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 --> 00:00:10,080 continue to offer high quality educational resources for free. 5 00:00:10,080 --> 00:00:12,670 To make a donation or to view additional materials 6 00:00:12,670 --> 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 00:00:46,972 --> 00:00:48,930 CATHERINE DRENNAN: Yeah, let's show the answer. 9 00:00:51,630 --> 00:00:53,800 All right. 10 00:00:53,800 --> 00:01:01,241 For a special treat, special benefit-- 11 00:01:01,241 --> 00:01:03,490 I'm not sure what to call it-- something special later 12 00:01:03,490 --> 00:01:05,900 in the class, does someone want to tell me 13 00:01:05,900 --> 00:01:07,164 why that's the right answer? 14 00:01:14,080 --> 00:01:15,820 AUDIENCE: Well as we did last lecture, 15 00:01:15,820 --> 00:01:23,580 we crossed off the k2 because that's the slow step. 16 00:01:23,580 --> 00:01:26,201 So it doesn't matter as much. 17 00:01:26,201 --> 00:01:27,200 CATHERINE DRENNAN: Yeah. 18 00:01:27,200 --> 00:01:30,770 And so when you get rid of that, when the k minus 1 19 00:01:30,770 --> 00:01:34,730 is really fast compared to k2, k2 20 00:01:34,730 --> 00:01:36,466 is very small compared to that. 21 00:01:36,466 --> 00:01:37,340 So you get rid of it. 22 00:01:37,340 --> 00:01:39,700 And that simplifies the expression. 23 00:01:39,700 --> 00:01:47,950 So temperature-- today's lecture is largely about temperature. 24 00:01:47,950 --> 00:01:49,800 But we're going to tie in all sorts 25 00:01:49,800 --> 00:01:51,870 of other things we've learned over the course 26 00:01:51,870 --> 00:01:52,840 of the semester. 27 00:01:52,840 --> 00:01:53,760 So I'm very excited. 28 00:01:53,760 --> 00:01:59,170 Some of my very favorite topics are coming back today. 29 00:01:59,170 --> 00:02:02,650 So effective temperature, we talked about this a little bit 30 00:02:02,650 --> 00:02:05,170 when we were talking about making bread, 31 00:02:05,170 --> 00:02:08,402 adding your baking soda and about rates 32 00:02:08,402 --> 00:02:09,860 and why you put things in the oven. 33 00:02:09,860 --> 00:02:13,840 And we talked about it has to do with also the spontaneity, 34 00:02:13,840 --> 00:02:15,260 whether you change the temperature 35 00:02:15,260 --> 00:02:17,610 and what your delta H and delta S are. 36 00:02:17,610 --> 00:02:19,450 But we also talked about rates and that 37 00:02:19,450 --> 00:02:21,850 increasing the temperature often increases the rate. 38 00:02:21,850 --> 00:02:23,700 So many of us have observed this. 39 00:02:23,700 --> 00:02:26,246 You increase the temperature, you increase the rate. 40 00:02:26,246 --> 00:02:27,870 But today we're going to talk about how 41 00:02:27,870 --> 00:02:33,570 you can quantitatively say how much the rate might 42 00:02:33,570 --> 00:02:35,930 be changed-- the rate constant might be changed if you 43 00:02:35,930 --> 00:02:37,810 increase the temperature. 44 00:02:37,810 --> 00:02:42,260 So in 1889, it was a wonderful day for Arrhenius. 45 00:02:42,260 --> 00:02:46,130 He had been trying to plot different values 46 00:02:46,130 --> 00:02:49,110 of rate constants versus temperature 47 00:02:49,110 --> 00:02:50,710 to see what would happen. 48 00:02:50,710 --> 00:02:54,060 And then he tried natural log of the rate constant k 49 00:02:54,060 --> 00:02:55,670 versus inverse temperature. 50 00:02:55,670 --> 00:02:57,150 And he got a straight line. 51 00:02:57,150 --> 00:02:59,850 And scientists always get very excited when your data 52 00:02:59,850 --> 00:03:01,290 falls on a straight line. 53 00:03:01,290 --> 00:03:03,830 It means you figured out some relationship 54 00:03:03,830 --> 00:03:06,230 between two values. 55 00:03:06,230 --> 00:03:09,020 So here is the Arrhenius plot. 56 00:03:09,020 --> 00:03:10,590 And we'll introduce some terms. 57 00:03:10,590 --> 00:03:14,560 So again, we're plotting the natural log of k versus 1 58 00:03:14,560 --> 00:03:18,880 over temperature, 1 over inverse kelvin over here. 59 00:03:18,880 --> 00:03:21,460 And here is our plot of this straight line 60 00:03:21,460 --> 00:03:22,830 that Arrhenius found. 61 00:03:22,830 --> 00:03:28,510 Natural log of k on the y-axis versus 1/T 62 00:03:28,510 --> 00:03:33,470 gives you a slope then of minus the activation energy-- 63 00:03:33,470 --> 00:03:37,615 so E to the little a is activation energy-- over R, 64 00:03:37,615 --> 00:03:40,260 our good friend the gas constant. 65 00:03:40,260 --> 00:03:43,320 And then the y-intercept over here 66 00:03:43,320 --> 00:03:45,420 is the natural log of something called 67 00:03:45,420 --> 00:03:49,160 A. We have a lot of different A's in this particular unit 68 00:03:49,160 --> 00:03:50,090 of kinetics. 69 00:03:50,090 --> 00:03:52,260 This one is factor A, sometimes called 70 00:03:52,260 --> 00:03:55,822 the Arrhenius factor, A for Arrhenius. 71 00:03:55,822 --> 00:03:57,280 It has a couple of different names. 72 00:03:57,280 --> 00:04:01,000 But importantly, it has the same units as k, our rate constants. 73 00:04:01,000 --> 00:04:05,510 So what this plot told Arrhenius back in 1889 74 00:04:05,510 --> 00:04:08,080 was that rate constants vary exponentially 75 00:04:08,080 --> 00:04:10,380 with inverse temperature. 76 00:04:10,380 --> 00:04:12,210 So this was the first kind of connection 77 00:04:12,210 --> 00:04:14,080 between rate constants and temperature 78 00:04:14,080 --> 00:04:15,930 that could be used to kind of come up 79 00:04:15,930 --> 00:04:18,640 with quantitative numbers. 80 00:04:18,640 --> 00:04:23,690 So factor A, this Arrhenius factor, and the activation 81 00:04:23,690 --> 00:04:27,685 energy, E to the sub a, depend on the reaction being studied. 82 00:04:27,685 --> 00:04:30,670 So they have to be measured for the particular reaction. 83 00:04:30,670 --> 00:04:32,110 So it's a clicker competition. 84 00:04:32,110 --> 00:04:34,840 So we'll have a bunch of clicker questions today. 85 00:04:34,840 --> 00:04:39,217 And why don't you tell me if you think factor A is temperature 86 00:04:39,217 --> 00:04:39,716 dependent? 87 00:04:49,840 --> 00:04:50,340 All right. 88 00:04:50,340 --> 00:04:51,006 10 more seconds. 89 00:05:06,070 --> 00:05:07,090 Yep. 90 00:05:07,090 --> 00:05:10,250 The answer is no. 91 00:05:10,250 --> 00:05:14,860 And so what is this factor A? 92 00:05:14,860 --> 00:05:20,160 If we think about this in terms of the plot, what it is 93 00:05:20,160 --> 00:05:26,910 is the rate constant when 1/T is equal to 0. 94 00:05:26,910 --> 00:05:28,640 Because it is the y-intercept. 95 00:05:28,640 --> 00:05:34,982 And when 1/T is equal to 0, what has to be true about T? 96 00:05:34,982 --> 00:05:36,320 Yeah. 97 00:05:36,320 --> 00:05:37,215 Infinitely large. 98 00:05:37,215 --> 00:05:42,500 So factor A is the rate constant at 99 00:05:42,500 --> 00:05:46,330 an infinitely large temperature, at a huge temperature. 100 00:05:46,330 --> 00:05:49,520 So it's the fastest that particular reaction could ever 101 00:05:49,520 --> 00:05:52,800 go at this infinitely huge temperature. 102 00:05:52,800 --> 00:05:54,275 So that's what factor A is. 103 00:05:54,275 --> 00:05:56,940 And of course, we can't plug things in 104 00:05:56,940 --> 00:06:00,530 and say, how fast is this at an infinitely huge temperature? 105 00:06:00,530 --> 00:06:03,820 So conveniently, you can get that value out 106 00:06:03,820 --> 00:06:05,310 of plotting your data. 107 00:06:05,310 --> 00:06:08,930 You measure a bunch of rate constants versus temperature. 108 00:06:08,930 --> 00:06:10,320 And you plot it this way, and you 109 00:06:10,320 --> 00:06:14,210 can calculate this sort of maximum rate constant possible 110 00:06:14,210 --> 00:06:16,790 for this reaction if you had this infinitely 111 00:06:16,790 --> 00:06:19,460 huge temperature. 112 00:06:19,460 --> 00:06:22,245 What about activation energy? 113 00:06:22,245 --> 00:06:23,995 Do you think that's temperature dependent? 114 00:06:38,790 --> 00:06:39,300 All right. 115 00:06:39,300 --> 00:06:40,195 10 more seconds. 116 00:06:55,200 --> 00:06:58,120 So no, it isn't. 117 00:06:58,120 --> 00:07:00,670 And so if we sort of just think about it back here, 118 00:07:00,670 --> 00:07:02,600 the answer is no. 119 00:07:02,600 --> 00:07:04,780 Again, you plot the rate constants 120 00:07:04,780 --> 00:07:08,720 over all these temperatures to get one value out of the slope. 121 00:07:08,720 --> 00:07:12,830 So it is largely independent of any kind of temperature. 122 00:07:12,830 --> 00:07:15,946 You get one activation energy for the reaction in question. 123 00:07:15,946 --> 00:07:17,820 But it does depend on the reaction, for sure. 124 00:07:17,820 --> 00:07:21,360 There isn't just one value for this for everything. 125 00:07:21,360 --> 00:07:23,930 It depends on the materials. 126 00:07:23,930 --> 00:07:24,560 All right. 127 00:07:24,560 --> 00:07:27,090 So let's look at some other ways that we can 128 00:07:27,090 --> 00:07:30,260 express the Arrhenius equation. 129 00:07:30,260 --> 00:07:33,170 So we have the Arrhenius equation 130 00:07:33,170 --> 00:07:36,320 written as a straight line. 131 00:07:36,320 --> 00:07:41,350 And we can also do something very, very simple to it, 132 00:07:41,350 --> 00:07:43,820 which is switch these two terms. 133 00:07:43,820 --> 00:07:46,330 And that gives us what is officially known 134 00:07:46,330 --> 00:07:47,980 as the Arrhenius equation. 135 00:07:47,980 --> 00:07:51,040 So natural log of k equals natural log 136 00:07:51,040 --> 00:07:53,750 of A, this Arrhenius or factor A, 137 00:07:53,750 --> 00:07:57,350 minus the activation energy over the gas 138 00:07:57,350 --> 00:07:59,280 constant times temperature. 139 00:07:59,280 --> 00:08:02,670 And of course, if you want to solve for k, 140 00:08:02,670 --> 00:08:05,560 you take the inverse log of both sides. 141 00:08:05,560 --> 00:08:09,830 And so then k is going to be equal to this factor A-- again, 142 00:08:09,830 --> 00:08:14,200 measured for every reaction in question-- e to the minus 143 00:08:14,200 --> 00:08:18,830 Ea, the activation energy, over RT. 144 00:08:18,830 --> 00:08:20,410 So here is our equations. 145 00:08:20,410 --> 00:08:21,980 These will be on equation sheets, 146 00:08:21,980 --> 00:08:23,580 so you don't have to memorize them. 147 00:08:23,580 --> 00:08:26,430 But you already, if you've been studying your equation sheet 148 00:08:26,430 --> 00:08:29,450 for exam four, realize that there's 149 00:08:29,450 --> 00:08:32,409 a lot of symbols that are very similar in these units. 150 00:08:32,409 --> 00:08:37,760 So we have a, in nuclear chemistry, activity. 151 00:08:37,760 --> 00:08:41,474 So just keep in mind what equation is what. 152 00:08:41,474 --> 00:08:44,059 When you're talking about nuclear decay, 153 00:08:44,059 --> 00:08:46,300 you don't have your activation energy term. 154 00:08:46,300 --> 00:08:48,600 So that should give you somewhat of a clue. 155 00:08:48,600 --> 00:08:50,520 So one of the challenges in the final 156 00:08:50,520 --> 00:08:54,590 is making sure you know which equation is which. 157 00:08:54,590 --> 00:08:56,770 So let's look at an example now where 158 00:08:56,770 --> 00:08:58,400 if we're given some of the information, 159 00:08:58,400 --> 00:09:00,300 we can solve for other things. 160 00:09:00,300 --> 00:09:03,430 So an example, this is classes at lunch time. 161 00:09:03,430 --> 00:09:08,360 So we can think about the hydrolysis of an average lunch 162 00:09:08,360 --> 00:09:11,280 of sucrose to form a molecule of glucose 163 00:09:11,280 --> 00:09:14,600 and a molecule of fructose as part of the digestive process. 164 00:09:14,600 --> 00:09:16,810 So some of you might have eaten already. 165 00:09:16,810 --> 00:09:19,730 You might be digesting sucrose-- it 166 00:09:19,730 --> 00:09:23,230 comes in many different forms-- right now. 167 00:09:23,230 --> 00:09:27,440 So using the information about activation energy, 168 00:09:27,440 --> 00:09:29,620 we can predict what the rate constant 169 00:09:29,620 --> 00:09:32,840 is going to be at a different temperature. 170 00:09:32,840 --> 00:09:37,000 So some kind person measured the activation energy 171 00:09:37,000 --> 00:09:41,550 for this digestive process at 108 kilojoules per mole 172 00:09:41,550 --> 00:09:46,730 and also figured out that the k observed, observed 173 00:09:46,730 --> 00:09:53,165 rate constant, for this reaction is 1.0 times 10 to the minus 3 174 00:09:53,165 --> 00:09:57,080 per mole or per at normal body temperature. 175 00:09:57,080 --> 00:09:59,750 And so now we're asked to calculate what 176 00:09:59,750 --> 00:10:04,050 that rate constant should be at lower temperature, 35 177 00:10:04,050 --> 00:10:08,180 degrees Celsius, somewhat below body temperature. 178 00:10:08,180 --> 00:10:09,540 So how are we going to do that? 179 00:10:09,540 --> 00:10:12,320 Well, let's remind ourselves of our equations, our Arrhenius 180 00:10:12,320 --> 00:10:13,390 equation. 181 00:10:13,390 --> 00:10:15,570 So we know what Ea is. 182 00:10:15,570 --> 00:10:19,150 We were not told what our Arrhenius factor A is. 183 00:10:19,150 --> 00:10:20,330 We don't know that. 184 00:10:20,330 --> 00:10:22,370 But we do know one of the rate constants 185 00:10:22,370 --> 00:10:23,780 at one of the temperatures. 186 00:10:23,780 --> 00:10:27,662 So if we combine these two equations, 187 00:10:27,662 --> 00:10:30,410 then we have an appropriate number 188 00:10:30,410 --> 00:10:33,030 of variables to solve for the rate 189 00:10:33,030 --> 00:10:35,490 constant at the new temperature. 190 00:10:35,490 --> 00:10:37,980 So let's combine those two equations. 191 00:10:37,980 --> 00:10:42,780 So we have natural log of k2, our rate constant, 192 00:10:42,780 --> 00:10:45,320 at temperature 2 minus the natural log 193 00:10:45,320 --> 00:10:47,310 of the rate constant at temperature 1, 194 00:10:47,310 --> 00:10:51,230 which can also be expressed as natural log rate constant 2 195 00:10:51,230 --> 00:10:53,080 over rate constant 1. 196 00:10:53,080 --> 00:10:58,010 And our natural log of the Arrhenius factor drops out. 197 00:10:58,010 --> 00:11:01,310 And so we have minus Ea, our activation energy, 198 00:11:01,310 --> 00:11:05,910 over our gas constant times 1 over the temperature minus 1 199 00:11:05,910 --> 00:11:07,760 over the first temperature. 200 00:11:07,760 --> 00:11:11,900 So we can put in our values that we're given. 201 00:11:11,900 --> 00:11:15,900 So we have our first rate constant down here, 202 00:11:15,900 --> 00:11:18,060 the rate constant at the first temperature. 203 00:11:18,060 --> 00:11:19,910 We put in our temperatures, making 204 00:11:19,910 --> 00:11:24,070 sure we convert them to kelvin because our gas constant is 205 00:11:24,070 --> 00:11:25,410 in kelvin. 206 00:11:25,410 --> 00:11:29,470 And also, we are going to convert our activation energy 207 00:11:29,470 --> 00:11:32,390 from kilojoules to joules because we 208 00:11:32,390 --> 00:11:34,230 want to cancel our units. 209 00:11:34,230 --> 00:11:35,790 So now we can cancel joules. 210 00:11:35,790 --> 00:11:37,120 We can cancel moles. 211 00:11:37,120 --> 00:11:39,390 And we can cancel kelvin. 212 00:11:39,390 --> 00:11:43,360 And so that gives us a new rate constant, 213 00:11:43,360 --> 00:11:48,740 7.6 times 10 to the minus 4 per mole or per second. 214 00:11:48,740 --> 00:11:53,550 And this is a lower rate at a lower temperature. 215 00:11:53,550 --> 00:11:55,655 And in fact, that is often true. 216 00:11:55,655 --> 00:11:58,540 You have a lower rate at a lower temperature. 217 00:11:58,540 --> 00:12:02,180 And this is one of the reasons why it's a really good idea 218 00:12:02,180 --> 00:12:05,880 to keep your body temperature at body temperature. 219 00:12:05,880 --> 00:12:09,400 And around this time of year, people come in to MIT 220 00:12:09,400 --> 00:12:10,550 from warmer climates. 221 00:12:10,550 --> 00:12:13,250 And they do not have appropriate clothing. 222 00:12:13,250 --> 00:12:17,110 And so your body does not work well in the cold. 223 00:12:17,110 --> 00:12:18,350 The rates slow down. 224 00:12:18,350 --> 00:12:19,720 You're not digesting things. 225 00:12:19,720 --> 00:12:21,669 Your body is really not doing anything 226 00:12:21,669 --> 00:12:23,210 at the rate it's supposed to be doing 227 00:12:23,210 --> 00:12:25,260 these reactions because they're nicely 228 00:12:25,260 --> 00:12:27,800 tuned to the appropriate body temperature. 229 00:12:27,800 --> 00:12:32,070 So go out and buy a winter coat. 230 00:12:32,070 --> 00:12:37,400 So this now, this equation that we just derived 231 00:12:37,400 --> 00:12:40,500 shows us the relationship between rate constants 232 00:12:40,500 --> 00:12:42,030 and temperature. 233 00:12:42,030 --> 00:12:47,790 So again, natural log k2 over k1 minus the activation energy 234 00:12:47,790 --> 00:12:51,710 over the gas constant and our temperature term. 235 00:12:51,710 --> 00:12:53,310 And if we look at this equation, we'll 236 00:12:53,310 --> 00:12:58,630 realize that if we have a very large value for this activation 237 00:12:58,630 --> 00:13:01,850 energy, this Ea term here, that's going 238 00:13:01,850 --> 00:13:04,000 to mean that the rate constants are 239 00:13:04,000 --> 00:13:05,850 very sensitive to temperature. 240 00:13:05,850 --> 00:13:08,320 If this is small, there won't be a big difference 241 00:13:08,320 --> 00:13:10,090 between k1 and k2. 242 00:13:10,090 --> 00:13:13,190 But if the activation energy is really big, 243 00:13:13,190 --> 00:13:16,490 there will be a big difference between k1 and k2. 244 00:13:16,490 --> 00:13:19,810 The rate constants will be very sensitive to temperature. 245 00:13:19,810 --> 00:13:20,310 All right. 246 00:13:20,310 --> 00:13:23,230 So now let's think about rates and temperature. 247 00:13:23,230 --> 00:13:26,130 And let's think of a cold temperature. 248 00:13:26,130 --> 00:13:29,900 Let's think, for example, of liquid nitrogen temperatures. 249 00:13:29,900 --> 00:13:31,840 What do you think happens to an enzyme? 250 00:13:31,840 --> 00:13:34,070 You have enzymes in your digestive process 251 00:13:34,070 --> 00:13:36,940 that are hydrolyzing sucrose that you 252 00:13:36,940 --> 00:13:38,370 might have had for lunch. 253 00:13:38,370 --> 00:13:39,900 What do you think happens to enzymes 254 00:13:39,900 --> 00:13:41,858 if you put them at liquid nitrogen temperature? 255 00:13:45,750 --> 00:13:47,820 They will slow way down. 256 00:13:47,820 --> 00:13:50,560 One would say they would stop working entirely. 257 00:13:50,560 --> 00:13:56,180 They would just sort of be, we call it frozen. 258 00:13:56,180 --> 00:14:00,020 And in fact, I use this all the time in my research. 259 00:14:00,020 --> 00:14:03,990 So one thing we do, we study the structures of proteins. 260 00:14:03,990 --> 00:14:07,170 And so we grow these lovely protein crystals. 261 00:14:07,170 --> 00:14:10,020 And what we can do, we take these-- a lot of times, 262 00:14:10,020 --> 00:14:12,580 the proteins or enzymes in the crystals are active. 263 00:14:12,580 --> 00:14:15,140 They're able to convert reactants to products. 264 00:14:15,140 --> 00:14:17,760 And so we can soak in reactants and then take our crystals 265 00:14:17,760 --> 00:14:20,250 and dunk them in liquid nitrogen and then 266 00:14:20,250 --> 00:14:21,740 determine the structure of that. 267 00:14:21,740 --> 00:14:23,670 And if we do it at different time points, 268 00:14:23,670 --> 00:14:25,920 you can actually kind of walk through the mechanism 269 00:14:25,920 --> 00:14:26,890 of the enzyme. 270 00:14:26,890 --> 00:14:29,480 Watch what happens at various different states. 271 00:14:29,480 --> 00:14:31,850 So this is a very common thing that's used. 272 00:14:31,850 --> 00:14:34,460 We like to do things at liquid nitrogen 273 00:14:34,460 --> 00:14:37,640 to see things sort of-- we sort of pause it 274 00:14:37,640 --> 00:14:39,140 when it's dumped in liquid nitrogen. 275 00:14:39,140 --> 00:14:40,240 It stops what it's doing. 276 00:14:40,240 --> 00:14:42,680 And we can capture its structure. 277 00:14:42,680 --> 00:14:46,005 What do you think about non-enzymatic reactions? 278 00:14:46,005 --> 00:14:49,849 Do you think they're also going to be slowed down? 279 00:14:49,849 --> 00:14:50,390 I don't know. 280 00:14:50,390 --> 00:14:52,106 Should we do an experiment and find out? 281 00:14:52,106 --> 00:14:52,860 AUDIENCE: Yeah. 282 00:14:52,860 --> 00:14:54,830 CATHERINE DRENNAN: Yeah, I think we should. 283 00:14:54,830 --> 00:14:59,410 So we now have-- let's bring out our demo TAs. 284 00:14:59,410 --> 00:15:02,440 We now have glow sticks. 285 00:15:02,440 --> 00:15:07,090 Glow sticks are in fact chemicals in there 286 00:15:07,090 --> 00:15:11,390 that when you break them, you have a chemical reaction. 287 00:15:11,390 --> 00:15:13,647 So we're going to do this. 288 00:15:13,647 --> 00:15:15,730 And we're going to break them and watch them glow. 289 00:15:15,730 --> 00:15:18,630 Let's break them and put them in here first. 290 00:15:18,630 --> 00:15:21,790 And then we're going to cool them down and see what happens 291 00:15:21,790 --> 00:15:23,960 to the chemical reaction. 292 00:15:23,960 --> 00:15:27,000 Here, let's have a little help over here. 293 00:15:27,000 --> 00:15:28,770 Please break some of these. 294 00:15:28,770 --> 00:15:30,103 AUDIENCE: How do you break them? 295 00:15:31,252 --> 00:15:32,710 CATHERINE DRENNAN: You break-- see, 296 00:15:32,710 --> 00:15:38,340 you just kind of snap them and then shake them up. 297 00:15:38,340 --> 00:15:42,540 And I think we'll bring down the lights so you can all see these 298 00:15:42,540 --> 00:15:43,180 pretty well. 299 00:15:53,740 --> 00:15:56,590 So now the chemical reaction is going. 300 00:15:56,590 --> 00:15:58,460 And so we can see the chemical reaction. 301 00:15:58,460 --> 00:16:00,500 That's what's wonderful about glow sticks. 302 00:16:00,500 --> 00:16:02,380 You can observe the chemical reaction 303 00:16:02,380 --> 00:16:03,895 because it causes it to glow. 304 00:16:09,010 --> 00:16:11,040 Move so everyone can see. 305 00:16:11,040 --> 00:16:13,380 Now we're going to see what happens to the chemical 306 00:16:13,380 --> 00:16:15,110 reaction when it slows again. 307 00:16:15,110 --> 00:16:17,236 When the chemical reaction is slowed down, 308 00:16:17,236 --> 00:16:19,070 you won't see it anymore. 309 00:16:19,070 --> 00:16:20,120 So it'll stop glowing. 310 00:16:27,144 --> 00:16:27,977 That should be good. 311 00:16:34,621 --> 00:16:36,120 We'll do all of them, and we'll see. 312 00:16:36,120 --> 00:16:37,630 It'll take a little bit of time. 313 00:16:37,630 --> 00:16:40,060 But it should not be too long. 314 00:16:48,500 --> 00:16:51,340 And hopefully, it'll slow down faster than the liquid nitrogen 315 00:16:51,340 --> 00:16:53,012 destroys the plastic cups. 316 00:16:55,324 --> 00:16:57,490 Yeah, I think you can see it with the first one now. 317 00:17:04,619 --> 00:17:06,129 Mary, why don't you come down here? 318 00:17:06,129 --> 00:17:07,670 We have something else for you to do. 319 00:17:19,480 --> 00:17:21,207 Let's put some liquid nitrogen in here. 320 00:17:27,849 --> 00:17:30,000 Let's bring up the lights because our glowing 321 00:17:30,000 --> 00:17:32,630 has stopped. 322 00:17:32,630 --> 00:17:35,180 So another thing that liquid nitrogen 323 00:17:35,180 --> 00:17:37,080 changes the properties. 324 00:17:37,080 --> 00:17:41,520 So Mary will demonstrate to you what happens to a flower. 325 00:17:41,520 --> 00:17:44,575 And just like right now, the flower does this. 326 00:17:44,575 --> 00:17:50,010 But let's put it in liquid nitrogen and see what happens. 327 00:17:50,010 --> 00:17:51,095 Give it a decent soak. 328 00:18:02,300 --> 00:18:04,010 And now smash it. 329 00:18:04,010 --> 00:18:08,690 [LAUGHTER] 330 00:18:08,690 --> 00:18:10,718 Thank you very much, our lovely assistant. 331 00:18:16,790 --> 00:18:18,620 I will brush off my computer later. 332 00:18:18,620 --> 00:18:20,910 We have more flowers, and apparently there's 333 00:18:20,910 --> 00:18:21,750 a broom somewhere. 334 00:18:21,750 --> 00:18:26,170 So if we finish class, for some lucky other individuals, 335 00:18:26,170 --> 00:18:28,930 you can come down and smash the rest of the flowers. 336 00:18:28,930 --> 00:18:33,200 So liquid nitrogen really changes the property of things. 337 00:18:33,200 --> 00:18:38,160 Sometimes it's used by dermatologists 338 00:18:38,160 --> 00:18:40,070 to remove warts or other things. 339 00:18:40,070 --> 00:18:43,400 They'll dab some liquid nitrogen on you. 340 00:18:43,400 --> 00:18:46,410 And I always feel like paying a lot of money 341 00:18:46,410 --> 00:18:49,142 to go have that done-- I have liquid nitrogen around my lab. 342 00:18:49,142 --> 00:18:50,600 But then I'm like, yeah, I probably 343 00:18:50,600 --> 00:18:52,590 have enough liquid nitrogen burns on my figures 344 00:18:52,590 --> 00:18:54,490 already that I don't want. 345 00:18:54,490 --> 00:18:56,850 Crystallographers who use this a lot, 346 00:18:56,850 --> 00:18:59,030 they'll often be in a situation where 347 00:18:59,030 --> 00:19:00,780 you have this liquid nitrogen. And it's 348 00:19:00,780 --> 00:19:02,280 sort of dripping on you. 349 00:19:02,280 --> 00:19:05,440 But you have your crystal, and if you let go, 350 00:19:05,440 --> 00:19:07,930 then you'll destroy your crystal experiment. 351 00:19:07,930 --> 00:19:10,770 So you'll have liquid nitrogen sitting in your palm. 352 00:19:10,770 --> 00:19:12,870 And you're like, OK, I'm going to get a burn. 353 00:19:12,870 --> 00:19:15,400 Or I'm going to lose my crystal. 354 00:19:15,400 --> 00:19:17,410 And so you decide how important that is. 355 00:19:17,410 --> 00:19:18,868 And so sometimes a crystallographer 356 00:19:18,868 --> 00:19:22,010 will walk up to you and go, see that scar? 357 00:19:22,010 --> 00:19:27,130 That's 1.2 angstrom data, baby, right there. 358 00:19:27,130 --> 00:19:30,990 So we suffer for our science sometimes. 359 00:19:30,990 --> 00:19:34,630 So liquid nitrogen is very-- changes the property. 360 00:19:34,630 --> 00:19:38,400 Cold things are different than warm things. 361 00:19:38,400 --> 00:19:40,330 Buy a winter coat. 362 00:19:40,330 --> 00:19:43,830 So everything slows down in the cold, 363 00:19:43,830 --> 00:19:46,320 in terms of elementary reactions. 364 00:19:46,320 --> 00:19:49,740 Our rate constants slow down. 365 00:19:49,740 --> 00:19:51,910 So now let's think about a reaction 366 00:19:51,910 --> 00:19:54,180 and what's called a reaction coordinate 367 00:19:54,180 --> 00:19:57,216 and consider what's happening in a reaction. 368 00:19:57,216 --> 00:19:58,590 And then we're going to come back 369 00:19:58,590 --> 00:20:00,750 to thinking about the effect of temperature, 370 00:20:00,750 --> 00:20:02,601 where temperature really makes a difference. 371 00:20:09,100 --> 00:20:11,790 We'll quiet down a little. 372 00:20:11,790 --> 00:20:15,870 So considering the reaction coordinate, reaction coordinate 373 00:20:15,870 --> 00:20:19,190 the reactions of-- you bring your reactants together 374 00:20:19,190 --> 00:20:20,620 and form your product. 375 00:20:20,620 --> 00:20:23,450 So two things coming together to form something else, 376 00:20:23,450 --> 00:20:24,410 that is a reaction. 377 00:20:24,410 --> 00:20:26,140 And as you go along in the reaction, 378 00:20:26,140 --> 00:20:27,716 that's your reaction coordinate. 379 00:20:27,716 --> 00:20:29,090 And we're going to also introduce 380 00:20:29,090 --> 00:20:34,190 this term of activation complex or transition state. 381 00:20:34,190 --> 00:20:37,020 So two molecules can collide. 382 00:20:37,020 --> 00:20:40,130 Two molecules colliding, bimolecular. 383 00:20:40,130 --> 00:20:43,610 But every time those two molecules come together, 384 00:20:43,610 --> 00:20:47,220 they're not necessarily going to form a product. 385 00:20:47,220 --> 00:20:49,140 Why? 386 00:20:49,140 --> 00:20:54,570 So only when that collision energy is greater 387 00:20:54,570 --> 00:20:58,570 than some critical energy-- which is sometimes called Emin, 388 00:20:58,570 --> 00:21:01,900 this sort of minimum energy to get this reaction to go, 389 00:21:01,900 --> 00:21:05,580 or as I like to call it, the activation energy, Ea, 390 00:21:05,580 --> 00:21:07,240 that's what we've been talking about, 391 00:21:07,240 --> 00:21:11,560 our activation energy-- will you get a reaction. 392 00:21:11,560 --> 00:21:14,390 So you need to have enough energy in your two 393 00:21:14,390 --> 00:21:16,310 things coming together. 394 00:21:16,310 --> 00:21:19,530 Those two things coming together need to have a critical energy 395 00:21:19,530 --> 00:21:20,431 for them to react. 396 00:21:25,870 --> 00:21:28,990 So why is this true? 397 00:21:28,990 --> 00:21:30,900 You need to have that critical energy. 398 00:21:30,900 --> 00:21:31,486 But why? 399 00:21:31,486 --> 00:21:34,420 Why is this necessary? 400 00:21:34,420 --> 00:21:39,010 So it's necessary because before this reaction takes place, 401 00:21:39,010 --> 00:21:45,040 even if it's a very happy reaction, things need to occur. 402 00:21:45,040 --> 00:21:50,290 So the two things coming together need to often distort. 403 00:21:50,290 --> 00:21:52,170 Bonds might have to be broken. 404 00:21:52,170 --> 00:21:54,350 And new bonds need to be formed. 405 00:21:54,350 --> 00:21:58,030 And while that is occurring, while there is distortion 406 00:21:58,030 --> 00:22:00,340 of the bond or bonds are breaking, 407 00:22:00,340 --> 00:22:02,850 you need to have some potential energy to do that. 408 00:22:02,850 --> 00:22:06,710 You need some energy to make that reaction go. 409 00:22:06,710 --> 00:22:08,980 So the potential energy of the system 410 00:22:08,980 --> 00:22:14,910 increases first while these distortions are happening. 411 00:22:14,910 --> 00:22:17,620 So the encounter then between them, you 412 00:22:17,620 --> 00:22:20,600 form some activated complex, or what's 413 00:22:20,600 --> 00:22:23,430 known as a transition state. 414 00:22:23,430 --> 00:22:31,550 And that activated complex, it can go on to form a molecule. 415 00:22:31,550 --> 00:22:35,690 Or from that activated complex, the two molecules 416 00:22:35,690 --> 00:22:38,310 may just go apart again. 417 00:22:38,310 --> 00:22:39,910 So what determines whether they're 418 00:22:39,910 --> 00:22:43,330 going to go on happily to form their complex 419 00:22:43,330 --> 00:22:46,400 or going to depart from each other, 420 00:22:46,400 --> 00:22:49,850 never to form a larger molecule? 421 00:22:49,850 --> 00:22:52,770 And the thing that determines the fate 422 00:22:52,770 --> 00:22:54,810 is that critical energy. 423 00:22:54,810 --> 00:22:58,050 So only those molecules with that sufficient energy 424 00:22:58,050 --> 00:23:02,000 to allow for those bond distortions and rearrangements 425 00:23:02,000 --> 00:23:03,820 will be able to go on and make this. 426 00:23:03,820 --> 00:23:05,403 And you can think about this, I think, 427 00:23:05,403 --> 00:23:07,770 in terms of a couple, a relationship, 428 00:23:07,770 --> 00:23:10,380 that there's always some work that has to go into it. 429 00:23:10,380 --> 00:23:11,310 It's never perfect. 430 00:23:11,310 --> 00:23:14,650 And if you put in this effort and this work, you can go on. 431 00:23:14,650 --> 00:23:17,670 And if you don't really have it in you and you're like, ah, 432 00:23:17,670 --> 00:23:18,850 you walk away. 433 00:23:18,850 --> 00:23:20,350 You don't have that critical energy. 434 00:23:20,350 --> 00:23:23,790 You don't have that special overcome, that activation 435 00:23:23,790 --> 00:23:25,284 energy, and you go back. 436 00:23:25,284 --> 00:23:27,450 So let's just take a look at some molecules checking 437 00:23:27,450 --> 00:23:30,180 each other out and figuring out if they have 438 00:23:30,180 --> 00:23:32,700 what it takes, if they have that critical energy. 439 00:23:32,700 --> 00:23:34,330 Let's watch and see what happens. 440 00:23:34,330 --> 00:23:35,280 Here they come. 441 00:23:35,280 --> 00:23:36,610 They're finding each other. 442 00:23:36,610 --> 00:23:37,890 They're circling each other. 443 00:23:37,890 --> 00:23:39,540 And oh my goodness. 444 00:23:39,540 --> 00:23:42,690 They had the critical energy necessary. 445 00:23:42,690 --> 00:23:44,660 And they formed a bigger yellow molecule. 446 00:23:44,660 --> 00:23:45,650 There they go. 447 00:23:45,650 --> 00:23:48,320 What a happy ending. 448 00:23:48,320 --> 00:23:53,260 So only those molecules that have that critical energy 449 00:23:53,260 --> 00:23:56,030 can go on and form their complex. 450 00:23:56,030 --> 00:24:00,380 And so here is where temperature comes into play. 451 00:24:00,380 --> 00:24:04,920 Here is where temperature becomes really important. 452 00:24:04,920 --> 00:24:08,450 Because they need to have a certain amount of energy. 453 00:24:08,450 --> 00:24:12,160 And there's a relationship between the kinetic energy 454 00:24:12,160 --> 00:24:15,330 of molecules and their temperature. 455 00:24:15,330 --> 00:24:18,170 So let's take a look at this plot over here. 456 00:24:18,170 --> 00:24:21,790 So this is, on one axis, fraction of molecules. 457 00:24:21,790 --> 00:24:25,180 And on the other axis, we have kinetic energy. 458 00:24:25,180 --> 00:24:28,340 So if you're at low temperature over here, 459 00:24:28,340 --> 00:24:32,820 most of your molecules are going to have a low kinetic energy. 460 00:24:32,820 --> 00:24:34,470 But it will tail off. 461 00:24:34,470 --> 00:24:38,140 And there will be some molecules that will have a higher energy. 462 00:24:38,140 --> 00:24:41,240 And if this line here represents that minimum energy, 463 00:24:41,240 --> 00:24:45,490 that critical energy or that activation energy, in blue 464 00:24:45,490 --> 00:24:48,630 here we have those low temperature molecules. 465 00:24:48,630 --> 00:24:50,700 Only a very small fraction of those 466 00:24:50,700 --> 00:24:53,360 will have the energy to react, will be able to overcome 467 00:24:53,360 --> 00:24:56,320 that minimum energy, that critical energy needed. 468 00:24:56,320 --> 00:24:58,220 Now if you're at high temperature, 469 00:24:58,220 --> 00:25:01,190 more molecules have a higher kinetic energy. 470 00:25:01,190 --> 00:25:06,410 And if you see over here shaded in orange, way more molecules 471 00:25:06,410 --> 00:25:08,960 have that critical energy, have the energy 472 00:25:08,960 --> 00:25:14,060 necessary to distort those bonds and go on to make a molecule. 473 00:25:14,060 --> 00:25:16,150 So here's where temperature is important. 474 00:25:16,150 --> 00:25:19,112 So kinetic energy, pass it on. 475 00:25:22,390 --> 00:25:24,844 So let's draw some reaction coordinates and think 476 00:25:24,844 --> 00:25:25,760 about what's going on. 477 00:25:25,760 --> 00:25:26,915 Yeah. 478 00:25:26,915 --> 00:25:28,740 AUDIENCE: For this idea of collision, 479 00:25:28,740 --> 00:25:31,386 could it also apply to the association reactions? 480 00:25:31,386 --> 00:25:33,362 Because that's [INAUDIBLE] molecule. 481 00:25:33,362 --> 00:25:35,529 Or are [? they still into ?] each other? [INAUDIBLE] 482 00:25:35,529 --> 00:25:36,528 CATHERINE DRENNAN: Yeah. 483 00:25:36,528 --> 00:25:38,680 So if you're talking about the forward reaction 484 00:25:38,680 --> 00:25:40,285 and the backward reaction of things, 485 00:25:40,285 --> 00:25:42,410 whether it's two molecules that are coming together 486 00:25:42,410 --> 00:25:45,950 to form things or molecules breaking apart, 487 00:25:45,950 --> 00:25:48,442 there's a critical energy both directions. 488 00:25:48,442 --> 00:25:49,650 And we'll see that, actually. 489 00:25:49,650 --> 00:25:50,525 It's a good question. 490 00:25:50,525 --> 00:25:55,070 That kind of leads us in to our reaction coordinate diagram. 491 00:25:55,070 --> 00:25:57,820 So most of this is in your notes. 492 00:25:57,820 --> 00:25:59,820 There's a few things that are not. 493 00:25:59,820 --> 00:26:03,060 But let's look at a reaction coordinate diagram. 494 00:26:03,060 --> 00:26:08,470 And by that, I mean we have Potential Energy, or PE, 495 00:26:08,470 --> 00:26:10,390 on one side. 496 00:26:10,390 --> 00:26:12,640 And on the other axis, we have what's 497 00:26:12,640 --> 00:26:14,465 called just the reaction coordinate. 498 00:26:21,220 --> 00:26:25,130 So if you're asked to draw a reaction coordinate diagram 499 00:26:25,130 --> 00:26:28,740 on a problem set or an exam, asking for potential energy 500 00:26:28,740 --> 00:26:31,370 versus reaction coordinate. 501 00:26:31,370 --> 00:26:35,640 Now our reactants are going to have a particular amount 502 00:26:35,640 --> 00:26:37,770 of potential energy. 503 00:26:37,770 --> 00:26:39,520 And in this case, it's up here. 504 00:26:43,810 --> 00:26:48,820 And our products are going to have some potential energy 505 00:26:48,820 --> 00:26:49,530 down here. 506 00:26:54,980 --> 00:27:01,750 And there will be a difference in energy between these, 507 00:27:01,750 --> 00:27:06,590 our delta E. But these reactants aren't just 508 00:27:06,590 --> 00:27:11,200 going to be able to go to products without overcoming 509 00:27:11,200 --> 00:27:13,310 some kind of critical energy. 510 00:27:13,310 --> 00:27:19,490 So before they can go down there, they need to go up here. 511 00:27:19,490 --> 00:27:21,790 So we have the activation energy, 512 00:27:21,790 --> 00:27:25,840 f for forward direction. 513 00:27:25,840 --> 00:27:28,990 Also, if you're down in products and you 514 00:27:28,990 --> 00:27:32,067 want to go back to reactants, there's also a barrier 515 00:27:32,067 --> 00:27:33,150 that you have to overcome. 516 00:27:33,150 --> 00:27:35,550 And it's not just this. 517 00:27:35,550 --> 00:27:37,590 It's all the way up here. 518 00:27:41,170 --> 00:27:43,290 And so this would be the activation energy 519 00:27:43,290 --> 00:27:45,660 of the reverse reaction. 520 00:27:45,660 --> 00:27:52,060 And this dashed line on the top is our transition state 521 00:27:52,060 --> 00:27:54,160 or our activated complex. 522 00:27:54,160 --> 00:27:57,610 Both are two expressions that are 523 00:27:57,610 --> 00:27:58,830 used kind of interchangeably. 524 00:28:02,390 --> 00:28:04,755 So this transition states, some kind of weird mixture 525 00:28:04,755 --> 00:28:06,630 where they've come together, and they're sort 526 00:28:06,630 --> 00:28:08,110 of breaking and distorting. 527 00:28:08,110 --> 00:28:10,170 It's not our final products. 528 00:28:10,170 --> 00:28:11,340 That's down here. 529 00:28:11,340 --> 00:28:14,870 But you need to go up in energy, overcome an activation energy 530 00:28:14,870 --> 00:28:15,670 barrier. 531 00:28:15,670 --> 00:28:21,640 So the transition here, you need to go up, and then you go down. 532 00:28:21,640 --> 00:28:27,410 So now let's think about all of these energy terms. 533 00:28:27,410 --> 00:28:32,570 And what's true is that the change in energy-- this change 534 00:28:32,570 --> 00:28:35,180 in energy between reactants and products 535 00:28:35,180 --> 00:28:40,190 is equal to our activation energy for our forward reaction 536 00:28:40,190 --> 00:28:47,010 minus the activation energy of the reverse reaction. 537 00:28:47,010 --> 00:28:51,020 And this delta E over here can be measured 538 00:28:51,020 --> 00:28:53,230 from a calorimetry experiment. 539 00:28:53,230 --> 00:28:54,955 And you might recall back when we 540 00:28:54,955 --> 00:28:58,010 were talking about thermodynamics that E 541 00:28:58,010 --> 00:29:01,380 is kind of closely related to delta H. 542 00:29:01,380 --> 00:29:06,120 So we had this equation at one point 543 00:29:06,120 --> 00:29:16,660 that delta H is equal to delta E plus the change in PV. 544 00:29:16,660 --> 00:29:22,290 And we said that for gases, there's about a 1% to 2% 545 00:29:22,290 --> 00:29:24,960 difference between delta H and delta E. 546 00:29:24,960 --> 00:29:27,990 But for solids and liquids, it's pretty much the same. 547 00:29:27,990 --> 00:29:30,380 So you can think about this delta E 548 00:29:30,380 --> 00:29:35,210 like you were thinking about delta H, for the most part. 549 00:29:35,210 --> 00:29:40,000 So now for this, if we have some of the numbers-- 550 00:29:40,000 --> 00:29:43,180 if you know activation energy in the forward and the reverse, 551 00:29:43,180 --> 00:29:44,720 you can calculate delta E. If you 552 00:29:44,720 --> 00:29:47,370 know delta E and one of the activation energies, 553 00:29:47,370 --> 00:29:49,220 you can calculate the other. 554 00:29:49,220 --> 00:29:52,810 So I'll tell you for this particular reaction 555 00:29:52,810 --> 00:29:53,896 what these values are. 556 00:29:53,896 --> 00:29:55,520 And I was going to bring colored chalk, 557 00:29:55,520 --> 00:29:57,620 and somehow that didn't happen. 558 00:29:57,620 --> 00:30:00,100 So the activation energy in this is not in your notes. 559 00:30:00,100 --> 00:30:01,640 So I'll write it down. 560 00:30:01,640 --> 00:30:06,640 Is 132 kilojoules per mole. 561 00:30:06,640 --> 00:30:09,320 And I'll also put it here, a little bigger, 562 00:30:09,320 --> 00:30:14,140 132 kilojoules per mole. 563 00:30:14,140 --> 00:30:22,760 And for the reverse reaction, it's 358 kilojoules per mole 564 00:30:22,760 --> 00:30:28,660 and minus 358 kilojoules per mole. 565 00:30:32,700 --> 00:30:35,490 And so if we do the math here, we 566 00:30:35,490 --> 00:30:45,780 will calculate that delta E is equal to minus 226 kilojoules 567 00:30:45,780 --> 00:30:48,350 per mole. 568 00:30:48,350 --> 00:30:51,900 So would you expect that to be an endothermic or exothermic 569 00:30:51,900 --> 00:30:53,890 reaction? 570 00:30:53,890 --> 00:30:54,770 Exothermic. 571 00:30:54,770 --> 00:30:55,400 Right. 572 00:30:55,400 --> 00:30:57,840 And we can put that value also in here, 573 00:30:57,840 --> 00:31:02,750 minus 226 kilojoules per mole. 574 00:31:02,750 --> 00:31:06,870 So what's really important here, even though it's 575 00:31:06,870 --> 00:31:09,220 exothermic reaction, it's not going 576 00:31:09,220 --> 00:31:13,620 to just go without having those molecules have 577 00:31:13,620 --> 00:31:17,030 some critical energy necessary to overcome 578 00:31:17,030 --> 00:31:19,090 this activation energy barrier. 579 00:31:19,090 --> 00:31:21,170 You still need energy. 580 00:31:21,170 --> 00:31:26,460 You need to go up here before you can go down there. 581 00:31:26,460 --> 00:31:30,800 So just sort of thinking about this in layman's terms, 582 00:31:30,800 --> 00:31:35,740 this idea of these activation energy barriers, 583 00:31:35,740 --> 00:31:42,820 for me whenever I sit down to write a grant or write a paper, 584 00:31:42,820 --> 00:31:44,320 I think, wow. 585 00:31:44,320 --> 00:31:47,904 I sit down at my computer, have my cup of coffee. 586 00:31:47,904 --> 00:31:50,070 I know that we're about to get scooped on this data. 587 00:31:50,070 --> 00:31:52,830 So I really should write this paper, a little stressed 588 00:31:52,830 --> 00:31:53,340 about it. 589 00:31:53,340 --> 00:31:54,720 Then I think, wow. 590 00:31:54,720 --> 00:31:57,290 I could write a lot better if my office was clean. 591 00:31:57,290 --> 00:31:59,750 Now I hate cleaning my office. 592 00:31:59,750 --> 00:32:03,470 But compared to writing a paper, it's pretty good. 593 00:32:03,470 --> 00:32:05,540 I don't mind doing it. 594 00:32:05,540 --> 00:32:09,260 So then clean the office. 595 00:32:09,260 --> 00:32:10,950 Then walk the dog. 596 00:32:10,950 --> 00:32:14,180 Maybe do later some laundry. 597 00:32:14,180 --> 00:32:15,810 Lots of things happen. 598 00:32:15,810 --> 00:32:18,940 And then eventually, you hear from the collaborators. 599 00:32:18,940 --> 00:32:20,830 They're like, I need this paper now. 600 00:32:20,830 --> 00:32:23,010 And you're like, oh man. 601 00:32:23,010 --> 00:32:28,500 And so that stress gets you over that activation energy barrier, 602 00:32:28,500 --> 00:32:30,970 just catapults you over. 603 00:32:30,970 --> 00:32:34,320 Or you wait to a grant-- you start six months early. 604 00:32:34,320 --> 00:32:36,100 And then the week before it's due, 605 00:32:36,100 --> 00:32:41,500 suddenly you have that nervous energy that gets you over that. 606 00:32:41,500 --> 00:32:44,520 So many of you have probably experienced this. 607 00:32:44,520 --> 00:32:49,950 And you know, if you need some extra help with an activation 608 00:32:49,950 --> 00:32:53,490 energy barrier to do the rest of those extra problems for exam 609 00:32:53,490 --> 00:32:55,670 four, you can come talk to me. 610 00:32:55,670 --> 00:32:59,570 And I'll stress you out and get you right over that activation 611 00:32:59,570 --> 00:33:00,510 energy barrier. 612 00:33:00,510 --> 00:33:04,630 But the thing to keep in mind is that look at this slope 613 00:33:04,630 --> 00:33:06,270 in this case down here. 614 00:33:06,270 --> 00:33:10,800 So often it's just that little something to get you over. 615 00:33:10,800 --> 00:33:13,090 And then it's smooth sailing. 616 00:33:13,090 --> 00:33:15,415 So you can work these games with yourself. 617 00:33:15,415 --> 00:33:17,040 You've just got to get over it and know 618 00:33:17,040 --> 00:33:18,980 that once you're over that barrier, 619 00:33:18,980 --> 00:33:20,330 it's all going to be good. 620 00:33:20,330 --> 00:33:22,780 So you can get yourself over. 621 00:33:22,780 --> 00:33:26,150 You should never forget about activation energy barriers. 622 00:33:26,150 --> 00:33:27,220 There's always a barrier. 623 00:33:27,220 --> 00:33:28,030 Am I right? 624 00:33:28,030 --> 00:33:29,940 There's always some kind of barrier. 625 00:33:29,940 --> 00:33:32,180 Anything you're going to do that's worthwhile 626 00:33:32,180 --> 00:33:37,230 has some barrier associated with it. 627 00:33:37,230 --> 00:33:40,240 So now let's think about the results of these barriers, 628 00:33:40,240 --> 00:33:43,630 again, coming back to this idea of temperature. 629 00:33:43,630 --> 00:33:46,000 So for an elementary reaction-- again, 630 00:33:46,000 --> 00:33:48,210 that's a reaction that occurs exactly as written. 631 00:33:48,210 --> 00:33:51,670 It's a step in an overall reaction mechanism. 632 00:33:51,670 --> 00:33:54,555 There's always a barrier, always a barrier. 633 00:33:54,555 --> 00:33:56,400 Barrier is always positive. 634 00:33:56,400 --> 00:33:58,550 It's always there. 635 00:33:58,550 --> 00:34:02,640 And so if you increase the temperature, 636 00:34:02,640 --> 00:34:05,890 you're always going to increase the rate of that reaction. 637 00:34:05,890 --> 00:34:10,100 It's always going to help get over that barrier. 638 00:34:10,100 --> 00:34:14,400 But increase the temperature, increase the rate. 639 00:34:14,400 --> 00:34:17,980 But for an overall reaction, temperature 640 00:34:17,980 --> 00:34:20,940 is a little more complicated to predict. 641 00:34:20,940 --> 00:34:23,510 So if you increase the temperature, 642 00:34:23,510 --> 00:34:26,529 it's not always as clear what's going to happen to the rate. 643 00:34:26,529 --> 00:34:28,820 Because you can have a lot of different steps involved. 644 00:34:28,820 --> 00:34:30,449 It can be exothermic and endothermic. 645 00:34:30,449 --> 00:34:32,219 A lot can be happening. 646 00:34:32,219 --> 00:34:35,000 Changing the temperature, you're changing k. 647 00:34:35,000 --> 00:34:38,190 So to think about an overall reaction, 648 00:34:38,190 --> 00:34:40,326 we need to understand reaction mechanisms. 649 00:34:40,326 --> 00:34:42,159 And that's really convenient because we just 650 00:34:42,159 --> 00:34:44,980 talked about reaction mechanisms on Monday. 651 00:34:44,980 --> 00:34:48,250 So you all know how to write out reaction mechanisms. 652 00:34:48,250 --> 00:34:50,430 So let's just practice. 653 00:34:50,430 --> 00:34:52,180 So for this example, we have two molecules 654 00:34:52,180 --> 00:34:55,775 of N O plus O2 going to 2NO2. 655 00:34:55,775 --> 00:34:58,830 Step one, fast and reversible. 656 00:34:58,830 --> 00:35:02,320 N O plus N O, going to an intermediate. 657 00:35:02,320 --> 00:35:03,840 Then the intermediate is reacting 658 00:35:03,840 --> 00:35:08,260 with oxygen going to two molecules of our product. 659 00:35:08,260 --> 00:35:10,420 So we can write the rate of formation 660 00:35:10,420 --> 00:35:12,670 of product based on the slow step, 661 00:35:12,670 --> 00:35:14,420 or if we didn't know the slow set, 662 00:35:14,420 --> 00:35:17,370 on the second step or the last step. 663 00:35:17,370 --> 00:35:20,360 So two molecules, again, two molecules are being formed. 664 00:35:20,360 --> 00:35:23,960 k2 times the concentration of the intermediate times 665 00:35:23,960 --> 00:35:27,060 the concentration of O2. 666 00:35:27,060 --> 00:35:30,439 But we have an intermediate. 667 00:35:30,439 --> 00:35:32,480 So we need to solve for the intermediate in terms 668 00:35:32,480 --> 00:35:36,610 of rate constants, reactants, and products. 669 00:35:36,610 --> 00:35:39,690 But now we're told we have a fast step followed 670 00:35:39,690 --> 00:35:40,800 by a slow step. 671 00:35:40,800 --> 00:35:43,390 So we know how to do this. 672 00:35:43,390 --> 00:35:46,100 And remember, when you have a fast, reversible step followed 673 00:35:46,100 --> 00:35:51,680 by a slow step, this first step approximates an equilibrium 674 00:35:51,680 --> 00:35:52,540 reaction. 675 00:35:52,540 --> 00:35:54,720 There is not much of the intermediate that's being 676 00:35:54,720 --> 00:35:56,187 siphoned off in the slow step. 677 00:35:56,187 --> 00:35:58,145 So pretty much, when you form the intermediate, 678 00:35:58,145 --> 00:36:00,330 it's going back and forth just like you have 679 00:36:00,330 --> 00:36:03,050 in an equilibrium situation. 680 00:36:03,050 --> 00:36:06,690 And just to emphasize this, I'll share a little picture 681 00:36:06,690 --> 00:36:08,890 here of the beach in the summer when you 682 00:36:08,890 --> 00:36:10,680 don't need your winter coat. 683 00:36:10,680 --> 00:36:14,440 And here we have my daughter and her best friend. 684 00:36:14,440 --> 00:36:16,400 And they are trying to empty the ocean 685 00:36:16,400 --> 00:36:18,950 with this princess bucket. 686 00:36:18,950 --> 00:36:22,590 And one can ask the question, is this going to affect the tides? 687 00:36:22,590 --> 00:36:24,480 Is the ocean going to be different? 688 00:36:24,480 --> 00:36:26,790 But the rate at which my daughter and Aiden 689 00:36:26,790 --> 00:36:30,460 fill up their buckets and bring them over here, 690 00:36:30,460 --> 00:36:33,370 that's a really slow step compared to everything 691 00:36:33,370 --> 00:36:35,230 that's going on in the ocean. 692 00:36:35,230 --> 00:36:38,400 So when you have a very slow step like a kid 693 00:36:38,400 --> 00:36:41,050 and a long distance with a princess pail, 694 00:36:41,050 --> 00:36:43,070 you don't really need to worry about that. 695 00:36:43,070 --> 00:36:46,230 And you can think about the fast step as being an equilibrium. 696 00:36:46,230 --> 00:36:49,480 This is not being siphoned off enough to worry about it. 697 00:36:49,480 --> 00:36:54,350 And so that makes it easier to solve for your intermediate 698 00:36:54,350 --> 00:36:57,470 because then you can just do it by an equilibrium expression. 699 00:36:57,470 --> 00:37:01,160 So in the clicker question, tell me how I can do that. 700 00:37:13,960 --> 00:37:15,210 AUDIENCE: I think you're good. 701 00:37:15,210 --> 00:37:15,850 CATHERINE DRENNAN: OK. 702 00:37:15,850 --> 00:37:16,742 10 more seconds. 703 00:37:32,860 --> 00:37:36,770 So let's just take a look at that. 704 00:37:36,770 --> 00:37:41,070 So again, we're writing our equilibrium expression 705 00:37:41,070 --> 00:37:45,460 for the first step, products over reactants. 706 00:37:45,460 --> 00:37:50,300 And then we can rearrange it to solve because the product here 707 00:37:50,300 --> 00:37:51,650 is our intermediate. 708 00:37:51,650 --> 00:37:54,800 So we can solve for the intermediate in terms 709 00:37:54,800 --> 00:37:58,490 of K1 and our reactant. 710 00:37:58,490 --> 00:38:01,500 And then we can substitute it in. 711 00:38:01,500 --> 00:38:03,170 And that gives us this. 712 00:38:03,170 --> 00:38:06,020 So we had the 2k2 here. 713 00:38:06,020 --> 00:38:11,210 Now we have our K1 and our N O squared. 714 00:38:11,210 --> 00:38:14,160 And here we have our oxygen there. 715 00:38:14,160 --> 00:38:17,220 So now we have a rate law that doesn't 716 00:38:17,220 --> 00:38:18,450 include any intermediates. 717 00:38:21,274 --> 00:38:23,190 I'm just going to put that back up here if you 718 00:38:23,190 --> 00:38:25,940 didn't get it written down. 719 00:38:25,940 --> 00:38:29,690 Now let's think of what happens with temperature. 720 00:38:29,690 --> 00:38:33,300 So this is an elementary rate constant, little k2. 721 00:38:33,300 --> 00:38:36,330 So you increase the rate when you increase temperature. 722 00:38:39,180 --> 00:38:41,150 And here is our expression again that 723 00:38:41,150 --> 00:38:44,490 tells us about that change. 724 00:38:44,490 --> 00:38:48,920 So now for the equilibrium constant, 725 00:38:48,920 --> 00:38:51,500 the effect of temperature depends on 726 00:38:51,500 --> 00:38:55,350 whether the reaction is exothermic or endothermic. 727 00:38:55,350 --> 00:39:00,230 And does anyone remember what the name of the equation 728 00:39:00,230 --> 00:39:03,330 is that tells us about temperature effect 729 00:39:03,330 --> 00:39:06,360 with equilibrium constants? 730 00:39:06,360 --> 00:39:07,760 Yes. 731 00:39:07,760 --> 00:39:09,480 The van 't Hoff equation. 732 00:39:09,480 --> 00:39:11,820 I told you that some time in the future I'd 733 00:39:11,820 --> 00:39:13,640 ask you for the name of this equation. 734 00:39:13,640 --> 00:39:15,446 And that time is now. 735 00:39:15,446 --> 00:39:16,820 You don't really need to know it. 736 00:39:16,820 --> 00:39:17,570 But it's just fun. 737 00:39:17,570 --> 00:39:21,590 There's not many names for equations in general chemistry. 738 00:39:21,590 --> 00:39:23,890 Look how similar these equations are. 739 00:39:23,890 --> 00:39:25,920 These are very similar equations. 740 00:39:25,920 --> 00:39:29,180 So we have the natural log of rate constants here. 741 00:39:29,180 --> 00:39:32,230 Here we have the natural log of equilibrium constants. 742 00:39:32,230 --> 00:39:35,060 Here we have our activation energy. 743 00:39:35,060 --> 00:39:38,670 Here we have delta H. 744 00:39:38,670 --> 00:39:45,910 So if the reaction is exothermic, 745 00:39:45,910 --> 00:39:48,800 if we increase the temperature, what 746 00:39:48,800 --> 00:39:50,620 happens to our equilibrium constant? 747 00:39:50,620 --> 00:39:51,842 Does it increase or decrease? 748 00:39:51,842 --> 00:39:54,940 You can just yell it out. 749 00:39:54,940 --> 00:39:57,740 It decreases. 750 00:39:57,740 --> 00:40:02,040 Now let's think about what happens here. 751 00:40:02,040 --> 00:40:07,680 So our kobs now has an elementary rate constant term, 752 00:40:07,680 --> 00:40:10,040 which is going to increase with temperature. 753 00:40:10,040 --> 00:40:12,840 And it has an equilibrium constant term, 754 00:40:12,840 --> 00:40:15,120 which is going to decrease with temperature 755 00:40:15,120 --> 00:40:19,050 for an exothermic reaction. 756 00:40:19,050 --> 00:40:22,750 So we have these going in opposite directions. 757 00:40:22,750 --> 00:40:27,500 So overall, we want to think about the magnitude 758 00:40:27,500 --> 00:40:32,220 of our activation energy term for our rate constants. 759 00:40:32,220 --> 00:40:35,930 And we want to think about the magnitude of our delta H term 760 00:40:35,930 --> 00:40:39,880 in terms of the equilibrium constants. 761 00:40:39,880 --> 00:40:45,140 So for the particular reaction in question, you can look up. 762 00:40:45,140 --> 00:40:48,540 The activation energy is a small number. 763 00:40:48,540 --> 00:40:51,570 And delta H, again, it's exothermic. 764 00:40:51,570 --> 00:40:54,070 And it's a big number. 765 00:40:54,070 --> 00:40:55,690 So then if you're thinking about that, 766 00:40:55,690 --> 00:40:59,640 if Ea is small and positive-- Ea is always positive. 767 00:40:59,640 --> 00:41:03,980 There is always a barrier, always positive. 768 00:41:03,980 --> 00:41:08,440 Then the rate constant is only going to increase a little bit. 769 00:41:08,440 --> 00:41:10,590 But delta H is a big negative number, 770 00:41:10,590 --> 00:41:14,090 so the equilibrium constant is going to decrease a lot. 771 00:41:14,090 --> 00:41:17,600 So for this particular reaction, increasing the temperature 772 00:41:17,600 --> 00:41:20,370 actually decreases the kobs. 773 00:41:20,370 --> 00:41:23,670 And so for any reaction, it depends 774 00:41:23,670 --> 00:41:27,430 on the magnitude of Ea and the magnitude 775 00:41:27,430 --> 00:41:33,070 and the sign of delta H. So again, large Ea 776 00:41:33,070 --> 00:41:35,600 means very sensitive to temperature. 777 00:41:35,600 --> 00:41:39,700 Large delta H means that the equilibrium constant is 778 00:41:39,700 --> 00:41:42,930 very sensitive to temperature. 779 00:41:42,930 --> 00:41:45,860 For an elementary rate constant, it's 780 00:41:45,860 --> 00:41:49,430 always going to increase with temperature. 781 00:41:49,430 --> 00:41:51,460 Because Ea is always positive. 782 00:41:51,460 --> 00:41:54,110 There is always a barrier to overcome. 783 00:41:54,110 --> 00:41:58,170 Temperature always increases a rate constant. 784 00:41:58,170 --> 00:42:00,400 But for an equilibrium constant, it 785 00:42:00,400 --> 00:42:05,060 can increase or decrease because delta H isn't always positive. 786 00:42:05,060 --> 00:42:06,720 Like the activation energy barrier, 787 00:42:06,720 --> 00:42:09,990 it can be positive or negative. 788 00:42:09,990 --> 00:42:14,570 So the magnitude of delta H, how big a number it is, 789 00:42:14,570 --> 00:42:16,630 tells you about the magnitude of the change, 790 00:42:16,630 --> 00:42:20,080 how much the equilibrium constant will change. 791 00:42:20,080 --> 00:42:22,630 Will k1 and k2 be almost like each other 792 00:42:22,630 --> 00:42:25,440 or really, really different from each other, those equilibrium 793 00:42:25,440 --> 00:42:26,440 constants? 794 00:42:26,440 --> 00:42:29,550 And the sign of delta H, whether it's positive or negative, 795 00:42:29,550 --> 00:42:31,400 tells you the direction of the change. 796 00:42:31,400 --> 00:42:34,920 Will it increase or decrease? 797 00:42:34,920 --> 00:42:38,930 So I just want to show where we're 798 00:42:38,930 --> 00:42:43,860 going with this because it's just super exciting. 799 00:42:43,860 --> 00:42:45,240 Yes. 800 00:42:45,240 --> 00:42:47,560 We're back to Le Chatelier. 801 00:42:47,560 --> 00:42:49,260 This is what I was so excited about. 802 00:42:49,260 --> 00:42:52,510 Remember, Le Chatelier told us, when we apply a stress 803 00:42:52,510 --> 00:42:56,450 to the system, the system responds in such a way 804 00:42:56,450 --> 00:42:59,970 to minimize that stress. 805 00:42:59,970 --> 00:43:04,840 So if we increase the temperature, 806 00:43:04,840 --> 00:43:07,810 according to Le Chatelier, what direction 807 00:43:07,810 --> 00:43:10,566 will the reaction shift, in the exothermic 808 00:43:10,566 --> 00:43:13,940 or the endothermic direction? 809 00:43:13,940 --> 00:43:14,680 What is it? 810 00:43:18,411 --> 00:43:19,357 The endothermic. 811 00:43:19,357 --> 00:43:20,920 So you increase the temperature. 812 00:43:20,920 --> 00:43:21,950 You add heat. 813 00:43:21,950 --> 00:43:26,420 It shifts in a direction to minimize that 814 00:43:26,420 --> 00:43:29,270 or to use up that heat. 815 00:43:29,270 --> 00:43:31,680 So we'll end with one last clicker question. 816 00:43:31,680 --> 00:43:34,710 And we'll finish this concept on Monday. 817 00:43:34,710 --> 00:43:39,120 But we want to think about how Le Chatelier and what we've 818 00:43:39,120 --> 00:43:41,760 known already applies here. 819 00:43:41,760 --> 00:43:46,970 So why don't you tell me which of these diagrams is exothermic 820 00:43:46,970 --> 00:43:48,582 and which is endothermic? 821 00:43:48,582 --> 00:43:50,040 Because we're going to tie this all 822 00:43:50,040 --> 00:43:53,077 back to temperature and Le Chatelier. 823 00:43:53,077 --> 00:43:54,660 AUDIENCE: We don't have that question. 824 00:43:54,660 --> 00:43:56,618 CATHERINE DRENNAN: We don't have that question. 825 00:43:56,618 --> 00:43:57,210 OK. 826 00:43:57,210 --> 00:43:58,652 Then we will not do that question. 827 00:43:58,652 --> 00:44:00,110 AUDIENCE: [INAUDIBLE] the question. 828 00:44:00,110 --> 00:44:01,026 CATHERINE DRENNAN: OK. 829 00:44:01,026 --> 00:44:04,290 So I'll just give you the answer to that. 830 00:44:04,290 --> 00:44:06,450 And then we'll see who the winners are. 831 00:44:06,450 --> 00:44:09,720 So that one is endothermic. 832 00:44:09,720 --> 00:44:13,430 And this one is exothermic. 833 00:44:13,430 --> 00:44:18,370 And we'll finish the rest of this on Monday. 834 00:44:18,370 --> 00:44:21,336 But for now, can you tell us who the winner is? 835 00:44:28,090 --> 00:44:31,590 No, it's not coming up. 836 00:44:31,590 --> 00:44:33,573 AUDIENCE: It's Dan. 837 00:44:33,573 --> 00:44:34,698 CATHERINE DRENNAN: Top two. 838 00:44:34,698 --> 00:44:36,610 AUDIENCE: Dan and Jay. 839 00:44:36,610 --> 00:44:39,750 CATHERINE DRENNAN: Dan and Jay have beat out the other folks. 840 00:44:39,750 --> 00:44:42,130 So we know who's in the playoffs. 841 00:44:42,130 --> 00:44:43,230 All right. 842 00:44:43,230 --> 00:44:44,370 Friday exam. 843 00:44:44,370 --> 00:44:47,130 See you Monday in class to finish the handout. 844 00:44:54,570 --> 00:44:55,070 All right. 845 00:44:55,070 --> 00:44:58,230 Let's take 10 more seconds on the clicker question. 846 00:45:17,204 --> 00:45:20,060 So let's take a look at this. 847 00:45:20,060 --> 00:45:24,100 56% do you have the right answer. 848 00:45:24,100 --> 00:45:26,610 So rate constants always increase 849 00:45:26,610 --> 00:45:27,835 with increase of temperature. 850 00:45:27,835 --> 00:45:30,450 That's our little k's. 851 00:45:30,450 --> 00:45:33,250 Because there is always some activation energy 852 00:45:33,250 --> 00:45:34,980 barrier to overcome. 853 00:45:34,980 --> 00:45:37,380 There is always a positive barrier. 854 00:45:37,380 --> 00:45:41,100 Whereas, for equilibrium constants, K, 855 00:45:41,100 --> 00:45:44,450 you can have a reaction that's exothermic or endothermic. 856 00:45:44,450 --> 00:45:45,900 And so that will change. 857 00:45:45,900 --> 00:45:49,730 It either increases or decreases because delta H 858 00:45:49,730 --> 00:45:51,260 can be positive or negative. 859 00:45:51,260 --> 00:45:53,930 But activation energy barriers are always positive. 860 00:45:53,930 --> 00:45:56,260 There's always a barrier. 861 00:45:56,260 --> 00:45:59,280 Always a barrier to doing anything that's important. 862 00:45:59,280 --> 00:46:01,620 I've got to calm down. 863 00:46:01,620 --> 00:46:08,900 Le Chatelier tells me that when a stress is applied 864 00:46:08,900 --> 00:46:12,760 to the system, I should respond in such a way 865 00:46:12,760 --> 00:46:15,120 to minimize the stress. 866 00:46:15,120 --> 00:46:20,120 It's a very calming rule in chemistry. 867 00:46:20,120 --> 00:46:22,680 Reactions don't like stress. 868 00:46:22,680 --> 00:46:26,140 And they'll respond in a way to minimize it. 869 00:46:26,140 --> 00:46:31,320 So we've been talking about Le Chatelier 870 00:46:31,320 --> 00:46:34,910 for a lot of the semester. 871 00:46:34,910 --> 00:46:37,580 And we've talked about the effect of temperature 872 00:46:37,580 --> 00:46:40,610 on reactions previously. 873 00:46:40,610 --> 00:46:45,140 So if you add heat to a reaction, 874 00:46:45,140 --> 00:46:47,990 it will want to respond in such a way to minimize the heat 875 00:46:47,990 --> 00:46:49,780 or absorb the heat. 876 00:46:49,780 --> 00:46:53,222 So it shifts in the endothermic direction. 877 00:46:53,222 --> 00:46:54,680 So we've already talked about this. 878 00:46:54,680 --> 00:46:56,780 And nothing is new here today. 879 00:46:56,780 --> 00:46:59,640 But today, I'm going to give you a different way 880 00:46:59,640 --> 00:47:03,120 to rationalize why that happens. 881 00:47:03,120 --> 00:47:04,971 So we're not changing what happens. 882 00:47:04,971 --> 00:47:06,720 We're just going to come up with a new way 883 00:47:06,720 --> 00:47:09,340 to rationalize the why. 884 00:47:09,340 --> 00:47:13,930 So we ended last time with these two diagrams. 885 00:47:13,930 --> 00:47:17,500 And we identified one as endothermic and one 886 00:47:17,500 --> 00:47:19,190 as exothermic. 887 00:47:19,190 --> 00:47:23,020 So these, again, are our reaction coordinate diagrams. 888 00:47:23,020 --> 00:47:24,580 We have PE on one side. 889 00:47:24,580 --> 00:47:27,050 What does PE stand for again? 890 00:47:27,050 --> 00:47:29,030 Potential energy. 891 00:47:29,030 --> 00:47:31,530 It stands for other things too, but in a reaction coordinate 892 00:47:31,530 --> 00:47:34,470 diagram, that's what it is. 893 00:47:34,470 --> 00:47:36,630 And then the reaction coordinate going this way. 894 00:47:36,630 --> 00:47:39,840 So we go from reactants to products along the reaction 895 00:47:39,840 --> 00:47:41,490 coordinate. 896 00:47:41,490 --> 00:47:44,430 So here in this endothermic reaction, 897 00:47:44,430 --> 00:47:47,210 we have our reactants down here, our products up here, 898 00:47:47,210 --> 00:47:50,440 and a very large activation energy barrier 899 00:47:50,440 --> 00:47:54,260 for the forward direction, a much smaller activation energy 900 00:47:54,260 --> 00:47:56,540 barrier for the reverse direction. 901 00:47:56,540 --> 00:47:59,580 With the exothermic reaction here, we 902 00:47:59,580 --> 00:48:01,910 have a smaller activation energy barrier 903 00:48:01,910 --> 00:48:04,230 for the forward direction and a bigger one 904 00:48:04,230 --> 00:48:07,130 for the reverse direction. 905 00:48:07,130 --> 00:48:09,930 So let's look at the one equation that 906 00:48:09,930 --> 00:48:13,170 will be on the exam that will not be on your equation sheet. 907 00:48:13,170 --> 00:48:16,000 I think this is-- for this unit, this 908 00:48:16,000 --> 00:48:20,210 is the one that will be not on your equation sheet. 909 00:48:20,210 --> 00:48:24,000 This change in energy is equal to the activation 910 00:48:24,000 --> 00:48:25,740 energy for the forward direction the 911 00:48:25,740 --> 00:48:28,960 of reaction minus the activation energy 912 00:48:28,960 --> 00:48:31,150 for the reverse reaction. 913 00:48:31,150 --> 00:48:34,080 And again, this is our delta E here. 914 00:48:34,080 --> 00:48:35,940 And this is our delta E here. 915 00:48:35,940 --> 00:48:40,210 Delta E is like delta H. If you're talking about a gas, 916 00:48:40,210 --> 00:48:41,920 it's 1% to 2% different. 917 00:48:41,920 --> 00:48:44,900 If you're talking about a solid or a liquid, 918 00:48:44,900 --> 00:48:47,760 it's negligibly different. 919 00:48:47,760 --> 00:48:52,020 So if we look at this endothermic reaction here, 920 00:48:52,020 --> 00:48:54,790 we have a big forward activation energy 921 00:48:54,790 --> 00:49:00,270 barrier minus a small reverse activation energy barrier. 922 00:49:00,270 --> 00:49:03,600 So that's going to give us a positive value for delta E. 923 00:49:03,600 --> 00:49:06,720 And it'll be an endothermic reaction. 924 00:49:06,720 --> 00:49:08,880 And if we look at this equation again and fill it 925 00:49:08,880 --> 00:49:12,220 in for the exothermic reaction, we 926 00:49:12,220 --> 00:49:14,790 have a small barrier in the forward direction, 927 00:49:14,790 --> 00:49:17,290 a big barrier in the reverse direction, 928 00:49:17,290 --> 00:49:20,190 and a negative value for delta E and delta H. 929 00:49:20,190 --> 00:49:23,385 It's an exothermic reaction. 930 00:49:23,385 --> 00:49:25,010 Now let's think about what happens when 931 00:49:25,010 --> 00:49:28,260 we increase the temperature. 932 00:49:28,260 --> 00:49:31,210 So if we increase temperature, it's 933 00:49:31,210 --> 00:49:36,420 a lot easier to overcome this forward activation energy 934 00:49:36,420 --> 00:49:38,470 barrier, the big one. 935 00:49:38,470 --> 00:49:40,990 And that's going to shift it to products. 936 00:49:40,990 --> 00:49:43,914 And this is because the small activation energy barrier, 937 00:49:43,914 --> 00:49:45,580 it's not that hard to get over something 938 00:49:45,580 --> 00:49:47,690 that's a small activation energy barrier. 939 00:49:47,690 --> 00:49:48,670 It's not hard. 940 00:49:48,670 --> 00:49:50,820 But if it's a really big barrier, 941 00:49:50,820 --> 00:49:52,740 it's hard to get over that barrier. 942 00:49:52,740 --> 00:49:54,580 You need a lot of kinetic energy. 943 00:49:54,580 --> 00:49:57,700 Increasing the temperature will give that kinetic energy 944 00:49:57,700 --> 00:49:59,180 to overcome that barrier. 945 00:49:59,180 --> 00:50:01,590 Remember that when molecules come together, 946 00:50:01,590 --> 00:50:03,460 bonds are distorted and formed. 947 00:50:03,460 --> 00:50:05,460 The potential energy goes up first. 948 00:50:05,460 --> 00:50:08,120 And only those molecules with that critical energy 949 00:50:08,120 --> 00:50:10,710 that can overcome that activation energy barrier 950 00:50:10,710 --> 00:50:12,100 can go on to products. 951 00:50:12,100 --> 00:50:14,410 Increase the temperature, that allows you 952 00:50:14,410 --> 00:50:18,210 to overcome the big barrier. 953 00:50:18,210 --> 00:50:21,407 Now if we look in this side, this is a small barrier. 954 00:50:21,407 --> 00:50:22,990 They weren't really having-- molecules 955 00:50:22,990 --> 00:50:25,031 were probably not having a huge amount of trouble 956 00:50:25,031 --> 00:50:25,890 with that barrier. 957 00:50:25,890 --> 00:50:29,070 The barrier that was hard was for the reverse direction. 958 00:50:29,070 --> 00:50:30,590 So now you increase the temperature 959 00:50:30,590 --> 00:50:32,320 of an exothermic reaction. 960 00:50:32,320 --> 00:50:36,130 It's easier to overcome this big barrier, the reverse barrier. 961 00:50:36,130 --> 00:50:38,800 And you have a shift toward reactants. 962 00:50:38,800 --> 00:50:40,500 So this is what we had seen before. 963 00:50:40,500 --> 00:50:43,380 You increase the temperature of an endothermic reaction. 964 00:50:43,380 --> 00:50:45,290 You go to products. 965 00:50:45,290 --> 00:50:48,780 You increase the temperature of an exothermic reaction. 966 00:50:48,780 --> 00:50:50,330 It shifts to reactants. 967 00:50:50,330 --> 00:50:52,540 So these are the same things we saw before. 968 00:50:52,540 --> 00:50:55,380 But now there's a new rationalization behind it. 969 00:50:55,380 --> 00:50:58,620 Now we can think about this in terms of activation energy 970 00:50:58,620 --> 00:51:00,410 barriers. 971 00:51:00,410 --> 00:51:03,290 So the important points to remember, 972 00:51:03,290 --> 00:51:07,190 big activation energy barrier, rate constant 973 00:51:07,190 --> 00:51:10,170 very sensitive to temperature. 974 00:51:10,170 --> 00:51:14,710 If you have a big barrier, increasing the temperature 975 00:51:14,710 --> 00:51:17,120 makes a big difference. 976 00:51:17,120 --> 00:51:19,820 If you have a small barrier, doesn't really 977 00:51:19,820 --> 00:51:21,040 matter that much. 978 00:51:21,040 --> 00:51:24,280 Most of your molecules can get over that small barrier. 979 00:51:24,280 --> 00:51:27,190 So big barrier, increasing the temperature, 980 00:51:27,190 --> 00:51:29,000 more molecules can go. 981 00:51:29,000 --> 00:51:31,230 And so you're going to shift in the direction 982 00:51:31,230 --> 00:51:33,460 of the big barrier.