WEBVTT 00:00:00.030 --> 00:00:02.400 The following content is provided under a Creative 00:00:02.400 --> 00:00:03.780 Commons license. 00:00:03.780 --> 00:00:06.020 Your support will help MIT OpenCourseWare 00:00:06.020 --> 00:00:10.090 continue to offer high quality educational resources for free. 00:00:10.090 --> 00:00:12.670 To make a donation or to view additional materials 00:00:12.670 --> 00:00:16.580 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.580 --> 00:00:17.963 at ocw.mit.edu. 00:00:26.204 --> 00:00:27.620 CATHY DRENNAN: So now, we're going 00:00:27.620 --> 00:00:34.220 to move on to talk about spontaneous change. 00:00:34.220 --> 00:00:36.076 And so this is today's handout. 00:00:43.670 --> 00:00:46.790 So spontaneous reaction is a reaction 00:00:46.790 --> 00:00:50.010 that proceeds in the forward direction 00:00:50.010 --> 00:00:52.600 without any kind of outside intervention, 00:00:52.600 --> 00:00:55.180 like heat being added, for example. 00:00:55.180 --> 00:01:00.630 It just goes in that direction. 00:01:04.819 --> 00:01:09.170 So we can talk about the following reactions 00:01:09.170 --> 00:01:13.880 are spontaneous at constant pressure. 00:01:13.880 --> 00:01:15.440 And we'll see later that temperature 00:01:15.440 --> 00:01:18.180 can make a difference between whether something's spontaneous 00:01:18.180 --> 00:01:19.570 or not. 00:01:19.570 --> 00:01:24.850 But constant pressure, here's an example, iron plus oxygen. 00:01:24.850 --> 00:01:28.310 And what is this in layman's term an example of? 00:01:28.310 --> 00:01:29.030 AUDIENCE: Rust. 00:01:29.030 --> 00:01:31.080 CATHY DRENNAN: Rust, yes. 00:01:31.080 --> 00:01:33.100 And many of you are probably aware of this, 00:01:33.100 --> 00:01:35.850 rust is a spontaneous process. 00:01:35.850 --> 00:01:38.270 It's something that people try to do something about. 00:01:38.270 --> 00:01:40.110 You don't want your car to rust. 00:01:40.110 --> 00:01:42.740 If you're new to New England and you're 00:01:42.740 --> 00:01:48.750 from a part of the country that doesn't get so cold, 00:01:48.750 --> 00:01:51.000 you'll look at people's cars and you're like, wow, 00:01:51.000 --> 00:01:52.500 look at all that rust all over them. 00:01:52.500 --> 00:01:55.720 Yes, rust happens, especially in New England. 00:01:55.720 --> 00:01:58.640 And delta H here is negative. 00:01:58.640 --> 00:02:00.575 Is this endothermic or exothermic? 00:02:00.575 --> 00:02:01.450 AUDIENCE: Exothermic. 00:02:01.450 --> 00:02:05.840 CATHY DRENNAN: Exothermic, minus 824. 00:02:05.840 --> 00:02:09.990 Here is another spontaneous process. 00:02:09.990 --> 00:02:13.270 This molecule is ATP. 00:02:13.270 --> 00:02:17.890 And it will hydrolyze, which means react with water, 00:02:17.890 --> 00:02:19.440 forming ADP. 00:02:19.440 --> 00:02:22.070 So ATP is triphosphate. 00:02:22.070 --> 00:02:24.140 ADP is diphosphate. 00:02:24.140 --> 00:02:25.890 So one of the phosphates-- and here's 00:02:25.890 --> 00:02:27.930 the phosphate-- comes off. 00:02:27.930 --> 00:02:29.860 It hydrolyzes off. 00:02:29.860 --> 00:02:34.780 And this is a spontaneous process. 00:02:34.780 --> 00:02:40.700 And we also have a delta H0 of minus 24 kilojoules per mole. 00:02:40.700 --> 00:02:43.380 And remember, when we oxidize glucose in our body, 00:02:43.380 --> 00:02:45.940 we store that energy in ATP. 00:02:45.940 --> 00:02:49.150 And we want that ATP to be around. 00:02:49.150 --> 00:02:53.190 And then when we break ATP apart, it releases the energy. 00:02:53.190 --> 00:02:56.160 So this is a very important biological process. 00:02:56.160 --> 00:02:59.690 And you have a negative value, exothermic reaction. 00:02:59.690 --> 00:03:04.570 But there's a few other examples of spontaneous reactions. 00:03:04.570 --> 00:03:06.910 One of them is this one. 00:03:06.910 --> 00:03:09.164 And we've probably all experienced this. 00:03:09.164 --> 00:03:10.580 If you're from New England, you've 00:03:10.580 --> 00:03:16.110 seen snow melt or ice melt, solid to liquid. 00:03:16.110 --> 00:03:18.150 If you're from a hot part of the world, 00:03:18.150 --> 00:03:21.470 you probably had ice cubes in your nice, refreshing drink 00:03:21.470 --> 00:03:23.850 with maybe a little umbrella on the top. 00:03:23.850 --> 00:03:28.220 Anyway, everyone, I think, has seen ice melt. 00:03:28.220 --> 00:03:31.820 But here, delta H0 is of positive value. 00:03:31.820 --> 00:03:34.710 It's endothermic. 00:03:34.710 --> 00:03:37.370 Also, if you have ammonium nitrate, 00:03:37.370 --> 00:03:41.680 this will just come apart in a spontaneous reaction. 00:03:41.680 --> 00:03:46.110 Delta H here is plus 28 kilojoules per mole. 00:03:46.110 --> 00:03:52.880 So is delta H the key to spontaneity? 00:03:52.880 --> 00:03:58.550 It is not-- plays into it, but it is not 00:03:58.550 --> 00:04:01.440 the determining factor. 00:04:01.440 --> 00:04:06.170 So if delta H0 is not the key to spontaneity, what is? 00:04:08.790 --> 00:04:13.680 It is free energy, yes, particularly Gibbs free energy, 00:04:13.680 --> 00:04:19.760 or delta G. And I'm really happy they decided 00:04:19.760 --> 00:04:24.020 to add the Gibbs free energy, because another thing of energy 00:04:24.020 --> 00:04:26.190 would be a lot. 00:04:26.190 --> 00:04:29.435 So having this free energy having abbreviation of G, 00:04:29.435 --> 00:04:33.320 I think is a good thing-- so Gibbs free energy. 00:04:33.320 --> 00:04:37.260 So Gibbs free energy depends on delta H. 00:04:37.260 --> 00:04:40.150 But it also depends on another term, 00:04:40.150 --> 00:04:48.070 which is T delta S-- temperature and delta S, change in entropy. 00:04:48.070 --> 00:04:52.180 So delta G is the predictor of whether a reaction will 00:04:52.180 --> 00:04:57.980 go in the forward direction in a spontaneous fashion or not. 00:04:57.980 --> 00:05:00.840 So let's just think about the sign of delta G 00:05:00.840 --> 00:05:03.120 and what it means. 00:05:03.120 --> 00:05:06.230 So again, at constant temperature and pressure 00:05:06.230 --> 00:05:12.410 here, delta G less than 0, negative delta G, 00:05:12.410 --> 00:05:13.960 is that spontaneous or not? 00:05:13.960 --> 00:05:15.196 AUDIENCE: Spontaneous. 00:05:15.196 --> 00:05:18.480 CATHY DRENNAN: Spontaneous. 00:05:18.480 --> 00:05:24.750 Positive delta G is not spontaneous, non-spontaneous. 00:05:24.750 --> 00:05:27.110 And delta G equals 0. 00:05:27.110 --> 00:05:29.840 It's one of the other things that I am very fond of, 00:05:29.840 --> 00:05:31.642 which is equilibrium. 00:05:34.200 --> 00:05:38.530 So delta G indicates whether something is spontaneous 00:05:38.530 --> 00:05:39.070 or not. 00:05:39.070 --> 00:05:42.020 Negative value, spontaneous in the forward direction. 00:05:42.020 --> 00:05:45.870 Positive value, not spontaneous in the forward direction. 00:05:45.870 --> 00:05:51.400 And equilibrium, the thing we all try to reach in our lives. 00:05:51.400 --> 00:05:54.800 So let's look at an example and calculate 00:05:54.800 --> 00:05:56.950 what delta G is going to be. 00:05:56.950 --> 00:06:00.100 So we saw this equation already. 00:06:00.100 --> 00:06:02.810 We have a positive delta H0. 00:06:02.810 --> 00:06:05.980 And now, I'm telling you that delta S0 is also 00:06:05.980 --> 00:06:07.940 a positive value. 00:06:07.940 --> 00:06:10.100 So we can use this equation. 00:06:10.100 --> 00:06:13.840 And this is really one of the most important equations 00:06:13.840 --> 00:06:15.240 in chemistry. 00:06:15.240 --> 00:06:20.520 Figuring out this equation was really a crowning achievement. 00:06:20.520 --> 00:06:22.370 And you'll be using it a lot. 00:06:22.370 --> 00:06:24.450 Not just in this unit, but pretty much 00:06:24.450 --> 00:06:28.630 in every unit from now on, you will be using this equation. 00:06:28.630 --> 00:06:31.459 So room temperature, pretty much we're not doing-- occasionally, 00:06:31.459 --> 00:06:33.250 we'll do something not at room temperature, 00:06:33.250 --> 00:06:35.560 but we like room temperature. 00:06:35.560 --> 00:06:38.360 And we like it in Kelvin. 00:06:38.360 --> 00:06:42.030 So delta G0, so we plug in our delta H value. 00:06:42.030 --> 00:06:45.940 So it's going to equal delta H minus the temperature. 00:06:45.940 --> 00:06:48.320 And if the temperature isn't given in a problem, 00:06:48.320 --> 00:06:52.130 you can assume that it's 298. 00:06:52.130 --> 00:06:54.380 And now we need to plug in delta S. 00:06:54.380 --> 00:06:57.280 But I left a blank here to make a point, which 00:06:57.280 --> 00:07:01.400 is that delta S's are almost always given 00:07:01.400 --> 00:07:04.930 in joules per kelvin per mole. 00:07:04.930 --> 00:07:07.620 But everything else is given in kilojoules. 00:07:07.620 --> 00:07:10.005 So you want to make sure you convert your units, 00:07:10.005 --> 00:07:13.470 or you're going to come up with very funky answers at the end. 00:07:13.470 --> 00:07:20.000 So from joules to kilojoules, so now plus 0.109 kilojoules 00:07:20.000 --> 00:07:23.110 per kelvin per mole. 00:07:23.110 --> 00:07:24.750 And we can do this out now. 00:07:24.750 --> 00:07:29.830 So we have plus 28 minus 32.48. 00:07:29.830 --> 00:07:32.700 And why don't you tell me how many significant figures 00:07:32.700 --> 00:07:33.881 this answer has. 00:07:52.300 --> 00:07:53.190 10 more seconds. 00:08:11.000 --> 00:08:14.630 So at least some people got it right. 00:08:14.630 --> 00:08:20.680 We've identified once again a weakness, so rules 00:08:20.680 --> 00:08:23.060 of adding and subtracting. 00:08:23.060 --> 00:08:26.510 So we have 28 here minus 32. 00:08:26.510 --> 00:08:30.260 There are no significant figures after the decimal point here. 00:08:30.260 --> 00:08:33.720 So we're just left with 4. 00:08:33.720 --> 00:08:37.010 So when we're doing multiplication or division, 00:08:37.010 --> 00:08:40.520 we consider the total number of significant figures. 00:08:40.520 --> 00:08:42.539 But with addition and subtraction, 00:08:42.539 --> 00:08:46.950 you gotta pay attention to where the decimal point is. 00:08:46.950 --> 00:08:51.880 And when we get into the next unit, there are logs. 00:08:51.880 --> 00:08:56.410 And those have special rules of significant figures. 00:08:56.410 --> 00:09:00.050 Yes, very exciting. 00:09:00.050 --> 00:09:05.780 So delta G0 is negative here, although delta H is positive. 00:09:05.780 --> 00:09:08.650 So this reaction is spontaneous. 00:09:08.650 --> 00:09:09.900 It's not hugely. 00:09:09.900 --> 00:09:15.270 It's a pretty small number, but still, it's spontaneous. 00:09:15.270 --> 00:09:17.230 So let's consider our friend over 00:09:17.230 --> 00:09:20.990 here that we've been talking about-- glucose being oxidized 00:09:20.990 --> 00:09:22.510 to CO2 and water. 00:09:22.510 --> 00:09:25.180 You practically should have the delta H memorized 00:09:25.180 --> 00:09:26.560 for that at this point. 00:09:26.560 --> 00:09:29.370 Now, I'm telling you what the delta S0 is. 00:09:29.370 --> 00:09:34.740 And it's positive 233 joules per kelvin per mole. 00:09:34.740 --> 00:09:37.070 And we can plug this into our equation 00:09:37.070 --> 00:09:42.520 to calculate a delta G0, again remembering to convert joules 00:09:42.520 --> 00:09:45.730 to kilojoules to do this. 00:09:45.730 --> 00:09:54.970 And so now, we see that it has a very negative delta G0 here, 00:09:54.970 --> 00:10:01.110 minus 2,885 kilojoules per mole at room temperature. 00:10:01.110 --> 00:10:03.450 So at room temperature, this reaction 00:10:03.450 --> 00:10:05.300 is spontaneous but slow. 00:10:05.300 --> 00:10:09.200 We saw that with the candies that had glucose in it. 00:10:09.200 --> 00:10:12.820 We opened them up, and no water or CO2 00:10:12.820 --> 00:10:16.890 were obviously being liberated in this reaction, 00:10:16.890 --> 00:10:19.230 because it is slow. 00:10:19.230 --> 00:10:20.930 And now, a clicker question. 00:10:20.930 --> 00:10:23.470 I want you to tell me whether it would 00:10:23.470 --> 00:10:27.345 be spontaneous at different temperatures or not? 00:10:45.860 --> 00:10:46.820 10 more seconds. 00:11:02.936 --> 00:11:03.920 Yep. 00:11:03.920 --> 00:11:07.300 So it is spontaneous at all temperatures. 00:11:07.300 --> 00:11:10.470 Not all reactions are, but this one is. 00:11:10.470 --> 00:11:13.060 So if we go back here, the reaction 00:11:13.060 --> 00:11:15.550 is spontaneous at all temperatures. 00:11:15.550 --> 00:11:18.220 And that's because, to be spontaneous, 00:11:18.220 --> 00:11:20.720 you want a delta G that's negative. 00:11:20.720 --> 00:11:25.150 If delta H is negative and delta S is positive, 00:11:25.150 --> 00:11:27.590 then you'll have a negative minus a negative. 00:11:27.590 --> 00:11:29.740 So it doesn't matter what temperature is. 00:11:29.740 --> 00:11:33.940 This will always yield a negative delta G0. 00:11:33.940 --> 00:11:36.980 So other reactions, that might not be the case. 00:11:36.980 --> 00:11:42.510 But if you have negative delta H and a positive delta S, 00:11:42.510 --> 00:11:43.660 it will be spontaneous. 00:11:43.660 --> 00:11:47.080 So negative delta H, again, exothermic, heat release. 00:11:47.080 --> 00:11:50.280 And a positive entropy is a favorable thing. 00:11:50.280 --> 00:11:52.640 Entropy is always increasing. 00:11:52.640 --> 00:11:56.110 So if this reaction has increased in entropy, 00:11:56.110 --> 00:11:59.790 it will be much more likely to be spontaneous. 00:11:59.790 --> 00:12:02.560 So let's talk about entropy. 00:12:02.560 --> 00:12:06.300 So entropy is a measure of disorder of a system. 00:12:06.300 --> 00:12:10.100 Delta S is the change in entropy. 00:12:10.100 --> 00:12:14.090 And delta S, again, is a state function. 00:12:14.090 --> 00:12:18.090 So one example of entropy in New England 00:12:18.090 --> 00:12:23.320 are these stone walls that do not look absolutely beautiful. 00:12:23.320 --> 00:12:26.550 There are often stones falling everywhere. 00:12:26.550 --> 00:12:29.860 And it doesn't matter if these stone walls that were probably 00:12:29.860 --> 00:12:35.080 built in 1600s or 1700s in New England fell totally apart 00:12:35.080 --> 00:12:40.270 and were rebuilt, now we just care about how the wall is 00:12:40.270 --> 00:12:42.080 compared to the way it started. 00:12:42.080 --> 00:12:44.990 So delta S, again, is a state function. 00:12:44.990 --> 00:12:47.310 It doesn't depend on path. 00:12:47.310 --> 00:12:49.470 And so if you get out and walk around 00:12:49.470 --> 00:12:51.870 and go like on the Minuteman Trail 00:12:51.870 --> 00:12:54.230 and see some of the historical sites 00:12:54.230 --> 00:12:58.490 where Paul Revere rode his horse along a lot of stone walls, 00:12:58.490 --> 00:13:01.030 there's a New England poet who writes 00:13:01.030 --> 00:13:02.480 about this, Robert Frost. 00:13:02.480 --> 00:13:07.310 And he said, "something there is that doesn't love a wall." 00:13:07.310 --> 00:13:09.497 And that something is entropy. 00:13:09.497 --> 00:13:10.330 AUDIENCE: [LAUGHTER] 00:13:10.330 --> 00:13:13.450 CATHY DRENNAN: Entropy does not love a wall. 00:13:13.450 --> 00:13:16.840 Entropy does not like order. 00:13:16.840 --> 00:13:20.760 Another example, those of you who are learning more about me 00:13:20.760 --> 00:13:26.050 as a person know that I am a fan of dogs. 00:13:26.050 --> 00:13:28.560 This is my dog Shep. 00:13:28.560 --> 00:13:32.460 Shep does not like going to the groomers, does not like it. 00:13:32.460 --> 00:13:35.620 And I think that this is because he's been at my office hours 00:13:35.620 --> 00:13:39.550 and he knows that increasing entropy is favorable, 00:13:39.550 --> 00:13:41.700 decreasing entropy is not. 00:13:41.700 --> 00:13:45.050 And he says, really, this violates 00:13:45.050 --> 00:13:47.460 the laws of thermodynamics, what you're doing to. 00:13:47.460 --> 00:13:50.490 Me and you should cease and desist. 00:13:50.490 --> 00:13:54.970 But anyway, he still get haircuts. 00:13:54.970 --> 00:13:57.190 So entropy, again, is this measure 00:13:57.190 --> 00:13:59.250 of disorder of a system. 00:13:59.250 --> 00:14:02.570 You have a positive delta S, which 00:14:02.570 --> 00:14:05.880 is going to be an increase in disorder. 00:14:05.880 --> 00:14:12.220 And a negative delta S is going to be a decrease in disorder. 00:14:12.220 --> 00:14:14.970 And disorder, you can be thinking 00:14:14.970 --> 00:14:18.540 about this as internal degrees of freedom in your molecule, 00:14:18.540 --> 00:14:20.650 thinking about this as vibrations. 00:14:20.650 --> 00:14:22.400 All sorts of different things can 00:14:22.400 --> 00:14:27.800 lead to increase or decrease in entropy. 00:14:27.800 --> 00:14:31.160 But we often think about changes in entropy 00:14:31.160 --> 00:14:34.570 depending on if the reaction is changing in phase. 00:14:34.570 --> 00:14:40.300 So gas molecules have greater disorder than liquid. 00:14:40.300 --> 00:14:44.110 And liquid has greater disorder than solids. 00:14:44.110 --> 00:14:48.070 And so a solid has all its molecules lined up. 00:14:48.070 --> 00:14:50.160 And liquid can move around a little bit more. 00:14:50.160 --> 00:14:52.930 But gas really can spread all out. 00:14:52.930 --> 00:14:56.630 So in terms of entropy and changes in entropy, 00:14:56.630 --> 00:14:59.910 we can think about the phase change that's happening 00:14:59.910 --> 00:15:02.280 and even predict if something's going to be 00:15:02.280 --> 00:15:04.640 an increase in entropy or not. 00:15:04.640 --> 00:15:07.270 So let's just look at one example. 00:15:07.270 --> 00:15:11.890 So without a calculation, predict the sign of delta S. 00:15:11.890 --> 00:15:13.208 And this is a clicker question. 00:15:37.930 --> 00:15:39.690 Let's just take 10 more seconds. 00:15:39.690 --> 00:15:41.800 And can our demo TAs come down? 00:15:55.700 --> 00:15:58.860 Yep, good. 00:15:58.860 --> 00:16:02.880 So you predicted positive, which is the correct answer. 00:16:02.880 --> 00:16:07.570 And so here, we're going from a liquid to a liquid and a gas. 00:16:07.570 --> 00:16:10.950 And so going to the gas, that will increase 00:16:10.950 --> 00:16:12.410 the disorder of the system. 00:16:12.410 --> 00:16:14.370 So delta S will be positive. 00:16:14.370 --> 00:16:16.620 So now, we're actually going to do a demo 00:16:16.620 --> 00:16:19.130 of this particular reaction. 00:16:19.130 --> 00:16:21.710 And so we have hydrogen peroxide, 00:16:21.710 --> 00:16:26.630 which can just be bought at a CVS or local drugstore. 00:16:26.630 --> 00:16:32.380 And it will go to liquid water and also oxygen gas. 00:16:32.380 --> 00:16:34.680 And so how do you see a gas? 00:16:34.680 --> 00:16:38.800 And you can see it by putting it in with soap bubbles. 00:16:38.800 --> 00:16:42.210 So as bubbles of oxygen form, the soap bubbles 00:16:42.210 --> 00:16:43.530 will bubble out. 00:16:43.530 --> 00:16:44.840 And so you can see it. 00:16:44.840 --> 00:16:48.540 And you can also add some kind of food color. 00:16:48.540 --> 00:16:50.750 And we have yeast as a catalyst to make 00:16:50.750 --> 00:16:52.530 it go a little bit faster. 00:16:52.530 --> 00:16:56.415 So let's see if we can actually see disorder increase. 00:17:01.420 --> 00:17:02.590 I don't want the mic. 00:17:06.720 --> 00:17:10.288 If you want to just say-- if you want to talk at the same time, 00:17:10.288 --> 00:17:10.829 here's a mic. 00:17:10.829 --> 00:17:11.749 AUDIENCE: I might do that. 00:17:11.749 --> 00:17:13.130 CATHY DRENNAN: You're not going to do that, OK. 00:17:13.130 --> 00:17:13.260 AUDIENCE: Yeah, we will. 00:17:13.260 --> 00:17:13.589 CATHY DRENNAN: Oh, you do. 00:17:13.589 --> 00:17:14.089 OK. 00:17:14.089 --> 00:17:15.910 AUDIENCE: Is this on? 00:17:15.910 --> 00:17:16.440 This on? 00:17:16.440 --> 00:17:16.990 Yes, it is. 00:17:16.990 --> 00:17:17.630 OK, great. 00:17:17.630 --> 00:17:22.722 So what we have going on here is we've got this container. 00:17:22.722 --> 00:17:23.680 It's filled with water. 00:17:23.680 --> 00:17:28.470 And what I did was I added about 4 teaspoons of yeast. 00:17:28.470 --> 00:17:31.550 The yeast, as Cathy said, is going to act as a catalyst. 00:17:31.550 --> 00:17:37.129 It's actually a biological species. 00:17:37.129 --> 00:17:38.920 It's a living species that's actually going 00:17:38.920 --> 00:17:40.190 to catalyze this reaction. 00:17:40.190 --> 00:17:43.280 What Erik is doing is Erik is pouring some hydrogen peroxide. 00:17:43.280 --> 00:17:44.870 He added some soap. 00:17:44.870 --> 00:17:47.120 So as you see in the reaction, the H2O2 00:17:47.120 --> 00:17:50.420 is going to break down into water and gas-- 00:17:50.420 --> 00:17:51.582 the gas being oxygen. 00:17:51.582 --> 00:17:53.040 And what we don't want to happen is 00:17:53.040 --> 00:17:54.910 we don't want just the gas to escape, 00:17:54.910 --> 00:17:56.368 because then you guys can't see it. 00:17:56.368 --> 00:17:58.780 So what Erik is doing right now is he's adding some soap. 00:17:58.780 --> 00:18:01.500 The soap is actually going to catch, if you will, 00:18:01.500 --> 00:18:06.740 the escaping gas and turn it into a foam. 00:18:06.740 --> 00:18:08.290 And what we should be able to see 00:18:08.290 --> 00:18:13.250 is the foam kind of escape from this container. 00:18:13.250 --> 00:18:14.351 You ready? 00:18:14.351 --> 00:18:14.850 OK. 00:18:14.850 --> 00:18:16.058 So hopefully, this will work. 00:18:16.058 --> 00:18:17.722 We should put on our goggles. 00:18:17.722 --> 00:18:19.310 [LAUGHTER] 00:18:19.310 --> 00:18:20.430 Smells really bad. 00:18:20.430 --> 00:18:22.250 OK, ready? 00:18:22.250 --> 00:18:28.500 And-- get out of there, look at that. 00:18:28.500 --> 00:18:29.190 Hey! 00:18:29.190 --> 00:18:30.731 Wow, that worked a lot better than we 00:18:30.731 --> 00:18:32.105 thought it was going to work. 00:18:32.105 --> 00:18:33.730 CATHY DRENNAN: And so this is sometimes 00:18:33.730 --> 00:18:36.240 called the elephant toothpaste demo, 00:18:36.240 --> 00:18:39.620 because that is sort of, if you were an elephant, what 00:18:39.620 --> 00:18:43.720 you would probably be brushing your teeth with, I don't know. 00:18:43.720 --> 00:18:44.460 Yes. 00:18:44.460 --> 00:18:47.314 So this is-- 00:18:47.314 --> 00:18:48.070 [APPLAUSE] 00:18:48.070 --> 00:18:49.644 --entropy increasing. 00:18:55.080 --> 00:18:58.830 So let's just see if we can quickly 00:18:58.830 --> 00:19:01.980 talk a little more about entropy and then we'll end. 00:19:01.980 --> 00:19:05.340 So entropy of reactions can be calculated 00:19:05.340 --> 00:19:07.400 from absolute values. 00:19:07.400 --> 00:19:12.280 And again, we can use this equation here. 00:19:12.280 --> 00:19:19.800 So we have a delta S for a particular reaction, 00:19:19.800 --> 00:19:22.400 can be calculated from the delta S's 00:19:22.400 --> 00:19:24.560 of the product minus reactants. 00:19:24.560 --> 00:19:27.250 So again, we have products minus reactants. 00:19:27.250 --> 00:19:32.130 The absolute value, or an absolute delta S, S 00:19:32.130 --> 00:19:37.910 equals 0 for a perfect crystal at a temperature of 0 kelvin. 00:19:37.910 --> 00:19:40.180 You never really talk about S by itself. 00:19:40.180 --> 00:19:45.171 It's always really delta S. And S of 0, 00:19:45.171 --> 00:19:47.420 this is like the saddest thing for a crystallographer, 00:19:47.420 --> 00:19:48.878 because you know you're never going 00:19:48.878 --> 00:19:52.290 to have a perfect crystal, even if you go to 0 kelvin, 00:19:52.290 --> 00:19:54.760 I feel like at least experimentally. 00:19:54.760 --> 00:19:58.960 So S equals 0, to me that's kind of sad. 00:19:58.960 --> 00:20:04.860 So if we just put in for this reaction that we just did, 00:20:04.860 --> 00:20:08.700 we can put in our values here. 00:20:08.700 --> 00:20:13.570 And we can put in we're forming liquid water. 00:20:13.570 --> 00:20:16.200 And we're forming O2 gas. 00:20:16.200 --> 00:20:24.140 And we're using two molecules of hydrogen peroxide-- H2 O2 here. 00:20:24.140 --> 00:20:29.990 And so now, we can calculate what that delta S0 is. 00:20:29.990 --> 00:20:35.590 And it's a value of 125 joules per kelvin per mole. 00:20:35.590 --> 00:20:39.950 So again, products, water and gas, minus reactants, 00:20:39.950 --> 00:20:42.240 pay attention to the stoichiometry, 00:20:42.240 --> 00:20:45.590 and you can get your delta S value. 00:20:45.590 --> 00:20:47.120 And why is it positive? 00:20:47.120 --> 00:20:49.380 Again, we already talked about this. 00:20:49.380 --> 00:20:54.850 It's because it's going from liquid to a liquid plus a gas. 00:20:54.850 --> 00:20:58.390 And then, if we plug these values in again 00:20:58.390 --> 00:21:00.690 to see if it's spontaneous, we can 00:21:00.690 --> 00:21:03.990 use this equation and plug in our values, 00:21:03.990 --> 00:21:07.480 making sure we change our units. 00:21:07.480 --> 00:21:12.310 And we can see that, in fact, this is a spontaneous reaction, 00:21:12.310 --> 00:21:14.150 because it's negative here. 00:21:14.150 --> 00:21:16.180 But you already knew that, because you 00:21:16.180 --> 00:21:18.510 watched it go spontaneously. 00:21:18.510 --> 00:21:21.690 So most of the time, you can't do the demo. 00:21:21.690 --> 00:21:25.840 So then you can use this awesome equation right here. 00:21:25.840 --> 00:21:29.040 So that's where we're stopping for today. 00:21:29.040 --> 00:21:32.330 And we'll see you all on Friday. 00:21:32.330 --> 00:21:39.180 So if you take out your Lecture 16 notes, the bottom of page 3, 00:21:39.180 --> 00:21:42.630 we had an example about the melting of ice 00:21:42.630 --> 00:21:45.950 at room temperature. 00:21:45.950 --> 00:21:49.930 So we did a little demo for you at the end of class last time 00:21:49.930 --> 00:21:54.000 and calculated that the reaction was 00:21:54.000 --> 00:21:57.700 spontaneous for hydrogen-- hydrogen 00:21:57.700 --> 00:21:59.580 peroxide is pretty reactive. 00:21:59.580 --> 00:22:02.090 And we watched the O2 bubble go. 00:22:02.090 --> 00:22:03.680 And we did that calculation. 00:22:03.680 --> 00:22:05.920 So we're thinking about, not just delta H, 00:22:05.920 --> 00:22:07.800 but we're thinking about delta S. 00:22:07.800 --> 00:22:09.820 And we're now thinking about delta G as well 00:22:09.820 --> 00:22:11.610 and how they all play together. 00:22:11.610 --> 00:22:15.380 So when we started last lecture, we 00:22:15.380 --> 00:22:17.920 had talked about the fact of some reactions that 00:22:17.920 --> 00:22:20.170 were spontaneous where delta H was negative, 00:22:20.170 --> 00:22:22.790 where it was exothermic, where heat was released. 00:22:22.790 --> 00:22:24.630 But then we also gave some examples 00:22:24.630 --> 00:22:26.860 where delta H was positive and said, 00:22:26.860 --> 00:22:29.220 but these are also spontaneous reactions. 00:22:29.220 --> 00:22:32.060 We all know that at room temperature ice will melt. 00:22:32.060 --> 00:22:34.610 We know that that's a spontaneous reaction. 00:22:34.610 --> 00:22:36.550 But the delta H for that reaction 00:22:36.550 --> 00:22:38.220 is actually a positive value. 00:22:38.220 --> 00:22:40.360 It's an endothermic reaction. 00:22:40.360 --> 00:22:42.300 So when we're thinking about these reactions 00:22:42.300 --> 00:22:45.295 and spontaneity, we have to be thinking about delta G, 00:22:45.295 --> 00:22:50.850 no just delta H. And delta G has to do with delta H and delta S. 00:22:50.850 --> 00:22:53.470 So sometimes, delta S is the driving force 00:22:53.470 --> 00:22:56.330 behind whether a reaction is going to be spontaneous. 00:22:56.330 --> 00:22:59.430 Whether the delta G will be negative or positive, 00:22:59.430 --> 00:23:02.320 delta S is making that determination. 00:23:02.320 --> 00:23:05.820 So we can calculate what a delta S for reaction 00:23:05.820 --> 00:23:10.440 is if we know the entropy values for the products. 00:23:10.440 --> 00:23:12.740 And it's the sum of the entropy values 00:23:12.740 --> 00:23:16.270 for the products minus the sum for the reactants. 00:23:16.270 --> 00:23:20.000 So when we're doing heats of formation, 00:23:20.000 --> 00:23:22.210 we also had products minus reactants. 00:23:22.210 --> 00:23:24.320 But we have one exception to this products 00:23:24.320 --> 00:23:27.705 minus reactant rule, and that's when we're using what? 00:23:27.705 --> 00:23:32.038 What thing are we going to do reactant minus products? 00:23:32.038 --> 00:23:32.986 AUDIENCE: Bond-- 00:23:32.986 --> 00:23:33.819 CATHY DRENNAN: Bond? 00:23:33.819 --> 00:23:34.882 AUDIENCE: Bond enthalpy. 00:23:34.882 --> 00:23:37.540 CATHY DRENNAN: Enthalpy, right. 00:23:37.540 --> 00:23:39.740 But here, we're products minus reactants. 00:23:39.740 --> 00:23:42.370 So we can plug those numbers in. 00:23:42.370 --> 00:23:45.500 Our product is our liquid water. 00:23:45.500 --> 00:23:50.050 Our reactant is our solid water, or our ice. 00:23:50.050 --> 00:23:55.950 And we can calculate what the delta S0 is for this reaction. 00:23:55.950 --> 00:23:57.390 We can put in our values. 00:23:57.390 --> 00:24:03.470 And we get a positive value, positive 28.59 joules 00:24:03.470 --> 00:24:05.850 per kelvin per mole. 00:24:05.850 --> 00:24:09.510 And delta S's tend to be in joules. 00:24:09.510 --> 00:24:12.070 Everything else is in kilojoules. 00:24:12.070 --> 00:24:14.340 So keep that in mind. 00:24:14.340 --> 00:24:20.470 And why do you think this reaction has a positive value? 00:24:20.470 --> 00:24:23.360 Why is delta S greater than 0? 00:24:23.360 --> 00:24:24.790 What would be your guess for that? 00:24:28.640 --> 00:24:30.276 What's happening? 00:24:30.276 --> 00:24:32.168 AUDIENCE: [INAUDIBLE]. 00:24:32.168 --> 00:24:35.730 CATHY DRENNAN: Yeah, so we're going from a solid to a liquid. 00:24:35.730 --> 00:24:39.210 So we're increasing the internal degrees of freedom. 00:24:39.210 --> 00:24:42.680 The molecules of water can move around more in a liquid 00:24:42.680 --> 00:24:44.270 than they can in a solid. 00:24:44.270 --> 00:24:46.885 So this is increasing the disorder of the system. 00:24:50.050 --> 00:24:53.270 You're increasing entropy here, because the water molecules 00:24:53.270 --> 00:24:54.600 can move around more. 00:24:54.600 --> 00:24:56.750 There's more freedom of motion. 00:24:56.750 --> 00:24:58.850 So delta S is positive. 00:24:58.850 --> 00:25:01.000 It's increasing. 00:25:01.000 --> 00:25:05.010 And then we can use that to calculate delta G0, 00:25:05.010 --> 00:25:06.470 Gibbs free energy. 00:25:06.470 --> 00:25:11.820 We can plug in our delta H value minus T, room temperature, 00:25:11.820 --> 00:25:15.170 times delta S, which we just calculated, making sure 00:25:15.170 --> 00:25:17.860 that we convert from joules to kilojoules. 00:25:17.860 --> 00:25:23.820 And then our units will be kilojoules per mole. 00:25:23.820 --> 00:25:26.650 And here, delta G0 is a negative value. 00:25:26.650 --> 00:25:27.950 So it is spontaneous. 00:25:27.950 --> 00:25:29.400 We all know it's spontaneous. 00:25:29.400 --> 00:25:31.700 We've observed this happening. 00:25:31.700 --> 00:25:34.460 So even though delta H0 is positive, 00:25:34.460 --> 00:25:36.640 it's an endothermic reaction. 00:25:36.640 --> 00:25:41.050 Ice melts at room temperature, because the delta G 00:25:41.050 --> 00:25:43.300 is negative. 00:25:43.300 --> 00:25:45.640 So let's talk a little more about delta G. 00:25:45.640 --> 00:25:49.520 So let's talk about free energy of formation 00:25:49.520 --> 00:25:51.380 and the last page of this handout. 00:25:51.380 --> 00:25:56.540 So free energy of formation, delta G sub f. 00:25:56.540 --> 00:26:00.630 And so this is analogous to delta H of formation-- 00:26:00.630 --> 00:26:04.890 so the change in enthalpy of formation. 00:26:04.890 --> 00:26:09.330 So again, when you have a little value here, 00:26:09.330 --> 00:26:13.320 this is standard Gibbs free energy of formation for the f 00:26:13.320 --> 00:26:13.970 here. 00:26:13.970 --> 00:26:16.860 And that's the formation of 1 mole 00:26:16.860 --> 00:26:21.540 of a molecule from its elements in most stable state 00:26:21.540 --> 00:26:24.200 and in their standard states. 00:26:24.200 --> 00:26:28.355 So we can have tables in your book of these values. 00:26:28.355 --> 00:26:30.480 So your book, in the back, if you haven't explored, 00:26:30.480 --> 00:26:33.500 the back of your book gives lots of tables of things, 00:26:33.500 --> 00:26:38.890 including information about delta G's and delta H's 00:26:38.890 --> 00:26:41.240 and bond enthalpies and all sorts of other things. 00:26:41.240 --> 00:26:43.320 Redox potentials, we haven't talked about yet, 00:26:43.320 --> 00:26:44.500 lots of tables. 00:26:44.500 --> 00:26:47.090 So you can look this up. 00:26:47.090 --> 00:26:50.280 Or if you have already, say, looked up 00:26:50.280 --> 00:26:55.490 your delta H of formation, you can use this handy equation-- 00:26:55.490 --> 00:26:58.930 delta G equals delta H minus T delta S. 00:26:58.930 --> 00:27:02.350 But if we plug in our delta H's of formation, 00:27:02.350 --> 00:27:05.510 we can get our delta G's of formation. 00:27:05.510 --> 00:27:09.290 So how you're going to calculate delta G of formation 00:27:09.290 --> 00:27:12.500 depends on what information you're given. 00:27:12.500 --> 00:27:14.480 So let's think a little bit about what 00:27:14.480 --> 00:27:18.080 it means for particular delta G's of formation-- 00:27:18.080 --> 00:27:22.330 if they're positive or if they're negative. 00:27:22.330 --> 00:27:24.040 So let's look at an example. 00:27:24.040 --> 00:27:25.850 And we saw this before. 00:27:25.850 --> 00:27:30.330 This is the formation of carbon dioxide from elements 00:27:30.330 --> 00:27:35.302 in its most stable state, which is graphite carbon and O2 gas. 00:27:35.302 --> 00:27:37.510 So these are the elements in their most stable state, 00:27:37.510 --> 00:27:39.190 forming CO2. 00:27:39.190 --> 00:27:40.780 Now, I'm telling you that the delta 00:27:40.780 --> 00:27:46.850 G0 is minus 394.36 kilojoules per mole. 00:27:46.850 --> 00:27:51.160 And we can think about what this information tells us, 00:27:51.160 --> 00:27:54.180 that this is a fairly large negative number. 00:27:54.180 --> 00:28:00.410 So if delta G of formation is less than 0, 00:28:00.410 --> 00:28:02.884 what's going to be true thermodynamically? 00:28:02.884 --> 00:28:04.175 And this is a clicker question. 00:28:27.577 --> 00:28:29.394 Let's just take 10 more seconds. 00:28:44.580 --> 00:28:47.100 Interesting. 00:28:47.100 --> 00:28:50.290 So let's think about why this is true. 00:28:50.290 --> 00:28:54.030 This might be a deciding clicker question. 00:28:54.030 --> 00:28:55.550 We'll see. 00:28:55.550 --> 00:29:01.470 So if it is negative value for delta G, 00:29:01.470 --> 00:29:03.770 a negative value for delta G means 00:29:03.770 --> 00:29:07.060 that it's spontaneous in its forward direction. 00:29:07.060 --> 00:29:10.720 So here, the formation from the elements in their most stable 00:29:10.720 --> 00:29:15.260 state, if this is spontaneous in the forward direction, 00:29:15.260 --> 00:29:17.580 it also means that it's non-spontaneous 00:29:17.580 --> 00:29:19.120 in the reverse direction. 00:29:19.120 --> 00:29:22.440 That means once CO2 forms, it's going 00:29:22.440 --> 00:29:26.380 to be stable compared to the elements from which it came, 00:29:26.380 --> 00:29:29.930 because it's non-spontaneous going in the reverse direction, 00:29:29.930 --> 00:29:32.320 or at least that's the way that I like to think about it. 00:29:32.320 --> 00:29:37.130 So relative to its elements, it's stable-- spontaneous 00:29:37.130 --> 00:29:41.350 forward, non-spontaneous in reverse. 00:29:41.350 --> 00:29:43.810 So this is kind of bad news for us, 00:29:43.810 --> 00:29:47.820 because there's too much CO2 in our environment right now. 00:29:47.820 --> 00:29:49.420 It's a greenhouse gas. 00:29:49.420 --> 00:29:53.270 And wouldn't it be awesome if we could just encourage it all 00:29:53.270 --> 00:29:56.710 to go back to its elements, form more oxygen, 00:29:56.710 --> 00:29:57.850 which we could breathe. 00:29:57.850 --> 00:29:58.510 How lovely? 00:29:58.510 --> 00:30:00.160 Make some nice graphite. 00:30:00.160 --> 00:30:02.300 Maybe compress it, make some diamonds. 00:30:02.300 --> 00:30:06.930 But no, it is quite stable compared to its elements. 00:30:06.930 --> 00:30:11.050 So CO2 is in our environment causing global warming. 00:30:11.050 --> 00:30:13.610 And it's going to be hard to solve that problem, not 00:30:13.610 --> 00:30:15.050 easy to solve that problem. 00:30:15.050 --> 00:30:19.700 So this is unfortunate news that thermodynamics gives us. 00:30:19.700 --> 00:30:21.130 So then we can look at the other. 00:30:21.130 --> 00:30:26.400 If you have a positive value for delta G of formation, 00:30:26.400 --> 00:30:29.930 then it's thermodynamically unstable 00:30:29.930 --> 00:30:31.360 compared to its elements. 00:30:31.360 --> 00:30:35.450 So it's spontaneous going in the reverse direction. 00:30:35.450 --> 00:30:38.160 So it's unstable. 00:30:38.160 --> 00:30:40.780 So thermodynamics tells us whether something 00:30:40.780 --> 00:30:44.210 is stable or unstable. 00:30:44.210 --> 00:30:47.450 And kinetics tells us about whether things 00:30:47.450 --> 00:30:49.150 will react quickly or not. 00:30:49.150 --> 00:30:52.350 So something can be kinetically inert-- it might 00:30:52.350 --> 00:30:54.290 take a long time to react. 00:30:54.290 --> 00:30:57.620 But thermodynamics tells us stable, unstable. 00:30:57.620 --> 00:31:01.740 So thermodynamics is great, but it doesn't tell us anything 00:31:01.740 --> 00:31:03.850 about the rates of reactions. 00:31:03.850 --> 00:31:06.210 So nothing about the rates, and that's kinetics. 00:31:06.210 --> 00:31:08.670 So really thermodynamics and kinetics 00:31:08.670 --> 00:31:11.980 are very important for explaining reactions. 00:31:11.980 --> 00:31:14.950 And we'll talk about more kinetics at the end. 00:31:14.950 --> 00:31:19.020 So to calculate a delta G for a reaction, 00:31:19.020 --> 00:31:21.010 it depends, again, what you're given. 00:31:21.010 --> 00:31:26.460 You can sum up the delta G of formation of your products 00:31:26.460 --> 00:31:27.950 minus your reactants. 00:31:27.950 --> 00:31:29.510 Or you might use this. 00:31:29.510 --> 00:31:32.150 You'll find yourself using this equation a lot. 00:31:32.150 --> 00:31:36.290 This, again, was a crowning achievement of thermodynamics, 00:31:36.290 --> 00:31:40.840 that delta G equals delta H minus T delta S. So, again, 00:31:40.840 --> 00:31:43.330 whatever information you're given, 00:31:43.330 --> 00:31:47.550 you can use that to find these values. 00:31:47.550 --> 00:31:49.250 So we're not done with this equation. 00:31:49.250 --> 00:31:50.830 We're going to switch handouts. 00:31:50.830 --> 00:31:55.590 But we're going to continue with that exact same equation.