1 00:00:00,030 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,780 Commons license. 3 00:00:03,780 --> 00:00:06,020 Your support will help MIT OpenCourseWare 4 00:00:06,020 --> 00:00:10,090 continue to offer high quality educational resources for free. 5 00:00:10,090 --> 00:00:12,660 To make a donation or to view additional materials 6 00:00:12,660 --> 00:00:16,405 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,405 --> 00:00:17,030 at ocw.mit.edu. 8 00:00:26,490 --> 00:00:29,020 PROFESSOR: So collective knowledge 9 00:00:29,020 --> 00:00:32,490 got us a bunch more points. 10 00:00:32,490 --> 00:00:36,230 So the rules for significant figures 11 00:00:36,230 --> 00:00:39,290 are on the back page of the handout from Friday. 12 00:00:39,290 --> 00:00:41,430 And so we want to think about how many are 13 00:00:41,430 --> 00:00:43,692 after the decimal point here. 14 00:00:43,692 --> 00:00:46,150 So there are three after the decimal point, which gives you 15 00:00:46,150 --> 00:00:49,470 three here in your answer. 16 00:00:49,470 --> 00:00:53,200 Log significant figure rules-- yes. 17 00:00:53,200 --> 00:00:56,700 So in base, and some in chemical equilibrium, which 18 00:00:56,700 --> 00:00:58,900 are all kind of the same thing, you 19 00:00:58,900 --> 00:01:06,080 can have subtraction and addition, then multiplication 20 00:01:06,080 --> 00:01:09,000 and division, and then end your answer 21 00:01:09,000 --> 00:01:15,130 with logs-- three significant figure rolls in one problem. 22 00:01:15,130 --> 00:01:17,450 Yes, it's coming. 23 00:01:17,450 --> 00:01:23,370 So you want to get those get those rolls down. 24 00:01:23,370 --> 00:01:27,750 OK, so today we're going to talk about solubility. 25 00:01:27,750 --> 00:01:33,610 And this is really a direct kind of growth out of thermodynamics 26 00:01:33,610 --> 00:01:35,110 and chemical equilibrium. 27 00:01:35,110 --> 00:01:37,820 You're going to be seeing a lot of the same ideas 28 00:01:37,820 --> 00:01:40,220 in solubility. 29 00:01:40,220 --> 00:01:41,850 And then at the end of the class we're 30 00:01:41,850 --> 00:01:42,933 going to start acid-bases. 31 00:01:45,420 --> 00:01:49,740 So far, we have been discussing mostly pure substances, 32 00:01:49,740 --> 00:01:52,340 and that is all changing right now. 33 00:01:52,340 --> 00:01:55,200 Because most substances are actually mixtures. 34 00:01:55,200 --> 00:01:59,710 So most of the time, you have two things mixed together. 35 00:01:59,710 --> 00:02:02,900 So mixtures-- you have a homogeneous mixture 36 00:02:02,900 --> 00:02:07,260 of something, something's dissolved in something else, 37 00:02:07,260 --> 00:02:12,970 and a solution is a homogeneous mixture. 38 00:02:12,970 --> 00:02:17,260 And so in your solution you have a solvent, 39 00:02:17,260 --> 00:02:20,040 which is the thing that is dissolving the other thing, 40 00:02:20,040 --> 00:02:21,770 and the other thing is the solute. 41 00:02:21,770 --> 00:02:25,670 So the solute is any dissolved substance in a solution, 42 00:02:25,670 --> 00:02:29,170 and the solvent is the substance that does the dissolving. 43 00:02:29,170 --> 00:02:31,520 And water is a great solvent. 44 00:02:31,520 --> 00:02:34,160 One of the most common solvents is water. 45 00:02:34,160 --> 00:02:38,260 So when you have your solute in your solvent 46 00:02:38,260 --> 00:02:42,040 and it's nicely dissolved, then you have a solution. 47 00:02:42,040 --> 00:02:43,860 So let's look at examples of things 48 00:02:43,860 --> 00:02:46,750 that dissolve in other things, and first we're 49 00:02:46,750 --> 00:02:50,660 going to talk about NaCl, common table salt, 50 00:02:50,660 --> 00:02:52,720 dissolving in water. 51 00:02:52,720 --> 00:02:55,780 So any NaCl is held together by an ionic bond 52 00:02:55,780 --> 00:02:57,650 and it forms crystals where you have 53 00:02:57,650 --> 00:03:01,230 these beautiful arrangement of our sodium plus 54 00:03:01,230 --> 00:03:03,670 and our Cl minus. 55 00:03:03,670 --> 00:03:08,480 And a polar solvent, like water, will, what's called, 56 00:03:08,480 --> 00:03:13,060 hydrate the ions and pry them away from the surface. 57 00:03:13,060 --> 00:03:15,940 So if we look at this picture, and we have our sodium 58 00:03:15,940 --> 00:03:19,130 and our chloride surrounded in our salt crystal, 59 00:03:19,130 --> 00:03:21,440 we have water, which is red with those two 60 00:03:21,440 --> 00:03:25,360 little white dots, which are the hydrogens on the water. 61 00:03:25,360 --> 00:03:29,620 And so we have our bent geometry for our polar water molecule. 62 00:03:29,620 --> 00:03:33,060 And the water is coming in, and it's hydrating the ions. 63 00:03:33,060 --> 00:03:36,570 And so here, you see the edge of the salt crystal, 64 00:03:36,570 --> 00:03:40,650 and now the water is kind of pulling one of the ions apart. 65 00:03:40,650 --> 00:03:44,280 And here you see the ions are coming out of the salt crystal 66 00:03:44,280 --> 00:03:46,790 and dissolving in your solution. 67 00:03:46,790 --> 00:03:50,130 So a polar solvent like water can just 68 00:03:50,130 --> 00:03:53,830 pry that crystal apart. 69 00:03:53,830 --> 00:03:57,770 So stirring can speed up this process. 70 00:03:57,770 --> 00:04:01,070 It doesn't often change the properties. 71 00:04:01,070 --> 00:04:02,610 It doesn't make things more soluble. 72 00:04:02,610 --> 00:04:04,860 But it will speed it up, because if you stir it, 73 00:04:04,860 --> 00:04:06,490 you're going to get more of that water 74 00:04:06,490 --> 00:04:09,830 having access to the edges of the salt crystals, 75 00:04:09,830 --> 00:04:15,160 and helping to pry apart and hydrate those ions so that they 76 00:04:15,160 --> 00:04:17,470 solubilize. 77 00:04:17,470 --> 00:04:21,550 We have an equilibrium expression for solubility, 78 00:04:21,550 --> 00:04:23,230 so we're just following right along 79 00:04:23,230 --> 00:04:24,840 with chemical equilibrium. 80 00:04:24,840 --> 00:04:30,780 And so we have our solid NaCl going to our hydrated ions, 81 00:04:30,780 --> 00:04:35,870 our Na plus, that was pulled out by water, and our CL minus. 82 00:04:35,870 --> 00:04:38,160 And this in here, it says aqueous, 83 00:04:38,160 --> 00:04:41,690 abbreviation for aqueous, so we have our dissolved ions. 84 00:04:41,690 --> 00:04:43,580 That's how we would interpret that. 85 00:04:43,580 --> 00:04:45,980 And S here as for solid. 86 00:04:45,980 --> 00:04:48,290 So we've also not paid much attention 87 00:04:48,290 --> 00:04:50,540 to phase of things at this point-- a little bit 88 00:04:50,540 --> 00:04:52,710 when we were talking about increasing entropy. 89 00:04:52,710 --> 00:04:55,760 We were talking about gases going to liquids and solids, 90 00:04:55,760 --> 00:04:59,820 and vice versa, and predicting the change in entropy 91 00:04:59,820 --> 00:05:01,190 for those processes. 92 00:05:01,190 --> 00:05:02,940 But now we're going to spend a lot of time 93 00:05:02,940 --> 00:05:05,150 paying attention to the phase. 94 00:05:05,150 --> 00:05:09,070 And so solubility, something solid, going to its aqueous 95 00:05:09,070 --> 00:05:10,470 ions. 96 00:05:10,470 --> 00:05:13,500 So we can write an equilibrium expression for this. 97 00:05:13,500 --> 00:05:17,270 And it has a special name-- KSP, SP 98 00:05:17,270 --> 00:05:19,840 stands for Solubility Product. 99 00:05:19,840 --> 00:05:25,520 And so KSP would be equal to the concentration of sodium ions 100 00:05:25,520 --> 00:05:29,780 and the concentration times the concentration of chloride ions. 101 00:05:29,780 --> 00:05:35,290 And equilibrium constants are products over reactants. 102 00:05:35,290 --> 00:05:38,070 We don't have our reactant in this equation 103 00:05:38,070 --> 00:05:39,400 because it is a solid. 104 00:05:39,400 --> 00:05:43,980 So it doesn't appear in the equilibrium expression. 105 00:05:43,980 --> 00:05:46,070 So our equilibrium expression, which 106 00:05:46,070 --> 00:05:49,290 here has the set special name of KSP, 107 00:05:49,290 --> 00:05:53,280 is just those ions in solution, the concentration 108 00:05:53,280 --> 00:05:56,220 of each ion times each other. 109 00:05:56,220 --> 00:05:58,290 All right, so let's have a little practice 110 00:05:58,290 --> 00:06:00,400 with writing KSPs. 111 00:06:00,400 --> 00:06:03,720 And so we have a clicker question on that. 112 00:06:19,490 --> 00:06:20,800 All right, 10 more seconds. 113 00:06:35,520 --> 00:06:36,480 Great. 114 00:06:36,480 --> 00:06:39,650 Yup, so again, we don't want to have 115 00:06:39,650 --> 00:06:42,230 the solid in the expression, and we 116 00:06:42,230 --> 00:06:43,910 need to remember the stoichiometry, 117 00:06:43,910 --> 00:06:46,580 always need to remember the stoichiometry. 118 00:06:46,580 --> 00:06:53,110 OK, so they're solubility product. 119 00:06:53,110 --> 00:06:57,910 OK, so now let's think about something that is not ionic. 120 00:06:57,910 --> 00:07:00,226 Let's think about glucose. 121 00:07:00,226 --> 00:07:01,600 We've talked a lot about glucose. 122 00:07:01,600 --> 00:07:04,860 Let's think about glucose dissolving in water. 123 00:07:04,860 --> 00:07:06,890 And here I have a picture of two beakers, 124 00:07:06,890 --> 00:07:09,340 a beaker where it's clear, and a beaker 125 00:07:09,340 --> 00:07:13,060 where there's a lot of glucose sitting on the bottom. 126 00:07:13,060 --> 00:07:15,530 And so we can think about the solubility here. 127 00:07:15,530 --> 00:07:17,140 So how can this dissolve? 128 00:07:17,140 --> 00:07:18,460 It can't form ions. 129 00:07:18,460 --> 00:07:20,870 It's not made up of ions like sodium chloride. 130 00:07:20,870 --> 00:07:22,310 So how does it dissolve? 131 00:07:22,310 --> 00:07:25,840 Well, it can dissolve, again, water molecules-- 132 00:07:25,840 --> 00:07:28,840 water's amazing-- water molecules form hydrogen 133 00:07:28,840 --> 00:07:32,740 bonds with the glucose and pull one of the glucose molecules 134 00:07:32,740 --> 00:07:36,360 that is at the edge of the solid piece of glucose, 135 00:07:36,360 --> 00:07:40,060 pulls it out into solution by hydrogen bonding to it. 136 00:07:40,060 --> 00:07:42,790 So again, it's solubilizing the glucose 137 00:07:42,790 --> 00:07:46,500 that's in this solid form by interacting at the surface 138 00:07:46,500 --> 00:07:49,080 and prying layers of glucose molecules 139 00:07:49,080 --> 00:07:51,890 off by hydrogen bonding to them. 140 00:07:51,890 --> 00:07:54,310 Luckily, you know all about hydrogen bonds, 141 00:07:54,310 --> 00:07:58,280 so just to do a little clicker practice, why don't you tell me 142 00:07:58,280 --> 00:08:00,760 what kind of hydrogen bonds glucose 143 00:08:00,760 --> 00:08:04,470 is capable of forming if it is, in fact, capable of forming. 144 00:08:04,470 --> 00:08:06,895 But I kind of gave away one of the answers here. 145 00:08:32,309 --> 00:08:33,009 10 more seconds. 146 00:08:47,220 --> 00:08:48,980 All right, good. 147 00:08:48,980 --> 00:08:50,320 Zero put four. 148 00:08:50,320 --> 00:08:51,490 That's awesome. 149 00:08:51,490 --> 00:08:54,520 You were listening to what I was saying. 150 00:08:54,520 --> 00:08:56,640 Yes, so that is correct. 151 00:08:56,640 --> 00:09:01,300 And if we look back at the molecule for a minute-- 152 00:09:01,300 --> 00:09:06,490 so it's a hydrogen bond donor because of these OH groups 153 00:09:06,490 --> 00:09:08,040 all along here. 154 00:09:08,040 --> 00:09:11,450 And so OH, there's an electron negativity difference 155 00:09:11,450 --> 00:09:14,990 of greater than 0.4, so it's capable of being a hydrogen 156 00:09:14,990 --> 00:09:16,190 bond donor. 157 00:09:16,190 --> 00:09:18,490 But this double bonded O here, it's 158 00:09:18,490 --> 00:09:20,010 not a complete Lewis structure. 159 00:09:20,010 --> 00:09:21,930 It's missing its lone pairs. 160 00:09:21,930 --> 00:09:25,010 But it has lone pairs on that oxygen, 161 00:09:25,010 --> 00:09:27,760 and that could be a hydrogen bond acceptor. 162 00:09:27,760 --> 00:09:31,370 So we have hydrogen bond donors and hydrogen bond acceptors 163 00:09:31,370 --> 00:09:35,460 in this molecule, and water can also be a hydrogen bond 164 00:09:35,460 --> 00:09:36,820 donor and acceptor. 165 00:09:36,820 --> 00:09:41,340 It has two hydrogens that can form hydrogen bond. 166 00:09:41,340 --> 00:09:44,620 It also has to lone pairs, which gives it its bent shape. 167 00:09:44,620 --> 00:09:46,530 So it can be a donor and acceptor. 168 00:09:46,530 --> 00:09:49,210 So it's capable of interacting with glucose 169 00:09:49,210 --> 00:09:53,110 and pulling that solid glucose into solution. 170 00:09:53,110 --> 00:09:55,580 So if we look at these two pictures, 171 00:09:55,580 --> 00:09:58,030 this could be glucose [INAUDIBLE] in solution. 172 00:09:58,030 --> 00:10:01,230 You can't see any glucose, but here you see a level of glucose 173 00:10:01,230 --> 00:10:02,630 at the bottom. 174 00:10:02,630 --> 00:10:05,810 And a solution is considered saturated 175 00:10:05,810 --> 00:10:09,760 when all of the solvent has dissolved as much as it can, 176 00:10:09,760 --> 00:10:12,650 but a little bit of undissolved solute remains. 177 00:10:12,650 --> 00:10:17,080 So this over saturated. 178 00:10:17,080 --> 00:10:20,380 Often it's just saturated when you have just a little bit more 179 00:10:20,380 --> 00:10:21,430 than can be dissolved. 180 00:10:21,430 --> 00:10:23,370 You've gone past that point. 181 00:10:23,370 --> 00:10:26,800 You can't get any more dissolved in that volume of water. 182 00:10:26,800 --> 00:10:29,540 This has a pretty large excess. 183 00:10:29,540 --> 00:10:31,900 So sometimes when you get your solution, 184 00:10:31,900 --> 00:10:35,530 you get your solution clear, your glucose has dissolved 185 00:10:35,530 --> 00:10:37,820 or whatever it is you're working on has dissolved, 186 00:10:37,820 --> 00:10:39,980 and then you maybe add a little bit too much, 187 00:10:39,980 --> 00:10:42,960 and it all crashes out of solution again, 188 00:10:42,960 --> 00:10:45,220 you have the precipitant just coming out. 189 00:10:45,220 --> 00:10:48,350 So undissolved solute, another name 190 00:10:48,350 --> 00:10:51,640 for that, the name that's used more often, is precipitate. 191 00:10:51,640 --> 00:10:53,980 So it precipitates out of solution 192 00:10:53,980 --> 00:10:55,930 if it has too much in it. 193 00:10:55,930 --> 00:10:59,490 So if you are not part of the solution, 194 00:10:59,490 --> 00:11:00,855 you are part of the precipitate. 195 00:11:04,472 --> 00:11:05,222 That's my t-shirt. 196 00:11:07,820 --> 00:11:12,340 OK, so dissolved and undissolved solute-- they're not 197 00:11:12,340 --> 00:11:13,460 doing nothing. 198 00:11:13,460 --> 00:11:15,010 They're in dynamic equilibrium. 199 00:11:15,010 --> 00:11:18,790 Just like all other kind of chemical equilibrium, 200 00:11:18,790 --> 00:11:21,672 there is no net change, but the process is still 201 00:11:21,672 --> 00:11:22,630 going forward and back. 202 00:11:22,630 --> 00:11:24,660 In fact, the rate going forward is 203 00:11:24,660 --> 00:11:26,390 equal to the rate going back. 204 00:11:26,390 --> 00:11:27,790 The same is true here. 205 00:11:27,790 --> 00:11:30,790 The dissolved and undissolved solute 206 00:11:30,790 --> 00:11:34,580 are in dynamic equilibrium with each other. 207 00:11:34,580 --> 00:11:36,640 So how much can you get in there? 208 00:11:36,640 --> 00:11:41,420 How much solute can you get into your solvent? 209 00:11:41,420 --> 00:11:44,560 And that depends on the particular substance 210 00:11:44,560 --> 00:11:47,000 and its molar solubility. 211 00:11:47,000 --> 00:11:50,970 And molar solubility is defined as S. Either little s 212 00:11:50,970 --> 00:11:54,760 or a big S. I've seen it both ways in textbooks. 213 00:11:54,760 --> 00:11:59,040 So the molar solubility, S, is the molar concentration 214 00:11:59,040 --> 00:12:01,280 in that saturated solution, where 215 00:12:01,280 --> 00:12:04,630 you've put just a little more in than can 216 00:12:04,630 --> 00:12:06,670 be completely dissolved. 217 00:12:06,670 --> 00:12:10,210 And so that represents the limit of the ability 218 00:12:10,210 --> 00:12:14,180 of that solvent to dissolve that solute, that particular 219 00:12:14,180 --> 00:12:15,450 compound. 220 00:12:15,450 --> 00:12:19,840 And its units are, perhaps unsurprisingly given its name, 221 00:12:19,840 --> 00:12:22,470 is molar, moles per liter. 222 00:12:25,590 --> 00:12:31,220 So molar solubility is related to our friend KSP, 223 00:12:31,220 --> 00:12:37,310 but it's not the same thing, or at least most of the time. 224 00:12:37,310 --> 00:12:39,830 I think none of the time exactly the same. 225 00:12:39,830 --> 00:12:42,830 So when you're talking about the molar solubility of either ion, 226 00:12:42,830 --> 00:12:46,350 you're talking about that concentration of sodium plus 227 00:12:46,350 --> 00:12:47,960 or chlorine minus. 228 00:12:47,960 --> 00:12:50,300 So you're talking about the moles per liter 229 00:12:50,300 --> 00:12:54,120 that can be dissolved in that particular amount of solvent 230 00:12:54,120 --> 00:12:55,350 or kind of solvent. 231 00:12:55,350 --> 00:12:59,380 And so those concentrations are the molar solubility. 232 00:12:59,380 --> 00:13:02,210 And again, our KSP is going to be 233 00:13:02,210 --> 00:13:06,580 equal to the concentration of the sodium in solution 234 00:13:06,580 --> 00:13:09,850 and the concentration of that chloride that's dissolved. 235 00:13:09,850 --> 00:13:12,020 So in this case, the relationship 236 00:13:12,020 --> 00:13:15,610 is that KSP would equal the molar solubility squared, 237 00:13:15,610 --> 00:13:17,570 the molar solubility of this times the molar 238 00:13:17,570 --> 00:13:19,420 solubility of that. 239 00:13:19,420 --> 00:13:24,330 So if you know one, you can derive the other, 240 00:13:24,330 --> 00:13:27,530 and you can do that on a problem set, in particular. 241 00:13:27,530 --> 00:13:31,610 So those are the definitions of those terms. 242 00:13:31,610 --> 00:13:33,740 So molar solubility, how much of something 243 00:13:33,740 --> 00:13:36,480 dissolves in another, whether it dissolves it all 244 00:13:36,480 --> 00:13:39,350 will depend on, again, the nature of the material 245 00:13:39,350 --> 00:13:40,800 and the nature of the solvent. 246 00:13:40,800 --> 00:13:42,830 Are they miscible? 247 00:13:42,830 --> 00:13:45,740 Is there a way, like water, getting in there 248 00:13:45,740 --> 00:13:48,420 and forming hydrogen bonds, for example. 249 00:13:48,420 --> 00:13:52,890 And this leads to a rule that a lot of people tend to know, 250 00:13:52,890 --> 00:13:55,560 which is like dissolves like rule. 251 00:13:55,560 --> 00:13:59,690 So if it's a non-ionic substance, 252 00:13:59,690 --> 00:14:04,580 then you need to pull it out of the solid by hydrogen bonding, 253 00:14:04,580 --> 00:14:06,810 if that material has nothing to hydrogen bond 254 00:14:06,810 --> 00:14:08,970 to, if it's completely non-polar, 255 00:14:08,970 --> 00:14:11,550 water is not going to be able to solubilize it. 256 00:14:11,550 --> 00:14:15,700 So polar things, polar liquids like water, 257 00:14:15,700 --> 00:14:18,890 are generally the best solvents for things that are ionic, 258 00:14:18,890 --> 00:14:21,950 like salts, or polar compounds. 259 00:14:21,950 --> 00:14:23,670 So it doesn't have to be a salt. It 260 00:14:23,670 --> 00:14:26,160 can be a compound made of carbon, 261 00:14:26,160 --> 00:14:28,680 but it needs to be a polar compound 262 00:14:28,680 --> 00:14:30,200 so that the water can get in there 263 00:14:30,200 --> 00:14:31,790 and form those hydrogen bonds. 264 00:14:31,790 --> 00:14:34,730 So you can see that knowing what is capable of forming hydrogen 265 00:14:34,730 --> 00:14:36,750 bonds can be really useful to think 266 00:14:36,750 --> 00:14:38,260 about what's going to be dissolved 267 00:14:38,260 --> 00:14:40,890 in what other type of material. 268 00:14:40,890 --> 00:14:46,890 So non-polar liquids, like hexane, for example, 269 00:14:46,890 --> 00:14:48,650 are better for non-polar. 270 00:14:48,650 --> 00:14:51,250 So non-polar like dissolves likes. 271 00:14:51,250 --> 00:14:54,240 And so that's why people go to dry cleaners, 272 00:14:54,240 --> 00:14:59,536 because you don't want to use water to wash certain things. 273 00:14:59,536 --> 00:15:00,940 You want to be dry. 274 00:15:00,940 --> 00:15:04,930 You want to get your non-polar stains out. 275 00:15:04,930 --> 00:15:07,410 So this is very important. 276 00:15:07,410 --> 00:15:11,250 There's a number of applications of these principles. 277 00:15:11,250 --> 00:15:14,890 This is important in the pharmaceutical industry. 278 00:15:14,890 --> 00:15:18,090 So we talked about some of the molecules that 279 00:15:18,090 --> 00:15:22,080 are used as pharmaceutical, in terms 280 00:15:22,080 --> 00:15:25,970 of being polar and non-polar, and vitamins the same way. 281 00:15:25,970 --> 00:15:27,500 So when you're designing a molecule 282 00:15:27,500 --> 00:15:30,860 that you want to get into the body and react with a target, 283 00:15:30,860 --> 00:15:34,500 if you design something that is really non-polar, 284 00:15:34,500 --> 00:15:37,550 the human body-- there's a lot of water parts. 285 00:15:37,550 --> 00:15:39,240 And it's not going to necessarily get 286 00:15:39,240 --> 00:15:41,340 to where you want it to go. 287 00:15:41,340 --> 00:15:46,730 But if it's too polar, it might just get washed out. 288 00:15:46,730 --> 00:15:49,440 So you want the right amount of polar non-polar 289 00:15:49,440 --> 00:15:52,020 to stay in the body long enough, but also 290 00:15:52,020 --> 00:15:53,900 to get to where it needs to go. 291 00:15:53,900 --> 00:15:56,770 So a lot of people who are designing molecules 292 00:15:56,770 --> 00:15:59,220 to treat disease are very concerned about, 293 00:15:59,220 --> 00:16:01,560 can we make this molecule more polar? 294 00:16:01,560 --> 00:16:04,230 Can we make it more soluble? 295 00:16:04,230 --> 00:16:06,120 It hits our target beautifully. 296 00:16:06,120 --> 00:16:08,190 It really destroys that enzyme. 297 00:16:08,190 --> 00:16:11,110 It would be a fantastic chemotherapeutic agent. 298 00:16:11,110 --> 00:16:14,560 But we need to figure out how to make it get into the cell 299 00:16:14,560 --> 00:16:16,670 that it needs to target. 300 00:16:16,670 --> 00:16:19,320 And of course, so if you're going to go into drug designer 301 00:16:19,320 --> 00:16:21,082 medicine, you care about this. 302 00:16:21,082 --> 00:16:22,540 If you're going to become a parent, 303 00:16:22,540 --> 00:16:24,660 you care a lot about cleaning. 304 00:16:24,660 --> 00:16:27,700 And it's a daily event. 305 00:16:27,700 --> 00:16:31,000 How am I going to get this stain out of my child's clothes? 306 00:16:31,000 --> 00:16:33,610 Every day this is a question. 307 00:16:33,610 --> 00:16:35,510 So this is important. 308 00:16:35,510 --> 00:16:36,480 All right. 309 00:16:36,480 --> 00:16:39,960 And one other example in the longs of cleaning-- I just 310 00:16:39,960 --> 00:16:43,410 want to share with you a short story about a chemist, 311 00:16:43,410 --> 00:16:46,610 Robert H. Black, and a life altering 312 00:16:46,610 --> 00:16:48,670 event that happened to him. 313 00:16:48,670 --> 00:16:57,270 So one day, Mrs. Black said to Mr. Black, clean the bath tub. 314 00:16:57,270 --> 00:17:02,280 This was an unprecedented event in the Black household. 315 00:17:02,280 --> 00:17:06,160 So Robert Black went in to clean his bath tub, 316 00:17:06,160 --> 00:17:07,150 and it was really hard. 317 00:17:07,150 --> 00:17:08,609 And he scrubbed, and he scrubbed, and he scrubbed, 318 00:17:08,609 --> 00:17:09,780 and he scrubbed. 319 00:17:09,780 --> 00:17:13,030 And he was a chemist, so he knew what kind of chemicals 320 00:17:13,030 --> 00:17:15,950 you would need to clean a bath tub. 321 00:17:15,950 --> 00:17:20,280 You want to have things to dissolve your likes, 322 00:17:20,280 --> 00:17:22,300 so you want to have your polar and non-polar. 323 00:17:22,300 --> 00:17:24,810 You want to have a chelating agent 324 00:17:24,810 --> 00:17:27,950 to pull the heavy metals out of the tub scum, 325 00:17:27,950 --> 00:17:32,050 a surfactant, to make them bead up so you can wash them away, 326 00:17:32,050 --> 00:17:35,390 alcohols, to remove sort of the more greasy kind of stains. 327 00:17:35,390 --> 00:17:36,870 So he knew all of this. 328 00:17:36,870 --> 00:17:40,500 But he realized that if you didn't apply cleaner 329 00:17:40,500 --> 00:17:45,510 on a regular basis, the tub scum got so thick and so nasty, 330 00:17:45,510 --> 00:17:48,680 that the cleaner couldn't penetrate the scum, 331 00:17:48,680 --> 00:17:50,170 and you need surface area. 332 00:17:50,170 --> 00:17:53,060 These things work by dissolving out. 333 00:17:53,060 --> 00:17:54,130 They touch the surface. 334 00:17:54,130 --> 00:17:55,450 So you need to have a surface. 335 00:17:55,450 --> 00:18:00,610 So he had this idea that instead of having this tub scum build 336 00:18:00,610 --> 00:18:03,890 up so much that it was a real problem to clean it, 337 00:18:03,890 --> 00:18:06,090 that you could take all of these things 338 00:18:06,090 --> 00:18:08,880 that chemists knew would be useful 339 00:18:08,880 --> 00:18:13,430 and package them in something that was a daily cleaner. 340 00:18:13,430 --> 00:18:16,220 So he advertised these things that, every time you 341 00:18:16,220 --> 00:18:19,340 take a shower, you just spray the shower or the tub, 342 00:18:19,340 --> 00:18:22,967 and then you never have to scrub your tub again. 343 00:18:22,967 --> 00:18:24,550 Because you do a little bit every day, 344 00:18:24,550 --> 00:18:26,530 where there's not a lot of surface, 345 00:18:26,530 --> 00:18:29,230 so it gets it right off, right away. 346 00:18:29,230 --> 00:18:34,330 And Mr. Black and his wife never had to clean their tub again, 347 00:18:34,330 --> 00:18:38,870 because this resulted in sales of about $70 million per year. 348 00:18:38,870 --> 00:18:40,740 And it's important to point out that he 349 00:18:40,740 --> 00:18:42,740 wasn't using anything new. 350 00:18:42,740 --> 00:18:47,200 He was just packaging his chemicals for daily use, 351 00:18:47,200 --> 00:18:50,380 rather than, say, for a weekly or monthly use. 352 00:18:50,380 --> 00:18:52,940 So I think there's a couple important lessons from this. 353 00:18:52,940 --> 00:18:56,030 One, it's always important to clean your bathroom. 354 00:18:56,030 --> 00:18:59,770 Two, you never know where chemistry knowledge is going 355 00:18:59,770 --> 00:19:02,520 to come, what life altering event 356 00:19:02,520 --> 00:19:06,450 will cause you to realize that you have in your mind chemistry 357 00:19:06,450 --> 00:19:10,100 knowledge that can make you $70 million a year. 358 00:19:10,100 --> 00:19:12,600 And when that life altering event occurs, 359 00:19:12,600 --> 00:19:17,800 remember that I take cash and checks and stock options. 360 00:19:17,800 --> 00:19:18,790 I added the last one. 361 00:19:18,790 --> 00:19:21,240 I thought, cash and checks-- I should be more flexible. 362 00:19:21,240 --> 00:19:24,230 So I'm also going to take stock options. 363 00:19:24,230 --> 00:19:27,610 And this is why I teach you all the fundamentals of chemistry, 364 00:19:27,610 --> 00:19:30,860 cover everything that's really important for you to know, 365 00:19:30,860 --> 00:19:35,400 because then if you use any of it, you've learned it from me. 366 00:19:38,010 --> 00:19:39,690 So just keep that in mind. 367 00:19:39,690 --> 00:19:43,416 One never knows what life event is ahead of you. 368 00:19:45,930 --> 00:19:49,580 So what about gas solubility? 369 00:19:49,580 --> 00:19:51,300 We talked about solids. 370 00:19:51,300 --> 00:19:54,330 Now let's talk about gases. 371 00:19:54,330 --> 00:19:57,520 So whether a gas is going to be soluble 372 00:19:57,520 --> 00:20:00,090 is going to depend on Henry's Law, 373 00:20:00,090 --> 00:20:02,320 or we can find out about it solubility. 374 00:20:02,320 --> 00:20:04,460 So a solubility of a gas-- we're going 375 00:20:04,460 --> 00:20:06,850 to call that little s, that's our solubility-- 376 00:20:06,850 --> 00:20:09,480 is directly proportional to the partial pressure 377 00:20:09,480 --> 00:20:13,370 of the gas and a constant called Henry's Constant. 378 00:20:13,370 --> 00:20:15,580 And that depends on the nature of the gas 379 00:20:15,580 --> 00:20:18,380 and the solvent and the temperature. 380 00:20:18,380 --> 00:20:24,240 So let's think about this rule in this plot in a clicker 381 00:20:24,240 --> 00:20:24,740 question. 382 00:20:42,540 --> 00:20:44,170 All right, just take 10 more seconds. 383 00:20:58,060 --> 00:20:59,530 Awesome. 384 00:20:59,530 --> 00:21:00,336 That is correct. 385 00:21:03,550 --> 00:21:05,870 So let's just take a look at that. 386 00:21:05,870 --> 00:21:08,370 So one of the clicker questions, which 387 00:21:08,370 --> 00:21:11,430 is kind of getting you to read the graph, 388 00:21:11,430 --> 00:21:16,150 and so it was asking, say at 0.5 atmospheres, 389 00:21:16,150 --> 00:21:18,990 it said oxygen is more soluble than helium. 390 00:21:18,990 --> 00:21:22,120 This is molar solubility versus partial pressure. 391 00:21:22,120 --> 00:21:26,070 At point 0.5, sure enough, oxygen is more soluble. 392 00:21:26,070 --> 00:21:28,350 So that was just reading the plot. 393 00:21:28,350 --> 00:21:32,840 The other one was just interpreting Henry's Law again, 394 00:21:32,840 --> 00:21:37,830 that solubility would increase as partial pressure increases. 395 00:21:37,830 --> 00:21:39,930 You can see that from the equation. 396 00:21:39,930 --> 00:21:42,960 But you can also think about why that's true, and why it's true 397 00:21:42,960 --> 00:21:46,430 is the answer to the three, that the solubility of a gas 398 00:21:46,430 --> 00:21:49,020 is proportional to its partial pressure. 399 00:21:49,020 --> 00:21:51,330 An increase in pressure corresponds 400 00:21:51,330 --> 00:21:55,480 to an increase in rate at which the gas molecules strike 401 00:21:55,480 --> 00:21:56,140 the surface. 402 00:21:56,140 --> 00:21:57,750 And that makes them more soluble. 403 00:21:57,750 --> 00:21:59,720 So again, solubility really has to do 404 00:21:59,720 --> 00:22:01,830 with getting at that surface. 405 00:22:01,830 --> 00:22:06,910 If you get at the surface, then you can dissolve your material. 406 00:22:06,910 --> 00:22:08,290 All right, so why should you care 407 00:22:08,290 --> 00:22:10,930 about the solubility of gases? 408 00:22:10,930 --> 00:22:14,570 And I'm going to give you an example. 409 00:22:14,570 --> 00:22:16,100 This is another in your own words, 410 00:22:16,100 --> 00:22:17,800 so we'll watch the video. 411 00:22:17,800 --> 00:22:20,850 And I think tonight there's some kind of big climate change 412 00:22:20,850 --> 00:22:21,350 thing. 413 00:22:21,350 --> 00:22:22,740 I've seen fliers around. 414 00:22:22,740 --> 00:22:25,150 I haven't paid enough attention to what's going on. 415 00:22:25,150 --> 00:22:29,590 But climate change is certainly a very hot topic, 416 00:22:29,590 --> 00:22:32,721 if you'll excuse that right now. 417 00:22:32,721 --> 00:22:33,720 I didn't really mean it. 418 00:22:33,720 --> 00:22:35,620 It just kind of came out. 419 00:22:35,620 --> 00:22:38,140 But CO2 is a greenhouse gas. 420 00:22:38,140 --> 00:22:39,530 It's a big problem. 421 00:22:39,530 --> 00:22:41,870 And so a number of researchers here at MIT 422 00:22:41,870 --> 00:22:46,170 are thinking about ways that you can capture and store it 423 00:22:46,170 --> 00:22:49,070 somewhere, get it out of our atmosphere. 424 00:22:49,070 --> 00:22:51,010 Other people are thinking about ways 425 00:22:51,010 --> 00:22:54,660 that you can change industrial processes to make less CO2. 426 00:22:54,660 --> 00:22:56,480 There's a lot of research going on. 427 00:22:56,480 --> 00:22:59,080 So today you'll hear from Hector Hernandez, who 428 00:22:59,080 --> 00:23:01,850 had a Ph.D. in chemistry from the chemistry department 429 00:23:01,850 --> 00:23:05,640 here at MIT, did his post-doc here at MIT in chemical 430 00:23:05,640 --> 00:23:09,330 engineering, and is now a professor of chemical 431 00:23:09,330 --> 00:23:14,610 engineering at Masdar Institute in the United Arab Emirates. 432 00:23:14,610 --> 00:23:18,020 So this is one of the MIT satellite universities 433 00:23:18,020 --> 00:23:20,470 that are popping up all over the world. 434 00:23:20,470 --> 00:23:23,120 So Hector was born, I think, in Honduras, 435 00:23:23,120 --> 00:23:25,530 and then lived in Florida, ended up in MIT, 436 00:23:25,530 --> 00:23:27,800 and now he's at the United Arab Emirates. 437 00:23:27,800 --> 00:23:29,680 So that's just another example of, 438 00:23:29,680 --> 00:23:32,020 one never knows what's going to happen to you. 439 00:23:32,020 --> 00:23:35,745 All right, so you can hear from Hector in his own words. 440 00:23:38,390 --> 00:23:44,710 So Hector's personal video's online. 441 00:23:44,710 --> 00:23:46,076 I think it's a fun one to watch. 442 00:23:46,076 --> 00:23:47,700 There's a couple things that are really 443 00:23:47,700 --> 00:23:52,140 interesting about his personal story, including the fact 444 00:23:52,140 --> 00:23:55,760 that as he mentioned in past life he did some construction, 445 00:23:55,760 --> 00:23:59,630 he started undergrad at age 30, I think, 446 00:23:59,630 --> 00:24:03,810 where he was basically building houses and fixing cars 447 00:24:03,810 --> 00:24:07,820 and he realized that his back might not survive this 448 00:24:07,820 --> 00:24:09,500 being his permanent career. 449 00:24:09,500 --> 00:24:12,490 So he decided to use his brain instead of his back. 450 00:24:12,490 --> 00:24:15,280 And so started undergraduate, now he's a professor. 451 00:24:15,280 --> 00:24:19,550 So it just shows there's a lot of different paths to success. 452 00:24:19,550 --> 00:24:23,240 All right so C02-- big problem. 453 00:24:23,240 --> 00:24:24,859 We were trying to use, in that case, 454 00:24:24,859 --> 00:24:26,775 he was interested in turning it into biofuels, 455 00:24:26,775 --> 00:24:28,880 they were interested in ways to store it. 456 00:24:28,880 --> 00:24:30,780 You care about solubility of gases 457 00:24:30,780 --> 00:24:32,850 and you care about Le Chatelier's principle 458 00:24:32,850 --> 00:24:35,010 of driving things the direction that you want 459 00:24:35,010 --> 00:24:36,890 to make the products you want. 460 00:24:36,890 --> 00:24:40,680 OK, so let's think more about these factors that 461 00:24:40,680 --> 00:24:42,610 affect solubility. 462 00:24:42,610 --> 00:24:44,670 And one thing that's very important, 463 00:24:44,670 --> 00:24:47,450 for sure, is temperature. 464 00:24:47,450 --> 00:24:53,980 So temperature-- most substances dissolve more quickly 465 00:24:53,980 --> 00:24:56,950 at higher temperature, which is often what you want. 466 00:24:56,950 --> 00:25:00,890 But it doesn't always mean if you increase the temperature 467 00:25:00,890 --> 00:25:03,210 that you're going to make it more soluble. 468 00:25:03,210 --> 00:25:05,460 You may just make it dissolve faster, 469 00:25:05,460 --> 00:25:08,270 but not actually change the end product. 470 00:25:08,270 --> 00:25:11,520 Some things are more sensitive to temperature than others. 471 00:25:11,520 --> 00:25:16,340 So most gases, for example, are less soluble in warm water 472 00:25:16,340 --> 00:25:19,390 than in cold water, but with solids, it's 473 00:25:19,390 --> 00:25:23,450 much harder to predict what effect temperature might have 474 00:25:23,450 --> 00:25:26,670 on their solubility, even though for many things 475 00:25:26,670 --> 00:25:29,940 it will make it faster. 476 00:25:29,940 --> 00:25:34,070 So let's think now about how to drive a reaction, 477 00:25:34,070 --> 00:25:37,820 and what factors are involved, and what's favorable 478 00:25:37,820 --> 00:25:39,410 and what's not favorable. 479 00:25:39,410 --> 00:25:41,690 And when we're talking about those things, 480 00:25:41,690 --> 00:25:44,200 we're going to be back to our thermodynamics, 481 00:25:44,200 --> 00:25:49,440 back to enthalpy, entropy, and Gibb's free energy. 482 00:25:49,440 --> 00:25:52,120 So all these have little special sub names. 483 00:25:52,120 --> 00:25:55,090 So when we're talking about delta H 484 00:25:55,090 --> 00:25:58,510 and we're talking about solutions, things dissolving, 485 00:25:58,510 --> 00:26:02,110 we're talking about delta H sub sol, 486 00:26:02,110 --> 00:26:07,710 for solution, or if you could think of it as solubility. 487 00:26:07,710 --> 00:26:09,850 And one point that I'll make-- a lot of times 488 00:26:09,850 --> 00:26:12,180 when people are learning a new field, they're like, 489 00:26:12,180 --> 00:26:14,130 oh, there's so much to learn. 490 00:26:14,130 --> 00:26:16,000 But you get to some point, everything 491 00:26:16,000 --> 00:26:18,750 is just kind of like a subtle difference in something 492 00:26:18,750 --> 00:26:19,880 you've already learned. 493 00:26:19,880 --> 00:26:21,820 So if you already learned about delta H, 494 00:26:21,820 --> 00:26:23,680 this is just a slight modification 495 00:26:23,680 --> 00:26:25,130 of what you learned. 496 00:26:25,130 --> 00:26:27,020 You don't have to learn a whole new thing. 497 00:26:27,020 --> 00:26:29,210 So that's one of the good things about the course. 498 00:26:29,210 --> 00:26:32,880 You start recognizing, I already pretty much know this. 499 00:26:32,880 --> 00:26:36,680 I just need to add a little sol to the end. 500 00:26:36,680 --> 00:26:40,540 So delta H of solution can be measured. 501 00:26:40,540 --> 00:26:43,860 You can measure it from heat released when something 502 00:26:43,860 --> 00:26:47,720 dissolves, or measure it by the heat absorbed 503 00:26:47,720 --> 00:26:50,550 and at constant pressure. 504 00:26:50,550 --> 00:26:55,390 And so the sign of delta H sol will tell you 505 00:26:55,390 --> 00:27:00,190 about whether heat is released or heat is required 506 00:27:00,190 --> 00:27:01,920 when something dissolves. 507 00:27:01,920 --> 00:27:04,760 So a negative enthalpy of solution 508 00:27:04,760 --> 00:27:08,390 tells us that heat is released when something dissolves. 509 00:27:08,390 --> 00:27:11,660 And many of you may have just experienced this. 510 00:27:11,660 --> 00:27:13,382 If you're mixing something together 511 00:27:13,382 --> 00:27:15,590 and you're holding the container, and all of a sudden 512 00:27:15,590 --> 00:27:21,760 it heats up, then you know delta H sol is a negative value. 513 00:27:21,760 --> 00:27:24,590 And of course a positive value tells you 514 00:27:24,590 --> 00:27:28,930 that that energy is absorbed, and sometimes things 515 00:27:28,930 --> 00:27:32,890 can get very cold when they're dissolving. 516 00:27:32,890 --> 00:27:38,400 So you already kind of knew that that negative delta H tells you 517 00:27:38,400 --> 00:27:41,730 that heat is released, and positive delta H tells you 518 00:27:41,730 --> 00:27:43,380 that heat is absorbed. 519 00:27:43,380 --> 00:27:47,070 So this is basically the same thing that you already learned. 520 00:27:47,070 --> 00:27:50,290 It just applies to things dissolving just the same 521 00:27:50,290 --> 00:27:52,470 as it does to any reaction. 522 00:27:52,470 --> 00:27:52,970 All right. 523 00:27:52,970 --> 00:27:57,530 Now, suppose we want to know whether the reaction is 524 00:27:57,530 --> 00:28:00,140 going to be spontaneous at constant pressure 525 00:28:00,140 --> 00:28:01,590 or temperature. 526 00:28:01,590 --> 00:28:07,060 If we want to know whether it will dissolve spontaneously, 527 00:28:07,060 --> 00:28:08,430 what am I asking about? 528 00:28:08,430 --> 00:28:09,900 What term do I want to know? 529 00:28:14,190 --> 00:28:17,460 I want to know about delta G. I sure do. 530 00:28:17,460 --> 00:28:21,290 Delta G tells us whether things are spontaneous or not. 531 00:28:21,290 --> 00:28:24,890 It does with thermodynamics, it does with solubility. 532 00:28:24,890 --> 00:28:27,320 Delta G is to predictor of whether something 533 00:28:27,320 --> 00:28:30,060 is going to be spontaneous or non-spontaneous 534 00:28:30,060 --> 00:28:35,370 by the sine of delta G. 535 00:28:35,370 --> 00:28:38,160 So entropy-- don't want to leave out entropy. 536 00:28:38,160 --> 00:28:39,840 Never leave out entropy. 537 00:28:39,840 --> 00:28:43,050 My t-shirt's still unaccounted for, 538 00:28:43,050 --> 00:28:45,800 after they got back to Massachusetts, 539 00:28:45,800 --> 00:28:48,320 they left for New Jersey and Cincinnati again. 540 00:28:48,320 --> 00:28:52,000 Last time I checked, they were still in Ohio. 541 00:28:52,000 --> 00:28:54,780 Entropy. 542 00:28:54,780 --> 00:28:57,980 So since disorder typically increases 543 00:28:57,980 --> 00:29:01,400 when a solvent dissolves, typically, entropy 544 00:29:01,400 --> 00:29:02,390 should increase. 545 00:29:02,390 --> 00:29:03,889 And there's a little star, and we'll 546 00:29:03,889 --> 00:29:06,840 get back to the star, because it doesn't always increase. 547 00:29:06,840 --> 00:29:10,240 But typically, it should increase. 548 00:29:10,240 --> 00:29:12,700 So if we're looking at these pictures here, 549 00:29:12,700 --> 00:29:15,950 over here you have your nice molecules all 550 00:29:15,950 --> 00:29:20,280 lined up in your solid, and it's going to then dissolve. 551 00:29:20,280 --> 00:29:22,470 And you can see that when it's dissolved, 552 00:29:22,470 --> 00:29:24,550 that's a lot more entropy over there. 553 00:29:24,550 --> 00:29:26,600 There are a lot more freedom moving around. 554 00:29:26,600 --> 00:29:28,600 This is more constrained. 555 00:29:28,600 --> 00:29:33,650 So typically, entropy increases when things dissolve. 556 00:29:33,650 --> 00:29:39,020 So if delta H is negative, and delta S increases 557 00:29:39,020 --> 00:29:41,620 when a solutes dissolves, then what 558 00:29:41,620 --> 00:29:44,996 do we expect about the dissolving process? 559 00:29:44,996 --> 00:29:46,245 And that's a clicker question. 560 00:30:03,720 --> 00:30:04,350 Yeah. 561 00:30:04,350 --> 00:30:06,630 So most people have, so we'll take 10 more seconds. 562 00:30:21,590 --> 00:30:23,910 So let's look at both of them, because we're 563 00:30:23,910 --> 00:30:27,300 going to do this one in a few minutes. 564 00:30:27,300 --> 00:30:29,832 So let's just continue with our notes, 565 00:30:29,832 --> 00:30:31,540 and we're going to come back to that one. 566 00:30:31,540 --> 00:30:32,081 I don't know. 567 00:30:32,081 --> 00:30:35,384 Can we just leave that one up, maybe for a minute? 568 00:30:35,384 --> 00:30:36,800 I don't know how long it was going 569 00:30:36,800 --> 00:30:38,260 to take to me to get there. 570 00:30:38,260 --> 00:30:44,652 All right, so when we're talking about this-- 571 00:30:44,652 --> 00:30:46,110 we'll just keep things in the notes 572 00:30:46,110 --> 00:30:47,443 and we'll come back to that one. 573 00:30:47,443 --> 00:30:51,810 So if this is negative and delta S 574 00:30:51,810 --> 00:30:56,480 is increasing-- so if this is negative and this is positive, 575 00:30:56,480 --> 00:30:59,990 then you're going to have a spontaneous process here. 576 00:30:59,990 --> 00:31:03,520 So if you can put in your notes that this should be spontaneous 577 00:31:03,520 --> 00:31:06,490 under these circumstances. 578 00:31:06,490 --> 00:31:10,240 Now let's continue on and think about getting around 579 00:31:10,240 --> 00:31:13,010 to the next one. 580 00:31:13,010 --> 00:31:18,840 So in some cases-- I don't know, hopefully people 581 00:31:18,840 --> 00:31:22,330 can see this-- in some cases, entropy of the system 582 00:31:22,330 --> 00:31:24,860 is actually lowered when something dissolves. 583 00:31:24,860 --> 00:31:27,760 And this is because of what's known as this cage effect. 584 00:31:27,760 --> 00:31:30,860 If you have water molecules, when something dissolves, 585 00:31:30,860 --> 00:31:33,460 sometimes order around the thing. 586 00:31:33,460 --> 00:31:38,970 And so the water entropy is determining the process. 587 00:31:38,970 --> 00:31:45,070 And have this new order because of the water structure. 588 00:31:45,070 --> 00:31:46,380 So this can change. 589 00:31:46,380 --> 00:31:50,430 So it's not always the case that entropy of the overall system 590 00:31:50,430 --> 00:31:52,780 is going to increase. 591 00:31:52,780 --> 00:31:55,700 So here, even if this is a negative value, 592 00:31:55,700 --> 00:31:59,110 delta H solution is negative, delta G might be positive. 593 00:31:59,110 --> 00:32:01,790 It might not be a spontaneous system. 594 00:32:01,790 --> 00:32:04,860 And so the cage effect is why some things that 595 00:32:04,860 --> 00:32:07,960 are hydrophobic, just you're having a hard time 596 00:32:07,960 --> 00:32:12,250 dissolving them, even if they have a negative enthalpy, even 597 00:32:12,250 --> 00:32:14,210 if you would just look at the negative enthalpy 598 00:32:14,210 --> 00:32:16,500 and predict that it should be spontaneous. 599 00:32:16,500 --> 00:32:19,820 Because even if this is negative, if delta 600 00:32:19,820 --> 00:32:21,880 S is also negative because you have 601 00:32:21,880 --> 00:32:24,400 more order due to the water molecules, 602 00:32:24,400 --> 00:32:26,770 then the overall term can be positive. 603 00:32:26,770 --> 00:32:29,940 It would depend on the magnitude of these 604 00:32:29,940 --> 00:32:30,962 and on the temperature. 605 00:32:35,366 --> 00:32:37,240 This is actually more slides than I remember. 606 00:32:37,240 --> 00:32:39,600 Before we get to the next-- oh, this is the last one. 607 00:32:39,600 --> 00:32:45,776 OK, so when gases are dissolved in a liquid, 608 00:32:45,776 --> 00:32:46,900 there is much less freedom. 609 00:32:46,900 --> 00:32:49,890 Gases not dissolved can be anywhere. 610 00:32:49,890 --> 00:32:54,320 But when they're in a liquid, then there's much less freedom. 611 00:32:54,320 --> 00:32:58,140 And so the entropy then is going to be negative, 612 00:32:58,140 --> 00:33:03,480 and the solubility will decrease as the temperature rises. 613 00:33:03,480 --> 00:33:08,540 And so that brings us then to our question about delta 614 00:33:08,540 --> 00:33:12,500 H of solution being positive. 615 00:33:12,500 --> 00:33:14,587 And now we can look at the answer to that. 616 00:33:18,720 --> 00:33:21,830 Now I guess we can go maybe to the other slide. 617 00:33:21,830 --> 00:33:24,470 So that was the question that you had here, 618 00:33:24,470 --> 00:33:27,220 and I've just put up the answers. 619 00:33:27,220 --> 00:33:30,990 So here, delta H is positive. 620 00:33:30,990 --> 00:33:35,850 And so we have positive minus T delta S again. 621 00:33:35,850 --> 00:33:39,930 We weren't told anything about delta S in this particular. 622 00:33:39,930 --> 00:33:43,390 Just ask what if it's positive. 623 00:33:43,390 --> 00:33:46,530 And so for the first one, it said dissolving is never 624 00:33:46,530 --> 00:33:47,880 spontaneous. 625 00:33:47,880 --> 00:33:51,230 And that isn't necessarily true, because you 626 00:33:51,230 --> 00:33:53,540 don't know what delta S is. 627 00:33:53,540 --> 00:33:56,750 Dissolving is only spontaneous if T delta S 628 00:33:56,750 --> 00:34:01,160 is positive and larger than delta H solution. 629 00:34:01,160 --> 00:34:02,790 And that was the correct answer. 630 00:34:02,790 --> 00:34:06,950 If this is a big term here, it's larger than that term, 631 00:34:06,950 --> 00:34:09,320 and this is a positive value, then you 632 00:34:09,320 --> 00:34:11,880 would get a negative delta G, and it 633 00:34:11,880 --> 00:34:14,969 would be a spontaneous process. 634 00:34:14,969 --> 00:34:19,110 If we look at number three here, dissolving is only spontaneous 635 00:34:19,110 --> 00:34:24,030 if this is negative-- that does not help us at all. 636 00:34:24,030 --> 00:34:26,520 And the rate will be slower. 637 00:34:26,520 --> 00:34:29,870 Delta H is not going to tell us anything about rates. 638 00:34:29,870 --> 00:34:32,790 Rates are determined by kinetics. 639 00:34:32,790 --> 00:34:34,710 Temperature can affect the rates, 640 00:34:34,710 --> 00:34:37,489 for sure, stirring can affect the rates for sure, 641 00:34:37,489 --> 00:34:44,500 but delta H being positive is not telling us about the rates. 642 00:34:44,500 --> 00:34:48,600 So you'll see in this particular unit, 643 00:34:48,600 --> 00:34:50,530 when you're thinking about solubility, 644 00:34:50,530 --> 00:34:53,370 you're thinking about equilibrium expressions, 645 00:34:53,370 --> 00:34:56,120 especially equilibrium expression KSP, 646 00:34:56,120 --> 00:35:00,270 you're thinking about partial pressures again, 647 00:35:00,270 --> 00:35:04,550 you're thinking about effects on the solubility that 648 00:35:04,550 --> 00:35:06,100 have to do with temperatures, you're 649 00:35:06,100 --> 00:35:09,810 thinking about delta H, delta S, and delta G. 650 00:35:09,810 --> 00:35:14,470 So it's really an outgrowth of thermodynamics and chemical 651 00:35:14,470 --> 00:35:17,100 equilibrium, but you're just applying, really, 652 00:35:17,100 --> 00:35:19,240 what you've already learned to thinking 653 00:35:19,240 --> 00:35:22,470 about a solute dissolving in a solvent. 654 00:35:22,470 --> 00:35:24,500 So that's one of the things that sets really 655 00:35:24,500 --> 00:35:27,270 great about the course and the next couple units, 656 00:35:27,270 --> 00:35:29,830 is that everything's very connected. 657 00:35:29,830 --> 00:35:33,070 So we can just briefly introduce you to the next unit, 658 00:35:33,070 --> 00:35:36,310 and we're going to be talking about acids and bases now 659 00:35:36,310 --> 00:35:38,440 for several lectures. 660 00:35:38,440 --> 00:35:42,435 And I have to say that this is one of the units when 661 00:35:42,435 --> 00:35:44,820 we get to kind of acid-base titrations 662 00:35:44,820 --> 00:35:47,200 that MIT students seem to struggle with. 663 00:35:47,200 --> 00:35:51,380 So we're going to slow this down and really 664 00:35:51,380 --> 00:35:55,343 go over it, and have you be awesome at acids and bases. 665 00:35:58,570 --> 00:36:02,550 And I'll mention that one of the reasons why I spent time 666 00:36:02,550 --> 00:36:06,010 on solubility and acids, bases, because these are topics that 667 00:36:06,010 --> 00:36:09,340 are really important for medical school, and I know a lot of you 668 00:36:09,340 --> 00:36:11,230 are not pre-med, but I want everybody 669 00:36:11,230 --> 00:36:14,620 in this class to have that background that they could 670 00:36:14,620 --> 00:36:16,610 go to med school if they wanted to, 671 00:36:16,610 --> 00:36:21,240 or they could just advocate for their own health 672 00:36:21,240 --> 00:36:24,800 with a doctor who doesn't know about acids and bases. 673 00:36:24,800 --> 00:36:29,230 And I'll tell you a story about that a little later. 674 00:36:29,230 --> 00:36:33,480 Anyway, OK, so acids and bases-- we're 675 00:36:33,480 --> 00:36:35,820 just going to some definitions. 676 00:36:35,820 --> 00:36:40,910 So the simplest definition of this 677 00:36:40,910 --> 00:36:44,960 is that an acid is a substance that when dissolved in water, 678 00:36:44,960 --> 00:36:50,540 increases the concentration of hydrogen ions, H plus. 679 00:36:50,540 --> 00:36:53,060 Whereas a base is a substance that 680 00:36:53,060 --> 00:36:58,800 increases the hydroxide concentration, OH minus. 681 00:36:58,800 --> 00:37:01,040 So there are acids and bases that definitely 682 00:37:01,040 --> 00:37:04,940 fit this description, but this is a very narrow description 683 00:37:04,940 --> 00:37:07,910 of acids and bases. 684 00:37:07,910 --> 00:37:10,600 The next one, the Bronsted-Lowry, 685 00:37:10,600 --> 00:37:12,910 is a bit broader. 686 00:37:12,910 --> 00:37:15,390 So an acid is something that donates 687 00:37:15,390 --> 00:37:20,390 a hydrogen ion, H plus, and a Bronsted-Lowry base 688 00:37:20,390 --> 00:37:23,750 is something that accepts a hydrogen ion. 689 00:37:23,750 --> 00:37:25,220 And this is really the definition 690 00:37:25,220 --> 00:37:28,990 we'll be spending the most time with in the class. 691 00:37:28,990 --> 00:37:33,730 So let's look at some examples of this. 692 00:37:33,730 --> 00:37:38,620 So here, we have an acid. 693 00:37:38,620 --> 00:37:44,070 And if it's an acid, it's going to be donating a hydrogen ion, 694 00:37:44,070 --> 00:37:47,090 and it's going to donate it here to the water. 695 00:37:47,090 --> 00:37:52,360 The water acts as a base in that it accepts the hydrogen ion. 696 00:37:52,360 --> 00:37:54,400 When it accepts the hydrogen ion, 697 00:37:54,400 --> 00:38:01,030 it has one extra hydrogen ion, so it's H3O plus. 698 00:38:01,030 --> 00:38:04,350 And when the acid loses its hydrogen ion, 699 00:38:04,350 --> 00:38:08,070 it becomes this base over here, the same molecule 700 00:38:08,070 --> 00:38:11,760 minus the hydrogen ion, so now it has a negative charge. 701 00:38:11,760 --> 00:38:15,200 And so you'll will see hydronium ions 702 00:38:15,200 --> 00:38:17,660 quite a bit, which is H3O plus. 703 00:38:17,660 --> 00:38:19,590 That's really the more accurate form, 704 00:38:19,590 --> 00:38:23,110 rather than just writing H plus somewhere. 705 00:38:23,110 --> 00:38:25,750 So that's the kind of true nature of things. 706 00:38:25,750 --> 00:38:28,680 So let me just show you a little animation of this happening. 707 00:38:28,680 --> 00:38:29,790 I think it's kind of cute. 708 00:38:33,930 --> 00:38:40,155 So here we have our water molecules and we have our acid. 709 00:38:40,155 --> 00:38:43,000 It donated a hydrogen ion to these, 710 00:38:43,000 --> 00:38:45,420 and then another water came and stole it away. 711 00:38:45,420 --> 00:38:48,330 And so there's our H3O plus. 712 00:38:48,330 --> 00:38:51,370 And so in solution, you have this exchange 713 00:38:51,370 --> 00:38:52,590 of hydrogen atoms. 714 00:38:52,590 --> 00:38:55,040 The acid is donating, the base is accepting. 715 00:38:55,040 --> 00:38:57,340 And as they donate and accept, they 716 00:38:57,340 --> 00:38:59,410 become different molecules. 717 00:38:59,410 --> 00:39:04,920 So it gives rise to this idea of conjugate acid-base pairs. 718 00:39:04,920 --> 00:39:10,440 So here you see that this acid is paired 719 00:39:10,440 --> 00:39:12,170 with this base over here. 720 00:39:12,170 --> 00:39:15,780 They're the same, except that one has the hydrogen ion 721 00:39:15,780 --> 00:39:17,690 and one does not. 722 00:39:17,690 --> 00:39:20,600 And then the other pair is here. 723 00:39:20,600 --> 00:39:22,370 It's another acid-base pair. 724 00:39:22,370 --> 00:39:27,300 We have water and hydronium ion that also differ by H plus. 725 00:39:27,300 --> 00:39:32,250 So every time an acid donates a hydrogen ion or proton, 726 00:39:32,250 --> 00:39:34,050 it becomes its conjugate base. 727 00:39:34,050 --> 00:39:37,560 Every time a base accepts a hydrogen ion or proton, 728 00:39:37,560 --> 00:39:41,140 it becomes its conjugate acid. 729 00:39:41,140 --> 00:39:44,140 So the conjugate base of any acid 730 00:39:44,140 --> 00:39:48,480 is the base that is formed when the acid has donated a hydrogen 731 00:39:48,480 --> 00:39:50,030 ion or proton. 732 00:39:50,030 --> 00:39:52,440 The conjugate acid of a base is the acid 733 00:39:52,440 --> 00:39:58,500 that forms when the base accepts a hydrogen ion or proton. 734 00:39:58,500 --> 00:40:01,780 So let's take a look at one more example. 735 00:40:01,780 --> 00:40:06,080 If this over here-- is this acting as an acid or base, 736 00:40:06,080 --> 00:40:07,021 this molecule here? 737 00:40:07,021 --> 00:40:07,770 What do you think? 738 00:40:07,770 --> 00:40:10,550 You can just yell it out. 739 00:40:10,550 --> 00:40:12,650 It's acting as an acid, right? 740 00:40:12,650 --> 00:40:15,430 This is acting, then, as the base. 741 00:40:15,430 --> 00:40:18,140 When this base accepts the hydrogen ion, 742 00:40:18,140 --> 00:40:20,400 it forms its conjugate acid. 743 00:40:20,400 --> 00:40:23,380 And this then forms its conjugate base, 744 00:40:23,380 --> 00:40:27,910 which differs by H plus. 745 00:40:27,910 --> 00:40:32,638 So why don't you try one of these, then, on your own? 746 00:40:43,710 --> 00:40:44,410 10 more seconds. 747 00:41:02,750 --> 00:41:07,780 OK, so we can go back to that over there. 748 00:41:07,780 --> 00:41:11,420 So you most people got this right, 749 00:41:11,420 --> 00:41:14,830 and so you're looking at what's acting as the base 750 00:41:14,830 --> 00:41:16,920 and what's acting as the acid. 751 00:41:16,920 --> 00:41:20,600 Again, the base is going to be accepting a hydrogen ion. 752 00:41:20,600 --> 00:41:22,340 So this is acting as the acid. 753 00:41:22,340 --> 00:41:25,240 This forms it's conjugate it and this. 754 00:41:25,240 --> 00:41:32,880 And quickly, with the idea of something that's amphoteric, 755 00:41:32,880 --> 00:41:35,920 which is molecule that can act as an acid or a base. 756 00:41:35,920 --> 00:41:39,030 You just saw some nice examples of that. 757 00:41:39,030 --> 00:41:45,110 And finally, just one last definition for acid-bases. 758 00:41:45,110 --> 00:41:52,300 And we'll just put this up and that's kind of the end. 759 00:41:52,300 --> 00:41:55,060 So the final definition, the Lewis Base, 760 00:41:55,060 --> 00:41:57,700 donates a lone pair, whereas a Lewis Acid 761 00:41:57,700 --> 00:42:01,170 accepts such electrons.