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 --> 00:00:12,670 To make a donation or to view additional materials 6 00:00:12,670 --> 00:00:16,580 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,580 --> 00:00:17,963 at ocw.mit.edu. 8 00:00:26,204 --> 00:00:27,620 CATHY DRENNAN: So now, we're going 9 00:00:27,620 --> 00:00:34,220 to move on to talk about spontaneous change. 10 00:00:34,220 --> 00:00:36,076 And so this is today's handout. 11 00:00:43,670 --> 00:00:46,790 So spontaneous reaction is a reaction 12 00:00:46,790 --> 00:00:50,010 that proceeds in the forward direction 13 00:00:50,010 --> 00:00:52,600 without any kind of outside intervention, 14 00:00:52,600 --> 00:00:55,180 like heat being added, for example. 15 00:00:55,180 --> 00:01:00,630 It just goes in that direction. 16 00:01:04,819 --> 00:01:09,170 So we can talk about the following reactions 17 00:01:09,170 --> 00:01:13,880 are spontaneous at constant pressure. 18 00:01:13,880 --> 00:01:15,440 And we'll see later that temperature 19 00:01:15,440 --> 00:01:18,180 can make a difference between whether something's spontaneous 20 00:01:18,180 --> 00:01:19,570 or not. 21 00:01:19,570 --> 00:01:24,850 But constant pressure, here's an example, iron plus oxygen. 22 00:01:24,850 --> 00:01:28,310 And what is this in layman's term an example of? 23 00:01:28,310 --> 00:01:29,030 AUDIENCE: Rust. 24 00:01:29,030 --> 00:01:31,080 CATHY DRENNAN: Rust, yes. 25 00:01:31,080 --> 00:01:33,100 And many of you are probably aware of this, 26 00:01:33,100 --> 00:01:35,850 rust is a spontaneous process. 27 00:01:35,850 --> 00:01:38,270 It's something that people try to do something about. 28 00:01:38,270 --> 00:01:40,110 You don't want your car to rust. 29 00:01:40,110 --> 00:01:42,740 If you're new to New England and you're 30 00:01:42,740 --> 00:01:48,750 from a part of the country that doesn't get so cold, 31 00:01:48,750 --> 00:01:51,000 you'll look at people's cars and you're like, wow, 32 00:01:51,000 --> 00:01:52,500 look at all that rust all over them. 33 00:01:52,500 --> 00:01:55,720 Yes, rust happens, especially in New England. 34 00:01:55,720 --> 00:01:58,640 And delta H here is negative. 35 00:01:58,640 --> 00:02:00,575 Is this endothermic or exothermic? 36 00:02:00,575 --> 00:02:01,450 AUDIENCE: Exothermic. 37 00:02:01,450 --> 00:02:05,840 CATHY DRENNAN: Exothermic, minus 824. 38 00:02:05,840 --> 00:02:09,990 Here is another spontaneous process. 39 00:02:09,990 --> 00:02:13,270 This molecule is ATP. 40 00:02:13,270 --> 00:02:17,890 And it will hydrolyze, which means react with water, 41 00:02:17,890 --> 00:02:19,440 forming ADP. 42 00:02:19,440 --> 00:02:22,070 So ATP is triphosphate. 43 00:02:22,070 --> 00:02:24,140 ADP is diphosphate. 44 00:02:24,140 --> 00:02:25,890 So one of the phosphates-- and here's 45 00:02:25,890 --> 00:02:27,930 the phosphate-- comes off. 46 00:02:27,930 --> 00:02:29,860 It hydrolyzes off. 47 00:02:29,860 --> 00:02:34,780 And this is a spontaneous process. 48 00:02:34,780 --> 00:02:40,700 And we also have a delta H0 of minus 24 kilojoules per mole. 49 00:02:40,700 --> 00:02:43,380 And remember, when we oxidize glucose in our body, 50 00:02:43,380 --> 00:02:45,940 we store that energy in ATP. 51 00:02:45,940 --> 00:02:49,150 And we want that ATP to be around. 52 00:02:49,150 --> 00:02:53,190 And then when we break ATP apart, it releases the energy. 53 00:02:53,190 --> 00:02:56,160 So this is a very important biological process. 54 00:02:56,160 --> 00:02:59,690 And you have a negative value, exothermic reaction. 55 00:02:59,690 --> 00:03:04,570 But there's a few other examples of spontaneous reactions. 56 00:03:04,570 --> 00:03:06,910 One of them is this one. 57 00:03:06,910 --> 00:03:09,164 And we've probably all experienced this. 58 00:03:09,164 --> 00:03:10,580 If you're from New England, you've 59 00:03:10,580 --> 00:03:16,110 seen snow melt or ice melt, solid to liquid. 60 00:03:16,110 --> 00:03:18,150 If you're from a hot part of the world, 61 00:03:18,150 --> 00:03:21,470 you probably had ice cubes in your nice, refreshing drink 62 00:03:21,470 --> 00:03:23,850 with maybe a little umbrella on the top. 63 00:03:23,850 --> 00:03:28,220 Anyway, everyone, I think, has seen ice melt. 64 00:03:28,220 --> 00:03:31,820 But here, delta H0 is of positive value. 65 00:03:31,820 --> 00:03:34,710 It's endothermic. 66 00:03:34,710 --> 00:03:37,370 Also, if you have ammonium nitrate, 67 00:03:37,370 --> 00:03:41,680 this will just come apart in a spontaneous reaction. 68 00:03:41,680 --> 00:03:46,110 Delta H here is plus 28 kilojoules per mole. 69 00:03:46,110 --> 00:03:52,880 So is delta H the key to spontaneity? 70 00:03:52,880 --> 00:03:58,550 It is not-- plays into it, but it is not 71 00:03:58,550 --> 00:04:01,440 the determining factor. 72 00:04:01,440 --> 00:04:06,170 So if delta H0 is not the key to spontaneity, what is? 73 00:04:08,790 --> 00:04:13,680 It is free energy, yes, particularly Gibbs free energy, 74 00:04:13,680 --> 00:04:19,760 or delta G. And I'm really happy they decided 75 00:04:19,760 --> 00:04:24,020 to add the Gibbs free energy, because another thing of energy 76 00:04:24,020 --> 00:04:26,190 would be a lot. 77 00:04:26,190 --> 00:04:29,435 So having this free energy having abbreviation of G, 78 00:04:29,435 --> 00:04:33,320 I think is a good thing-- so Gibbs free energy. 79 00:04:33,320 --> 00:04:37,260 So Gibbs free energy depends on delta H. 80 00:04:37,260 --> 00:04:40,150 But it also depends on another term, 81 00:04:40,150 --> 00:04:48,070 which is T delta S-- temperature and delta S, change in entropy. 82 00:04:48,070 --> 00:04:52,180 So delta G is the predictor of whether a reaction will 83 00:04:52,180 --> 00:04:57,980 go in the forward direction in a spontaneous fashion or not. 84 00:04:57,980 --> 00:05:00,840 So let's just think about the sign of delta G 85 00:05:00,840 --> 00:05:03,120 and what it means. 86 00:05:03,120 --> 00:05:06,230 So again, at constant temperature and pressure 87 00:05:06,230 --> 00:05:12,410 here, delta G less than 0, negative delta G, 88 00:05:12,410 --> 00:05:13,960 is that spontaneous or not? 89 00:05:13,960 --> 00:05:15,196 AUDIENCE: Spontaneous. 90 00:05:15,196 --> 00:05:18,480 CATHY DRENNAN: Spontaneous. 91 00:05:18,480 --> 00:05:24,750 Positive delta G is not spontaneous, non-spontaneous. 92 00:05:24,750 --> 00:05:27,110 And delta G equals 0. 93 00:05:27,110 --> 00:05:29,840 It's one of the other things that I am very fond of, 94 00:05:29,840 --> 00:05:31,642 which is equilibrium. 95 00:05:34,200 --> 00:05:38,530 So delta G indicates whether something is spontaneous 96 00:05:38,530 --> 00:05:39,070 or not. 97 00:05:39,070 --> 00:05:42,020 Negative value, spontaneous in the forward direction. 98 00:05:42,020 --> 00:05:45,870 Positive value, not spontaneous in the forward direction. 99 00:05:45,870 --> 00:05:51,400 And equilibrium, the thing we all try to reach in our lives. 100 00:05:51,400 --> 00:05:54,800 So let's look at an example and calculate 101 00:05:54,800 --> 00:05:56,950 what delta G is going to be. 102 00:05:56,950 --> 00:06:00,100 So we saw this equation already. 103 00:06:00,100 --> 00:06:02,810 We have a positive delta H0. 104 00:06:02,810 --> 00:06:05,980 And now, I'm telling you that delta S0 is also 105 00:06:05,980 --> 00:06:07,940 a positive value. 106 00:06:07,940 --> 00:06:10,100 So we can use this equation. 107 00:06:10,100 --> 00:06:13,840 And this is really one of the most important equations 108 00:06:13,840 --> 00:06:15,240 in chemistry. 109 00:06:15,240 --> 00:06:20,520 Figuring out this equation was really a crowning achievement. 110 00:06:20,520 --> 00:06:22,370 And you'll be using it a lot. 111 00:06:22,370 --> 00:06:24,450 Not just in this unit, but pretty much 112 00:06:24,450 --> 00:06:28,630 in every unit from now on, you will be using this equation. 113 00:06:28,630 --> 00:06:31,459 So room temperature, pretty much we're not doing-- occasionally, 114 00:06:31,459 --> 00:06:33,250 we'll do something not at room temperature, 115 00:06:33,250 --> 00:06:35,560 but we like room temperature. 116 00:06:35,560 --> 00:06:38,360 And we like it in Kelvin. 117 00:06:38,360 --> 00:06:42,030 So delta G0, so we plug in our delta H value. 118 00:06:42,030 --> 00:06:45,940 So it's going to equal delta H minus the temperature. 119 00:06:45,940 --> 00:06:48,320 And if the temperature isn't given in a problem, 120 00:06:48,320 --> 00:06:52,130 you can assume that it's 298. 121 00:06:52,130 --> 00:06:54,380 And now we need to plug in delta S. 122 00:06:54,380 --> 00:06:57,280 But I left a blank here to make a point, which 123 00:06:57,280 --> 00:07:01,400 is that delta S's are almost always given 124 00:07:01,400 --> 00:07:04,930 in joules per kelvin per mole. 125 00:07:04,930 --> 00:07:07,620 But everything else is given in kilojoules. 126 00:07:07,620 --> 00:07:10,005 So you want to make sure you convert your units, 127 00:07:10,005 --> 00:07:13,470 or you're going to come up with very funky answers at the end. 128 00:07:13,470 --> 00:07:20,000 So from joules to kilojoules, so now plus 0.109 kilojoules 129 00:07:20,000 --> 00:07:23,110 per kelvin per mole. 130 00:07:23,110 --> 00:07:24,750 And we can do this out now. 131 00:07:24,750 --> 00:07:29,830 So we have plus 28 minus 32.48. 132 00:07:29,830 --> 00:07:32,700 And why don't you tell me how many significant figures 133 00:07:32,700 --> 00:07:33,881 this answer has. 134 00:07:52,300 --> 00:07:53,190 10 more seconds. 135 00:08:11,000 --> 00:08:14,630 So at least some people got it right. 136 00:08:14,630 --> 00:08:20,680 We've identified once again a weakness, so rules 137 00:08:20,680 --> 00:08:23,060 of adding and subtracting. 138 00:08:23,060 --> 00:08:26,510 So we have 28 here minus 32. 139 00:08:26,510 --> 00:08:30,260 There are no significant figures after the decimal point here. 140 00:08:30,260 --> 00:08:33,720 So we're just left with 4. 141 00:08:33,720 --> 00:08:37,010 So when we're doing multiplication or division, 142 00:08:37,010 --> 00:08:40,520 we consider the total number of significant figures. 143 00:08:40,520 --> 00:08:42,539 But with addition and subtraction, 144 00:08:42,539 --> 00:08:46,950 you gotta pay attention to where the decimal point is. 145 00:08:46,950 --> 00:08:51,880 And when we get into the next unit, there are logs. 146 00:08:51,880 --> 00:08:56,410 And those have special rules of significant figures. 147 00:08:56,410 --> 00:09:00,050 Yes, very exciting. 148 00:09:00,050 --> 00:09:05,780 So delta G0 is negative here, although delta H is positive. 149 00:09:05,780 --> 00:09:08,650 So this reaction is spontaneous. 150 00:09:08,650 --> 00:09:09,900 It's not hugely. 151 00:09:09,900 --> 00:09:15,270 It's a pretty small number, but still, it's spontaneous. 152 00:09:15,270 --> 00:09:17,230 So let's consider our friend over 153 00:09:17,230 --> 00:09:20,990 here that we've been talking about-- glucose being oxidized 154 00:09:20,990 --> 00:09:22,510 to CO2 and water. 155 00:09:22,510 --> 00:09:25,180 You practically should have the delta H memorized 156 00:09:25,180 --> 00:09:26,560 for that at this point. 157 00:09:26,560 --> 00:09:29,370 Now, I'm telling you what the delta S0 is. 158 00:09:29,370 --> 00:09:34,740 And it's positive 233 joules per kelvin per mole. 159 00:09:34,740 --> 00:09:37,070 And we can plug this into our equation 160 00:09:37,070 --> 00:09:42,520 to calculate a delta G0, again remembering to convert joules 161 00:09:42,520 --> 00:09:45,730 to kilojoules to do this. 162 00:09:45,730 --> 00:09:54,970 And so now, we see that it has a very negative delta G0 here, 163 00:09:54,970 --> 00:10:01,110 minus 2,885 kilojoules per mole at room temperature. 164 00:10:01,110 --> 00:10:03,450 So at room temperature, this reaction 165 00:10:03,450 --> 00:10:05,300 is spontaneous but slow. 166 00:10:05,300 --> 00:10:09,200 We saw that with the candies that had glucose in it. 167 00:10:09,200 --> 00:10:12,820 We opened them up, and no water or CO2 168 00:10:12,820 --> 00:10:16,890 were obviously being liberated in this reaction, 169 00:10:16,890 --> 00:10:19,230 because it is slow. 170 00:10:19,230 --> 00:10:20,930 And now, a clicker question. 171 00:10:20,930 --> 00:10:23,470 I want you to tell me whether it would 172 00:10:23,470 --> 00:10:27,345 be spontaneous at different temperatures or not? 173 00:10:45,860 --> 00:10:46,820 10 more seconds. 174 00:11:02,936 --> 00:11:03,920 Yep. 175 00:11:03,920 --> 00:11:07,300 So it is spontaneous at all temperatures. 176 00:11:07,300 --> 00:11:10,470 Not all reactions are, but this one is. 177 00:11:10,470 --> 00:11:13,060 So if we go back here, the reaction 178 00:11:13,060 --> 00:11:15,550 is spontaneous at all temperatures. 179 00:11:15,550 --> 00:11:18,220 And that's because, to be spontaneous, 180 00:11:18,220 --> 00:11:20,720 you want a delta G that's negative. 181 00:11:20,720 --> 00:11:25,150 If delta H is negative and delta S is positive, 182 00:11:25,150 --> 00:11:27,590 then you'll have a negative minus a negative. 183 00:11:27,590 --> 00:11:29,740 So it doesn't matter what temperature is. 184 00:11:29,740 --> 00:11:33,940 This will always yield a negative delta G0. 185 00:11:33,940 --> 00:11:36,980 So other reactions, that might not be the case. 186 00:11:36,980 --> 00:11:42,510 But if you have negative delta H and a positive delta S, 187 00:11:42,510 --> 00:11:43,660 it will be spontaneous. 188 00:11:43,660 --> 00:11:47,080 So negative delta H, again, exothermic, heat release. 189 00:11:47,080 --> 00:11:50,280 And a positive entropy is a favorable thing. 190 00:11:50,280 --> 00:11:52,640 Entropy is always increasing. 191 00:11:52,640 --> 00:11:56,110 So if this reaction has increased in entropy, 192 00:11:56,110 --> 00:11:59,790 it will be much more likely to be spontaneous. 193 00:11:59,790 --> 00:12:02,560 So let's talk about entropy. 194 00:12:02,560 --> 00:12:06,300 So entropy is a measure of disorder of a system. 195 00:12:06,300 --> 00:12:10,100 Delta S is the change in entropy. 196 00:12:10,100 --> 00:12:14,090 And delta S, again, is a state function. 197 00:12:14,090 --> 00:12:18,090 So one example of entropy in New England 198 00:12:18,090 --> 00:12:23,320 are these stone walls that do not look absolutely beautiful. 199 00:12:23,320 --> 00:12:26,550 There are often stones falling everywhere. 200 00:12:26,550 --> 00:12:29,860 And it doesn't matter if these stone walls that were probably 201 00:12:29,860 --> 00:12:35,080 built in 1600s or 1700s in New England fell totally apart 202 00:12:35,080 --> 00:12:40,270 and were rebuilt, now we just care about how the wall is 203 00:12:40,270 --> 00:12:42,080 compared to the way it started. 204 00:12:42,080 --> 00:12:44,990 So delta S, again, is a state function. 205 00:12:44,990 --> 00:12:47,310 It doesn't depend on path. 206 00:12:47,310 --> 00:12:49,470 And so if you get out and walk around 207 00:12:49,470 --> 00:12:51,870 and go like on the Minuteman Trail 208 00:12:51,870 --> 00:12:54,230 and see some of the historical sites 209 00:12:54,230 --> 00:12:58,490 where Paul Revere rode his horse along a lot of stone walls, 210 00:12:58,490 --> 00:13:01,030 there's a New England poet who writes 211 00:13:01,030 --> 00:13:02,480 about this, Robert Frost. 212 00:13:02,480 --> 00:13:07,310 And he said, "something there is that doesn't love a wall." 213 00:13:07,310 --> 00:13:09,497 And that something is entropy. 214 00:13:09,497 --> 00:13:10,330 AUDIENCE: [LAUGHTER] 215 00:13:10,330 --> 00:13:13,450 CATHY DRENNAN: Entropy does not love a wall. 216 00:13:13,450 --> 00:13:16,840 Entropy does not like order. 217 00:13:16,840 --> 00:13:20,760 Another example, those of you who are learning more about me 218 00:13:20,760 --> 00:13:26,050 as a person know that I am a fan of dogs. 219 00:13:26,050 --> 00:13:28,560 This is my dog Shep. 220 00:13:28,560 --> 00:13:32,460 Shep does not like going to the groomers, does not like it. 221 00:13:32,460 --> 00:13:35,620 And I think that this is because he's been at my office hours 222 00:13:35,620 --> 00:13:39,550 and he knows that increasing entropy is favorable, 223 00:13:39,550 --> 00:13:41,700 decreasing entropy is not. 224 00:13:41,700 --> 00:13:45,050 And he says, really, this violates 225 00:13:45,050 --> 00:13:47,460 the laws of thermodynamics, what you're doing to. 226 00:13:47,460 --> 00:13:50,490 Me and you should cease and desist. 227 00:13:50,490 --> 00:13:54,970 But anyway, he still get haircuts. 228 00:13:54,970 --> 00:13:57,190 So entropy, again, is this measure 229 00:13:57,190 --> 00:13:59,250 of disorder of a system. 230 00:13:59,250 --> 00:14:02,570 You have a positive delta S, which 231 00:14:02,570 --> 00:14:05,880 is going to be an increase in disorder. 232 00:14:05,880 --> 00:14:12,220 And a negative delta S is going to be a decrease in disorder. 233 00:14:12,220 --> 00:14:14,970 And disorder, you can be thinking 234 00:14:14,970 --> 00:14:18,540 about this as internal degrees of freedom in your molecule, 235 00:14:18,540 --> 00:14:20,650 thinking about this as vibrations. 236 00:14:20,650 --> 00:14:22,400 All sorts of different things can 237 00:14:22,400 --> 00:14:27,800 lead to increase or decrease in entropy. 238 00:14:27,800 --> 00:14:31,160 But we often think about changes in entropy 239 00:14:31,160 --> 00:14:34,570 depending on if the reaction is changing in phase. 240 00:14:34,570 --> 00:14:40,300 So gas molecules have greater disorder than liquid. 241 00:14:40,300 --> 00:14:44,110 And liquid has greater disorder than solids. 242 00:14:44,110 --> 00:14:48,070 And so a solid has all its molecules lined up. 243 00:14:48,070 --> 00:14:50,160 And liquid can move around a little bit more. 244 00:14:50,160 --> 00:14:52,930 But gas really can spread all out. 245 00:14:52,930 --> 00:14:56,630 So in terms of entropy and changes in entropy, 246 00:14:56,630 --> 00:14:59,910 we can think about the phase change that's happening 247 00:14:59,910 --> 00:15:02,280 and even predict if something's going to be 248 00:15:02,280 --> 00:15:04,640 an increase in entropy or not. 249 00:15:04,640 --> 00:15:07,270 So let's just look at one example. 250 00:15:07,270 --> 00:15:11,890 So without a calculation, predict the sign of delta S. 251 00:15:11,890 --> 00:15:13,208 And this is a clicker question. 252 00:15:37,930 --> 00:15:39,690 Let's just take 10 more seconds. 253 00:15:39,690 --> 00:15:41,800 And can our demo TAs come down? 254 00:15:55,700 --> 00:15:58,860 Yep, good. 255 00:15:58,860 --> 00:16:02,880 So you predicted positive, which is the correct answer. 256 00:16:02,880 --> 00:16:07,570 And so here, we're going from a liquid to a liquid and a gas. 257 00:16:07,570 --> 00:16:10,950 And so going to the gas, that will increase 258 00:16:10,950 --> 00:16:12,410 the disorder of the system. 259 00:16:12,410 --> 00:16:14,370 So delta S will be positive. 260 00:16:14,370 --> 00:16:16,620 So now, we're actually going to do a demo 261 00:16:16,620 --> 00:16:19,130 of this particular reaction. 262 00:16:19,130 --> 00:16:21,710 And so we have hydrogen peroxide, 263 00:16:21,710 --> 00:16:26,630 which can just be bought at a CVS or local drugstore. 264 00:16:26,630 --> 00:16:32,380 And it will go to liquid water and also oxygen gas. 265 00:16:32,380 --> 00:16:34,680 And so how do you see a gas? 266 00:16:34,680 --> 00:16:38,800 And you can see it by putting it in with soap bubbles. 267 00:16:38,800 --> 00:16:42,210 So as bubbles of oxygen form, the soap bubbles 268 00:16:42,210 --> 00:16:43,530 will bubble out. 269 00:16:43,530 --> 00:16:44,840 And so you can see it. 270 00:16:44,840 --> 00:16:48,540 And you can also add some kind of food color. 271 00:16:48,540 --> 00:16:50,750 And we have yeast as a catalyst to make 272 00:16:50,750 --> 00:16:52,530 it go a little bit faster. 273 00:16:52,530 --> 00:16:56,415 So let's see if we can actually see disorder increase. 274 00:17:01,420 --> 00:17:02,590 I don't want the mic. 275 00:17:06,720 --> 00:17:10,288 If you want to just say-- if you want to talk at the same time, 276 00:17:10,288 --> 00:17:10,829 here's a mic. 277 00:17:10,829 --> 00:17:11,749 AUDIENCE: I might do that. 278 00:17:11,749 --> 00:17:13,130 CATHY DRENNAN: You're not going to do that, OK. 279 00:17:13,130 --> 00:17:13,260 AUDIENCE: Yeah, we will. 280 00:17:13,260 --> 00:17:13,589 CATHY DRENNAN: Oh, you do. 281 00:17:13,589 --> 00:17:14,089 OK. 282 00:17:14,089 --> 00:17:15,910 AUDIENCE: Is this on? 283 00:17:15,910 --> 00:17:16,440 This on? 284 00:17:16,440 --> 00:17:16,990 Yes, it is. 285 00:17:16,990 --> 00:17:17,630 OK, great. 286 00:17:17,630 --> 00:17:22,722 So what we have going on here is we've got this container. 287 00:17:22,722 --> 00:17:23,680 It's filled with water. 288 00:17:23,680 --> 00:17:28,470 And what I did was I added about 4 teaspoons of yeast. 289 00:17:28,470 --> 00:17:31,550 The yeast, as Cathy said, is going to act as a catalyst. 290 00:17:31,550 --> 00:17:37,129 It's actually a biological species. 291 00:17:37,129 --> 00:17:38,920 It's a living species that's actually going 292 00:17:38,920 --> 00:17:40,190 to catalyze this reaction. 293 00:17:40,190 --> 00:17:43,280 What Erik is doing is Erik is pouring some hydrogen peroxide. 294 00:17:43,280 --> 00:17:44,870 He added some soap. 295 00:17:44,870 --> 00:17:47,120 So as you see in the reaction, the H2O2 296 00:17:47,120 --> 00:17:50,420 is going to break down into water and gas-- 297 00:17:50,420 --> 00:17:51,582 the gas being oxygen. 298 00:17:51,582 --> 00:17:53,040 And what we don't want to happen is 299 00:17:53,040 --> 00:17:54,910 we don't want just the gas to escape, 300 00:17:54,910 --> 00:17:56,368 because then you guys can't see it. 301 00:17:56,368 --> 00:17:58,780 So what Erik is doing right now is he's adding some soap. 302 00:17:58,780 --> 00:18:01,500 The soap is actually going to catch, if you will, 303 00:18:01,500 --> 00:18:06,740 the escaping gas and turn it into a foam. 304 00:18:06,740 --> 00:18:08,290 And what we should be able to see 305 00:18:08,290 --> 00:18:13,250 is the foam kind of escape from this container. 306 00:18:13,250 --> 00:18:14,351 You ready? 307 00:18:14,351 --> 00:18:14,850 OK. 308 00:18:14,850 --> 00:18:16,058 So hopefully, this will work. 309 00:18:16,058 --> 00:18:17,722 We should put on our goggles. 310 00:18:17,722 --> 00:18:19,310 [LAUGHTER] 311 00:18:19,310 --> 00:18:20,430 Smells really bad. 312 00:18:20,430 --> 00:18:22,250 OK, ready? 313 00:18:22,250 --> 00:18:28,500 And-- get out of there, look at that. 314 00:18:28,500 --> 00:18:29,190 Hey! 315 00:18:29,190 --> 00:18:30,731 Wow, that worked a lot better than we 316 00:18:30,731 --> 00:18:32,105 thought it was going to work. 317 00:18:32,105 --> 00:18:33,730 CATHY DRENNAN: And so this is sometimes 318 00:18:33,730 --> 00:18:36,240 called the elephant toothpaste demo, 319 00:18:36,240 --> 00:18:39,620 because that is sort of, if you were an elephant, what 320 00:18:39,620 --> 00:18:43,720 you would probably be brushing your teeth with, I don't know. 321 00:18:43,720 --> 00:18:44,460 Yes. 322 00:18:44,460 --> 00:18:47,314 So this is-- 323 00:18:47,314 --> 00:18:48,070 [APPLAUSE] 324 00:18:48,070 --> 00:18:49,644 --entropy increasing. 325 00:18:55,080 --> 00:18:58,830 So let's just see if we can quickly 326 00:18:58,830 --> 00:19:01,980 talk a little more about entropy and then we'll end. 327 00:19:01,980 --> 00:19:05,340 So entropy of reactions can be calculated 328 00:19:05,340 --> 00:19:07,400 from absolute values. 329 00:19:07,400 --> 00:19:12,280 And again, we can use this equation here. 330 00:19:12,280 --> 00:19:19,800 So we have a delta S for a particular reaction, 331 00:19:19,800 --> 00:19:22,400 can be calculated from the delta S's 332 00:19:22,400 --> 00:19:24,560 of the product minus reactants. 333 00:19:24,560 --> 00:19:27,250 So again, we have products minus reactants. 334 00:19:27,250 --> 00:19:32,130 The absolute value, or an absolute delta S, S 335 00:19:32,130 --> 00:19:37,910 equals 0 for a perfect crystal at a temperature of 0 kelvin. 336 00:19:37,910 --> 00:19:40,180 You never really talk about S by itself. 337 00:19:40,180 --> 00:19:45,171 It's always really delta S. And S of 0, 338 00:19:45,171 --> 00:19:47,420 this is like the saddest thing for a crystallographer, 339 00:19:47,420 --> 00:19:48,878 because you know you're never going 340 00:19:48,878 --> 00:19:52,290 to have a perfect crystal, even if you go to 0 kelvin, 341 00:19:52,290 --> 00:19:54,760 I feel like at least experimentally. 342 00:19:54,760 --> 00:19:58,960 So S equals 0, to me that's kind of sad. 343 00:19:58,960 --> 00:20:04,860 So if we just put in for this reaction that we just did, 344 00:20:04,860 --> 00:20:08,700 we can put in our values here. 345 00:20:08,700 --> 00:20:13,570 And we can put in we're forming liquid water. 346 00:20:13,570 --> 00:20:16,200 And we're forming O2 gas. 347 00:20:16,200 --> 00:20:24,140 And we're using two molecules of hydrogen peroxide-- H2 O2 here. 348 00:20:24,140 --> 00:20:29,990 And so now, we can calculate what that delta S0 is. 349 00:20:29,990 --> 00:20:35,590 And it's a value of 125 joules per kelvin per mole. 350 00:20:35,590 --> 00:20:39,950 So again, products, water and gas, minus reactants, 351 00:20:39,950 --> 00:20:42,240 pay attention to the stoichiometry, 352 00:20:42,240 --> 00:20:45,590 and you can get your delta S value. 353 00:20:45,590 --> 00:20:47,120 And why is it positive? 354 00:20:47,120 --> 00:20:49,380 Again, we already talked about this. 355 00:20:49,380 --> 00:20:54,850 It's because it's going from liquid to a liquid plus a gas. 356 00:20:54,850 --> 00:20:58,390 And then, if we plug these values in again 357 00:20:58,390 --> 00:21:00,690 to see if it's spontaneous, we can 358 00:21:00,690 --> 00:21:03,990 use this equation and plug in our values, 359 00:21:03,990 --> 00:21:07,480 making sure we change our units. 360 00:21:07,480 --> 00:21:12,310 And we can see that, in fact, this is a spontaneous reaction, 361 00:21:12,310 --> 00:21:14,150 because it's negative here. 362 00:21:14,150 --> 00:21:16,180 But you already knew that, because you 363 00:21:16,180 --> 00:21:18,510 watched it go spontaneously. 364 00:21:18,510 --> 00:21:21,690 So most of the time, you can't do the demo. 365 00:21:21,690 --> 00:21:25,840 So then you can use this awesome equation right here. 366 00:21:25,840 --> 00:21:29,040 So that's where we're stopping for today. 367 00:21:29,040 --> 00:21:32,330 And we'll see you all on Friday. 368 00:21:32,330 --> 00:21:39,180 So if you take out your Lecture 16 notes, the bottom of page 3, 369 00:21:39,180 --> 00:21:42,630 we had an example about the melting of ice 370 00:21:42,630 --> 00:21:45,950 at room temperature. 371 00:21:45,950 --> 00:21:49,930 So we did a little demo for you at the end of class last time 372 00:21:49,930 --> 00:21:54,000 and calculated that the reaction was 373 00:21:54,000 --> 00:21:57,700 spontaneous for hydrogen-- hydrogen 374 00:21:57,700 --> 00:21:59,580 peroxide is pretty reactive. 375 00:21:59,580 --> 00:22:02,090 And we watched the O2 bubble go. 376 00:22:02,090 --> 00:22:03,680 And we did that calculation. 377 00:22:03,680 --> 00:22:05,920 So we're thinking about, not just delta H, 378 00:22:05,920 --> 00:22:07,800 but we're thinking about delta S. 379 00:22:07,800 --> 00:22:09,820 And we're now thinking about delta G as well 380 00:22:09,820 --> 00:22:11,610 and how they all play together. 381 00:22:11,610 --> 00:22:15,380 So when we started last lecture, we 382 00:22:15,380 --> 00:22:17,920 had talked about the fact of some reactions that 383 00:22:17,920 --> 00:22:20,170 were spontaneous where delta H was negative, 384 00:22:20,170 --> 00:22:22,790 where it was exothermic, where heat was released. 385 00:22:22,790 --> 00:22:24,630 But then we also gave some examples 386 00:22:24,630 --> 00:22:26,860 where delta H was positive and said, 387 00:22:26,860 --> 00:22:29,220 but these are also spontaneous reactions. 388 00:22:29,220 --> 00:22:32,060 We all know that at room temperature ice will melt. 389 00:22:32,060 --> 00:22:34,610 We know that that's a spontaneous reaction. 390 00:22:34,610 --> 00:22:36,550 But the delta H for that reaction 391 00:22:36,550 --> 00:22:38,220 is actually a positive value. 392 00:22:38,220 --> 00:22:40,360 It's an endothermic reaction. 393 00:22:40,360 --> 00:22:42,300 So when we're thinking about these reactions 394 00:22:42,300 --> 00:22:45,295 and spontaneity, we have to be thinking about delta G, 395 00:22:45,295 --> 00:22:50,850 no just delta H. And delta G has to do with delta H and delta S. 396 00:22:50,850 --> 00:22:53,470 So sometimes, delta S is the driving force 397 00:22:53,470 --> 00:22:56,330 behind whether a reaction is going to be spontaneous. 398 00:22:56,330 --> 00:22:59,430 Whether the delta G will be negative or positive, 399 00:22:59,430 --> 00:23:02,320 delta S is making that determination. 400 00:23:02,320 --> 00:23:05,820 So we can calculate what a delta S for reaction 401 00:23:05,820 --> 00:23:10,440 is if we know the entropy values for the products. 402 00:23:10,440 --> 00:23:12,740 And it's the sum of the entropy values 403 00:23:12,740 --> 00:23:16,270 for the products minus the sum for the reactants. 404 00:23:16,270 --> 00:23:20,000 So when we're doing heats of formation, 405 00:23:20,000 --> 00:23:22,210 we also had products minus reactants. 406 00:23:22,210 --> 00:23:24,320 But we have one exception to this products 407 00:23:24,320 --> 00:23:27,705 minus reactant rule, and that's when we're using what? 408 00:23:27,705 --> 00:23:32,038 What thing are we going to do reactant minus products? 409 00:23:32,038 --> 00:23:32,986 AUDIENCE: Bond-- 410 00:23:32,986 --> 00:23:33,819 CATHY DRENNAN: Bond? 411 00:23:33,819 --> 00:23:34,882 AUDIENCE: Bond enthalpy. 412 00:23:34,882 --> 00:23:37,540 CATHY DRENNAN: Enthalpy, right. 413 00:23:37,540 --> 00:23:39,740 But here, we're products minus reactants. 414 00:23:39,740 --> 00:23:42,370 So we can plug those numbers in. 415 00:23:42,370 --> 00:23:45,500 Our product is our liquid water. 416 00:23:45,500 --> 00:23:50,050 Our reactant is our solid water, or our ice. 417 00:23:50,050 --> 00:23:55,950 And we can calculate what the delta S0 is for this reaction. 418 00:23:55,950 --> 00:23:57,390 We can put in our values. 419 00:23:57,390 --> 00:24:03,470 And we get a positive value, positive 28.59 joules 420 00:24:03,470 --> 00:24:05,850 per kelvin per mole. 421 00:24:05,850 --> 00:24:09,510 And delta S's tend to be in joules. 422 00:24:09,510 --> 00:24:12,070 Everything else is in kilojoules. 423 00:24:12,070 --> 00:24:14,340 So keep that in mind. 424 00:24:14,340 --> 00:24:20,470 And why do you think this reaction has a positive value? 425 00:24:20,470 --> 00:24:23,360 Why is delta S greater than 0? 426 00:24:23,360 --> 00:24:24,790 What would be your guess for that? 427 00:24:28,640 --> 00:24:30,276 What's happening? 428 00:24:30,276 --> 00:24:32,168 AUDIENCE: [INAUDIBLE]. 429 00:24:32,168 --> 00:24:35,730 CATHY DRENNAN: Yeah, so we're going from a solid to a liquid. 430 00:24:35,730 --> 00:24:39,210 So we're increasing the internal degrees of freedom. 431 00:24:39,210 --> 00:24:42,680 The molecules of water can move around more in a liquid 432 00:24:42,680 --> 00:24:44,270 than they can in a solid. 433 00:24:44,270 --> 00:24:46,885 So this is increasing the disorder of the system. 434 00:24:50,050 --> 00:24:53,270 You're increasing entropy here, because the water molecules 435 00:24:53,270 --> 00:24:54,600 can move around more. 436 00:24:54,600 --> 00:24:56,750 There's more freedom of motion. 437 00:24:56,750 --> 00:24:58,850 So delta S is positive. 438 00:24:58,850 --> 00:25:01,000 It's increasing. 439 00:25:01,000 --> 00:25:05,010 And then we can use that to calculate delta G0, 440 00:25:05,010 --> 00:25:06,470 Gibbs free energy. 441 00:25:06,470 --> 00:25:11,820 We can plug in our delta H value minus T, room temperature, 442 00:25:11,820 --> 00:25:15,170 times delta S, which we just calculated, making sure 443 00:25:15,170 --> 00:25:17,860 that we convert from joules to kilojoules. 444 00:25:17,860 --> 00:25:23,820 And then our units will be kilojoules per mole. 445 00:25:23,820 --> 00:25:26,650 And here, delta G0 is a negative value. 446 00:25:26,650 --> 00:25:27,950 So it is spontaneous. 447 00:25:27,950 --> 00:25:29,400 We all know it's spontaneous. 448 00:25:29,400 --> 00:25:31,700 We've observed this happening. 449 00:25:31,700 --> 00:25:34,460 So even though delta H0 is positive, 450 00:25:34,460 --> 00:25:36,640 it's an endothermic reaction. 451 00:25:36,640 --> 00:25:41,050 Ice melts at room temperature, because the delta G 452 00:25:41,050 --> 00:25:43,300 is negative. 453 00:25:43,300 --> 00:25:45,640 So let's talk a little more about delta G. 454 00:25:45,640 --> 00:25:49,520 So let's talk about free energy of formation 455 00:25:49,520 --> 00:25:51,380 and the last page of this handout. 456 00:25:51,380 --> 00:25:56,540 So free energy of formation, delta G sub f. 457 00:25:56,540 --> 00:26:00,630 And so this is analogous to delta H of formation-- 458 00:26:00,630 --> 00:26:04,890 so the change in enthalpy of formation. 459 00:26:04,890 --> 00:26:09,330 So again, when you have a little value here, 460 00:26:09,330 --> 00:26:13,320 this is standard Gibbs free energy of formation for the f 461 00:26:13,320 --> 00:26:13,970 here. 462 00:26:13,970 --> 00:26:16,860 And that's the formation of 1 mole 463 00:26:16,860 --> 00:26:21,540 of a molecule from its elements in most stable state 464 00:26:21,540 --> 00:26:24,200 and in their standard states. 465 00:26:24,200 --> 00:26:28,355 So we can have tables in your book of these values. 466 00:26:28,355 --> 00:26:30,480 So your book, in the back, if you haven't explored, 467 00:26:30,480 --> 00:26:33,500 the back of your book gives lots of tables of things, 468 00:26:33,500 --> 00:26:38,890 including information about delta G's and delta H's 469 00:26:38,890 --> 00:26:41,240 and bond enthalpies and all sorts of other things. 470 00:26:41,240 --> 00:26:43,320 Redox potentials, we haven't talked about yet, 471 00:26:43,320 --> 00:26:44,500 lots of tables. 472 00:26:44,500 --> 00:26:47,090 So you can look this up. 473 00:26:47,090 --> 00:26:50,280 Or if you have already, say, looked up 474 00:26:50,280 --> 00:26:55,490 your delta H of formation, you can use this handy equation-- 475 00:26:55,490 --> 00:26:58,930 delta G equals delta H minus T delta S. 476 00:26:58,930 --> 00:27:02,350 But if we plug in our delta H's of formation, 477 00:27:02,350 --> 00:27:05,510 we can get our delta G's of formation. 478 00:27:05,510 --> 00:27:09,290 So how you're going to calculate delta G of formation 479 00:27:09,290 --> 00:27:12,500 depends on what information you're given. 480 00:27:12,500 --> 00:27:14,480 So let's think a little bit about what 481 00:27:14,480 --> 00:27:18,080 it means for particular delta G's of formation-- 482 00:27:18,080 --> 00:27:22,330 if they're positive or if they're negative. 483 00:27:22,330 --> 00:27:24,040 So let's look at an example. 484 00:27:24,040 --> 00:27:25,850 And we saw this before. 485 00:27:25,850 --> 00:27:30,330 This is the formation of carbon dioxide from elements 486 00:27:30,330 --> 00:27:35,302 in its most stable state, which is graphite carbon and O2 gas. 487 00:27:35,302 --> 00:27:37,510 So these are the elements in their most stable state, 488 00:27:37,510 --> 00:27:39,190 forming CO2. 489 00:27:39,190 --> 00:27:40,780 Now, I'm telling you that the delta 490 00:27:40,780 --> 00:27:46,850 G0 is minus 394.36 kilojoules per mole. 491 00:27:46,850 --> 00:27:51,160 And we can think about what this information tells us, 492 00:27:51,160 --> 00:27:54,180 that this is a fairly large negative number. 493 00:27:54,180 --> 00:28:00,410 So if delta G of formation is less than 0, 494 00:28:00,410 --> 00:28:02,884 what's going to be true thermodynamically? 495 00:28:02,884 --> 00:28:04,175 And this is a clicker question. 496 00:28:27,577 --> 00:28:29,394 Let's just take 10 more seconds. 497 00:28:44,580 --> 00:28:47,100 Interesting. 498 00:28:47,100 --> 00:28:50,290 So let's think about why this is true. 499 00:28:50,290 --> 00:28:54,030 This might be a deciding clicker question. 500 00:28:54,030 --> 00:28:55,550 We'll see. 501 00:28:55,550 --> 00:29:01,470 So if it is negative value for delta G, 502 00:29:01,470 --> 00:29:03,770 a negative value for delta G means 503 00:29:03,770 --> 00:29:07,060 that it's spontaneous in its forward direction. 504 00:29:07,060 --> 00:29:10,720 So here, the formation from the elements in their most stable 505 00:29:10,720 --> 00:29:15,260 state, if this is spontaneous in the forward direction, 506 00:29:15,260 --> 00:29:17,580 it also means that it's non-spontaneous 507 00:29:17,580 --> 00:29:19,120 in the reverse direction. 508 00:29:19,120 --> 00:29:22,440 That means once CO2 forms, it's going 509 00:29:22,440 --> 00:29:26,380 to be stable compared to the elements from which it came, 510 00:29:26,380 --> 00:29:29,930 because it's non-spontaneous going in the reverse direction, 511 00:29:29,930 --> 00:29:32,320 or at least that's the way that I like to think about it. 512 00:29:32,320 --> 00:29:37,130 So relative to its elements, it's stable-- spontaneous 513 00:29:37,130 --> 00:29:41,350 forward, non-spontaneous in reverse. 514 00:29:41,350 --> 00:29:43,810 So this is kind of bad news for us, 515 00:29:43,810 --> 00:29:47,820 because there's too much CO2 in our environment right now. 516 00:29:47,820 --> 00:29:49,420 It's a greenhouse gas. 517 00:29:49,420 --> 00:29:53,270 And wouldn't it be awesome if we could just encourage it all 518 00:29:53,270 --> 00:29:56,710 to go back to its elements, form more oxygen, 519 00:29:56,710 --> 00:29:57,850 which we could breathe. 520 00:29:57,850 --> 00:29:58,510 How lovely? 521 00:29:58,510 --> 00:30:00,160 Make some nice graphite. 522 00:30:00,160 --> 00:30:02,300 Maybe compress it, make some diamonds. 523 00:30:02,300 --> 00:30:06,930 But no, it is quite stable compared to its elements. 524 00:30:06,930 --> 00:30:11,050 So CO2 is in our environment causing global warming. 525 00:30:11,050 --> 00:30:13,610 And it's going to be hard to solve that problem, not 526 00:30:13,610 --> 00:30:15,050 easy to solve that problem. 527 00:30:15,050 --> 00:30:19,700 So this is unfortunate news that thermodynamics gives us. 528 00:30:19,700 --> 00:30:21,130 So then we can look at the other. 529 00:30:21,130 --> 00:30:26,400 If you have a positive value for delta G of formation, 530 00:30:26,400 --> 00:30:29,930 then it's thermodynamically unstable 531 00:30:29,930 --> 00:30:31,360 compared to its elements. 532 00:30:31,360 --> 00:30:35,450 So it's spontaneous going in the reverse direction. 533 00:30:35,450 --> 00:30:38,160 So it's unstable. 534 00:30:38,160 --> 00:30:40,780 So thermodynamics tells us whether something 535 00:30:40,780 --> 00:30:44,210 is stable or unstable. 536 00:30:44,210 --> 00:30:47,450 And kinetics tells us about whether things 537 00:30:47,450 --> 00:30:49,150 will react quickly or not. 538 00:30:49,150 --> 00:30:52,350 So something can be kinetically inert-- it might 539 00:30:52,350 --> 00:30:54,290 take a long time to react. 540 00:30:54,290 --> 00:30:57,620 But thermodynamics tells us stable, unstable. 541 00:30:57,620 --> 00:31:01,740 So thermodynamics is great, but it doesn't tell us anything 542 00:31:01,740 --> 00:31:03,850 about the rates of reactions. 543 00:31:03,850 --> 00:31:06,210 So nothing about the rates, and that's kinetics. 544 00:31:06,210 --> 00:31:08,670 So really thermodynamics and kinetics 545 00:31:08,670 --> 00:31:11,980 are very important for explaining reactions. 546 00:31:11,980 --> 00:31:14,950 And we'll talk about more kinetics at the end. 547 00:31:14,950 --> 00:31:19,020 So to calculate a delta G for a reaction, 548 00:31:19,020 --> 00:31:21,010 it depends, again, what you're given. 549 00:31:21,010 --> 00:31:26,460 You can sum up the delta G of formation of your products 550 00:31:26,460 --> 00:31:27,950 minus your reactants. 551 00:31:27,950 --> 00:31:29,510 Or you might use this. 552 00:31:29,510 --> 00:31:32,150 You'll find yourself using this equation a lot. 553 00:31:32,150 --> 00:31:36,290 This, again, was a crowning achievement of thermodynamics, 554 00:31:36,290 --> 00:31:40,840 that delta G equals delta H minus T delta S. So, again, 555 00:31:40,840 --> 00:31:43,330 whatever information you're given, 556 00:31:43,330 --> 00:31:47,550 you can use that to find these values. 557 00:31:47,550 --> 00:31:49,250 So we're not done with this equation. 558 00:31:49,250 --> 00:31:50,830 We're going to switch handouts. 559 00:31:50,830 --> 00:31:55,590 But we're going to continue with that exact same equation.