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 --> 00:00:12.660 To make a donation or to view additional materials 00:00:12.660 --> 00:00:16.405 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.405 --> 00:00:17.030 at ocw.mit.edu. 00:00:26.490 --> 00:00:29.020 PROFESSOR: So collective knowledge 00:00:29.020 --> 00:00:32.490 got us a bunch more points. 00:00:32.490 --> 00:00:36.230 So the rules for significant figures 00:00:36.230 --> 00:00:39.290 are on the back page of the handout from Friday. 00:00:39.290 --> 00:00:41.430 And so we want to think about how many are 00:00:41.430 --> 00:00:43.692 after the decimal point here. 00:00:43.692 --> 00:00:46.150 So there are three after the decimal point, which gives you 00:00:46.150 --> 00:00:49.470 three here in your answer. 00:00:49.470 --> 00:00:53.200 Log significant figure rules-- yes. 00:00:53.200 --> 00:00:56.700 So in base, and some in chemical equilibrium, which 00:00:56.700 --> 00:00:58.900 are all kind of the same thing, you 00:00:58.900 --> 00:01:06.080 can have subtraction and addition, then multiplication 00:01:06.080 --> 00:01:09.000 and division, and then end your answer 00:01:09.000 --> 00:01:15.130 with logs-- three significant figure rolls in one problem. 00:01:15.130 --> 00:01:17.450 Yes, it's coming. 00:01:17.450 --> 00:01:23.370 So you want to get those get those rolls down. 00:01:23.370 --> 00:01:27.750 OK, so today we're going to talk about solubility. 00:01:27.750 --> 00:01:33.610 And this is really a direct kind of growth out of thermodynamics 00:01:33.610 --> 00:01:35.110 and chemical equilibrium. 00:01:35.110 --> 00:01:37.820 You're going to be seeing a lot of the same ideas 00:01:37.820 --> 00:01:40.220 in solubility. 00:01:40.220 --> 00:01:41.850 And then at the end of the class we're 00:01:41.850 --> 00:01:42.933 going to start acid-bases. 00:01:45.420 --> 00:01:49.740 So far, we have been discussing mostly pure substances, 00:01:49.740 --> 00:01:52.340 and that is all changing right now. 00:01:52.340 --> 00:01:55.200 Because most substances are actually mixtures. 00:01:55.200 --> 00:01:59.710 So most of the time, you have two things mixed together. 00:01:59.710 --> 00:02:02.900 So mixtures-- you have a homogeneous mixture 00:02:02.900 --> 00:02:07.260 of something, something's dissolved in something else, 00:02:07.260 --> 00:02:12.970 and a solution is a homogeneous mixture. 00:02:12.970 --> 00:02:17.260 And so in your solution you have a solvent, 00:02:17.260 --> 00:02:20.040 which is the thing that is dissolving the other thing, 00:02:20.040 --> 00:02:21.770 and the other thing is the solute. 00:02:21.770 --> 00:02:25.670 So the solute is any dissolved substance in a solution, 00:02:25.670 --> 00:02:29.170 and the solvent is the substance that does the dissolving. 00:02:29.170 --> 00:02:31.520 And water is a great solvent. 00:02:31.520 --> 00:02:34.160 One of the most common solvents is water. 00:02:34.160 --> 00:02:38.260 So when you have your solute in your solvent 00:02:38.260 --> 00:02:42.040 and it's nicely dissolved, then you have a solution. 00:02:42.040 --> 00:02:43.860 So let's look at examples of things 00:02:43.860 --> 00:02:46.750 that dissolve in other things, and first we're 00:02:46.750 --> 00:02:50.660 going to talk about NaCl, common table salt, 00:02:50.660 --> 00:02:52.720 dissolving in water. 00:02:52.720 --> 00:02:55.780 So any NaCl is held together by an ionic bond 00:02:55.780 --> 00:02:57.650 and it forms crystals where you have 00:02:57.650 --> 00:03:01.230 these beautiful arrangement of our sodium plus 00:03:01.230 --> 00:03:03.670 and our Cl minus. 00:03:03.670 --> 00:03:08.480 And a polar solvent, like water, will, what's called, 00:03:08.480 --> 00:03:13.060 hydrate the ions and pry them away from the surface. 00:03:13.060 --> 00:03:15.940 So if we look at this picture, and we have our sodium 00:03:15.940 --> 00:03:19.130 and our chloride surrounded in our salt crystal, 00:03:19.130 --> 00:03:21.440 we have water, which is red with those two 00:03:21.440 --> 00:03:25.360 little white dots, which are the hydrogens on the water. 00:03:25.360 --> 00:03:29.620 And so we have our bent geometry for our polar water molecule. 00:03:29.620 --> 00:03:33.060 And the water is coming in, and it's hydrating the ions. 00:03:33.060 --> 00:03:36.570 And so here, you see the edge of the salt crystal, 00:03:36.570 --> 00:03:40.650 and now the water is kind of pulling one of the ions apart. 00:03:40.650 --> 00:03:44.280 And here you see the ions are coming out of the salt crystal 00:03:44.280 --> 00:03:46.790 and dissolving in your solution. 00:03:46.790 --> 00:03:50.130 So a polar solvent like water can just 00:03:50.130 --> 00:03:53.830 pry that crystal apart. 00:03:53.830 --> 00:03:57.770 So stirring can speed up this process. 00:03:57.770 --> 00:04:01.070 It doesn't often change the properties. 00:04:01.070 --> 00:04:02.610 It doesn't make things more soluble. 00:04:02.610 --> 00:04:04.860 But it will speed it up, because if you stir it, 00:04:04.860 --> 00:04:06.490 you're going to get more of that water 00:04:06.490 --> 00:04:09.830 having access to the edges of the salt crystals, 00:04:09.830 --> 00:04:15.160 and helping to pry apart and hydrate those ions so that they 00:04:15.160 --> 00:04:17.470 solubilize. 00:04:17.470 --> 00:04:21.550 We have an equilibrium expression for solubility, 00:04:21.550 --> 00:04:23.230 so we're just following right along 00:04:23.230 --> 00:04:24.840 with chemical equilibrium. 00:04:24.840 --> 00:04:30.780 And so we have our solid NaCl going to our hydrated ions, 00:04:30.780 --> 00:04:35.870 our Na plus, that was pulled out by water, and our CL minus. 00:04:35.870 --> 00:04:38.160 And this in here, it says aqueous, 00:04:38.160 --> 00:04:41.690 abbreviation for aqueous, so we have our dissolved ions. 00:04:41.690 --> 00:04:43.580 That's how we would interpret that. 00:04:43.580 --> 00:04:45.980 And S here as for solid. 00:04:45.980 --> 00:04:48.290 So we've also not paid much attention 00:04:48.290 --> 00:04:50.540 to phase of things at this point-- a little bit 00:04:50.540 --> 00:04:52.710 when we were talking about increasing entropy. 00:04:52.710 --> 00:04:55.760 We were talking about gases going to liquids and solids, 00:04:55.760 --> 00:04:59.820 and vice versa, and predicting the change in entropy 00:04:59.820 --> 00:05:01.190 for those processes. 00:05:01.190 --> 00:05:02.940 But now we're going to spend a lot of time 00:05:02.940 --> 00:05:05.150 paying attention to the phase. 00:05:05.150 --> 00:05:09.070 And so solubility, something solid, going to its aqueous 00:05:09.070 --> 00:05:10.470 ions. 00:05:10.470 --> 00:05:13.500 So we can write an equilibrium expression for this. 00:05:13.500 --> 00:05:17.270 And it has a special name-- KSP, SP 00:05:17.270 --> 00:05:19.840 stands for Solubility Product. 00:05:19.840 --> 00:05:25.520 And so KSP would be equal to the concentration of sodium ions 00:05:25.520 --> 00:05:29.780 and the concentration times the concentration of chloride ions. 00:05:29.780 --> 00:05:35.290 And equilibrium constants are products over reactants. 00:05:35.290 --> 00:05:38.070 We don't have our reactant in this equation 00:05:38.070 --> 00:05:39.400 because it is a solid. 00:05:39.400 --> 00:05:43.980 So it doesn't appear in the equilibrium expression. 00:05:43.980 --> 00:05:46.070 So our equilibrium expression, which 00:05:46.070 --> 00:05:49.290 here has the set special name of KSP, 00:05:49.290 --> 00:05:53.280 is just those ions in solution, the concentration 00:05:53.280 --> 00:05:56.220 of each ion times each other. 00:05:56.220 --> 00:05:58.290 All right, so let's have a little practice 00:05:58.290 --> 00:06:00.400 with writing KSPs. 00:06:00.400 --> 00:06:03.720 And so we have a clicker question on that. 00:06:19.490 --> 00:06:20.800 All right, 10 more seconds. 00:06:35.520 --> 00:06:36.480 Great. 00:06:36.480 --> 00:06:39.650 Yup, so again, we don't want to have 00:06:39.650 --> 00:06:42.230 the solid in the expression, and we 00:06:42.230 --> 00:06:43.910 need to remember the stoichiometry, 00:06:43.910 --> 00:06:46.580 always need to remember the stoichiometry. 00:06:46.580 --> 00:06:53.110 OK, so they're solubility product. 00:06:53.110 --> 00:06:57.910 OK, so now let's think about something that is not ionic. 00:06:57.910 --> 00:07:00.226 Let's think about glucose. 00:07:00.226 --> 00:07:01.600 We've talked a lot about glucose. 00:07:01.600 --> 00:07:04.860 Let's think about glucose dissolving in water. 00:07:04.860 --> 00:07:06.890 And here I have a picture of two beakers, 00:07:06.890 --> 00:07:09.340 a beaker where it's clear, and a beaker 00:07:09.340 --> 00:07:13.060 where there's a lot of glucose sitting on the bottom. 00:07:13.060 --> 00:07:15.530 And so we can think about the solubility here. 00:07:15.530 --> 00:07:17.140 So how can this dissolve? 00:07:17.140 --> 00:07:18.460 It can't form ions. 00:07:18.460 --> 00:07:20.870 It's not made up of ions like sodium chloride. 00:07:20.870 --> 00:07:22.310 So how does it dissolve? 00:07:22.310 --> 00:07:25.840 Well, it can dissolve, again, water molecules-- 00:07:25.840 --> 00:07:28.840 water's amazing-- water molecules form hydrogen 00:07:28.840 --> 00:07:32.740 bonds with the glucose and pull one of the glucose molecules 00:07:32.740 --> 00:07:36.360 that is at the edge of the solid piece of glucose, 00:07:36.360 --> 00:07:40.060 pulls it out into solution by hydrogen bonding to it. 00:07:40.060 --> 00:07:42.790 So again, it's solubilizing the glucose 00:07:42.790 --> 00:07:46.500 that's in this solid form by interacting at the surface 00:07:46.500 --> 00:07:49.080 and prying layers of glucose molecules 00:07:49.080 --> 00:07:51.890 off by hydrogen bonding to them. 00:07:51.890 --> 00:07:54.310 Luckily, you know all about hydrogen bonds, 00:07:54.310 --> 00:07:58.280 so just to do a little clicker practice, why don't you tell me 00:07:58.280 --> 00:08:00.760 what kind of hydrogen bonds glucose 00:08:00.760 --> 00:08:04.470 is capable of forming if it is, in fact, capable of forming. 00:08:04.470 --> 00:08:06.895 But I kind of gave away one of the answers here. 00:08:32.309 --> 00:08:33.009 10 more seconds. 00:08:47.220 --> 00:08:48.980 All right, good. 00:08:48.980 --> 00:08:50.320 Zero put four. 00:08:50.320 --> 00:08:51.490 That's awesome. 00:08:51.490 --> 00:08:54.520 You were listening to what I was saying. 00:08:54.520 --> 00:08:56.640 Yes, so that is correct. 00:08:56.640 --> 00:09:01.300 And if we look back at the molecule for a minute-- 00:09:01.300 --> 00:09:06.490 so it's a hydrogen bond donor because of these OH groups 00:09:06.490 --> 00:09:08.040 all along here. 00:09:08.040 --> 00:09:11.450 And so OH, there's an electron negativity difference 00:09:11.450 --> 00:09:14.990 of greater than 0.4, so it's capable of being a hydrogen 00:09:14.990 --> 00:09:16.190 bond donor. 00:09:16.190 --> 00:09:18.490 But this double bonded O here, it's 00:09:18.490 --> 00:09:20.010 not a complete Lewis structure. 00:09:20.010 --> 00:09:21.930 It's missing its lone pairs. 00:09:21.930 --> 00:09:25.010 But it has lone pairs on that oxygen, 00:09:25.010 --> 00:09:27.760 and that could be a hydrogen bond acceptor. 00:09:27.760 --> 00:09:31.370 So we have hydrogen bond donors and hydrogen bond acceptors 00:09:31.370 --> 00:09:35.460 in this molecule, and water can also be a hydrogen bond 00:09:35.460 --> 00:09:36.820 donor and acceptor. 00:09:36.820 --> 00:09:41.340 It has two hydrogens that can form hydrogen bond. 00:09:41.340 --> 00:09:44.620 It also has to lone pairs, which gives it its bent shape. 00:09:44.620 --> 00:09:46.530 So it can be a donor and acceptor. 00:09:46.530 --> 00:09:49.210 So it's capable of interacting with glucose 00:09:49.210 --> 00:09:53.110 and pulling that solid glucose into solution. 00:09:53.110 --> 00:09:55.580 So if we look at these two pictures, 00:09:55.580 --> 00:09:58.030 this could be glucose [INAUDIBLE] in solution. 00:09:58.030 --> 00:10:01.230 You can't see any glucose, but here you see a level of glucose 00:10:01.230 --> 00:10:02.630 at the bottom. 00:10:02.630 --> 00:10:05.810 And a solution is considered saturated 00:10:05.810 --> 00:10:09.760 when all of the solvent has dissolved as much as it can, 00:10:09.760 --> 00:10:12.650 but a little bit of undissolved solute remains. 00:10:12.650 --> 00:10:17.080 So this over saturated. 00:10:17.080 --> 00:10:20.380 Often it's just saturated when you have just a little bit more 00:10:20.380 --> 00:10:21.430 than can be dissolved. 00:10:21.430 --> 00:10:23.370 You've gone past that point. 00:10:23.370 --> 00:10:26.800 You can't get any more dissolved in that volume of water. 00:10:26.800 --> 00:10:29.540 This has a pretty large excess. 00:10:29.540 --> 00:10:31.900 So sometimes when you get your solution, 00:10:31.900 --> 00:10:35.530 you get your solution clear, your glucose has dissolved 00:10:35.530 --> 00:10:37.820 or whatever it is you're working on has dissolved, 00:10:37.820 --> 00:10:39.980 and then you maybe add a little bit too much, 00:10:39.980 --> 00:10:42.960 and it all crashes out of solution again, 00:10:42.960 --> 00:10:45.220 you have the precipitant just coming out. 00:10:45.220 --> 00:10:48.350 So undissolved solute, another name 00:10:48.350 --> 00:10:51.640 for that, the name that's used more often, is precipitate. 00:10:51.640 --> 00:10:53.980 So it precipitates out of solution 00:10:53.980 --> 00:10:55.930 if it has too much in it. 00:10:55.930 --> 00:10:59.490 So if you are not part of the solution, 00:10:59.490 --> 00:11:00.855 you are part of the precipitate. 00:11:04.472 --> 00:11:05.222 That's my t-shirt. 00:11:07.820 --> 00:11:12.340 OK, so dissolved and undissolved solute-- they're not 00:11:12.340 --> 00:11:13.460 doing nothing. 00:11:13.460 --> 00:11:15.010 They're in dynamic equilibrium. 00:11:15.010 --> 00:11:18.790 Just like all other kind of chemical equilibrium, 00:11:18.790 --> 00:11:21.672 there is no net change, but the process is still 00:11:21.672 --> 00:11:22.630 going forward and back. 00:11:22.630 --> 00:11:24.660 In fact, the rate going forward is 00:11:24.660 --> 00:11:26.390 equal to the rate going back. 00:11:26.390 --> 00:11:27.790 The same is true here. 00:11:27.790 --> 00:11:30.790 The dissolved and undissolved solute 00:11:30.790 --> 00:11:34.580 are in dynamic equilibrium with each other. 00:11:34.580 --> 00:11:36.640 So how much can you get in there? 00:11:36.640 --> 00:11:41.420 How much solute can you get into your solvent? 00:11:41.420 --> 00:11:44.560 And that depends on the particular substance 00:11:44.560 --> 00:11:47.000 and its molar solubility. 00:11:47.000 --> 00:11:50.970 And molar solubility is defined as S. Either little s 00:11:50.970 --> 00:11:54.760 or a big S. I've seen it both ways in textbooks. 00:11:54.760 --> 00:11:59.040 So the molar solubility, S, is the molar concentration 00:11:59.040 --> 00:12:01.280 in that saturated solution, where 00:12:01.280 --> 00:12:04.630 you've put just a little more in than can 00:12:04.630 --> 00:12:06.670 be completely dissolved. 00:12:06.670 --> 00:12:10.210 And so that represents the limit of the ability 00:12:10.210 --> 00:12:14.180 of that solvent to dissolve that solute, that particular 00:12:14.180 --> 00:12:15.450 compound. 00:12:15.450 --> 00:12:19.840 And its units are, perhaps unsurprisingly given its name, 00:12:19.840 --> 00:12:22.470 is molar, moles per liter. 00:12:25.590 --> 00:12:31.220 So molar solubility is related to our friend KSP, 00:12:31.220 --> 00:12:37.310 but it's not the same thing, or at least most of the time. 00:12:37.310 --> 00:12:39.830 I think none of the time exactly the same. 00:12:39.830 --> 00:12:42.830 So when you're talking about the molar solubility of either ion, 00:12:42.830 --> 00:12:46.350 you're talking about that concentration of sodium plus 00:12:46.350 --> 00:12:47.960 or chlorine minus. 00:12:47.960 --> 00:12:50.300 So you're talking about the moles per liter 00:12:50.300 --> 00:12:54.120 that can be dissolved in that particular amount of solvent 00:12:54.120 --> 00:12:55.350 or kind of solvent. 00:12:55.350 --> 00:12:59.380 And so those concentrations are the molar solubility. 00:12:59.380 --> 00:13:02.210 And again, our KSP is going to be 00:13:02.210 --> 00:13:06.580 equal to the concentration of the sodium in solution 00:13:06.580 --> 00:13:09.850 and the concentration of that chloride that's dissolved. 00:13:09.850 --> 00:13:12.020 So in this case, the relationship 00:13:12.020 --> 00:13:15.610 is that KSP would equal the molar solubility squared, 00:13:15.610 --> 00:13:17.570 the molar solubility of this times the molar 00:13:17.570 --> 00:13:19.420 solubility of that. 00:13:19.420 --> 00:13:24.330 So if you know one, you can derive the other, 00:13:24.330 --> 00:13:27.530 and you can do that on a problem set, in particular. 00:13:27.530 --> 00:13:31.610 So those are the definitions of those terms. 00:13:31.610 --> 00:13:33.740 So molar solubility, how much of something 00:13:33.740 --> 00:13:36.480 dissolves in another, whether it dissolves it all 00:13:36.480 --> 00:13:39.350 will depend on, again, the nature of the material 00:13:39.350 --> 00:13:40.800 and the nature of the solvent. 00:13:40.800 --> 00:13:42.830 Are they miscible? 00:13:42.830 --> 00:13:45.740 Is there a way, like water, getting in there 00:13:45.740 --> 00:13:48.420 and forming hydrogen bonds, for example. 00:13:48.420 --> 00:13:52.890 And this leads to a rule that a lot of people tend to know, 00:13:52.890 --> 00:13:55.560 which is like dissolves like rule. 00:13:55.560 --> 00:13:59.690 So if it's a non-ionic substance, 00:13:59.690 --> 00:14:04.580 then you need to pull it out of the solid by hydrogen bonding, 00:14:04.580 --> 00:14:06.810 if that material has nothing to hydrogen bond 00:14:06.810 --> 00:14:08.970 to, if it's completely non-polar, 00:14:08.970 --> 00:14:11.550 water is not going to be able to solubilize it. 00:14:11.550 --> 00:14:15.700 So polar things, polar liquids like water, 00:14:15.700 --> 00:14:18.890 are generally the best solvents for things that are ionic, 00:14:18.890 --> 00:14:21.950 like salts, or polar compounds. 00:14:21.950 --> 00:14:23.670 So it doesn't have to be a salt. It 00:14:23.670 --> 00:14:26.160 can be a compound made of carbon, 00:14:26.160 --> 00:14:28.680 but it needs to be a polar compound 00:14:28.680 --> 00:14:30.200 so that the water can get in there 00:14:30.200 --> 00:14:31.790 and form those hydrogen bonds. 00:14:31.790 --> 00:14:34.730 So you can see that knowing what is capable of forming hydrogen 00:14:34.730 --> 00:14:36.750 bonds can be really useful to think 00:14:36.750 --> 00:14:38.260 about what's going to be dissolved 00:14:38.260 --> 00:14:40.890 in what other type of material. 00:14:40.890 --> 00:14:46.890 So non-polar liquids, like hexane, for example, 00:14:46.890 --> 00:14:48.650 are better for non-polar. 00:14:48.650 --> 00:14:51.250 So non-polar like dissolves likes. 00:14:51.250 --> 00:14:54.240 And so that's why people go to dry cleaners, 00:14:54.240 --> 00:14:59.536 because you don't want to use water to wash certain things. 00:14:59.536 --> 00:15:00.940 You want to be dry. 00:15:00.940 --> 00:15:04.930 You want to get your non-polar stains out. 00:15:04.930 --> 00:15:07.410 So this is very important. 00:15:07.410 --> 00:15:11.250 There's a number of applications of these principles. 00:15:11.250 --> 00:15:14.890 This is important in the pharmaceutical industry. 00:15:14.890 --> 00:15:18.090 So we talked about some of the molecules that 00:15:18.090 --> 00:15:22.080 are used as pharmaceutical, in terms 00:15:22.080 --> 00:15:25.970 of being polar and non-polar, and vitamins the same way. 00:15:25.970 --> 00:15:27.500 So when you're designing a molecule 00:15:27.500 --> 00:15:30.860 that you want to get into the body and react with a target, 00:15:30.860 --> 00:15:34.500 if you design something that is really non-polar, 00:15:34.500 --> 00:15:37.550 the human body-- there's a lot of water parts. 00:15:37.550 --> 00:15:39.240 And it's not going to necessarily get 00:15:39.240 --> 00:15:41.340 to where you want it to go. 00:15:41.340 --> 00:15:46.730 But if it's too polar, it might just get washed out. 00:15:46.730 --> 00:15:49.440 So you want the right amount of polar non-polar 00:15:49.440 --> 00:15:52.020 to stay in the body long enough, but also 00:15:52.020 --> 00:15:53.900 to get to where it needs to go. 00:15:53.900 --> 00:15:56.770 So a lot of people who are designing molecules 00:15:56.770 --> 00:15:59.220 to treat disease are very concerned about, 00:15:59.220 --> 00:16:01.560 can we make this molecule more polar? 00:16:01.560 --> 00:16:04.230 Can we make it more soluble? 00:16:04.230 --> 00:16:06.120 It hits our target beautifully. 00:16:06.120 --> 00:16:08.190 It really destroys that enzyme. 00:16:08.190 --> 00:16:11.110 It would be a fantastic chemotherapeutic agent. 00:16:11.110 --> 00:16:14.560 But we need to figure out how to make it get into the cell 00:16:14.560 --> 00:16:16.670 that it needs to target. 00:16:16.670 --> 00:16:19.320 And of course, so if you're going to go into drug designer 00:16:19.320 --> 00:16:21.082 medicine, you care about this. 00:16:21.082 --> 00:16:22.540 If you're going to become a parent, 00:16:22.540 --> 00:16:24.660 you care a lot about cleaning. 00:16:24.660 --> 00:16:27.700 And it's a daily event. 00:16:27.700 --> 00:16:31.000 How am I going to get this stain out of my child's clothes? 00:16:31.000 --> 00:16:33.610 Every day this is a question. 00:16:33.610 --> 00:16:35.510 So this is important. 00:16:35.510 --> 00:16:36.480 All right. 00:16:36.480 --> 00:16:39.960 And one other example in the longs of cleaning-- I just 00:16:39.960 --> 00:16:43.410 want to share with you a short story about a chemist, 00:16:43.410 --> 00:16:46.610 Robert H. Black, and a life altering 00:16:46.610 --> 00:16:48.670 event that happened to him. 00:16:48.670 --> 00:16:57.270 So one day, Mrs. Black said to Mr. Black, clean the bath tub. 00:16:57.270 --> 00:17:02.280 This was an unprecedented event in the Black household. 00:17:02.280 --> 00:17:06.160 So Robert Black went in to clean his bath tub, 00:17:06.160 --> 00:17:07.150 and it was really hard. 00:17:07.150 --> 00:17:08.609 And he scrubbed, and he scrubbed, and he scrubbed, 00:17:08.609 --> 00:17:09.780 and he scrubbed. 00:17:09.780 --> 00:17:13.030 And he was a chemist, so he knew what kind of chemicals 00:17:13.030 --> 00:17:15.950 you would need to clean a bath tub. 00:17:15.950 --> 00:17:20.280 You want to have things to dissolve your likes, 00:17:20.280 --> 00:17:22.300 so you want to have your polar and non-polar. 00:17:22.300 --> 00:17:24.810 You want to have a chelating agent 00:17:24.810 --> 00:17:27.950 to pull the heavy metals out of the tub scum, 00:17:27.950 --> 00:17:32.050 a surfactant, to make them bead up so you can wash them away, 00:17:32.050 --> 00:17:35.390 alcohols, to remove sort of the more greasy kind of stains. 00:17:35.390 --> 00:17:36.870 So he knew all of this. 00:17:36.870 --> 00:17:40.500 But he realized that if you didn't apply cleaner 00:17:40.500 --> 00:17:45.510 on a regular basis, the tub scum got so thick and so nasty, 00:17:45.510 --> 00:17:48.680 that the cleaner couldn't penetrate the scum, 00:17:48.680 --> 00:17:50.170 and you need surface area. 00:17:50.170 --> 00:17:53.060 These things work by dissolving out. 00:17:53.060 --> 00:17:54.130 They touch the surface. 00:17:54.130 --> 00:17:55.450 So you need to have a surface. 00:17:55.450 --> 00:18:00.610 So he had this idea that instead of having this tub scum build 00:18:00.610 --> 00:18:03.890 up so much that it was a real problem to clean it, 00:18:03.890 --> 00:18:06.090 that you could take all of these things 00:18:06.090 --> 00:18:08.880 that chemists knew would be useful 00:18:08.880 --> 00:18:13.430 and package them in something that was a daily cleaner. 00:18:13.430 --> 00:18:16.220 So he advertised these things that, every time you 00:18:16.220 --> 00:18:19.340 take a shower, you just spray the shower or the tub, 00:18:19.340 --> 00:18:22.967 and then you never have to scrub your tub again. 00:18:22.967 --> 00:18:24.550 Because you do a little bit every day, 00:18:24.550 --> 00:18:26.530 where there's not a lot of surface, 00:18:26.530 --> 00:18:29.230 so it gets it right off, right away. 00:18:29.230 --> 00:18:34.330 And Mr. Black and his wife never had to clean their tub again, 00:18:34.330 --> 00:18:38.870 because this resulted in sales of about $70 million per year. 00:18:38.870 --> 00:18:40.740 And it's important to point out that he 00:18:40.740 --> 00:18:42.740 wasn't using anything new. 00:18:42.740 --> 00:18:47.200 He was just packaging his chemicals for daily use, 00:18:47.200 --> 00:18:50.380 rather than, say, for a weekly or monthly use. 00:18:50.380 --> 00:18:52.940 So I think there's a couple important lessons from this. 00:18:52.940 --> 00:18:56.030 One, it's always important to clean your bathroom. 00:18:56.030 --> 00:18:59.770 Two, you never know where chemistry knowledge is going 00:18:59.770 --> 00:19:02.520 to come, what life altering event 00:19:02.520 --> 00:19:06.450 will cause you to realize that you have in your mind chemistry 00:19:06.450 --> 00:19:10.100 knowledge that can make you $70 million a year. 00:19:10.100 --> 00:19:12.600 And when that life altering event occurs, 00:19:12.600 --> 00:19:17.800 remember that I take cash and checks and stock options. 00:19:17.800 --> 00:19:18.790 I added the last one. 00:19:18.790 --> 00:19:21.240 I thought, cash and checks-- I should be more flexible. 00:19:21.240 --> 00:19:24.230 So I'm also going to take stock options. 00:19:24.230 --> 00:19:27.610 And this is why I teach you all the fundamentals of chemistry, 00:19:27.610 --> 00:19:30.860 cover everything that's really important for you to know, 00:19:30.860 --> 00:19:35.400 because then if you use any of it, you've learned it from me. 00:19:38.010 --> 00:19:39.690 So just keep that in mind. 00:19:39.690 --> 00:19:43.416 One never knows what life event is ahead of you. 00:19:45.930 --> 00:19:49.580 So what about gas solubility? 00:19:49.580 --> 00:19:51.300 We talked about solids. 00:19:51.300 --> 00:19:54.330 Now let's talk about gases. 00:19:54.330 --> 00:19:57.520 So whether a gas is going to be soluble 00:19:57.520 --> 00:20:00.090 is going to depend on Henry's Law, 00:20:00.090 --> 00:20:02.320 or we can find out about it solubility. 00:20:02.320 --> 00:20:04.460 So a solubility of a gas-- we're going 00:20:04.460 --> 00:20:06.850 to call that little s, that's our solubility-- 00:20:06.850 --> 00:20:09.480 is directly proportional to the partial pressure 00:20:09.480 --> 00:20:13.370 of the gas and a constant called Henry's Constant. 00:20:13.370 --> 00:20:15.580 And that depends on the nature of the gas 00:20:15.580 --> 00:20:18.380 and the solvent and the temperature. 00:20:18.380 --> 00:20:24.240 So let's think about this rule in this plot in a clicker 00:20:24.240 --> 00:20:24.740 question. 00:20:42.540 --> 00:20:44.170 All right, just take 10 more seconds. 00:20:58.060 --> 00:20:59.530 Awesome. 00:20:59.530 --> 00:21:00.336 That is correct. 00:21:03.550 --> 00:21:05.870 So let's just take a look at that. 00:21:05.870 --> 00:21:08.370 So one of the clicker questions, which 00:21:08.370 --> 00:21:11.430 is kind of getting you to read the graph, 00:21:11.430 --> 00:21:16.150 and so it was asking, say at 0.5 atmospheres, 00:21:16.150 --> 00:21:18.990 it said oxygen is more soluble than helium. 00:21:18.990 --> 00:21:22.120 This is molar solubility versus partial pressure. 00:21:22.120 --> 00:21:26.070 At point 0.5, sure enough, oxygen is more soluble. 00:21:26.070 --> 00:21:28.350 So that was just reading the plot. 00:21:28.350 --> 00:21:32.840 The other one was just interpreting Henry's Law again, 00:21:32.840 --> 00:21:37.830 that solubility would increase as partial pressure increases. 00:21:37.830 --> 00:21:39.930 You can see that from the equation. 00:21:39.930 --> 00:21:42.960 But you can also think about why that's true, and why it's true 00:21:42.960 --> 00:21:46.430 is the answer to the three, that the solubility of a gas 00:21:46.430 --> 00:21:49.020 is proportional to its partial pressure. 00:21:49.020 --> 00:21:51.330 An increase in pressure corresponds 00:21:51.330 --> 00:21:55.480 to an increase in rate at which the gas molecules strike 00:21:55.480 --> 00:21:56.140 the surface. 00:21:56.140 --> 00:21:57.750 And that makes them more soluble. 00:21:57.750 --> 00:21:59.720 So again, solubility really has to do 00:21:59.720 --> 00:22:01.830 with getting at that surface. 00:22:01.830 --> 00:22:06.910 If you get at the surface, then you can dissolve your material. 00:22:06.910 --> 00:22:08.290 All right, so why should you care 00:22:08.290 --> 00:22:10.930 about the solubility of gases? 00:22:10.930 --> 00:22:14.570 And I'm going to give you an example. 00:22:14.570 --> 00:22:16.100 This is another in your own words, 00:22:16.100 --> 00:22:17.800 so we'll watch the video. 00:22:17.800 --> 00:22:20.850 And I think tonight there's some kind of big climate change 00:22:20.850 --> 00:22:21.350 thing. 00:22:21.350 --> 00:22:22.740 I've seen fliers around. 00:22:22.740 --> 00:22:25.150 I haven't paid enough attention to what's going on. 00:22:25.150 --> 00:22:29.590 But climate change is certainly a very hot topic, 00:22:29.590 --> 00:22:32.721 if you'll excuse that right now. 00:22:32.721 --> 00:22:33.720 I didn't really mean it. 00:22:33.720 --> 00:22:35.620 It just kind of came out. 00:22:35.620 --> 00:22:38.140 But CO2 is a greenhouse gas. 00:22:38.140 --> 00:22:39.530 It's a big problem. 00:22:39.530 --> 00:22:41.870 And so a number of researchers here at MIT 00:22:41.870 --> 00:22:46.170 are thinking about ways that you can capture and store it 00:22:46.170 --> 00:22:49.070 somewhere, get it out of our atmosphere. 00:22:49.070 --> 00:22:51.010 Other people are thinking about ways 00:22:51.010 --> 00:22:54.660 that you can change industrial processes to make less CO2. 00:22:54.660 --> 00:22:56.480 There's a lot of research going on. 00:22:56.480 --> 00:22:59.080 So today you'll hear from Hector Hernandez, who 00:22:59.080 --> 00:23:01.850 had a Ph.D. in chemistry from the chemistry department 00:23:01.850 --> 00:23:05.640 here at MIT, did his post-doc here at MIT in chemical 00:23:05.640 --> 00:23:09.330 engineering, and is now a professor of chemical 00:23:09.330 --> 00:23:14.610 engineering at Masdar Institute in the United Arab Emirates. 00:23:14.610 --> 00:23:18.020 So this is one of the MIT satellite universities 00:23:18.020 --> 00:23:20.470 that are popping up all over the world. 00:23:20.470 --> 00:23:23.120 So Hector was born, I think, in Honduras, 00:23:23.120 --> 00:23:25.530 and then lived in Florida, ended up in MIT, 00:23:25.530 --> 00:23:27.800 and now he's at the United Arab Emirates. 00:23:27.800 --> 00:23:29.680 So that's just another example of, 00:23:29.680 --> 00:23:32.020 one never knows what's going to happen to you. 00:23:32.020 --> 00:23:35.745 All right, so you can hear from Hector in his own words. 00:23:38.390 --> 00:23:44.710 So Hector's personal video's online. 00:23:44.710 --> 00:23:46.076 I think it's a fun one to watch. 00:23:46.076 --> 00:23:47.700 There's a couple things that are really 00:23:47.700 --> 00:23:52.140 interesting about his personal story, including the fact 00:23:52.140 --> 00:23:55.760 that as he mentioned in past life he did some construction, 00:23:55.760 --> 00:23:59.630 he started undergrad at age 30, I think, 00:23:59.630 --> 00:24:03.810 where he was basically building houses and fixing cars 00:24:03.810 --> 00:24:07.820 and he realized that his back might not survive this 00:24:07.820 --> 00:24:09.500 being his permanent career. 00:24:09.500 --> 00:24:12.490 So he decided to use his brain instead of his back. 00:24:12.490 --> 00:24:15.280 And so started undergraduate, now he's a professor. 00:24:15.280 --> 00:24:19.550 So it just shows there's a lot of different paths to success. 00:24:19.550 --> 00:24:23.240 All right so C02-- big problem. 00:24:23.240 --> 00:24:24.859 We were trying to use, in that case, 00:24:24.859 --> 00:24:26.775 he was interested in turning it into biofuels, 00:24:26.775 --> 00:24:28.880 they were interested in ways to store it. 00:24:28.880 --> 00:24:30.780 You care about solubility of gases 00:24:30.780 --> 00:24:32.850 and you care about Le Chatelier's principle 00:24:32.850 --> 00:24:35.010 of driving things the direction that you want 00:24:35.010 --> 00:24:36.890 to make the products you want. 00:24:36.890 --> 00:24:40.680 OK, so let's think more about these factors that 00:24:40.680 --> 00:24:42.610 affect solubility. 00:24:42.610 --> 00:24:44.670 And one thing that's very important, 00:24:44.670 --> 00:24:47.450 for sure, is temperature. 00:24:47.450 --> 00:24:53.980 So temperature-- most substances dissolve more quickly 00:24:53.980 --> 00:24:56.950 at higher temperature, which is often what you want. 00:24:56.950 --> 00:25:00.890 But it doesn't always mean if you increase the temperature 00:25:00.890 --> 00:25:03.210 that you're going to make it more soluble. 00:25:03.210 --> 00:25:05.460 You may just make it dissolve faster, 00:25:05.460 --> 00:25:08.270 but not actually change the end product. 00:25:08.270 --> 00:25:11.520 Some things are more sensitive to temperature than others. 00:25:11.520 --> 00:25:16.340 So most gases, for example, are less soluble in warm water 00:25:16.340 --> 00:25:19.390 than in cold water, but with solids, it's 00:25:19.390 --> 00:25:23.450 much harder to predict what effect temperature might have 00:25:23.450 --> 00:25:26.670 on their solubility, even though for many things 00:25:26.670 --> 00:25:29.940 it will make it faster. 00:25:29.940 --> 00:25:34.070 So let's think now about how to drive a reaction, 00:25:34.070 --> 00:25:37.820 and what factors are involved, and what's favorable 00:25:37.820 --> 00:25:39.410 and what's not favorable. 00:25:39.410 --> 00:25:41.690 And when we're talking about those things, 00:25:41.690 --> 00:25:44.200 we're going to be back to our thermodynamics, 00:25:44.200 --> 00:25:49.440 back to enthalpy, entropy, and Gibb's free energy. 00:25:49.440 --> 00:25:52.120 So all these have little special sub names. 00:25:52.120 --> 00:25:55.090 So when we're talking about delta H 00:25:55.090 --> 00:25:58.510 and we're talking about solutions, things dissolving, 00:25:58.510 --> 00:26:02.110 we're talking about delta H sub sol, 00:26:02.110 --> 00:26:07.710 for solution, or if you could think of it as solubility. 00:26:07.710 --> 00:26:09.850 And one point that I'll make-- a lot of times 00:26:09.850 --> 00:26:12.180 when people are learning a new field, they're like, 00:26:12.180 --> 00:26:14.130 oh, there's so much to learn. 00:26:14.130 --> 00:26:16.000 But you get to some point, everything 00:26:16.000 --> 00:26:18.750 is just kind of like a subtle difference in something 00:26:18.750 --> 00:26:19.880 you've already learned. 00:26:19.880 --> 00:26:21.820 So if you already learned about delta H, 00:26:21.820 --> 00:26:23.680 this is just a slight modification 00:26:23.680 --> 00:26:25.130 of what you learned. 00:26:25.130 --> 00:26:27.020 You don't have to learn a whole new thing. 00:26:27.020 --> 00:26:29.210 So that's one of the good things about the course. 00:26:29.210 --> 00:26:32.880 You start recognizing, I already pretty much know this. 00:26:32.880 --> 00:26:36.680 I just need to add a little sol to the end. 00:26:36.680 --> 00:26:40.540 So delta H of solution can be measured. 00:26:40.540 --> 00:26:43.860 You can measure it from heat released when something 00:26:43.860 --> 00:26:47.720 dissolves, or measure it by the heat absorbed 00:26:47.720 --> 00:26:50.550 and at constant pressure. 00:26:50.550 --> 00:26:55.390 And so the sign of delta H sol will tell you 00:26:55.390 --> 00:27:00.190 about whether heat is released or heat is required 00:27:00.190 --> 00:27:01.920 when something dissolves. 00:27:01.920 --> 00:27:04.760 So a negative enthalpy of solution 00:27:04.760 --> 00:27:08.390 tells us that heat is released when something dissolves. 00:27:08.390 --> 00:27:11.660 And many of you may have just experienced this. 00:27:11.660 --> 00:27:13.382 If you're mixing something together 00:27:13.382 --> 00:27:15.590 and you're holding the container, and all of a sudden 00:27:15.590 --> 00:27:21.760 it heats up, then you know delta H sol is a negative value. 00:27:21.760 --> 00:27:24.590 And of course a positive value tells you 00:27:24.590 --> 00:27:28.930 that that energy is absorbed, and sometimes things 00:27:28.930 --> 00:27:32.890 can get very cold when they're dissolving. 00:27:32.890 --> 00:27:38.400 So you already kind of knew that that negative delta H tells you 00:27:38.400 --> 00:27:41.730 that heat is released, and positive delta H tells you 00:27:41.730 --> 00:27:43.380 that heat is absorbed. 00:27:43.380 --> 00:27:47.070 So this is basically the same thing that you already learned. 00:27:47.070 --> 00:27:50.290 It just applies to things dissolving just the same 00:27:50.290 --> 00:27:52.470 as it does to any reaction. 00:27:52.470 --> 00:27:52.970 All right. 00:27:52.970 --> 00:27:57.530 Now, suppose we want to know whether the reaction is 00:27:57.530 --> 00:28:00.140 going to be spontaneous at constant pressure 00:28:00.140 --> 00:28:01.590 or temperature. 00:28:01.590 --> 00:28:07.060 If we want to know whether it will dissolve spontaneously, 00:28:07.060 --> 00:28:08.430 what am I asking about? 00:28:08.430 --> 00:28:09.900 What term do I want to know? 00:28:14.190 --> 00:28:17.460 I want to know about delta G. I sure do. 00:28:17.460 --> 00:28:21.290 Delta G tells us whether things are spontaneous or not. 00:28:21.290 --> 00:28:24.890 It does with thermodynamics, it does with solubility. 00:28:24.890 --> 00:28:27.320 Delta G is to predictor of whether something 00:28:27.320 --> 00:28:30.060 is going to be spontaneous or non-spontaneous 00:28:30.060 --> 00:28:35.370 by the sine of delta G. 00:28:35.370 --> 00:28:38.160 So entropy-- don't want to leave out entropy. 00:28:38.160 --> 00:28:39.840 Never leave out entropy. 00:28:39.840 --> 00:28:43.050 My t-shirt's still unaccounted for, 00:28:43.050 --> 00:28:45.800 after they got back to Massachusetts, 00:28:45.800 --> 00:28:48.320 they left for New Jersey and Cincinnati again. 00:28:48.320 --> 00:28:52.000 Last time I checked, they were still in Ohio. 00:28:52.000 --> 00:28:54.780 Entropy. 00:28:54.780 --> 00:28:57.980 So since disorder typically increases 00:28:57.980 --> 00:29:01.400 when a solvent dissolves, typically, entropy 00:29:01.400 --> 00:29:02.390 should increase. 00:29:02.390 --> 00:29:03.889 And there's a little star, and we'll 00:29:03.889 --> 00:29:06.840 get back to the star, because it doesn't always increase. 00:29:06.840 --> 00:29:10.240 But typically, it should increase. 00:29:10.240 --> 00:29:12.700 So if we're looking at these pictures here, 00:29:12.700 --> 00:29:15.950 over here you have your nice molecules all 00:29:15.950 --> 00:29:20.280 lined up in your solid, and it's going to then dissolve. 00:29:20.280 --> 00:29:22.470 And you can see that when it's dissolved, 00:29:22.470 --> 00:29:24.550 that's a lot more entropy over there. 00:29:24.550 --> 00:29:26.600 There are a lot more freedom moving around. 00:29:26.600 --> 00:29:28.600 This is more constrained. 00:29:28.600 --> 00:29:33.650 So typically, entropy increases when things dissolve. 00:29:33.650 --> 00:29:39.020 So if delta H is negative, and delta S increases 00:29:39.020 --> 00:29:41.620 when a solutes dissolves, then what 00:29:41.620 --> 00:29:44.996 do we expect about the dissolving process? 00:29:44.996 --> 00:29:46.245 And that's a clicker question. 00:30:03.720 --> 00:30:04.350 Yeah. 00:30:04.350 --> 00:30:06.630 So most people have, so we'll take 10 more seconds. 00:30:21.590 --> 00:30:23.910 So let's look at both of them, because we're 00:30:23.910 --> 00:30:27.300 going to do this one in a few minutes. 00:30:27.300 --> 00:30:29.832 So let's just continue with our notes, 00:30:29.832 --> 00:30:31.540 and we're going to come back to that one. 00:30:31.540 --> 00:30:32.081 I don't know. 00:30:32.081 --> 00:30:35.384 Can we just leave that one up, maybe for a minute? 00:30:35.384 --> 00:30:36.800 I don't know how long it was going 00:30:36.800 --> 00:30:38.260 to take to me to get there. 00:30:38.260 --> 00:30:44.652 All right, so when we're talking about this-- 00:30:44.652 --> 00:30:46.110 we'll just keep things in the notes 00:30:46.110 --> 00:30:47.443 and we'll come back to that one. 00:30:47.443 --> 00:30:51.810 So if this is negative and delta S 00:30:51.810 --> 00:30:56.480 is increasing-- so if this is negative and this is positive, 00:30:56.480 --> 00:30:59.990 then you're going to have a spontaneous process here. 00:30:59.990 --> 00:31:03.520 So if you can put in your notes that this should be spontaneous 00:31:03.520 --> 00:31:06.490 under these circumstances. 00:31:06.490 --> 00:31:10.240 Now let's continue on and think about getting around 00:31:10.240 --> 00:31:13.010 to the next one. 00:31:13.010 --> 00:31:18.840 So in some cases-- I don't know, hopefully people 00:31:18.840 --> 00:31:22.330 can see this-- in some cases, entropy of the system 00:31:22.330 --> 00:31:24.860 is actually lowered when something dissolves. 00:31:24.860 --> 00:31:27.760 And this is because of what's known as this cage effect. 00:31:27.760 --> 00:31:30.860 If you have water molecules, when something dissolves, 00:31:30.860 --> 00:31:33.460 sometimes order around the thing. 00:31:33.460 --> 00:31:38.970 And so the water entropy is determining the process. 00:31:38.970 --> 00:31:45.070 And have this new order because of the water structure. 00:31:45.070 --> 00:31:46.380 So this can change. 00:31:46.380 --> 00:31:50.430 So it's not always the case that entropy of the overall system 00:31:50.430 --> 00:31:52.780 is going to increase. 00:31:52.780 --> 00:31:55.700 So here, even if this is a negative value, 00:31:55.700 --> 00:31:59.110 delta H solution is negative, delta G might be positive. 00:31:59.110 --> 00:32:01.790 It might not be a spontaneous system. 00:32:01.790 --> 00:32:04.860 And so the cage effect is why some things that 00:32:04.860 --> 00:32:07.960 are hydrophobic, just you're having a hard time 00:32:07.960 --> 00:32:12.250 dissolving them, even if they have a negative enthalpy, even 00:32:12.250 --> 00:32:14.210 if you would just look at the negative enthalpy 00:32:14.210 --> 00:32:16.500 and predict that it should be spontaneous. 00:32:16.500 --> 00:32:19.820 Because even if this is negative, if delta 00:32:19.820 --> 00:32:21.880 S is also negative because you have 00:32:21.880 --> 00:32:24.400 more order due to the water molecules, 00:32:24.400 --> 00:32:26.770 then the overall term can be positive. 00:32:26.770 --> 00:32:29.940 It would depend on the magnitude of these 00:32:29.940 --> 00:32:30.962 and on the temperature. 00:32:35.366 --> 00:32:37.240 This is actually more slides than I remember. 00:32:37.240 --> 00:32:39.600 Before we get to the next-- oh, this is the last one. 00:32:39.600 --> 00:32:45.776 OK, so when gases are dissolved in a liquid, 00:32:45.776 --> 00:32:46.900 there is much less freedom. 00:32:46.900 --> 00:32:49.890 Gases not dissolved can be anywhere. 00:32:49.890 --> 00:32:54.320 But when they're in a liquid, then there's much less freedom. 00:32:54.320 --> 00:32:58.140 And so the entropy then is going to be negative, 00:32:58.140 --> 00:33:03.480 and the solubility will decrease as the temperature rises. 00:33:03.480 --> 00:33:08.540 And so that brings us then to our question about delta 00:33:08.540 --> 00:33:12.500 H of solution being positive. 00:33:12.500 --> 00:33:14.587 And now we can look at the answer to that. 00:33:18.720 --> 00:33:21.830 Now I guess we can go maybe to the other slide. 00:33:21.830 --> 00:33:24.470 So that was the question that you had here, 00:33:24.470 --> 00:33:27.220 and I've just put up the answers. 00:33:27.220 --> 00:33:30.990 So here, delta H is positive. 00:33:30.990 --> 00:33:35.850 And so we have positive minus T delta S again. 00:33:35.850 --> 00:33:39.930 We weren't told anything about delta S in this particular. 00:33:39.930 --> 00:33:43.390 Just ask what if it's positive. 00:33:43.390 --> 00:33:46.530 And so for the first one, it said dissolving is never 00:33:46.530 --> 00:33:47.880 spontaneous. 00:33:47.880 --> 00:33:51.230 And that isn't necessarily true, because you 00:33:51.230 --> 00:33:53.540 don't know what delta S is. 00:33:53.540 --> 00:33:56.750 Dissolving is only spontaneous if T delta S 00:33:56.750 --> 00:34:01.160 is positive and larger than delta H solution. 00:34:01.160 --> 00:34:02.790 And that was the correct answer. 00:34:02.790 --> 00:34:06.950 If this is a big term here, it's larger than that term, 00:34:06.950 --> 00:34:09.320 and this is a positive value, then you 00:34:09.320 --> 00:34:11.880 would get a negative delta G, and it 00:34:11.880 --> 00:34:14.969 would be a spontaneous process. 00:34:14.969 --> 00:34:19.110 If we look at number three here, dissolving is only spontaneous 00:34:19.110 --> 00:34:24.030 if this is negative-- that does not help us at all. 00:34:24.030 --> 00:34:26.520 And the rate will be slower. 00:34:26.520 --> 00:34:29.870 Delta H is not going to tell us anything about rates. 00:34:29.870 --> 00:34:32.790 Rates are determined by kinetics. 00:34:32.790 --> 00:34:34.710 Temperature can affect the rates, 00:34:34.710 --> 00:34:37.489 for sure, stirring can affect the rates for sure, 00:34:37.489 --> 00:34:44.500 but delta H being positive is not telling us about the rates. 00:34:44.500 --> 00:34:48.600 So you'll see in this particular unit, 00:34:48.600 --> 00:34:50.530 when you're thinking about solubility, 00:34:50.530 --> 00:34:53.370 you're thinking about equilibrium expressions, 00:34:53.370 --> 00:34:56.120 especially equilibrium expression KSP, 00:34:56.120 --> 00:35:00.270 you're thinking about partial pressures again, 00:35:00.270 --> 00:35:04.550 you're thinking about effects on the solubility that 00:35:04.550 --> 00:35:06.100 have to do with temperatures, you're 00:35:06.100 --> 00:35:09.810 thinking about delta H, delta S, and delta G. 00:35:09.810 --> 00:35:14.470 So it's really an outgrowth of thermodynamics and chemical 00:35:14.470 --> 00:35:17.100 equilibrium, but you're just applying, really, 00:35:17.100 --> 00:35:19.240 what you've already learned to thinking 00:35:19.240 --> 00:35:22.470 about a solute dissolving in a solvent. 00:35:22.470 --> 00:35:24.500 So that's one of the things that sets really 00:35:24.500 --> 00:35:27.270 great about the course and the next couple units, 00:35:27.270 --> 00:35:29.830 is that everything's very connected. 00:35:29.830 --> 00:35:33.070 So we can just briefly introduce you to the next unit, 00:35:33.070 --> 00:35:36.310 and we're going to be talking about acids and bases now 00:35:36.310 --> 00:35:38.440 for several lectures. 00:35:38.440 --> 00:35:42.435 And I have to say that this is one of the units when 00:35:42.435 --> 00:35:44.820 we get to kind of acid-base titrations 00:35:44.820 --> 00:35:47.200 that MIT students seem to struggle with. 00:35:47.200 --> 00:35:51.380 So we're going to slow this down and really 00:35:51.380 --> 00:35:55.343 go over it, and have you be awesome at acids and bases. 00:35:58.570 --> 00:36:02.550 And I'll mention that one of the reasons why I spent time 00:36:02.550 --> 00:36:06.010 on solubility and acids, bases, because these are topics that 00:36:06.010 --> 00:36:09.340 are really important for medical school, and I know a lot of you 00:36:09.340 --> 00:36:11.230 are not pre-med, but I want everybody 00:36:11.230 --> 00:36:14.620 in this class to have that background that they could 00:36:14.620 --> 00:36:16.610 go to med school if they wanted to, 00:36:16.610 --> 00:36:21.240 or they could just advocate for their own health 00:36:21.240 --> 00:36:24.800 with a doctor who doesn't know about acids and bases. 00:36:24.800 --> 00:36:29.230 And I'll tell you a story about that a little later. 00:36:29.230 --> 00:36:33.480 Anyway, OK, so acids and bases-- we're 00:36:33.480 --> 00:36:35.820 just going to some definitions. 00:36:35.820 --> 00:36:40.910 So the simplest definition of this 00:36:40.910 --> 00:36:44.960 is that an acid is a substance that when dissolved in water, 00:36:44.960 --> 00:36:50.540 increases the concentration of hydrogen ions, H plus. 00:36:50.540 --> 00:36:53.060 Whereas a base is a substance that 00:36:53.060 --> 00:36:58.800 increases the hydroxide concentration, OH minus. 00:36:58.800 --> 00:37:01.040 So there are acids and bases that definitely 00:37:01.040 --> 00:37:04.940 fit this description, but this is a very narrow description 00:37:04.940 --> 00:37:07.910 of acids and bases. 00:37:07.910 --> 00:37:10.600 The next one, the Bronsted-Lowry, 00:37:10.600 --> 00:37:12.910 is a bit broader. 00:37:12.910 --> 00:37:15.390 So an acid is something that donates 00:37:15.390 --> 00:37:20.390 a hydrogen ion, H plus, and a Bronsted-Lowry base 00:37:20.390 --> 00:37:23.750 is something that accepts a hydrogen ion. 00:37:23.750 --> 00:37:25.220 And this is really the definition 00:37:25.220 --> 00:37:28.990 we'll be spending the most time with in the class. 00:37:28.990 --> 00:37:33.730 So let's look at some examples of this. 00:37:33.730 --> 00:37:38.620 So here, we have an acid. 00:37:38.620 --> 00:37:44.070 And if it's an acid, it's going to be donating a hydrogen ion, 00:37:44.070 --> 00:37:47.090 and it's going to donate it here to the water. 00:37:47.090 --> 00:37:52.360 The water acts as a base in that it accepts the hydrogen ion. 00:37:52.360 --> 00:37:54.400 When it accepts the hydrogen ion, 00:37:54.400 --> 00:38:01.030 it has one extra hydrogen ion, so it's H3O plus. 00:38:01.030 --> 00:38:04.350 And when the acid loses its hydrogen ion, 00:38:04.350 --> 00:38:08.070 it becomes this base over here, the same molecule 00:38:08.070 --> 00:38:11.760 minus the hydrogen ion, so now it has a negative charge. 00:38:11.760 --> 00:38:15.200 And so you'll will see hydronium ions 00:38:15.200 --> 00:38:17.660 quite a bit, which is H3O plus. 00:38:17.660 --> 00:38:19.590 That's really the more accurate form, 00:38:19.590 --> 00:38:23.110 rather than just writing H plus somewhere. 00:38:23.110 --> 00:38:25.750 So that's the kind of true nature of things. 00:38:25.750 --> 00:38:28.680 So let me just show you a little animation of this happening. 00:38:28.680 --> 00:38:29.790 I think it's kind of cute. 00:38:33.930 --> 00:38:40.155 So here we have our water molecules and we have our acid. 00:38:40.155 --> 00:38:43.000 It donated a hydrogen ion to these, 00:38:43.000 --> 00:38:45.420 and then another water came and stole it away. 00:38:45.420 --> 00:38:48.330 And so there's our H3O plus. 00:38:48.330 --> 00:38:51.370 And so in solution, you have this exchange 00:38:51.370 --> 00:38:52.590 of hydrogen atoms. 00:38:52.590 --> 00:38:55.040 The acid is donating, the base is accepting. 00:38:55.040 --> 00:38:57.340 And as they donate and accept, they 00:38:57.340 --> 00:38:59.410 become different molecules. 00:38:59.410 --> 00:39:04.920 So it gives rise to this idea of conjugate acid-base pairs. 00:39:04.920 --> 00:39:10.440 So here you see that this acid is paired 00:39:10.440 --> 00:39:12.170 with this base over here. 00:39:12.170 --> 00:39:15.780 They're the same, except that one has the hydrogen ion 00:39:15.780 --> 00:39:17.690 and one does not. 00:39:17.690 --> 00:39:20.600 And then the other pair is here. 00:39:20.600 --> 00:39:22.370 It's another acid-base pair. 00:39:22.370 --> 00:39:27.300 We have water and hydronium ion that also differ by H plus. 00:39:27.300 --> 00:39:32.250 So every time an acid donates a hydrogen ion or proton, 00:39:32.250 --> 00:39:34.050 it becomes its conjugate base. 00:39:34.050 --> 00:39:37.560 Every time a base accepts a hydrogen ion or proton, 00:39:37.560 --> 00:39:41.140 it becomes its conjugate acid. 00:39:41.140 --> 00:39:44.140 So the conjugate base of any acid 00:39:44.140 --> 00:39:48.480 is the base that is formed when the acid has donated a hydrogen 00:39:48.480 --> 00:39:50.030 ion or proton. 00:39:50.030 --> 00:39:52.440 The conjugate acid of a base is the acid 00:39:52.440 --> 00:39:58.500 that forms when the base accepts a hydrogen ion or proton. 00:39:58.500 --> 00:40:01.780 So let's take a look at one more example. 00:40:01.780 --> 00:40:06.080 If this over here-- is this acting as an acid or base, 00:40:06.080 --> 00:40:07.021 this molecule here? 00:40:07.021 --> 00:40:07.770 What do you think? 00:40:07.770 --> 00:40:10.550 You can just yell it out. 00:40:10.550 --> 00:40:12.650 It's acting as an acid, right? 00:40:12.650 --> 00:40:15.430 This is acting, then, as the base. 00:40:15.430 --> 00:40:18.140 When this base accepts the hydrogen ion, 00:40:18.140 --> 00:40:20.400 it forms its conjugate acid. 00:40:20.400 --> 00:40:23.380 And this then forms its conjugate base, 00:40:23.380 --> 00:40:27.910 which differs by H plus. 00:40:27.910 --> 00:40:32.638 So why don't you try one of these, then, on your own? 00:40:43.710 --> 00:40:44.410 10 more seconds. 00:41:02.750 --> 00:41:07.780 OK, so we can go back to that over there. 00:41:07.780 --> 00:41:11.420 So you most people got this right, 00:41:11.420 --> 00:41:14.830 and so you're looking at what's acting as the base 00:41:14.830 --> 00:41:16.920 and what's acting as the acid. 00:41:16.920 --> 00:41:20.600 Again, the base is going to be accepting a hydrogen ion. 00:41:20.600 --> 00:41:22.340 So this is acting as the acid. 00:41:22.340 --> 00:41:25.240 This forms it's conjugate it and this. 00:41:25.240 --> 00:41:32.880 And quickly, with the idea of something that's amphoteric, 00:41:32.880 --> 00:41:35.920 which is molecule that can act as an acid or a base. 00:41:35.920 --> 00:41:39.030 You just saw some nice examples of that. 00:41:39.030 --> 00:41:45.110 And finally, just one last definition for acid-bases. 00:41:45.110 --> 00:41:52.300 And we'll just put this up and that's kind of the end. 00:41:52.300 --> 00:41:55.060 So the final definition, the Lewis Base, 00:41:55.060 --> 00:41:57.700 donates a lone pair, whereas a Lewis Acid 00:41:57.700 --> 00:42:01.170 accepts such electrons.