WEBVTT 00:00:00.090 --> 00:00:02.430 The following content is provided under a Creative 00:00:02.430 --> 00:00:03.820 Commons license. 00:00:03.820 --> 00:00:06.060 Your support will help MIT OpenCourseWare 00:00:06.060 --> 00:00:10.150 continue to offer high quality educational resources for free. 00:00:10.150 --> 00:00:12.690 To make a donation or to view additional materials 00:00:12.690 --> 00:00:16.445 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.445 --> 00:00:17.070 at osw.mit.edu. 00:00:26.400 --> 00:00:30.150 CATHERINE DRENNAN: Next hand out-- thermodynamics. 00:00:30.150 --> 00:00:31.520 Yes. 00:00:31.520 --> 00:00:33.010 Yes. 00:00:33.010 --> 00:00:35.520 I love thermodynamics. 00:00:35.520 --> 00:00:36.062 All right. 00:00:39.540 --> 00:00:42.090 So what is thermodynamics? 00:00:44.610 --> 00:00:47.700 So thermodynamics and kinetics I feel go together, 00:00:47.700 --> 00:00:51.390 but for kind of weird reasons we do thermodynamics now 00:00:51.390 --> 00:00:55.260 and we do kinetics at the very last unit of the semester. 00:00:55.260 --> 00:00:59.040 Part of the reason for this is that kinetics is often a unit 00:00:59.040 --> 00:01:01.830 that students can pick up really fast, 00:01:01.830 --> 00:01:05.220 and so I like doing it at the end when everything-- 00:01:05.220 --> 00:01:07.560 your world is sort of crazy and you 00:01:07.560 --> 00:01:10.650 have something that you can get a grasp on pretty easily 00:01:10.650 --> 00:01:11.784 for the last unit. 00:01:11.784 --> 00:01:13.950 So anyway, I'll tell you a little bit about kinetics 00:01:13.950 --> 00:01:17.760 now because we won't get to a lot of it until later. 00:01:17.760 --> 00:01:22.230 So thermodynamics deals with energy change and spontaneity 00:01:22.230 --> 00:01:24.180 of reactions. 00:01:24.180 --> 00:01:28.500 And thermodynamics brings you three of my favorite things 00:01:28.500 --> 00:01:35.550 in chemistry, which are [MUSIC PLAYING] delta H, 00:01:35.550 --> 00:01:45.600 [CYMBAL CRASH] Delta S, and [DRUM ROLL] delta G. 00:01:45.600 --> 00:01:47.790 I love these. 00:01:47.790 --> 00:01:50.760 I live my life around these things. 00:01:50.760 --> 00:01:55.050 I believe entropy should always be increasing. 00:01:55.050 --> 00:02:00.150 And I love nothing more than free energy. 00:02:00.150 --> 00:02:01.860 That's great stuff. 00:02:01.860 --> 00:02:05.310 So today we're going to talk about delta H. Next week we 00:02:05.310 --> 00:02:09.479 have delta S and delta G. 00:02:09.479 --> 00:02:10.620 What about kinetics? 00:02:10.620 --> 00:02:12.030 What does kinetics bring us? 00:02:12.030 --> 00:02:17.970 Well, kinetics brings us the rate or speed of a reaction. 00:02:17.970 --> 00:02:23.070 It can bring us fast reactions and it 00:02:23.070 --> 00:02:31.370 can bring us slow reactions. 00:02:31.370 --> 00:02:33.190 I like kinetics too. 00:02:33.190 --> 00:02:36.200 I kind of like the fast to be honest with you. 00:02:36.200 --> 00:02:36.700 All right. 00:02:36.700 --> 00:02:39.190 So thermodynamics and kinetics. 00:02:39.190 --> 00:02:41.560 One. thermodynamics tells us whether something 00:02:41.560 --> 00:02:44.830 is going to happen spontaneously or not, 00:02:44.830 --> 00:02:47.860 but kinetics tells us the rate at which it happens. 00:02:47.860 --> 00:02:51.910 So let's just think of an example for a minute. 00:02:51.910 --> 00:02:54.220 You may have heard-- this commercial happens 00:02:54.220 --> 00:02:56.380 a lot around Valentine's Day. 00:02:56.380 --> 00:02:59.600 Diamonds are forever. 00:02:59.600 --> 00:03:05.860 So actually, thermodynamically, graphite 00:03:05.860 --> 00:03:08.080 is favorable to diamonds. 00:03:08.080 --> 00:03:10.210 It's more stable. 00:03:10.210 --> 00:03:15.520 So graphite or coal-- thermodynamically, 00:03:15.520 --> 00:03:17.680 this is the stuff. 00:03:17.680 --> 00:03:22.880 Diamonds-- diamond is not forever. 00:03:22.880 --> 00:03:26.800 That's really a kinetic statement. 00:03:26.800 --> 00:03:34.060 It's there for a very long time, but you know it isn't inert. 00:03:34.060 --> 00:03:36.650 So here's an important question. 00:03:36.650 --> 00:03:41.670 What is the best ring for one geek to give another geek? 00:03:41.670 --> 00:03:44.690 The thermodynamically stable one or one 00:03:44.690 --> 00:03:51.180 that is more kinetically slow to react, more stable, more inert. 00:03:51.180 --> 00:03:53.210 Inert is reaction. 00:03:53.210 --> 00:03:55.520 What do you-- I should have a clicker question on this, 00:03:55.520 --> 00:03:56.020 I know. 00:03:56.020 --> 00:03:57.430 I don't know what you think. 00:03:57.430 --> 00:04:06.520 But actually, the answer is, in my opinion, neither of these. 00:04:06.520 --> 00:04:07.416 Come on. 00:04:11.980 --> 00:04:16.810 There's only really one ring that any geek really wants. 00:04:16.810 --> 00:04:19.750 Green Lantern's ring has the power 00:04:19.750 --> 00:04:23.050 of chemistry at your fingertips. 00:04:23.050 --> 00:04:25.780 Who cares if something's inert? 00:04:25.780 --> 00:04:28.840 If you're the Green Lantern you can do whatever. 00:04:28.840 --> 00:04:31.780 So that's the ring. 00:04:31.780 --> 00:04:33.100 Anyway-- 00:04:33.100 --> 00:04:34.587 AUDIENCE: What about the one ring? 00:04:34.587 --> 00:04:35.920 CATHERINE DRENNAN: Oh, one ring. 00:04:35.920 --> 00:04:36.430 Yeah. 00:04:36.430 --> 00:04:41.470 I like the Green Lantern ring, but I guess next-- maybe 00:04:41.470 --> 00:04:43.910 we should open a blog on this. 00:04:43.910 --> 00:04:47.020 It's a really important question. 00:04:47.020 --> 00:04:49.930 So let's think about bonding for a minute. 00:04:49.930 --> 00:04:52.970 So thermodynamics is telling us about energy change. 00:04:52.970 --> 00:04:55.510 It's telling us about spontaneity. 00:04:55.510 --> 00:04:58.630 And so we need to think about energy that's going in. 00:04:58.630 --> 00:05:00.760 So kinetics is telling us about how fast. 00:05:00.760 --> 00:05:03.850 Thermodynamics is really telling us about stability. 00:05:03.850 --> 00:05:04.960 How stable is something? 00:05:04.960 --> 00:05:07.960 How much does it cost to break it apart? 00:05:07.960 --> 00:05:11.870 So we're back to bond association energies. 00:05:11.870 --> 00:05:14.750 We have the change in the dissociation 00:05:14.750 --> 00:05:16.720 energy E to the little d. 00:05:16.720 --> 00:05:18.650 It's the energy to break a bond. 00:05:18.650 --> 00:05:20.570 We've seen this plot before. 00:05:20.570 --> 00:05:22.130 Now we have methane. 00:05:22.130 --> 00:05:24.380 We're breaking off a hydrogen from it, which 00:05:24.380 --> 00:05:26.060 is actually very hard to do. 00:05:26.060 --> 00:05:29.180 Scientists would love to break apart methane and make 00:05:29.180 --> 00:05:32.090 methanol, but it's a hard thing to do. 00:05:32.090 --> 00:05:34.460 Up here we have unfavorable reactions. 00:05:34.460 --> 00:05:36.590 When the atoms are too close, then you 00:05:36.590 --> 00:05:39.140 have a sweet spot where the positions of the atoms 00:05:39.140 --> 00:05:40.790 are just right to form a bond. 00:05:40.790 --> 00:05:42.380 That's when you get methane. 00:05:42.380 --> 00:05:46.440 And then, if you put in energy, you can pull this off, 00:05:46.440 --> 00:05:49.450 is they go far apart with the radius this way. 00:05:49.450 --> 00:05:51.230 Your bond will dissociate. 00:05:51.230 --> 00:05:55.160 You always have to put energy in to get it to dissociate. 00:05:55.160 --> 00:05:57.500 So now we can think about it-- we thought about this 00:05:57.500 --> 00:05:59.150 in terms of bond dissociation. 00:05:59.150 --> 00:06:01.190 We've seen this before, but now we 00:06:01.190 --> 00:06:03.500 can think about it in terms of a new term, which 00:06:03.500 --> 00:06:06.550 is delta H B, or bond enthalpy. 00:06:06.550 --> 00:06:09.770 So bond enthalpy is the change in heat accompanying 00:06:09.770 --> 00:06:13.250 the dissociation of a bond and that's measured 00:06:13.250 --> 00:06:15.150 at a constant pressure. 00:06:15.150 --> 00:06:19.490 In fact, if we relate delta H to Delta E-- delta 00:06:19.490 --> 00:06:25.520 H equals delta E plus whatever change in pressure or volume. 00:06:25.520 --> 00:06:27.590 And often, this term is pretty small, 00:06:27.590 --> 00:06:29.990 so people often really think about reactions 00:06:29.990 --> 00:06:33.320 in terms of these being pretty similar to each other. 00:06:33.320 --> 00:06:38.240 So for gases, the difference really is 1% to 2%. 00:06:38.240 --> 00:06:42.110 And if you're talking about a liquid or a solid 00:06:42.110 --> 00:06:44.200 it's really a negligible difference. 00:06:44.200 --> 00:06:46.700 So we often really kind of think about these things 00:06:46.700 --> 00:06:47.420 in the same way. 00:06:47.420 --> 00:06:49.730 We think about the energy going into a system 00:06:49.730 --> 00:06:52.850 to break the bond, or we think about the bond enthalpies. 00:06:56.440 --> 00:06:59.980 And bond enthalpy, delta H, is often easier to measure, 00:06:59.980 --> 00:07:02.870 so it's very convenient. 00:07:02.870 --> 00:07:05.900 So again, bond enthalpies You always 00:07:05.900 --> 00:07:08.990 have to put energy in if you're going to break a bond. 00:07:08.990 --> 00:07:10.850 So it's always going to be positive. 00:07:10.850 --> 00:07:12.020 It always takes heat. 00:07:12.020 --> 00:07:16.370 It always takes something to break a bond. 00:07:16.370 --> 00:07:20.200 And so breaking a bond is endothermic. 00:07:20.200 --> 00:07:21.556 Heat must be added. 00:07:21.556 --> 00:07:24.560 Whereas bond formation is exothermic. 00:07:24.560 --> 00:07:26.270 Heat is being released. 00:07:26.270 --> 00:07:28.250 And so we can think about this-- again, 00:07:28.250 --> 00:07:33.500 when you form a bond those-- we saw with MO theory-- 00:07:33.500 --> 00:07:37.640 there's more electrons in lower energy in the bonding orbitals 00:07:37.640 --> 00:07:39.530 than in the antibonding orbitals. 00:07:39.530 --> 00:07:40.130 They're happy. 00:07:40.130 --> 00:07:42.600 This is a lower energy state. 00:07:42.600 --> 00:07:44.900 So if you're going to break that bond, 00:07:44.900 --> 00:07:46.270 breaking up is hard to do. 00:07:46.270 --> 00:07:48.527 And there's a song that verifies that statement. 00:07:48.527 --> 00:07:49.610 Breaking up is hard to do. 00:07:49.610 --> 00:07:51.500 You always have to have heat. 00:07:51.500 --> 00:07:56.710 But when you go from that stable stay 00:07:56.710 --> 00:08:00.470 out-- when you form those bonds, it's 00:08:00.470 --> 00:08:03.420 like kind of the married couple, often get a little boring. 00:08:03.420 --> 00:08:04.670 They're in a low energy state. 00:08:04.670 --> 00:08:07.250 It's hard to get them out of the house 00:08:07.250 --> 00:08:10.310 and so they release all of their energy 00:08:10.310 --> 00:08:12.980 and they form this nice, little, happy, stable couple. 00:08:12.980 --> 00:08:17.197 So when you do bond formation that's an exothermic process. 00:08:19.960 --> 00:08:23.720 So we can talk about standard bond enthalpies. 00:08:23.720 --> 00:08:26.240 When you see this little circle up there 00:08:26.240 --> 00:08:30.530 that means it's a value that's at standard conditions, where 00:08:30.530 --> 00:08:33.558 your reactants and products are in their standard states. 00:08:36.549 --> 00:08:41.230 And so we can think about what are some delta H or some bond 00:08:41.230 --> 00:08:45.900 enthalpies for different kinds of carbon hydrogen bonds. 00:08:45.900 --> 00:08:47.607 So here's our friend methane. 00:08:47.607 --> 00:08:49.190 If you're going to pull off a hydrogen 00:08:49.190 --> 00:08:51.320 you have to put energy in to do that, 00:08:51.320 --> 00:08:55.960 and the bond enthalpy for that is 438. 00:08:55.960 --> 00:08:58.790 Now we can think about some other kinds of carbon bonds. 00:08:58.790 --> 00:09:00.110 We talked about this one. 00:09:00.110 --> 00:09:03.480 We have it here in the classroom. 00:09:03.480 --> 00:09:08.980 So if we pull off a hydrogen from that, it's plus 410. 00:09:08.980 --> 00:09:11.660 So similar but not the same. 00:09:11.660 --> 00:09:15.470 If you now substitute three flourines for three 00:09:15.470 --> 00:09:17.930 of the hydrogens that changes your value 00:09:17.930 --> 00:09:19.700 a little bit, but not much. 00:09:19.700 --> 00:09:24.050 If you substitute chlorine it changes it a lot more. 00:09:24.050 --> 00:09:25.720 Same with bromine. 00:09:25.720 --> 00:09:26.930 So it depends. 00:09:26.930 --> 00:09:30.990 The bond enthalpy depends on what else is around that atom 00:09:30.990 --> 00:09:32.120 that you're pulling around. 00:09:32.120 --> 00:09:34.915 So it's not always the same value. 00:09:34.915 --> 00:09:35.540 It's different. 00:09:35.540 --> 00:09:37.820 It depends on what else is there. 00:09:37.820 --> 00:09:41.630 So often you'll have a table that will report mean bond 00:09:41.630 --> 00:09:42.700 enthalpies. 00:09:42.700 --> 00:09:44.540 And they take all the bonds enthalpies 00:09:44.540 --> 00:09:46.820 and they get the mean value, and they're usually 00:09:46.820 --> 00:09:48.820 within 8% of each other. 00:09:48.820 --> 00:09:50.960 And for a carbon hydrogen bond it's 00:09:50.960 --> 00:09:54.410 around 412 is the mean bond enthalpy. 00:09:54.410 --> 00:09:56.500 But when you're using mean bond enthalpies 00:09:56.500 --> 00:09:58.880 to calculate something, you have to realize 00:09:58.880 --> 00:10:02.870 that there can be some pretty big differences depending 00:10:02.870 --> 00:10:08.210 on what's around that bond that you're going to break. 00:10:08.210 --> 00:10:10.310 So those are some bond enthalpies. 00:10:10.310 --> 00:10:12.450 Why are they important? 00:10:12.450 --> 00:10:14.750 Well, they're important because the difference 00:10:14.750 --> 00:10:18.260 in bond enthalpies between a product and a reactant 00:10:18.260 --> 00:10:21.980 can tell you about the enthalpy of that reaction. 00:10:21.980 --> 00:10:24.680 The enthalpy of the reaction of breaking of bonds 00:10:24.680 --> 00:10:28.610 and forming new bonds, or the enthalpy of reaction, which 00:10:28.610 --> 00:10:34.670 is delta H R-- sub R. And that is in the standard state 00:10:34.670 --> 00:10:35.750 in that case. 00:10:35.750 --> 00:10:39.530 So let's talk about enthalpies of reactions. 00:10:39.530 --> 00:10:42.300 So we have this symbol again. 00:10:42.300 --> 00:10:45.690 Standard bond enthalpy for a reaction. 00:10:45.690 --> 00:10:47.750 So if it's a little B it's a bond enthalpy. 00:10:47.750 --> 00:10:50.780 If it's a little R it's a reaction enthalpy. 00:10:50.780 --> 00:10:55.830 If it's negative value it means it's an exothermic reaction, 00:10:55.830 --> 00:10:58.490 and if it's a positive value it means that it's 00:10:58.490 --> 00:11:00.840 an endothermic reaction. 00:11:00.840 --> 00:11:03.050 So we'll use these terms a lot and you'll 00:11:03.050 --> 00:11:04.890 get very familiar with them. 00:11:04.890 --> 00:11:08.720 So let's look at some examples of reactions. 00:11:08.720 --> 00:11:15.801 And here is one of my favorites, and it is yes. 00:11:15.801 --> 00:11:19.170 [MUSIC PLAYING] 00:11:19.170 --> 00:11:20.220 It has it's own song. 00:11:20.220 --> 00:11:24.552 - (SINGING) Photosynthesis Aah. 00:11:24.552 --> 00:11:27.426 Photosynthesis. 00:11:27.426 --> 00:11:29.342 Aah. 00:11:29.342 --> 00:11:31.591 Photosynthesis. 00:11:31.591 --> 00:11:32.090 Aah. 00:11:39.373 --> 00:11:42.810 Photosynthesis does not involve a camera or a synthesizer, 00:11:42.810 --> 00:11:44.283 although that's interesting too. 00:11:44.283 --> 00:11:46.901 Photosynthesis is how the plants take in light from the sun 00:11:46.901 --> 00:11:48.702 and turn it into energy. 00:11:48.702 --> 00:11:51.648 It's actually a thing on which most life depends here 00:11:51.648 --> 00:11:53.121 on the planet Earth. 00:11:53.121 --> 00:11:55.576 Photosynthesis. 00:11:55.576 --> 00:11:56.494 Aah. 00:11:56.494 --> 00:11:57.410 CATHERINE DRENNAN: OK. 00:11:57.410 --> 00:11:58.640 That gives you an idea. 00:11:58.640 --> 00:12:01.520 Unfortunately, every time you will hear the word 00:12:01.520 --> 00:12:04.100 photosynthesis you'll go Ah. 00:12:04.100 --> 00:12:05.180 It happens. 00:12:05.180 --> 00:12:07.340 I'm sorry about that. 00:12:07.340 --> 00:12:09.470 So photosynthesis. 00:12:09.470 --> 00:12:10.640 Amazing reaction. 00:12:10.640 --> 00:12:14.660 People right now are trying to duplicate it in industry 00:12:14.660 --> 00:12:16.200 to solve the energy problem. 00:12:16.200 --> 00:12:18.650 Good luck with that. 00:12:18.650 --> 00:12:20.060 But I know. 00:12:20.060 --> 00:12:21.330 I wish them good luck. 00:12:21.330 --> 00:12:23.480 That would be awesome. 00:12:23.480 --> 00:12:27.680 We use the opposite of that reaction for our energy. 00:12:27.680 --> 00:12:31.400 So we take sugar and use oxygen to break it down, 00:12:31.400 --> 00:12:34.670 which is an awesome thing because this 00:12:34.670 --> 00:12:39.470 has a really negative enthalpy of reaction 00:12:39.470 --> 00:12:43.560 minus 2816 kilojoules per mole. 00:12:43.560 --> 00:12:44.360 It's huge. 00:12:44.360 --> 00:12:47.610 And we store this in something called ATP. 00:12:47.610 --> 00:12:50.877 So since this is-- and I'm going to need the help of the TAs 00:12:50.877 --> 00:12:53.210 for a minute because we're going to do a very quick demo 00:12:53.210 --> 00:12:55.340 at the end of today's class. 00:12:55.340 --> 00:13:00.050 This reaction is exothermic big time. 00:13:00.050 --> 00:13:03.620 It's a big negative, which raises the question, 00:13:03.620 --> 00:13:06.950 if it's that exothermic-- really big value. 00:13:06.950 --> 00:13:09.590 We have sugar in air. 00:13:09.590 --> 00:13:11.900 Why we should feel heat? 00:13:11.900 --> 00:13:13.560 Heat should be released. 00:13:13.560 --> 00:13:17.360 So I think we should do this demo now and see 00:13:17.360 --> 00:13:18.710 whether that's true. 00:13:18.710 --> 00:13:24.620 So I have a bag of sugar and it is sealed under nitrogen 00:13:24.620 --> 00:13:27.020 so there's no oxygen in there. 00:13:27.020 --> 00:13:31.995 And I forgot my safety glasses but I'll try to-- sorry 00:13:31.995 --> 00:13:32.870 about the front room. 00:13:32.870 --> 00:13:34.610 I should have had a stellar announcement that you 00:13:34.610 --> 00:13:35.840 might want to sit back. 00:13:35.840 --> 00:13:40.010 But I'm going to cut this open and let O2 in. 00:13:40.010 --> 00:13:44.552 So there should be a lot of heat coming out. 00:13:44.552 --> 00:13:47.439 AUDIENCE: [INAUDIBLE] the things inside it individually wrapped? 00:13:47.439 --> 00:13:48.980 CATHERINE DRENNAN: Oh, you know what? 00:13:48.980 --> 00:13:50.060 They are individually wrapped. 00:13:50.060 --> 00:13:51.800 All right, so this is not going to work. 00:13:51.800 --> 00:13:55.850 So I need the TAs to come down here, please, 00:13:55.850 --> 00:13:58.250 and you've got to help me unwrap them. 00:13:58.250 --> 00:14:01.520 Has anyone done the experiment yet? 00:14:01.520 --> 00:14:04.410 Do you feel heat coming out? 00:14:04.410 --> 00:14:05.369 AUDIENCE: Yeah. 00:14:05.369 --> 00:14:06.452 CATHERINE DRENNAN: You do? 00:14:06.452 --> 00:14:10.110 [LAUGHTER] 00:14:10.110 --> 00:14:10.610 All right. 00:14:10.610 --> 00:14:13.610 I better try it up here. 00:14:13.610 --> 00:14:14.480 Let's see. 00:14:14.480 --> 00:14:17.630 I'm going or unwrap mine. 00:14:17.630 --> 00:14:20.730 It's not working very well. 00:14:20.730 --> 00:14:27.320 So it turns out that heat should be 00:14:27.320 --> 00:14:31.730 released but this is very slow. 00:14:31.730 --> 00:14:36.650 So we don't feel the heat when we unwrap our Hershey's Kisses. 00:14:36.650 --> 00:14:41.420 I encourage everyone to try this experiment at least once. 00:14:41.420 --> 00:14:46.880 But the way that we harness this energy in our bodies 00:14:46.880 --> 00:14:49.370 is that we have catalysts, which are enzymes, 00:14:49.370 --> 00:14:51.170 that speed up the reaction. 00:14:51.170 --> 00:14:55.910 And that's how we get the full force of this reaction out. 00:14:55.910 --> 00:15:02.170 So that is actually our introduction to thermodynamics. 00:15:02.170 --> 00:15:04.940 And next time we're going to talk 00:15:04.940 --> 00:15:11.600 about how we're going to calculate these delta Hr, 00:15:11.600 --> 00:15:15.140 these heats of reaction. 00:15:15.140 --> 00:15:18.140 So we were talking about delta H, 00:15:18.140 --> 00:15:22.360 and so we want to pull out the handouts from last time. 00:15:22.360 --> 00:15:26.980 And we were at the bottom of page two 00:15:26.980 --> 00:15:31.480 with three different ways to calculate delta H. 00:15:31.480 --> 00:15:37.120 So our delta H of reaction, delta Hr, the reaction 00:15:37.120 --> 00:15:38.920 enthalpy. 00:15:38.920 --> 00:15:41.620 So I introduced you to bond enthalpies, 00:15:41.620 --> 00:15:43.480 and today we're going to look at how 00:15:43.480 --> 00:15:49.170 you use bond enthalpies to calculate reaction enthalpies. 00:15:49.170 --> 00:15:52.030 And remember, bond enthalpies-- sometimes it has nothing. 00:15:52.030 --> 00:15:53.740 Just delta H. Sometimes it's delta 00:15:53.740 --> 00:15:58.030 H sub B. Capital B for bond. 00:15:58.030 --> 00:15:59.800 And we're going to look at that. 00:15:59.800 --> 00:16:02.200 Then we're going to look at how you can calculate 00:16:02.200 --> 00:16:06.490 delta H for reaction from the standard enthalpies 00:16:06.490 --> 00:16:09.770 of formation, and I'll introduce you to what that means. 00:16:09.770 --> 00:16:12.130 And then, also tell you about Hess's law 00:16:12.130 --> 00:16:15.430 where you can combine known reactions that 00:16:15.430 --> 00:16:19.090 have known delta H's to get a new equation 00:16:19.090 --> 00:16:22.400 and calculate a new delta H for that reaction. 00:16:22.400 --> 00:16:23.590 So three different ways. 00:16:23.590 --> 00:16:28.810 So we're going to start with way one, which is bond enthalpies. 00:16:28.810 --> 00:16:33.700 So here is the equation for calculating bond enthalpies. 00:16:33.700 --> 00:16:38.710 So we have the delta H0 of the reaction 00:16:38.710 --> 00:16:43.570 equals the sum of all of the reactants bond 00:16:43.570 --> 00:16:49.270 enthalpies minus the sum of all the product bond enthalpies. 00:16:49.270 --> 00:16:55.090 And so this is bonds broken minus bonds formed. 00:16:55.090 --> 00:16:56.950 And so let's think about this for a minute 00:16:56.950 --> 00:16:59.770 and think about what would be true. 00:16:59.770 --> 00:17:05.200 If you had stronger bonds in the products than in the reactants, 00:17:05.200 --> 00:17:06.040 what would be true? 00:17:06.040 --> 00:17:07.354 And this is a clicker question. 00:17:15.971 --> 00:17:16.470 All right. 00:17:16.470 --> 00:17:18.003 Let's just take 10 more seconds. 00:17:27.839 --> 00:17:30.180 OK Yep. 00:17:30.180 --> 00:17:33.790 So now let's think about why this is true. 00:17:33.790 --> 00:17:35.730 So it's good news that most of you 00:17:35.730 --> 00:17:38.850 know that negative means exothermic and positive 00:17:38.850 --> 00:17:40.710 means endothermic. 00:17:40.710 --> 00:17:42.593 And let's look at why this is true. 00:17:46.380 --> 00:17:48.310 So let's look at both of these. 00:17:48.310 --> 00:17:51.210 So if we have bonds stronger in the products-- 00:17:51.210 --> 00:17:53.650 you can just think about the equation. 00:17:53.650 --> 00:17:55.710 So if you are bond stronger in the products 00:17:55.710 --> 00:17:57.480 this is a bigger number and that's 00:17:57.480 --> 00:17:59.040 the smaller number, which is going 00:17:59.040 --> 00:18:01.050 to give you a negative answer. 00:18:01.050 --> 00:18:04.390 And a negative value is exothermic. 00:18:04.390 --> 00:18:07.620 And you can think about the equation stronger bonds here. 00:18:07.620 --> 00:18:09.690 A bigger number minus the smaller number. 00:18:09.690 --> 00:18:11.670 Positive or endothermic. 00:18:11.670 --> 00:18:14.820 But let's think for a minute about why this is the case 00:18:14.820 --> 00:18:17.610 and rationalize it because on an exam 00:18:17.610 --> 00:18:20.310 this is one of the equations that you're not given, 00:18:20.310 --> 00:18:23.635 so let's help you remember why this would be true. 00:18:26.580 --> 00:18:30.930 We can think about this-- if you're going to break bonds-- 00:18:30.930 --> 00:18:34.290 and this isn't in your notes but people get confused by this, 00:18:34.290 --> 00:18:37.950 so I'm just going to write a little bit on the board. 00:18:37.950 --> 00:18:39.970 So if you're going to break bonds 00:18:39.970 --> 00:18:44.820 you need to put energy into the system to break bonds. 00:18:44.820 --> 00:18:47.070 And we talked about this before. 00:18:47.070 --> 00:18:50.460 And since we have exam two coming up 00:18:50.460 --> 00:18:53.100 we'll just do a little review of some of the things 00:18:53.100 --> 00:18:54.580 that might be on the exam. 00:18:54.580 --> 00:18:58.140 So you don't have this in the handout we're doing right now, 00:18:58.140 --> 00:19:00.120 but you had this in the lecture nine handout. 00:19:00.120 --> 00:19:02.670 And something like this might be on the exam 00:19:02.670 --> 00:19:05.100 so we should be thinking about it. 00:19:05.100 --> 00:19:07.920 So remember, if there was no energy 00:19:07.920 --> 00:19:09.510 that you needed to put in to break 00:19:09.510 --> 00:19:12.540 a bond-- if breaking the bond required no energy there would 00:19:12.540 --> 00:19:13.230 be no bond. 00:19:13.230 --> 00:19:15.990 So when the energy is zero there's no bonds. 00:19:15.990 --> 00:19:19.630 These two are not-- these things are not bonded together. 00:19:19.630 --> 00:19:23.100 And when you do form a bond, you go down an energy 00:19:23.100 --> 00:19:25.440 here so it's at a lower state. 00:19:25.440 --> 00:19:26.610 It's more stable. 00:19:26.610 --> 00:19:28.470 That's why it forms a bond. 00:19:28.470 --> 00:19:30.990 If it was less stable it wouldn't be forming a bond. 00:19:30.990 --> 00:19:33.720 But if it's more stable, lower in energy, a larger 00:19:33.720 --> 00:19:37.530 negative number, then a bond forms. 00:19:37.530 --> 00:19:41.610 So to break this bond you have to put energy into the system. 00:19:41.610 --> 00:19:45.360 So breaking bonds always involves energy in. 00:19:45.360 --> 00:19:56.220 But forming bonds-- so if we're forming bonds then 00:19:56.220 --> 00:19:58.380 we're going to have energy out. 00:19:58.380 --> 00:20:06.580 So we're at a lower place here. 00:20:06.580 --> 00:20:09.440 So if we want to break bonds we have to put energy in, 00:20:09.440 --> 00:20:11.630 but if we're forming bonds then we're 00:20:11.630 --> 00:20:14.870 going to have energy that these guys had that is 00:20:14.870 --> 00:20:16.460 going to be released somewhere. 00:20:16.460 --> 00:20:19.040 So energy goes out of the system. 00:20:19.040 --> 00:20:23.090 And the farther down we have the stronger bonds, the more energy 00:20:23.090 --> 00:20:24.350 you have to put in to break. 00:20:24.350 --> 00:20:29.360 But also, the more energy that comes out when the bonds form. 00:20:29.360 --> 00:20:31.910 So energy in to break a bond, but when 00:20:31.910 --> 00:20:35.900 a bond is forming it goes to a lower state and that energy 00:20:35.900 --> 00:20:37.100 is released. 00:20:37.100 --> 00:20:40.160 So now we can think about what happens if you have 00:20:40.160 --> 00:20:43.610 a reactant with weak bonds. 00:20:43.610 --> 00:20:53.220 So if the reactant then has weak bonds, 00:20:53.220 --> 00:20:56.310 how much energy do you have to put in if it has weak bonds 00:20:56.310 --> 00:20:58.260 to break them? 00:20:58.260 --> 00:20:59.010 Not a lot. 00:20:59.010 --> 00:21:02.760 So we have just sort of a little bit of energy in. 00:21:02.760 --> 00:21:05.460 Little energy in to break those bonds. 00:21:05.460 --> 00:21:16.010 Now in the products, if we have strong bonds, 00:21:16.010 --> 00:21:21.040 how much energy goes out if we're forming strong bonds? 00:21:21.040 --> 00:21:22.970 A lot. 00:21:22.970 --> 00:21:25.850 So energy out. 00:21:25.850 --> 00:21:29.630 We have lots of energy out. 00:21:29.630 --> 00:21:33.140 So that was the first case that we had. 00:21:33.140 --> 00:21:35.420 So we had something where the bonds were stronger 00:21:35.420 --> 00:21:39.740 in the product and we said that this was negative. 00:21:39.740 --> 00:21:47.990 So net here we have heat or energy out is released, 00:21:47.990 --> 00:21:50.390 and so that's an exothermic system. 00:21:53.452 --> 00:21:55.016 Oh, the boards work today. 00:21:57.980 --> 00:22:02.930 And if we have the other-- if we have, say, strong bonds 00:22:02.930 --> 00:22:08.450 in the reactants, then we have to put a lot of energy in. 00:22:08.450 --> 00:22:10.190 Big energy in. 00:22:10.190 --> 00:22:13.910 And if we have weak bonds that are being formed, 00:22:13.910 --> 00:22:16.670 we're not getting much energy back 00:22:16.670 --> 00:22:22.595 so the net here is that you have heat in or heat absorbed. 00:22:28.160 --> 00:22:32.220 And it's an endothermic reaction. 00:22:32.220 --> 00:22:34.010 So this is just one way to think about it. 00:22:34.010 --> 00:22:35.966 Remember, whenever you are going to break bond 00:22:35.966 --> 00:22:38.090 you always have to put energy in to break the bond. 00:22:38.090 --> 00:22:41.120 And when a bond is formed that energy is released. 00:22:41.120 --> 00:22:45.080 So we are thinking about the net of the processes, 00:22:45.080 --> 00:22:49.250 and that's why this equation works for us. 00:22:49.250 --> 00:22:50.360 So keep this in mind. 00:22:50.360 --> 00:22:52.280 This is one of the points that people 00:22:52.280 --> 00:22:54.440 get confused on the exams. 00:22:54.440 --> 00:22:57.449 And sometimes like they say, oh, thermodynamics. 00:22:57.449 --> 00:22:59.240 I just don't understand it, and they're not 00:22:59.240 --> 00:23:01.460 keeping calm and sciencing on. 00:23:01.460 --> 00:23:05.150 They're getting all stressed by thermodynamics, 00:23:05.150 --> 00:23:06.860 and it's only this confusion. 00:23:06.860 --> 00:23:07.520 That's it. 00:23:07.520 --> 00:23:10.040 So if you work this out then thermodynamics 00:23:10.040 --> 00:23:12.920 will be your friend and you will love thermodynamics 00:23:12.920 --> 00:23:14.042 like I do forever. 00:23:17.630 --> 00:23:21.260 Just kind of keep this in mind and those diagrams in mind 00:23:21.260 --> 00:23:22.500 and you'll be all good. 00:23:22.500 --> 00:23:23.000 All right. 00:23:23.000 --> 00:23:25.340 So let's do an example now. 00:23:25.340 --> 00:23:30.860 So we can use these bond enthalpies in this equation 00:23:30.860 --> 00:23:32.690 where we're summing up all our reactants. 00:23:32.690 --> 00:23:35.710 And sometimes you see some of a little i 00:23:35.710 --> 00:23:39.430 here for i reactants minus j products. 00:23:39.430 --> 00:23:43.090 So the sum of all of the products. 00:23:43.090 --> 00:23:45.650 And it really is a lot here because we're talking 00:23:45.650 --> 00:23:48.370 about breaking every bond. 00:23:48.370 --> 00:23:51.050 We're talking about forming every bond. 00:23:51.050 --> 00:23:53.960 So this is not a huge molecule, but let's think 00:23:53.960 --> 00:23:55.640 about how many bonds we're actually 00:23:55.640 --> 00:23:57.450 going to be breaking here. 00:23:57.450 --> 00:24:00.890 So these are all the bonds that are broken. 00:24:00.890 --> 00:24:03.410 They're not quite as many being formed here. 00:24:03.410 --> 00:24:04.430 So bonds broken. 00:24:04.430 --> 00:24:10.190 We have carbon hydrogen bonds, and we have seven of those. 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, 00:24:14.900 --> 00:24:19.370 5, 6, 7. 00:24:19.370 --> 00:24:20.240 There it is. 00:24:20.240 --> 00:24:22.280 I need my glasses. 00:24:22.280 --> 00:24:23.180 OH bonds. 00:24:23.180 --> 00:24:25.190 We have these guys up here. 00:24:25.190 --> 00:24:27.380 One, two, three, four, five. 00:24:27.380 --> 00:24:33.480 We have CO bond over a one double bond over here. 00:24:33.480 --> 00:24:36.800 So we also are going to have these ones here. 00:24:36.800 --> 00:24:38.660 One, two, three, four, five. 00:24:38.660 --> 00:24:41.120 We have the double bond over there. 00:24:41.120 --> 00:24:43.820 We have five carbon bonds. 00:24:43.820 --> 00:24:45.920 The one single bond here. 00:24:45.920 --> 00:24:48.980 And the carbon bond is one, two, three, four, five. 00:24:48.980 --> 00:24:50.480 And we have OO bonds. 00:24:50.480 --> 00:24:51.506 We have six of those. 00:24:51.506 --> 00:24:52.130 Thank goodness. 00:24:52.130 --> 00:24:53.421 I didn't have to count anymore. 00:24:53.421 --> 00:24:54.530 It's already labeled. 00:24:54.530 --> 00:24:56.390 And then the bonds formed. 00:24:56.390 --> 00:24:57.730 So we're going to have these. 00:24:57.730 --> 00:25:01.310 So it's six of those, so we have 12 altogether 00:25:01.310 --> 00:25:03.930 and we have also 12 over here. 00:25:03.930 --> 00:25:05.390 So first you have to count. 00:25:05.390 --> 00:25:07.430 And counting is not one of my strengths, 00:25:07.430 --> 00:25:08.870 so I don't like doing it this way 00:25:08.870 --> 00:25:10.536 and I'm going to show you two other ways 00:25:10.536 --> 00:25:12.430 to calculate the same thing. 00:25:12.430 --> 00:25:15.950 But we can take this and sum these all up. 00:25:15.950 --> 00:25:19.280 We can look up the mean bond enthalpies for every single one 00:25:19.280 --> 00:25:21.950 of these types of bonds, multiply them 00:25:21.950 --> 00:25:24.770 by the appropriate coefficients, and come up 00:25:24.770 --> 00:25:29.360 with a sum for all the bonds for i number of bonds 00:25:29.360 --> 00:25:31.100 that you have in the reactants. 00:25:31.100 --> 00:25:34.790 And you can do the same in the products for j number of bonds 00:25:34.790 --> 00:25:39.330 that you have in the products and come up with these numbers. 00:25:39.330 --> 00:25:41.750 So if you were told that you have to do it 00:25:41.750 --> 00:25:45.080 this way-- use bond enthalpies and you know how to do it-- 00:25:45.080 --> 00:25:47.330 or if it's an easier problem and you're only, say, 00:25:47.330 --> 00:25:50.690 breaking two things and forming two things this isn't a bad way 00:25:50.690 --> 00:25:51.470 to do it. 00:25:51.470 --> 00:25:55.180 For big molecules this is definitely a nuisance. 00:25:55.180 --> 00:25:59.830 And if we sum all of this up together-- and so 00:25:59.830 --> 00:26:03.400 for the total number we have reactants minus products. 00:26:03.400 --> 00:26:08.890 And so if we subtract this we get minus 2,740. 00:26:08.890 --> 00:26:15.550 And the actual value is minus 2,816. 00:26:15.550 --> 00:26:17.560 So it's not even the best agreement 00:26:17.560 --> 00:26:18.940 when you do it this way. 00:26:18.940 --> 00:26:21.130 And the reason was, if you remember last time, 00:26:21.130 --> 00:26:23.320 we were talking about the bonds. 00:26:23.320 --> 00:26:27.540 Mean bond enthalpy is about 8% difference. 00:26:27.540 --> 00:26:30.370 So if you had, say, CH in a system that 00:26:30.370 --> 00:26:33.910 has all the rest of the atoms on carbon or H, 00:26:33.910 --> 00:26:35.290 that's a somewhat different value 00:26:35.290 --> 00:26:37.750 than if all of those other H atoms 00:26:37.750 --> 00:26:41.260 were substituted with bromine or if all those other H atoms were 00:26:41.260 --> 00:26:43.110 substituted with carbon. 00:26:43.110 --> 00:26:47.650 Then the bond enthalpy for that CH-- it depends on what else 00:26:47.650 --> 00:26:51.430 is bonded to the C. And so there's about 8% difference 00:26:51.430 --> 00:26:53.750 usually in the values. 00:26:53.750 --> 00:26:56.830 And so overall, you're not going to get much better. 00:26:56.830 --> 00:26:59.200 You certainly are not going to get better than eight. 00:26:59.200 --> 00:27:01.510 So agreement of 3% is pretty good, 00:27:01.510 --> 00:27:03.670 but it's not all that precise because we're 00:27:03.670 --> 00:27:06.310 using these mean bond enthalpies, which 00:27:06.310 --> 00:27:11.530 don't depend on the actual value in that particular system. 00:27:11.530 --> 00:27:16.180 So we can do better than this, and it can be also easier. 00:27:16.180 --> 00:27:20.710 And it'll be easier if we use standard heats of formation. 00:27:20.710 --> 00:27:26.920 So this is delta H sub f, f for formation. 00:27:26.920 --> 00:27:33.970 So the delta H sub f not for standard value 00:27:33.970 --> 00:27:38.380 is equal to the reaction, delta H, 00:27:38.380 --> 00:27:41.500 if you're talking about a reaction that involves 00:27:41.500 --> 00:27:46.750 one mole of compound being derived from its pure elements 00:27:46.750 --> 00:27:52.000 in their most stable state and in their standards state. 00:27:52.000 --> 00:27:57.310 So this is standard state 1 bar and room temperature. 00:27:57.310 --> 00:28:01.600 So let's calculate for the same reaction glucose plus oxygen 00:28:01.600 --> 00:28:05.860 going to CO2 plus water and see if we 00:28:05.860 --> 00:28:09.430 can get a little bit more accurate value that way. 00:28:09.430 --> 00:28:12.650 So let's think about what's happening in this reaction. 00:28:12.650 --> 00:28:16.720 So every time we oxidize glucose we're forming water. 00:28:16.720 --> 00:28:19.840 And so we can think about the heat 00:28:19.840 --> 00:28:23.320 of formation for liquid water. 00:28:23.320 --> 00:28:28.090 So again, this would be one mole coming from pure elements 00:28:28.090 --> 00:28:30.190 in their most standard state. 00:28:30.190 --> 00:28:33.070 So we have to think about where the hydrogen is coming from 00:28:33.070 --> 00:28:35.350 and where the oxygen is coming from. 00:28:35.350 --> 00:28:40.060 So hydrogen in its most stable form is H2 gas, 00:28:40.060 --> 00:28:44.800 and oxygen in its most stable form is O2 gas. 00:28:44.800 --> 00:28:48.940 So that's then the equation balanced for one mole 00:28:48.940 --> 00:28:52.690 of H2O liquid being formed. 00:28:52.690 --> 00:28:57.520 And we can look up the delta H for this-- 00:28:57.520 --> 00:29:01.180 that delta H of formation-- for this reaction as written 00:29:01.180 --> 00:29:06.940 is the delta H of formation, and it's minus 285.83 kilojoules 00:29:06.940 --> 00:29:08.260 per mole. 00:29:08.260 --> 00:29:12.190 So now let's consider what else that we're forming, water. 00:29:12.190 --> 00:29:15.610 And we're also forming CO2. 00:29:15.610 --> 00:29:20.890 So CO2 is derived from carbon in its most stable state, which 00:29:20.890 --> 00:29:26.080 is graphite as we discussed before, and also O2, oxygen. 00:29:26.080 --> 00:29:29.890 And O2 oxygen gas is the most stable state there. 00:29:29.890 --> 00:29:32.920 So for this reaction as written that 00:29:32.920 --> 00:29:36.160 is the delta H of formation of CO2 gas 00:29:36.160 --> 00:29:41.210 and it's minus 393.5 kilojoules per mole. 00:29:41.210 --> 00:29:42.740 So those are our products. 00:29:42.740 --> 00:29:44.890 We also have two reactants. 00:29:44.890 --> 00:29:47.830 One of our reactants is O2. 00:29:47.830 --> 00:29:50.740 So it's what's doing the oxidation. 00:29:50.740 --> 00:29:55.570 And we're going from O2 gas to O2 gas. 00:29:55.570 --> 00:29:57.680 This is the most stable state. 00:29:57.680 --> 00:29:59.610 So what do you think the value is here? 00:29:59.610 --> 00:30:00.490 AUDIENCE: 0. 00:30:00.490 --> 00:30:01.420 CATHERINE DRENNAN: 0. 00:30:01.420 --> 00:30:03.490 Yes. 00:30:03.490 --> 00:30:06.400 So if you have an element already 00:30:06.400 --> 00:30:09.160 in its most stable state, its heat of formation 00:30:09.160 --> 00:30:10.450 is going to be 0. 00:30:10.450 --> 00:30:12.910 Because it's already the most stable state, 00:30:12.910 --> 00:30:14.950 so the heat of formation is 0. 00:30:14.950 --> 00:30:17.380 And every year, I think, on an exam someone's 00:30:17.380 --> 00:30:19.600 trying to see if they can calculate 00:30:19.600 --> 00:30:22.720 a delta H of a reaction and they're looking 00:30:22.720 --> 00:30:25.060 and they're like, oh, I want to use heats of formation 00:30:25.060 --> 00:30:28.810 because I know that's a lot easier but a value is missing 00:30:28.810 --> 00:30:30.010 from my table. 00:30:30.010 --> 00:30:32.650 And they're like, the value is missing from the table. 00:30:32.650 --> 00:30:35.440 And the TA doesn't know how much information or whatever 00:30:35.440 --> 00:30:36.200 to give. 00:30:36.200 --> 00:30:38.325 And if you think you should have a value on an exam 00:30:38.325 --> 00:30:41.230 and you don't think about, is that element already 00:30:41.230 --> 00:30:43.090 in its most standard state? 00:30:43.090 --> 00:30:46.720 Perhaps it's zero and that's why it's not listed on the table. 00:30:46.720 --> 00:30:48.160 So keep this in mind. 00:30:48.160 --> 00:30:51.291 This can be very useful to remember. 00:30:51.291 --> 00:30:51.790 All right. 00:30:51.790 --> 00:30:55.250 One more thing is involved in the equation. 00:30:55.250 --> 00:30:56.860 We have glucose. 00:30:56.860 --> 00:30:59.980 So we can think about the reaction that forms glucose 00:30:59.980 --> 00:31:02.170 from elements in its most stable state, 00:31:02.170 --> 00:31:04.900 and we've actually talked about all these already. 00:31:04.900 --> 00:31:05.860 We have O2. 00:31:05.860 --> 00:31:07.660 That's in its most stable state. 00:31:07.660 --> 00:31:09.160 Carbon graphite. 00:31:09.160 --> 00:31:10.690 H2 gas. 00:31:10.690 --> 00:31:13.660 And so this reaction as written-- 00:31:13.660 --> 00:31:19.250 it has a heat of formation of minus 1,260 kilojoules 00:31:19.250 --> 00:31:20.940 per mole. 00:31:20.940 --> 00:31:24.140 So now we can calculate the delta H 00:31:24.140 --> 00:31:26.800 for the oxidation of glucose. 00:31:26.800 --> 00:31:33.020 The delta H of the reaction from these delta H's of formation. 00:31:33.020 --> 00:31:35.180 And here's the equation. 00:31:35.180 --> 00:31:38.810 Delta H of the reaction is equal to the sum 00:31:38.810 --> 00:31:42.020 of all of the delta H's of formation 00:31:42.020 --> 00:31:47.180 of the products minus the sum of the delta H's of formation 00:31:47.180 --> 00:31:50.250 of the reactants. 00:31:50.250 --> 00:31:54.620 So this now is one of the sources of confusion 00:31:54.620 --> 00:31:57.380 because if you're using bond enthalpies 00:31:57.380 --> 00:31:59.780 it's reactants minus products. 00:31:59.780 --> 00:32:02.300 If you're using delta H of formation 00:32:02.300 --> 00:32:04.880 it's products minus reactants. 00:32:04.880 --> 00:32:06.950 So that's why I spend a little time 00:32:06.950 --> 00:32:11.460 over here thinking about what's going on with the bond 00:32:11.460 --> 00:32:16.040 enthalpies, so hopefully no one will fall into this delta H 00:32:16.040 --> 00:32:18.890 pitfall over here and you'll keep 00:32:18.890 --> 00:32:22.520 the reactions-- the equations straight. 00:32:22.520 --> 00:32:24.140 So now we can plug it in. 00:32:24.140 --> 00:32:27.860 If you remember the equations this is pretty easy. 00:32:27.860 --> 00:32:31.880 So we have our delta H of reaction. 00:32:31.880 --> 00:32:35.750 We have 6 times the heat of formation 00:32:35.750 --> 00:32:37.280 of our products over here. 00:32:37.280 --> 00:32:38.420 CO2. 00:32:38.420 --> 00:32:41.450 6 times the first product and then 00:32:41.450 --> 00:32:44.840 6 times the second product, which is water, 00:32:44.840 --> 00:32:49.310 minus the first reactant, which is our glucose, 00:32:49.310 --> 00:32:54.150 and we have one of those, and we have 6 oxygens. 00:32:54.150 --> 00:32:56.960 So products minus reactants. 00:32:56.960 --> 00:32:59.330 Pay attention to the stoichiometry. 00:32:59.330 --> 00:33:02.210 You need to multiply the heats of formation 00:33:02.210 --> 00:33:06.860 by the number of molecules so then we can put in the values 00:33:06.860 --> 00:33:08.180 that we just saw. 00:33:08.180 --> 00:33:14.810 C02 minus 392 for our water minus 285 minus-- 00:33:14.810 --> 00:33:19.490 and here we have a minus 1,260 for glucose. 00:33:19.490 --> 00:33:22.550 And again, 6 times 0 because the oxygen is already 00:33:22.550 --> 00:33:24.380 in its most stable state. 00:33:24.380 --> 00:33:26.570 And if we do the math correctly, you 00:33:26.570 --> 00:33:31.050 get minus 2,816 kilojoules per mole, 00:33:31.050 --> 00:33:34.070 and that is exactly the experimental value. 00:33:34.070 --> 00:33:36.500 And it's because the heats of formation 00:33:36.500 --> 00:33:41.360 are also experimental, so this is a very precise number. 00:33:41.360 --> 00:33:43.190 When you use the heats of formation 00:33:43.190 --> 00:33:50.180 you're going to get a much closer value to experimental. 00:33:50.180 --> 00:33:52.280 And this was a bit easier than thinking 00:33:52.280 --> 00:33:54.530 about every bond that would be broken 00:33:54.530 --> 00:33:58.310 and every bond that would be formed. 00:33:58.310 --> 00:34:01.550 One more way that you can do this. 00:34:01.550 --> 00:34:03.800 And this takes advantage of something 00:34:03.800 --> 00:34:08.900 known as Hess's Law and the fact that enthalpy 00:34:08.900 --> 00:34:14.270 is a state function, which means that it's independent of path. 00:34:14.270 --> 00:34:16.429 So if you were climbing a mountain 00:34:16.429 --> 00:34:19.159 and you wanted to go from point A to point B, 00:34:19.159 --> 00:34:22.130 you could climb all the way up to the top and go back down 00:34:22.130 --> 00:34:26.830 or you could just go right from A to B and it wouldn't matter. 00:34:26.830 --> 00:34:30.230 Your delta H would be the same in both cases 00:34:30.230 --> 00:34:32.540 because it's independent of path. 00:34:32.540 --> 00:34:35.960 So it only matters what the values 00:34:35.960 --> 00:34:38.510 are for your reactants and your final products. 00:34:38.510 --> 00:34:41.030 It doesn't matter how you get from the reactants 00:34:41.030 --> 00:34:42.290 to the products. 00:34:42.290 --> 00:34:45.050 Delta H is going to be the same. 00:34:45.050 --> 00:34:48.800 And because of this, you can take different routes 00:34:48.800 --> 00:34:52.969 if their equations for different parts of your reaction 00:34:52.969 --> 00:34:55.010 that are already known with values of delta 00:34:55.010 --> 00:34:59.360 H-- you can add those equations together and then add together 00:34:59.360 --> 00:35:02.450 the delta H's to get a new value. 00:35:02.450 --> 00:35:06.380 So Hess's Law-- if there are two or more equations that 00:35:06.380 --> 00:35:09.260 are added to give another chemical equations 00:35:09.260 --> 00:35:13.520 then you can add up the delta H for the reactions of each 00:35:13.520 --> 00:35:16.040 of the individual equations to get 00:35:16.040 --> 00:35:19.140 the sum for your new equation. 00:35:19.140 --> 00:35:25.170 So let's do this now, again, for glucose and oxygen. 00:35:25.170 --> 00:35:27.880 So if we have these three equations 00:35:27.880 --> 00:35:30.260 here-- this one is showing glucose 00:35:30.260 --> 00:35:33.470 plus oxygen being broken down to the elements that 00:35:33.470 --> 00:35:35.090 are in the most stable state. 00:35:35.090 --> 00:35:38.900 So graphite, H2 and O2 for glucose. 00:35:38.900 --> 00:35:41.730 And then our 6 O2s are there on both sides 00:35:41.730 --> 00:35:44.400 because it's already in the most stable state. 00:35:44.400 --> 00:35:47.690 We're going to be forming CO2 from the elements in the most 00:35:47.690 --> 00:35:50.210 stable state and also water. 00:35:50.210 --> 00:35:53.300 So we can add these together, paying attention 00:35:53.300 --> 00:35:55.520 to the stoichiometry. 00:35:55.520 --> 00:35:59.810 So we need to multiply this equation by 6 and this equation 00:35:59.810 --> 00:36:04.000 by 6, and then we should be able to do some canceling 00:36:04.000 --> 00:36:07.630 and make sure that we're getting our equation of interest. 00:36:07.630 --> 00:36:12.080 So we can cancel these 6 O2s with these, 00:36:12.080 --> 00:36:16.130 we can cancel these O2s with these, 00:36:16.130 --> 00:36:19.850 and we can cancel this H2 with this. 00:36:19.850 --> 00:36:24.770 And that leaves us with glucose plus 6 oxygens going 00:36:24.770 --> 00:36:29.360 to 6 CO2s and 6 waters. 00:36:29.360 --> 00:36:31.820 So this is going to work now. 00:36:31.820 --> 00:36:35.300 And now, since we added this together to get this, 00:36:35.300 --> 00:36:38.360 we can add our delta H of reactions 00:36:38.360 --> 00:36:42.440 together to get a new delta H of reaction. 00:36:42.440 --> 00:36:43.610 Oh, sorry. 00:36:43.610 --> 00:36:45.190 I forgot to cancel my graphites. 00:36:45.190 --> 00:36:45.870 There we go. 00:36:45.870 --> 00:36:47.420 Now we're good. 00:36:47.420 --> 00:36:49.310 Didn't notice them there. 00:36:49.310 --> 00:36:52.210 So our delta H for reaction. 00:36:52.210 --> 00:36:57.710 We saw before that the formation of CO2 from elements 00:36:57.710 --> 00:36:59.534 in its most stable state was minus, 00:36:59.534 --> 00:37:01.700 so now we've just changed the sign because now we're 00:37:01.700 --> 00:37:03.660 going the opposite direction. 00:37:03.660 --> 00:37:07.880 So we have a positive value for that delta H of reaction. 00:37:07.880 --> 00:37:11.210 Now we have 6 times the heat of formation 00:37:11.210 --> 00:37:16.070 of CO2 and 6 times the heat of formation of water 00:37:16.070 --> 00:37:17.870 because that's what those equations are. 00:37:17.870 --> 00:37:20.170 Those are the heat of formation reactions. 00:37:20.170 --> 00:37:25.770 And if we add this all together then we get the number 00:37:25.770 --> 00:37:28.170 that we saw before. 00:37:28.170 --> 00:37:30.470 So it doesn't matter what path we take. 00:37:30.470 --> 00:37:32.570 We're going to get to that same answer. 00:37:32.570 --> 00:37:34.740 And this one, since we're using information 00:37:34.740 --> 00:37:36.770 that all has to do with heat of formation, 00:37:36.770 --> 00:37:39.560 it's not really very different from the one we did before. 00:37:39.560 --> 00:37:42.350 But you can use Hess's Law for delta H 00:37:42.350 --> 00:37:46.520 of reactions that are not heats of formation. 00:37:46.520 --> 00:37:49.940 If equations are available that can be added 00:37:49.940 --> 00:37:52.310 or summed to get your net reaction, 00:37:52.310 --> 00:37:56.840 then you can add or subtract these values to get a new delta 00:37:56.840 --> 00:38:03.980 H. Don't forget kilojoules per mole. 00:38:03.980 --> 00:38:07.130 So we have our three different ways-- bond enthalpies, 00:38:07.130 --> 00:38:10.450 heat of formation, and Hess's Law.