:42:11,200 --> 00:42:14,280 familiar with because a lot of us do have hydrogen peroxide 865 00:42:14,280 --> 00:42:16,000 just in our medicine cabinet. 866 00:42:16,000 --> 00:42:18,960 And if you go and look at the expiration date on your 867 00:42:18,960 --> 00:42:20,870 hydrogen peroxide, you will see that it 868 00:42:20,870 --> 00:42:22,710 does, in fact, expire. 869 00:42:22,710 --> 00:42:25,900 And you might think oh no, what happens when it expires, 870 00:42:25,900 --> 00:42:27,030 when it goes bad? 871 00:42:27,030 --> 00:42:30,560 All that's happening is it's turning into water. 872 00:42:30,560 --> 00:42:33,700 So I wouldn't drink it after its expiration date, but 873 00:42:33,700 --> 00:42:35,550 probably there's more water than hydrogen 874 00:42:35,550 --> 00:42:37,450 peroxide at that point. 875 00:42:37,450 --> 00:42:40,030 But it also actually brings out a really important point 876 00:42:40,030 --> 00:42:41,740 about thermodynamics, which is that thermodynamics, in fact, 877 00:42:41,740 --> 00:42:46,480 tells us if a reaction happening in the forward 878 00:42:46,480 --> 00:42:49,250 direction is spontaneous or non-spontaneous. 879 00:42:49,250 --> 00:42:51,530 So, for example, we see that yes, it is 880 00:42:51,530 --> 00:42:53,320 a spontaneous reaction. 881 00:42:53,320 --> 00:42:55,350 But you'll note that thermodynamics tells us 882 00:42:55,350 --> 00:42:58,820 absolutely nothing about the timeframe of that reaction. 883 00:42:58,820 --> 00:43:02,160 So some reactions that are spontaneous take place in a 884 00:43:02,160 --> 00:43:04,870 matter of seconds or microseconds or less. 885 00:43:04,870 --> 00:43:08,110 Other reactions, such as this, to have an appreciable amount 886 00:43:08,110 --> 00:43:11,220 of that reaction go forward, it actually takes years. 887 00:43:11,220 --> 00:43:13,590 You'll notice that it's not just one day before that 888 00:43:13,590 --> 00:43:15,480 hydrogen peroxide expires. 889 00:43:15,480 --> 00:43:18,290 It's takes a really long time to get a significant amount of 890 00:43:18,290 --> 00:43:19,850 this reaction to go. 891 00:43:19,850 --> 00:43:23,250 So just make sure, we will get to talking about kinetics, 892 00:43:23,250 --> 00:43:24,510 which does tell us about the 893 00:43:24,510 --> 00:43:26,680 timeframe of chemical reactions. 894 00:43:26,680 --> 00:43:28,810 But don't confuse thermochemistry or 895 00:43:28,810 --> 00:43:30,770 thermodynamics with kinetics. 896 00:43:30,770 --> 00:43:34,370 It doesn't matter how huge our delta g is in terms of being 897 00:43:34,370 --> 00:43:37,280 if it's negative two or if it's negative two million, 898 00:43:37,280 --> 00:43:39,070 that still doesn't tell us how fast the 899 00:43:39,070 --> 00:43:40,210 reaction's going to go. 900 00:43:40,210 --> 00:43:42,520 All right. 901 00:43:42,520 --> 00:43:45,690 So let's think about one more example here in terms of 902 00:43:45,690 --> 00:43:47,110 thinking about entropy. 903 00:43:47,110 --> 00:43:50,050 So another example we talked about was the melting of ice 904 00:43:50,050 --> 00:43:51,440 at room temperature. 905 00:43:51,440 --> 00:43:55,330 I think all of us agree that this is spontaneous, but that 906 00:43:55,330 --> 00:43:58,490 the enthalpy change is positive, it's 907 00:43:58,490 --> 00:44:01,670 an endothermic reaction. 908 00:44:01,670 --> 00:44:07,350 So we can think about calculating the entropy here, 909 00:44:07,350 --> 00:44:11,080 and I'm happy to see that I did, in fact, put the products 910 00:44:11,080 --> 00:44:15,780 h 2 o here as a liquid, subtracting the reactant, 911 00:44:15,780 --> 00:44:19,010 which is h 2 o as a solid. 912 00:44:19,010 --> 00:44:22,480 If we go ahead and plug in those values here, what we end 913 00:44:22,480 --> 00:44:26,530 up with is a delta s of this reaction of 28 . 914 00:44:26,530 --> 00:44:28,120 59 joules. 915 00:44:28,120 --> 00:44:31,020 This makes sense because we're increasing the disorder of the 916 00:44:31,020 --> 00:44:37,110 system, we're seeing a positive delta s here. 917 00:44:37,110 --> 00:44:38,880 And I just answered that question for you. 918 00:44:38,880 --> 00:44:42,010 We're going why is delta s positive? 919 00:44:42,010 --> 00:44:43,820 We're going from a solid to liquid, 920 00:44:43,820 --> 00:44:46,570 we're increasing disorder. 921 00:44:46,570 --> 00:44:50,250 So, we can calculate delta g as well, and if we do this we 922 00:44:50,250 --> 00:44:51,810 plug in these numbers here. 923 00:44:51,810 --> 00:44:55,260 What we end up with is that delta g equals negative 1 . 924 00:44:55,260 --> 00:44:57,710 57 kilojoules per mole. 925 00:44:57,710 --> 00:45:01,150 So it is spontaneous, delta g is just barely negative. 926 00:45:01,150 --> 00:45:04,330 And what actually is the case, which we'll talk about more on 927 00:45:04,330 --> 00:45:07,040 Monday and thinking more and more about how temperature 928 00:45:07,040 --> 00:45:09,570 actually affects these reactions, but what we know 929 00:45:09,570 --> 00:45:12,270 about melting of ice is that it is, in fact, temperature 930 00:45:12,270 --> 00:45:13,160 dependent, right. 931 00:45:13,160 --> 00:45:16,000 Ice doesn't melt at just any old temperature. 932 00:45:16,000 --> 00:45:18,820 So that's where temperature actually comes into play here. 933 00:45:18,820 --> 00:45:21,350 All right. 934 00:45:21,350 --> 00:45:24,020 So we have a case -- spontaneous, even though delta 935 00:45:24,020 --> 00:45:24,990 h is positive. 936 00:45:24,990 --> 00:45:27,920 All right, so let's think also about how we can go about 937 00:45:27,920 --> 00:45:30,560 calculating the free energy of formation. 938 00:45:30,560 --> 00:45:34,170 So we know that we can use the equation that we saw in terms 939 00:45:34,170 --> 00:45:36,760 of the free energy the reaction, but what if we're 940 00:45:36,760 --> 00:45:39,510 actually thinking about the free energy of the formation 941 00:45:39,510 --> 00:45:41,210 of a particular molecule? 942 00:45:41,210 --> 00:45:43,830 So that's actually completely analogous to thinking about 943 00:45:43,830 --> 00:45:47,860 the enthalpy of formation, or it's basically the standard 944 00:45:47,860 --> 00:45:49,370 Gibbs free energy of formation. 945 00:45:49,370 --> 00:45:52,780 So this is talking about forming a mole of a compound 946 00:45:52,780 --> 00:45:55,500 from its most stable elements, from its elements in their 947 00:45:55,500 --> 00:45:59,140 most stable states at pressure of one bar and at room 948 00:45:59,140 --> 00:46:01,400 temperature. 949 00:46:01,400 --> 00:46:03,930 So we know that we can tabulate it must like we 950 00:46:03,930 --> 00:46:07,790 tabulated delta h formation, but we can also tabulate it 951 00:46:07,790 --> 00:46:10,690 from this reaction here, which it looks like I already told 952 00:46:10,690 --> 00:46:13,420 you this reaction, which I did but I told you a more general 953 00:46:13,420 --> 00:46:16,850 form, which was for the free energy of a reaction. 954 00:46:16,850 --> 00:46:19,180 This is the free energy of a reaction, which is the 955 00:46:19,180 --> 00:46:22,350 formation of a certain compound, and this is the free 956 00:46:22,350 --> 00:46:25,790 energy of formation equals the enthalpy of formation 957 00:46:25,790 --> 00:46:29,040 minus t delta s. 958 00:46:29,040 --> 00:46:33,620 So let's think about why it is important to be thinking about 959 00:46:33,620 --> 00:46:37,240 these changes of free energy in terms of forming a 960 00:46:37,240 --> 00:46:40,580 molecule, and it's important because the delta g 961 00:46:40,580 --> 00:46:43,680 information is actually a measure of how stable or 962 00:46:43,680 --> 00:46:48,120 unstable a molecule is relative to its element. 963 00:46:48,120 --> 00:46:51,670 So, for example, if something is really stable relative to 964 00:46:51,670 --> 00:46:55,025 its elements, we can think about whether delta g of 965 00:46:55,025 --> 00:46:56,770 formation should be negative or positive. 966 00:46:56,770 --> 00:46:59,820 So let's have you do one last clicker question here, and 967 00:46:59,820 --> 00:47:03,470 tell me what do you think, if delta g of formation, of 968 00:47:03,470 --> 00:47:06,990 actually forming a molecule is going to be negative, would 969 00:47:06,990 --> 00:47:11,330 you say that the compound is going to be stable or unstable 970 00:47:11,330 --> 00:47:14,150 relative to the elements that that compound is made up of? 971 00:47:14,150 --> 00:47:37,830 All right, let's take 10 seconds on this. 972 00:47:37,830 --> 00:47:38,990 OK, interesting. 973 00:47:38,990 --> 00:47:40,850 So this is going to be another tie breaker for us. 974 00:47:40,850 --> 00:47:46,040 So let's go back to the notes and think about this. 975 00:47:46,040 --> 00:47:50,100 So it turns out that it's going to be stable, relative 976 00:47:50,100 --> 00:47:54,440 to its -- the compound is stable 977 00:47:54,440 --> 00:47:55,810 relative to its elements. 978 00:47:55,810 --> 00:47:58,960 And the reason for this is if you think of the reaction of 979 00:47:58,960 --> 00:48:03,440 the elements forming the compound, if the delta g of 980 00:48:03,440 --> 00:48:06,030 that reaction is negative, that means it's going to 981 00:48:06,030 --> 00:48:08,440 spontaneously form the compound. 982 00:48:08,440 --> 00:48:11,730 It's going to go -- release energy when it actually forms 983 00:48:11,730 --> 00:48:12,930 that compound. 984 00:48:12,930 --> 00:48:15,340 So that's another way of telling us that the compound 985 00:48:15,340 --> 00:48:17,990 is more stable than its elements. 986 00:48:17,990 --> 00:48:22,270 So conversely, if we have delta g of formation being 987 00:48:22,270 --> 00:48:25,560 greater than zero, so delta g of formation is just delta g 988 00:48:25,560 --> 00:48:29,170 of the reaction where the compound is formed, then what 989 00:48:29,170 --> 00:48:31,770 we find is that it's thermodynamically unstable 990 00:48:31,770 --> 00:48:34,680 relative to its elements, because instead of releasing 991 00:48:34,680 --> 00:48:37,110 energy in that reaction, you actually have to put energy 992 00:48:37,110 --> 00:48:40,200 into that reaction in order to make it happen. 993 00:48:40,200 --> 00:48:43,180 So, delta g of formation can give us an indication of 994 00:48:43,180 --> 00:48:46,680 whether we have a stable or an unstable compound.