WEBVTT 00:00:00.030 --> 00:00:02.400 The following content is provided under a Creative 00:00:02.400 --> 00:00:03.780 Commons license. 00:00:03.780 --> 00:00:06.020 Your support will help MIT OpenCourseWare 00:00:06.020 --> 00:00:10.080 continue to offer high quality educational resources for free. 00:00:10.080 --> 00:00:12.670 To make a donation or to view additional materials 00:00:12.670 --> 00:00:16.405 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.405 --> 00:00:17.030 at ocw.mit.edu. 00:00:46.972 --> 00:00:48.930 CATHERINE DRENNAN: Yeah, let's show the answer. 00:00:51.630 --> 00:00:53.800 All right. 00:00:53.800 --> 00:01:01.241 For a special treat, special benefit-- 00:01:01.241 --> 00:01:03.490 I'm not sure what to call it-- something special later 00:01:03.490 --> 00:01:05.900 in the class, does someone want to tell me 00:01:05.900 --> 00:01:07.164 why that's the right answer? 00:01:14.080 --> 00:01:15.820 AUDIENCE: Well as we did last lecture, 00:01:15.820 --> 00:01:23.580 we crossed off the k2 because that's the slow step. 00:01:23.580 --> 00:01:26.201 So it doesn't matter as much. 00:01:26.201 --> 00:01:27.200 CATHERINE DRENNAN: Yeah. 00:01:27.200 --> 00:01:30.770 And so when you get rid of that, when the k minus 1 00:01:30.770 --> 00:01:34.730 is really fast compared to k2, k2 00:01:34.730 --> 00:01:36.466 is very small compared to that. 00:01:36.466 --> 00:01:37.340 So you get rid of it. 00:01:37.340 --> 00:01:39.700 And that simplifies the expression. 00:01:39.700 --> 00:01:47.950 So temperature-- today's lecture is largely about temperature. 00:01:47.950 --> 00:01:49.800 But we're going to tie in all sorts 00:01:49.800 --> 00:01:51.870 of other things we've learned over the course 00:01:51.870 --> 00:01:52.840 of the semester. 00:01:52.840 --> 00:01:53.760 So I'm very excited. 00:01:53.760 --> 00:01:59.170 Some of my very favorite topics are coming back today. 00:01:59.170 --> 00:02:02.650 So effective temperature, we talked about this a little bit 00:02:02.650 --> 00:02:05.170 when we were talking about making bread, 00:02:05.170 --> 00:02:08.402 adding your baking soda and about rates 00:02:08.402 --> 00:02:09.860 and why you put things in the oven. 00:02:09.860 --> 00:02:13.840 And we talked about it has to do with also the spontaneity, 00:02:13.840 --> 00:02:15.260 whether you change the temperature 00:02:15.260 --> 00:02:17.610 and what your delta H and delta S are. 00:02:17.610 --> 00:02:19.450 But we also talked about rates and that 00:02:19.450 --> 00:02:21.850 increasing the temperature often increases the rate. 00:02:21.850 --> 00:02:23.700 So many of us have observed this. 00:02:23.700 --> 00:02:26.246 You increase the temperature, you increase the rate. 00:02:26.246 --> 00:02:27.870 But today we're going to talk about how 00:02:27.870 --> 00:02:33.570 you can quantitatively say how much the rate might 00:02:33.570 --> 00:02:35.930 be changed-- the rate constant might be changed if you 00:02:35.930 --> 00:02:37.810 increase the temperature. 00:02:37.810 --> 00:02:42.260 So in 1889, it was a wonderful day for Arrhenius. 00:02:42.260 --> 00:02:46.130 He had been trying to plot different values 00:02:46.130 --> 00:02:49.110 of rate constants versus temperature 00:02:49.110 --> 00:02:50.710 to see what would happen. 00:02:50.710 --> 00:02:54.060 And then he tried natural log of the rate constant k 00:02:54.060 --> 00:02:55.670 versus inverse temperature. 00:02:55.670 --> 00:02:57.150 And he got a straight line. 00:02:57.150 --> 00:02:59.850 And scientists always get very excited when your data 00:02:59.850 --> 00:03:01.290 falls on a straight line. 00:03:01.290 --> 00:03:03.830 It means you figured out some relationship 00:03:03.830 --> 00:03:06.230 between two values. 00:03:06.230 --> 00:03:09.020 So here is the Arrhenius plot. 00:03:09.020 --> 00:03:10.590 And we'll introduce some terms. 00:03:10.590 --> 00:03:14.560 So again, we're plotting the natural log of k versus 1 00:03:14.560 --> 00:03:18.880 over temperature, 1 over inverse kelvin over here. 00:03:18.880 --> 00:03:21.460 And here is our plot of this straight line 00:03:21.460 --> 00:03:22.830 that Arrhenius found. 00:03:22.830 --> 00:03:28.510 Natural log of k on the y-axis versus 1/T 00:03:28.510 --> 00:03:33.470 gives you a slope then of minus the activation energy-- 00:03:33.470 --> 00:03:37.615 so E to the little a is activation energy-- over R, 00:03:37.615 --> 00:03:40.260 our good friend the gas constant. 00:03:40.260 --> 00:03:43.320 And then the y-intercept over here 00:03:43.320 --> 00:03:45.420 is the natural log of something called 00:03:45.420 --> 00:03:49.160 A. We have a lot of different A's in this particular unit 00:03:49.160 --> 00:03:50.090 of kinetics. 00:03:50.090 --> 00:03:52.260 This one is factor A, sometimes called 00:03:52.260 --> 00:03:55.822 the Arrhenius factor, A for Arrhenius. 00:03:55.822 --> 00:03:57.280 It has a couple of different names. 00:03:57.280 --> 00:04:01.000 But importantly, it has the same units as k, our rate constants. 00:04:01.000 --> 00:04:05.510 So what this plot told Arrhenius back in 1889 00:04:05.510 --> 00:04:08.080 was that rate constants vary exponentially 00:04:08.080 --> 00:04:10.380 with inverse temperature. 00:04:10.380 --> 00:04:12.210 So this was the first kind of connection 00:04:12.210 --> 00:04:14.080 between rate constants and temperature 00:04:14.080 --> 00:04:15.930 that could be used to kind of come up 00:04:15.930 --> 00:04:18.640 with quantitative numbers. 00:04:18.640 --> 00:04:23.690 So factor A, this Arrhenius factor, and the activation 00:04:23.690 --> 00:04:27.685 energy, E to the sub a, depend on the reaction being studied. 00:04:27.685 --> 00:04:30.670 So they have to be measured for the particular reaction. 00:04:30.670 --> 00:04:32.110 So it's a clicker competition. 00:04:32.110 --> 00:04:34.840 So we'll have a bunch of clicker questions today. 00:04:34.840 --> 00:04:39.217 And why don't you tell me if you think factor A is temperature 00:04:39.217 --> 00:04:39.716 dependent? 00:04:49.840 --> 00:04:50.340 All right. 00:04:50.340 --> 00:04:51.006 10 more seconds. 00:05:06.070 --> 00:05:07.090 Yep. 00:05:07.090 --> 00:05:10.250 The answer is no. 00:05:10.250 --> 00:05:14.860 And so what is this factor A? 00:05:14.860 --> 00:05:20.160 If we think about this in terms of the plot, what it is 00:05:20.160 --> 00:05:26.910 is the rate constant when 1/T is equal to 0. 00:05:26.910 --> 00:05:28.640 Because it is the y-intercept. 00:05:28.640 --> 00:05:34.982 And when 1/T is equal to 0, what has to be true about T? 00:05:34.982 --> 00:05:36.320 Yeah. 00:05:36.320 --> 00:05:37.215 Infinitely large. 00:05:37.215 --> 00:05:42.500 So factor A is the rate constant at 00:05:42.500 --> 00:05:46.330 an infinitely large temperature, at a huge temperature. 00:05:46.330 --> 00:05:49.520 So it's the fastest that particular reaction could ever 00:05:49.520 --> 00:05:52.800 go at this infinitely huge temperature. 00:05:52.800 --> 00:05:54.275 So that's what factor A is. 00:05:54.275 --> 00:05:56.940 And of course, we can't plug things in 00:05:56.940 --> 00:06:00.530 and say, how fast is this at an infinitely huge temperature? 00:06:00.530 --> 00:06:03.820 So conveniently, you can get that value out 00:06:03.820 --> 00:06:05.310 of plotting your data. 00:06:05.310 --> 00:06:08.930 You measure a bunch of rate constants versus temperature. 00:06:08.930 --> 00:06:10.320 And you plot it this way, and you 00:06:10.320 --> 00:06:14.210 can calculate this sort of maximum rate constant possible 00:06:14.210 --> 00:06:16.790 for this reaction if you had this infinitely 00:06:16.790 --> 00:06:19.460 huge temperature. 00:06:19.460 --> 00:06:22.245 What about activation energy? 00:06:22.245 --> 00:06:23.995 Do you think that's temperature dependent? 00:06:38.790 --> 00:06:39.300 All right. 00:06:39.300 --> 00:06:40.195 10 more seconds. 00:06:55.200 --> 00:06:58.120 So no, it isn't. 00:06:58.120 --> 00:07:00.670 And so if we sort of just think about it back here, 00:07:00.670 --> 00:07:02.600 the answer is no. 00:07:02.600 --> 00:07:04.780 Again, you plot the rate constants 00:07:04.780 --> 00:07:08.720 over all these temperatures to get one value out of the slope. 00:07:08.720 --> 00:07:12.830 So it is largely independent of any kind of temperature. 00:07:12.830 --> 00:07:15.946 You get one activation energy for the reaction in question. 00:07:15.946 --> 00:07:17.820 But it does depend on the reaction, for sure. 00:07:17.820 --> 00:07:21.360 There isn't just one value for this for everything. 00:07:21.360 --> 00:07:23.930 It depends on the materials. 00:07:23.930 --> 00:07:24.560 All right. 00:07:24.560 --> 00:07:27.090 So let's look at some other ways that we can 00:07:27.090 --> 00:07:30.260 express the Arrhenius equation. 00:07:30.260 --> 00:07:33.170 So we have the Arrhenius equation 00:07:33.170 --> 00:07:36.320 written as a straight line. 00:07:36.320 --> 00:07:41.350 And we can also do something very, very simple to it, 00:07:41.350 --> 00:07:43.820 which is switch these two terms. 00:07:43.820 --> 00:07:46.330 And that gives us what is officially known 00:07:46.330 --> 00:07:47.980 as the Arrhenius equation. 00:07:47.980 --> 00:07:51.040 So natural log of k equals natural log 00:07:51.040 --> 00:07:53.750 of A, this Arrhenius or factor A, 00:07:53.750 --> 00:07:57.350 minus the activation energy over the gas 00:07:57.350 --> 00:07:59.280 constant times temperature. 00:07:59.280 --> 00:08:02.670 And of course, if you want to solve for k, 00:08:02.670 --> 00:08:05.560 you take the inverse log of both sides. 00:08:05.560 --> 00:08:09.830 And so then k is going to be equal to this factor A-- again, 00:08:09.830 --> 00:08:14.200 measured for every reaction in question-- e to the minus 00:08:14.200 --> 00:08:18.830 Ea, the activation energy, over RT. 00:08:18.830 --> 00:08:20.410 So here is our equations. 00:08:20.410 --> 00:08:21.980 These will be on equation sheets, 00:08:21.980 --> 00:08:23.580 so you don't have to memorize them. 00:08:23.580 --> 00:08:26.430 But you already, if you've been studying your equation sheet 00:08:26.430 --> 00:08:29.450 for exam four, realize that there's 00:08:29.450 --> 00:08:32.409 a lot of symbols that are very similar in these units. 00:08:32.409 --> 00:08:37.760 So we have a, in nuclear chemistry, activity. 00:08:37.760 --> 00:08:41.474 So just keep in mind what equation is what. 00:08:41.474 --> 00:08:44.059 When you're talking about nuclear decay, 00:08:44.059 --> 00:08:46.300 you don't have your activation energy term. 00:08:46.300 --> 00:08:48.600 So that should give you somewhat of a clue. 00:08:48.600 --> 00:08:50.520 So one of the challenges in the final 00:08:50.520 --> 00:08:54.590 is making sure you know which equation is which. 00:08:54.590 --> 00:08:56.770 So let's look at an example now where 00:08:56.770 --> 00:08:58.400 if we're given some of the information, 00:08:58.400 --> 00:09:00.300 we can solve for other things. 00:09:00.300 --> 00:09:03.430 So an example, this is classes at lunch time. 00:09:03.430 --> 00:09:08.360 So we can think about the hydrolysis of an average lunch 00:09:08.360 --> 00:09:11.280 of sucrose to form a molecule of glucose 00:09:11.280 --> 00:09:14.600 and a molecule of fructose as part of the digestive process. 00:09:14.600 --> 00:09:16.810 So some of you might have eaten already. 00:09:16.810 --> 00:09:19.730 You might be digesting sucrose-- it 00:09:19.730 --> 00:09:23.230 comes in many different forms-- right now. 00:09:23.230 --> 00:09:27.440 So using the information about activation energy, 00:09:27.440 --> 00:09:29.620 we can predict what the rate constant 00:09:29.620 --> 00:09:32.840 is going to be at a different temperature. 00:09:32.840 --> 00:09:37.000 So some kind person measured the activation energy 00:09:37.000 --> 00:09:41.550 for this digestive process at 108 kilojoules per mole 00:09:41.550 --> 00:09:46.730 and also figured out that the k observed, observed 00:09:46.730 --> 00:09:53.165 rate constant, for this reaction is 1.0 times 10 to the minus 3 00:09:53.165 --> 00:09:57.080 per mole or per at normal body temperature. 00:09:57.080 --> 00:09:59.750 And so now we're asked to calculate what 00:09:59.750 --> 00:10:04.050 that rate constant should be at lower temperature, 35 00:10:04.050 --> 00:10:08.180 degrees Celsius, somewhat below body temperature. 00:10:08.180 --> 00:10:09.540 So how are we going to do that? 00:10:09.540 --> 00:10:12.320 Well, let's remind ourselves of our equations, our Arrhenius 00:10:12.320 --> 00:10:13.390 equation. 00:10:13.390 --> 00:10:15.570 So we know what Ea is. 00:10:15.570 --> 00:10:19.150 We were not told what our Arrhenius factor A is. 00:10:19.150 --> 00:10:20.330 We don't know that. 00:10:20.330 --> 00:10:22.370 But we do know one of the rate constants 00:10:22.370 --> 00:10:23.780 at one of the temperatures. 00:10:23.780 --> 00:10:27.662 So if we combine these two equations, 00:10:27.662 --> 00:10:30.410 then we have an appropriate number 00:10:30.410 --> 00:10:33.030 of variables to solve for the rate 00:10:33.030 --> 00:10:35.490 constant at the new temperature. 00:10:35.490 --> 00:10:37.980 So let's combine those two equations. 00:10:37.980 --> 00:10:42.780 So we have natural log of k2, our rate constant, 00:10:42.780 --> 00:10:45.320 at temperature 2 minus the natural log 00:10:45.320 --> 00:10:47.310 of the rate constant at temperature 1, 00:10:47.310 --> 00:10:51.230 which can also be expressed as natural log rate constant 2 00:10:51.230 --> 00:10:53.080 over rate constant 1. 00:10:53.080 --> 00:10:58.010 And our natural log of the Arrhenius factor drops out. 00:10:58.010 --> 00:11:01.310 And so we have minus Ea, our activation energy, 00:11:01.310 --> 00:11:05.910 over our gas constant times 1 over the temperature minus 1 00:11:05.910 --> 00:11:07.760 over the first temperature. 00:11:07.760 --> 00:11:11.900 So we can put in our values that we're given. 00:11:11.900 --> 00:11:15.900 So we have our first rate constant down here, 00:11:15.900 --> 00:11:18.060 the rate constant at the first temperature. 00:11:18.060 --> 00:11:19.910 We put in our temperatures, making 00:11:19.910 --> 00:11:24.070 sure we convert them to kelvin because our gas constant is 00:11:24.070 --> 00:11:25.410 in kelvin. 00:11:25.410 --> 00:11:29.470 And also, we are going to convert our activation energy 00:11:29.470 --> 00:11:32.390 from kilojoules to joules because we 00:11:32.390 --> 00:11:34.230 want to cancel our units. 00:11:34.230 --> 00:11:35.790 So now we can cancel joules. 00:11:35.790 --> 00:11:37.120 We can cancel moles. 00:11:37.120 --> 00:11:39.390 And we can cancel kelvin. 00:11:39.390 --> 00:11:43.360 And so that gives us a new rate constant, 00:11:43.360 --> 00:11:48.740 7.6 times 10 to the minus 4 per mole or per second. 00:11:48.740 --> 00:11:53.550 And this is a lower rate at a lower temperature. 00:11:53.550 --> 00:11:55.655 And in fact, that is often true. 00:11:55.655 --> 00:11:58.540 You have a lower rate at a lower temperature. 00:11:58.540 --> 00:12:02.180 And this is one of the reasons why it's a really good idea 00:12:02.180 --> 00:12:05.880 to keep your body temperature at body temperature. 00:12:05.880 --> 00:12:09.400 And around this time of year, people come in to MIT 00:12:09.400 --> 00:12:10.550 from warmer climates. 00:12:10.550 --> 00:12:13.250 And they do not have appropriate clothing. 00:12:13.250 --> 00:12:17.110 And so your body does not work well in the cold. 00:12:17.110 --> 00:12:18.350 The rates slow down. 00:12:18.350 --> 00:12:19.720 You're not digesting things. 00:12:19.720 --> 00:12:21.669 Your body is really not doing anything 00:12:21.669 --> 00:12:23.210 at the rate it's supposed to be doing 00:12:23.210 --> 00:12:25.260 these reactions because they're nicely 00:12:25.260 --> 00:12:27.800 tuned to the appropriate body temperature. 00:12:27.800 --> 00:12:32.070 So go out and buy a winter coat. 00:12:32.070 --> 00:12:37.400 So this now, this equation that we just derived 00:12:37.400 --> 00:12:40.500 shows us the relationship between rate constants 00:12:40.500 --> 00:12:42.030 and temperature. 00:12:42.030 --> 00:12:47.790 So again, natural log k2 over k1 minus the activation energy 00:12:47.790 --> 00:12:51.710 over the gas constant and our temperature term. 00:12:51.710 --> 00:12:53.310 And if we look at this equation, we'll 00:12:53.310 --> 00:12:58.630 realize that if we have a very large value for this activation 00:12:58.630 --> 00:13:01.850 energy, this Ea term here, that's going 00:13:01.850 --> 00:13:04.000 to mean that the rate constants are 00:13:04.000 --> 00:13:05.850 very sensitive to temperature. 00:13:05.850 --> 00:13:08.320 If this is small, there won't be a big difference 00:13:08.320 --> 00:13:10.090 between k1 and k2. 00:13:10.090 --> 00:13:13.190 But if the activation energy is really big, 00:13:13.190 --> 00:13:16.490 there will be a big difference between k1 and k2. 00:13:16.490 --> 00:13:19.810 The rate constants will be very sensitive to temperature. 00:13:19.810 --> 00:13:20.310 All right. 00:13:20.310 --> 00:13:23.230 So now let's think about rates and temperature. 00:13:23.230 --> 00:13:26.130 And let's think of a cold temperature. 00:13:26.130 --> 00:13:29.900 Let's think, for example, of liquid nitrogen temperatures. 00:13:29.900 --> 00:13:31.840 What do you think happens to an enzyme? 00:13:31.840 --> 00:13:34.070 You have enzymes in your digestive process 00:13:34.070 --> 00:13:36.940 that are hydrolyzing sucrose that you 00:13:36.940 --> 00:13:38.370 might have had for lunch. 00:13:38.370 --> 00:13:39.900 What do you think happens to enzymes 00:13:39.900 --> 00:13:41.858 if you put them at liquid nitrogen temperature? 00:13:45.750 --> 00:13:47.820 They will slow way down. 00:13:47.820 --> 00:13:50.560 One would say they would stop working entirely. 00:13:50.560 --> 00:13:56.180 They would just sort of be, we call it frozen. 00:13:56.180 --> 00:14:00.020 And in fact, I use this all the time in my research. 00:14:00.020 --> 00:14:03.990 So one thing we do, we study the structures of proteins. 00:14:03.990 --> 00:14:07.170 And so we grow these lovely protein crystals. 00:14:07.170 --> 00:14:10.020 And what we can do, we take these-- a lot of times, 00:14:10.020 --> 00:14:12.580 the proteins or enzymes in the crystals are active. 00:14:12.580 --> 00:14:15.140 They're able to convert reactants to products. 00:14:15.140 --> 00:14:17.760 And so we can soak in reactants and then take our crystals 00:14:17.760 --> 00:14:20.250 and dunk them in liquid nitrogen and then 00:14:20.250 --> 00:14:21.740 determine the structure of that. 00:14:21.740 --> 00:14:23.670 And if we do it at different time points, 00:14:23.670 --> 00:14:25.920 you can actually kind of walk through the mechanism 00:14:25.920 --> 00:14:26.890 of the enzyme. 00:14:26.890 --> 00:14:29.480 Watch what happens at various different states. 00:14:29.480 --> 00:14:31.850 So this is a very common thing that's used. 00:14:31.850 --> 00:14:34.460 We like to do things at liquid nitrogen 00:14:34.460 --> 00:14:37.640 to see things sort of-- we sort of pause it 00:14:37.640 --> 00:14:39.140 when it's dumped in liquid nitrogen. 00:14:39.140 --> 00:14:40.240 It stops what it's doing. 00:14:40.240 --> 00:14:42.680 And we can capture its structure. 00:14:42.680 --> 00:14:46.005 What do you think about non-enzymatic reactions? 00:14:46.005 --> 00:14:49.849 Do you think they're also going to be slowed down? 00:14:49.849 --> 00:14:50.390 I don't know. 00:14:50.390 --> 00:14:52.106 Should we do an experiment and find out? 00:14:52.106 --> 00:14:52.860 AUDIENCE: Yeah. 00:14:52.860 --> 00:14:54.830 CATHERINE DRENNAN: Yeah, I think we should. 00:14:54.830 --> 00:14:59.410 So we now have-- let's bring out our demo TAs. 00:14:59.410 --> 00:15:02.440 We now have glow sticks. 00:15:02.440 --> 00:15:07.090 Glow sticks are in fact chemicals in there 00:15:07.090 --> 00:15:11.390 that when you break them, you have a chemical reaction. 00:15:11.390 --> 00:15:13.647 So we're going to do this. 00:15:13.647 --> 00:15:15.730 And we're going to break them and watch them glow. 00:15:15.730 --> 00:15:18.630 Let's break them and put them in here first. 00:15:18.630 --> 00:15:21.790 And then we're going to cool them down and see what happens 00:15:21.790 --> 00:15:23.960 to the chemical reaction. 00:15:23.960 --> 00:15:27.000 Here, let's have a little help over here. 00:15:27.000 --> 00:15:28.770 Please break some of these. 00:15:28.770 --> 00:15:30.103 AUDIENCE: How do you break them? 00:15:31.252 --> 00:15:32.710 CATHERINE DRENNAN: You break-- see, 00:15:32.710 --> 00:15:38.340 you just kind of snap them and then shake them up. 00:15:38.340 --> 00:15:42.540 And I think we'll bring down the lights so you can all see these 00:15:42.540 --> 00:15:43.180 pretty well. 00:15:53.740 --> 00:15:56.590 So now the chemical reaction is going. 00:15:56.590 --> 00:15:58.460 And so we can see the chemical reaction. 00:15:58.460 --> 00:16:00.500 That's what's wonderful about glow sticks. 00:16:00.500 --> 00:16:02.380 You can observe the chemical reaction 00:16:02.380 --> 00:16:03.895 because it causes it to glow. 00:16:09.010 --> 00:16:11.040 Move so everyone can see. 00:16:11.040 --> 00:16:13.380 Now we're going to see what happens to the chemical 00:16:13.380 --> 00:16:15.110 reaction when it slows again. 00:16:15.110 --> 00:16:17.236 When the chemical reaction is slowed down, 00:16:17.236 --> 00:16:19.070 you won't see it anymore. 00:16:19.070 --> 00:16:20.120 So it'll stop glowing. 00:16:27.144 --> 00:16:27.977 That should be good. 00:16:34.621 --> 00:16:36.120 We'll do all of them, and we'll see. 00:16:36.120 --> 00:16:37.630 It'll take a little bit of time. 00:16:37.630 --> 00:16:40.060 But it should not be too long. 00:16:48.500 --> 00:16:51.340 And hopefully, it'll slow down faster than the liquid nitrogen 00:16:51.340 --> 00:16:53.012 destroys the plastic cups. 00:16:55.324 --> 00:16:57.490 Yeah, I think you can see it with the first one now. 00:17:04.619 --> 00:17:06.129 Mary, why don't you come down here? 00:17:06.129 --> 00:17:07.670 We have something else for you to do. 00:17:19.480 --> 00:17:21.207 Let's put some liquid nitrogen in here. 00:17:27.849 --> 00:17:30.000 Let's bring up the lights because our glowing 00:17:30.000 --> 00:17:32.630 has stopped. 00:17:32.630 --> 00:17:35.180 So another thing that liquid nitrogen 00:17:35.180 --> 00:17:37.080 changes the properties. 00:17:37.080 --> 00:17:41.520 So Mary will demonstrate to you what happens to a flower. 00:17:41.520 --> 00:17:44.575 And just like right now, the flower does this. 00:17:44.575 --> 00:17:50.010 But let's put it in liquid nitrogen and see what happens. 00:17:50.010 --> 00:17:51.095 Give it a decent soak. 00:18:02.300 --> 00:18:04.010 And now smash it. 00:18:04.010 --> 00:18:08.690 [LAUGHTER] 00:18:08.690 --> 00:18:10.718 Thank you very much, our lovely assistant. 00:18:16.790 --> 00:18:18.620 I will brush off my computer later. 00:18:18.620 --> 00:18:20.910 We have more flowers, and apparently there's 00:18:20.910 --> 00:18:21.750 a broom somewhere. 00:18:21.750 --> 00:18:26.170 So if we finish class, for some lucky other individuals, 00:18:26.170 --> 00:18:28.930 you can come down and smash the rest of the flowers. 00:18:28.930 --> 00:18:33.200 So liquid nitrogen really changes the property of things. 00:18:33.200 --> 00:18:38.160 Sometimes it's used by dermatologists 00:18:38.160 --> 00:18:40.070 to remove warts or other things. 00:18:40.070 --> 00:18:43.400 They'll dab some liquid nitrogen on you. 00:18:43.400 --> 00:18:46.410 And I always feel like paying a lot of money 00:18:46.410 --> 00:18:49.142 to go have that done-- I have liquid nitrogen around my lab. 00:18:49.142 --> 00:18:50.600 But then I'm like, yeah, I probably 00:18:50.600 --> 00:18:52.590 have enough liquid nitrogen burns on my figures 00:18:52.590 --> 00:18:54.490 already that I don't want. 00:18:54.490 --> 00:18:56.850 Crystallographers who use this a lot, 00:18:56.850 --> 00:18:59.030 they'll often be in a situation where 00:18:59.030 --> 00:19:00.780 you have this liquid nitrogen. And it's 00:19:00.780 --> 00:19:02.280 sort of dripping on you. 00:19:02.280 --> 00:19:05.440 But you have your crystal, and if you let go, 00:19:05.440 --> 00:19:07.930 then you'll destroy your crystal experiment. 00:19:07.930 --> 00:19:10.770 So you'll have liquid nitrogen sitting in your palm. 00:19:10.770 --> 00:19:12.870 And you're like, OK, I'm going to get a burn. 00:19:12.870 --> 00:19:15.400 Or I'm going to lose my crystal. 00:19:15.400 --> 00:19:17.410 And so you decide how important that is. 00:19:17.410 --> 00:19:18.868 And so sometimes a crystallographer 00:19:18.868 --> 00:19:22.010 will walk up to you and go, see that scar? 00:19:22.010 --> 00:19:27.130 That's 1.2 angstrom data, baby, right there. 00:19:27.130 --> 00:19:30.990 So we suffer for our science sometimes. 00:19:30.990 --> 00:19:34.630 So liquid nitrogen is very-- changes the property. 00:19:34.630 --> 00:19:38.400 Cold things are different than warm things. 00:19:38.400 --> 00:19:40.330 Buy a winter coat. 00:19:40.330 --> 00:19:43.830 So everything slows down in the cold, 00:19:43.830 --> 00:19:46.320 in terms of elementary reactions. 00:19:46.320 --> 00:19:49.740 Our rate constants slow down. 00:19:49.740 --> 00:19:51.910 So now let's think about a reaction 00:19:51.910 --> 00:19:54.180 and what's called a reaction coordinate 00:19:54.180 --> 00:19:57.216 and consider what's happening in a reaction. 00:19:57.216 --> 00:19:58.590 And then we're going to come back 00:19:58.590 --> 00:20:00.750 to thinking about the effect of temperature, 00:20:00.750 --> 00:20:02.601 where temperature really makes a difference. 00:20:09.100 --> 00:20:11.790 We'll quiet down a little. 00:20:11.790 --> 00:20:15.870 So considering the reaction coordinate, reaction coordinate 00:20:15.870 --> 00:20:19.190 the reactions of-- you bring your reactants together 00:20:19.190 --> 00:20:20.620 and form your product. 00:20:20.620 --> 00:20:23.450 So two things coming together to form something else, 00:20:23.450 --> 00:20:24.410 that is a reaction. 00:20:24.410 --> 00:20:26.140 And as you go along in the reaction, 00:20:26.140 --> 00:20:27.716 that's your reaction coordinate. 00:20:27.716 --> 00:20:29.090 And we're going to also introduce 00:20:29.090 --> 00:20:34.190 this term of activation complex or transition state. 00:20:34.190 --> 00:20:37.020 So two molecules can collide. 00:20:37.020 --> 00:20:40.130 Two molecules colliding, bimolecular. 00:20:40.130 --> 00:20:43.610 But every time those two molecules come together, 00:20:43.610 --> 00:20:47.220 they're not necessarily going to form a product. 00:20:47.220 --> 00:20:49.140 Why? 00:20:49.140 --> 00:20:54.570 So only when that collision energy is greater 00:20:54.570 --> 00:20:58.570 than some critical energy-- which is sometimes called Emin, 00:20:58.570 --> 00:21:01.900 this sort of minimum energy to get this reaction to go, 00:21:01.900 --> 00:21:05.580 or as I like to call it, the activation energy, Ea, 00:21:05.580 --> 00:21:07.240 that's what we've been talking about, 00:21:07.240 --> 00:21:11.560 our activation energy-- will you get a reaction. 00:21:11.560 --> 00:21:14.390 So you need to have enough energy in your two 00:21:14.390 --> 00:21:16.310 things coming together. 00:21:16.310 --> 00:21:19.530 Those two things coming together need to have a critical energy 00:21:19.530 --> 00:21:20.431 for them to react. 00:21:25.870 --> 00:21:28.990 So why is this true? 00:21:28.990 --> 00:21:30.900 You need to have that critical energy. 00:21:30.900 --> 00:21:31.486 But why? 00:21:31.486 --> 00:21:34.420 Why is this necessary? 00:21:34.420 --> 00:21:39.010 So it's necessary because before this reaction takes place, 00:21:39.010 --> 00:21:45.040 even if it's a very happy reaction, things need to occur. 00:21:45.040 --> 00:21:50.290 So the two things coming together need to often distort. 00:21:50.290 --> 00:21:52.170 Bonds might have to be broken. 00:21:52.170 --> 00:21:54.350 And new bonds need to be formed. 00:21:54.350 --> 00:21:58.030 And while that is occurring, while there is distortion 00:21:58.030 --> 00:22:00.340 of the bond or bonds are breaking, 00:22:00.340 --> 00:22:02.850 you need to have some potential energy to do that. 00:22:02.850 --> 00:22:06.710 You need some energy to make that reaction go. 00:22:06.710 --> 00:22:08.980 So the potential energy of the system 00:22:08.980 --> 00:22:14.910 increases first while these distortions are happening. 00:22:14.910 --> 00:22:17.620 So the encounter then between them, you 00:22:17.620 --> 00:22:20.600 form some activated complex, or what's 00:22:20.600 --> 00:22:23.430 known as a transition state. 00:22:23.430 --> 00:22:31.550 And that activated complex, it can go on to form a molecule. 00:22:31.550 --> 00:22:35.690 Or from that activated complex, the two molecules 00:22:35.690 --> 00:22:38.310 may just go apart again. 00:22:38.310 --> 00:22:39.910 So what determines whether they're 00:22:39.910 --> 00:22:43.330 going to go on happily to form their complex 00:22:43.330 --> 00:22:46.400 or going to depart from each other, 00:22:46.400 --> 00:22:49.850 never to form a larger molecule? 00:22:49.850 --> 00:22:52.770 And the thing that determines the fate 00:22:52.770 --> 00:22:54.810 is that critical energy. 00:22:54.810 --> 00:22:58.050 So only those molecules with that sufficient energy 00:22:58.050 --> 00:23:02.000 to allow for those bond distortions and rearrangements 00:23:02.000 --> 00:23:03.820 will be able to go on and make this. 00:23:03.820 --> 00:23:05.403 And you can think about this, I think, 00:23:05.403 --> 00:23:07.770 in terms of a couple, a relationship, 00:23:07.770 --> 00:23:10.380 that there's always some work that has to go into it. 00:23:10.380 --> 00:23:11.310 It's never perfect. 00:23:11.310 --> 00:23:14.650 And if you put in this effort and this work, you can go on. 00:23:14.650 --> 00:23:17.670 And if you don't really have it in you and you're like, ah, 00:23:17.670 --> 00:23:18.850 you walk away. 00:23:18.850 --> 00:23:20.350 You don't have that critical energy. 00:23:20.350 --> 00:23:23.790 You don't have that special overcome, that activation 00:23:23.790 --> 00:23:25.284 energy, and you go back. 00:23:25.284 --> 00:23:27.450 So let's just take a look at some molecules checking 00:23:27.450 --> 00:23:30.180 each other out and figuring out if they have 00:23:30.180 --> 00:23:32.700 what it takes, if they have that critical energy. 00:23:32.700 --> 00:23:34.330 Let's watch and see what happens. 00:23:34.330 --> 00:23:35.280 Here they come. 00:23:35.280 --> 00:23:36.610 They're finding each other. 00:23:36.610 --> 00:23:37.890 They're circling each other. 00:23:37.890 --> 00:23:39.540 And oh my goodness. 00:23:39.540 --> 00:23:42.690 They had the critical energy necessary. 00:23:42.690 --> 00:23:44.660 And they formed a bigger yellow molecule. 00:23:44.660 --> 00:23:45.650 There they go. 00:23:45.650 --> 00:23:48.320 What a happy ending. 00:23:48.320 --> 00:23:53.260 So only those molecules that have that critical energy 00:23:53.260 --> 00:23:56.030 can go on and form their complex. 00:23:56.030 --> 00:24:00.380 And so here is where temperature comes into play. 00:24:00.380 --> 00:24:04.920 Here is where temperature becomes really important. 00:24:04.920 --> 00:24:08.450 Because they need to have a certain amount of energy. 00:24:08.450 --> 00:24:12.160 And there's a relationship between the kinetic energy 00:24:12.160 --> 00:24:15.330 of molecules and their temperature. 00:24:15.330 --> 00:24:18.170 So let's take a look at this plot over here. 00:24:18.170 --> 00:24:21.790 So this is, on one axis, fraction of molecules. 00:24:21.790 --> 00:24:25.180 And on the other axis, we have kinetic energy. 00:24:25.180 --> 00:24:28.340 So if you're at low temperature over here, 00:24:28.340 --> 00:24:32.820 most of your molecules are going to have a low kinetic energy. 00:24:32.820 --> 00:24:34.470 But it will tail off. 00:24:34.470 --> 00:24:38.140 And there will be some molecules that will have a higher energy. 00:24:38.140 --> 00:24:41.240 And if this line here represents that minimum energy, 00:24:41.240 --> 00:24:45.490 that critical energy or that activation energy, in blue 00:24:45.490 --> 00:24:48.630 here we have those low temperature molecules. 00:24:48.630 --> 00:24:50.700 Only a very small fraction of those 00:24:50.700 --> 00:24:53.360 will have the energy to react, will be able to overcome 00:24:53.360 --> 00:24:56.320 that minimum energy, that critical energy needed. 00:24:56.320 --> 00:24:58.220 Now if you're at high temperature, 00:24:58.220 --> 00:25:01.190 more molecules have a higher kinetic energy. 00:25:01.190 --> 00:25:06.410 And if you see over here shaded in orange, way more molecules 00:25:06.410 --> 00:25:08.960 have that critical energy, have the energy 00:25:08.960 --> 00:25:14.060 necessary to distort those bonds and go on to make a molecule. 00:25:14.060 --> 00:25:16.150 So here's where temperature is important. 00:25:16.150 --> 00:25:19.112 So kinetic energy, pass it on. 00:25:22.390 --> 00:25:24.844 So let's draw some reaction coordinates and think 00:25:24.844 --> 00:25:25.760 about what's going on. 00:25:25.760 --> 00:25:26.915 Yeah. 00:25:26.915 --> 00:25:28.740 AUDIENCE: For this idea of collision, 00:25:28.740 --> 00:25:31.386 could it also apply to the association reactions? 00:25:31.386 --> 00:25:33.362 Because that's [INAUDIBLE] molecule. 00:25:33.362 --> 00:25:35.529 Or are [? they still into ?] each other? [INAUDIBLE] 00:25:35.529 --> 00:25:36.528 CATHERINE DRENNAN: Yeah. 00:25:36.528 --> 00:25:38.680 So if you're talking about the forward reaction 00:25:38.680 --> 00:25:40.285 and the backward reaction of things, 00:25:40.285 --> 00:25:42.410 whether it's two molecules that are coming together 00:25:42.410 --> 00:25:45.950 to form things or molecules breaking apart, 00:25:45.950 --> 00:25:48.442 there's a critical energy both directions. 00:25:48.442 --> 00:25:49.650 And we'll see that, actually. 00:25:49.650 --> 00:25:50.525 It's a good question. 00:25:50.525 --> 00:25:55.070 That kind of leads us in to our reaction coordinate diagram. 00:25:55.070 --> 00:25:57.820 So most of this is in your notes. 00:25:57.820 --> 00:25:59.820 There's a few things that are not. 00:25:59.820 --> 00:26:03.060 But let's look at a reaction coordinate diagram. 00:26:03.060 --> 00:26:08.470 And by that, I mean we have Potential Energy, or PE, 00:26:08.470 --> 00:26:10.390 on one side. 00:26:10.390 --> 00:26:12.640 And on the other axis, we have what's 00:26:12.640 --> 00:26:14.465 called just the reaction coordinate. 00:26:21.220 --> 00:26:25.130 So if you're asked to draw a reaction coordinate diagram 00:26:25.130 --> 00:26:28.740 on a problem set or an exam, asking for potential energy 00:26:28.740 --> 00:26:31.370 versus reaction coordinate. 00:26:31.370 --> 00:26:35.640 Now our reactants are going to have a particular amount 00:26:35.640 --> 00:26:37.770 of potential energy. 00:26:37.770 --> 00:26:39.520 And in this case, it's up here. 00:26:43.810 --> 00:26:48.820 And our products are going to have some potential energy 00:26:48.820 --> 00:26:49.530 down here. 00:26:54.980 --> 00:27:01.750 And there will be a difference in energy between these, 00:27:01.750 --> 00:27:06.590 our delta E. But these reactants aren't just 00:27:06.590 --> 00:27:11.200 going to be able to go to products without overcoming 00:27:11.200 --> 00:27:13.310 some kind of critical energy. 00:27:13.310 --> 00:27:19.490 So before they can go down there, they need to go up here. 00:27:19.490 --> 00:27:21.790 So we have the activation energy, 00:27:21.790 --> 00:27:25.840 f for forward direction. 00:27:25.840 --> 00:27:28.990 Also, if you're down in products and you 00:27:28.990 --> 00:27:32.067 want to go back to reactants, there's also a barrier 00:27:32.067 --> 00:27:33.150 that you have to overcome. 00:27:33.150 --> 00:27:35.550 And it's not just this. 00:27:35.550 --> 00:27:37.590 It's all the way up here. 00:27:41.170 --> 00:27:43.290 And so this would be the activation energy 00:27:43.290 --> 00:27:45.660 of the reverse reaction. 00:27:45.660 --> 00:27:52.060 And this dashed line on the top is our transition state 00:27:52.060 --> 00:27:54.160 or our activated complex. 00:27:54.160 --> 00:27:57.610 Both are two expressions that are 00:27:57.610 --> 00:27:58.830 used kind of interchangeably. 00:28:02.390 --> 00:28:04.755 So this transition states, some kind of weird mixture 00:28:04.755 --> 00:28:06.630 where they've come together, and they're sort 00:28:06.630 --> 00:28:08.110 of breaking and distorting. 00:28:08.110 --> 00:28:10.170 It's not our final products. 00:28:10.170 --> 00:28:11.340 That's down here. 00:28:11.340 --> 00:28:14.870 But you need to go up in energy, overcome an activation energy 00:28:14.870 --> 00:28:15.670 barrier. 00:28:15.670 --> 00:28:21.640 So the transition here, you need to go up, and then you go down. 00:28:21.640 --> 00:28:27.410 So now let's think about all of these energy terms. 00:28:27.410 --> 00:28:32.570 And what's true is that the change in energy-- this change 00:28:32.570 --> 00:28:35.180 in energy between reactants and products 00:28:35.180 --> 00:28:40.190 is equal to our activation energy for our forward reaction 00:28:40.190 --> 00:28:47.010 minus the activation energy of the reverse reaction. 00:28:47.010 --> 00:28:51.020 And this delta E over here can be measured 00:28:51.020 --> 00:28:53.230 from a calorimetry experiment. 00:28:53.230 --> 00:28:54.955 And you might recall back when we 00:28:54.955 --> 00:28:58.010 were talking about thermodynamics that E 00:28:58.010 --> 00:29:01.380 is kind of closely related to delta H. 00:29:01.380 --> 00:29:06.120 So we had this equation at one point 00:29:06.120 --> 00:29:16.660 that delta H is equal to delta E plus the change in PV. 00:29:16.660 --> 00:29:22.290 And we said that for gases, there's about a 1% to 2% 00:29:22.290 --> 00:29:24.960 difference between delta H and delta E. 00:29:24.960 --> 00:29:27.990 But for solids and liquids, it's pretty much the same. 00:29:27.990 --> 00:29:30.380 So you can think about this delta E 00:29:30.380 --> 00:29:35.210 like you were thinking about delta H, for the most part. 00:29:35.210 --> 00:29:40.000 So now for this, if we have some of the numbers-- 00:29:40.000 --> 00:29:43.180 if you know activation energy in the forward and the reverse, 00:29:43.180 --> 00:29:44.720 you can calculate delta E. If you 00:29:44.720 --> 00:29:47.370 know delta E and one of the activation energies, 00:29:47.370 --> 00:29:49.220 you can calculate the other. 00:29:49.220 --> 00:29:52.810 So I'll tell you for this particular reaction 00:29:52.810 --> 00:29:53.896 what these values are. 00:29:53.896 --> 00:29:55.520 And I was going to bring colored chalk, 00:29:55.520 --> 00:29:57.620 and somehow that didn't happen. 00:29:57.620 --> 00:30:00.100 So the activation energy in this is not in your notes. 00:30:00.100 --> 00:30:01.640 So I'll write it down. 00:30:01.640 --> 00:30:06.640 Is 132 kilojoules per mole. 00:30:06.640 --> 00:30:09.320 And I'll also put it here, a little bigger, 00:30:09.320 --> 00:30:14.140 132 kilojoules per mole. 00:30:14.140 --> 00:30:22.760 And for the reverse reaction, it's 358 kilojoules per mole 00:30:22.760 --> 00:30:28.660 and minus 358 kilojoules per mole. 00:30:32.700 --> 00:30:35.490 And so if we do the math here, we 00:30:35.490 --> 00:30:45.780 will calculate that delta E is equal to minus 226 kilojoules 00:30:45.780 --> 00:30:48.350 per mole. 00:30:48.350 --> 00:30:51.900 So would you expect that to be an endothermic or exothermic 00:30:51.900 --> 00:30:53.890 reaction? 00:30:53.890 --> 00:30:54.770 Exothermic. 00:30:54.770 --> 00:30:55.400 Right. 00:30:55.400 --> 00:30:57.840 And we can put that value also in here, 00:30:57.840 --> 00:31:02.750 minus 226 kilojoules per mole. 00:31:02.750 --> 00:31:06.870 So what's really important here, even though it's 00:31:06.870 --> 00:31:09.220 exothermic reaction, it's not going 00:31:09.220 --> 00:31:13.620 to just go without having those molecules have 00:31:13.620 --> 00:31:17.030 some critical energy necessary to overcome 00:31:17.030 --> 00:31:19.090 this activation energy barrier. 00:31:19.090 --> 00:31:21.170 You still need energy. 00:31:21.170 --> 00:31:26.460 You need to go up here before you can go down there. 00:31:26.460 --> 00:31:30.800 So just sort of thinking about this in layman's terms, 00:31:30.800 --> 00:31:35.740 this idea of these activation energy barriers, 00:31:35.740 --> 00:31:42.820 for me whenever I sit down to write a grant or write a paper, 00:31:42.820 --> 00:31:44.320 I think, wow. 00:31:44.320 --> 00:31:47.904 I sit down at my computer, have my cup of coffee. 00:31:47.904 --> 00:31:50.070 I know that we're about to get scooped on this data. 00:31:50.070 --> 00:31:52.830 So I really should write this paper, a little stressed 00:31:52.830 --> 00:31:53.340 about it. 00:31:53.340 --> 00:31:54.720 Then I think, wow. 00:31:54.720 --> 00:31:57.290 I could write a lot better if my office was clean. 00:31:57.290 --> 00:31:59.750 Now I hate cleaning my office. 00:31:59.750 --> 00:32:03.470 But compared to writing a paper, it's pretty good. 00:32:03.470 --> 00:32:05.540 I don't mind doing it. 00:32:05.540 --> 00:32:09.260 So then clean the office. 00:32:09.260 --> 00:32:10.950 Then walk the dog. 00:32:10.950 --> 00:32:14.180 Maybe do later some laundry. 00:32:14.180 --> 00:32:15.810 Lots of things happen. 00:32:15.810 --> 00:32:18.940 And then eventually, you hear from the collaborators. 00:32:18.940 --> 00:32:20.830 They're like, I need this paper now. 00:32:20.830 --> 00:32:23.010 And you're like, oh man. 00:32:23.010 --> 00:32:28.500 And so that stress gets you over that activation energy barrier, 00:32:28.500 --> 00:32:30.970 just catapults you over. 00:32:30.970 --> 00:32:34.320 Or you wait to a grant-- you start six months early. 00:32:34.320 --> 00:32:36.100 And then the week before it's due, 00:32:36.100 --> 00:32:41.500 suddenly you have that nervous energy that gets you over that. 00:32:41.500 --> 00:32:44.520 So many of you have probably experienced this. 00:32:44.520 --> 00:32:49.950 And you know, if you need some extra help with an activation 00:32:49.950 --> 00:32:53.490 energy barrier to do the rest of those extra problems for exam 00:32:53.490 --> 00:32:55.670 four, you can come talk to me. 00:32:55.670 --> 00:32:59.570 And I'll stress you out and get you right over that activation 00:32:59.570 --> 00:33:00.510 energy barrier. 00:33:00.510 --> 00:33:04.630 But the thing to keep in mind is that look at this slope 00:33:04.630 --> 00:33:06.270 in this case down here. 00:33:06.270 --> 00:33:10.800 So often it's just that little something to get you over. 00:33:10.800 --> 00:33:13.090 And then it's smooth sailing. 00:33:13.090 --> 00:33:15.415 So you can work these games with yourself. 00:33:15.415 --> 00:33:17.040 You've just got to get over it and know 00:33:17.040 --> 00:33:18.980 that once you're over that barrier, 00:33:18.980 --> 00:33:20.330 it's all going to be good. 00:33:20.330 --> 00:33:22.780 So you can get yourself over. 00:33:22.780 --> 00:33:26.150 You should never forget about activation energy barriers. 00:33:26.150 --> 00:33:27.220 There's always a barrier. 00:33:27.220 --> 00:33:28.030 Am I right? 00:33:28.030 --> 00:33:29.940 There's always some kind of barrier. 00:33:29.940 --> 00:33:32.180 Anything you're going to do that's worthwhile 00:33:32.180 --> 00:33:37.230 has some barrier associated with it. 00:33:37.230 --> 00:33:40.240 So now let's think about the results of these barriers, 00:33:40.240 --> 00:33:43.630 again, coming back to this idea of temperature. 00:33:43.630 --> 00:33:46.000 So for an elementary reaction-- again, 00:33:46.000 --> 00:33:48.210 that's a reaction that occurs exactly as written. 00:33:48.210 --> 00:33:51.670 It's a step in an overall reaction mechanism. 00:33:51.670 --> 00:33:54.555 There's always a barrier, always a barrier. 00:33:54.555 --> 00:33:56.400 Barrier is always positive. 00:33:56.400 --> 00:33:58.550 It's always there. 00:33:58.550 --> 00:34:02.640 And so if you increase the temperature, 00:34:02.640 --> 00:34:05.890 you're always going to increase the rate of that reaction. 00:34:05.890 --> 00:34:10.100 It's always going to help get over that barrier. 00:34:10.100 --> 00:34:14.400 But increase the temperature, increase the rate. 00:34:14.400 --> 00:34:17.980 But for an overall reaction, temperature 00:34:17.980 --> 00:34:20.940 is a little more complicated to predict. 00:34:20.940 --> 00:34:23.510 So if you increase the temperature, 00:34:23.510 --> 00:34:26.529 it's not always as clear what's going to happen to the rate. 00:34:26.529 --> 00:34:28.820 Because you can have a lot of different steps involved. 00:34:28.820 --> 00:34:30.449 It can be exothermic and endothermic. 00:34:30.449 --> 00:34:32.219 A lot can be happening. 00:34:32.219 --> 00:34:35.000 Changing the temperature, you're changing k. 00:34:35.000 --> 00:34:38.190 So to think about an overall reaction, 00:34:38.190 --> 00:34:40.326 we need to understand reaction mechanisms. 00:34:40.326 --> 00:34:42.159 And that's really convenient because we just 00:34:42.159 --> 00:34:44.980 talked about reaction mechanisms on Monday. 00:34:44.980 --> 00:34:48.250 So you all know how to write out reaction mechanisms. 00:34:48.250 --> 00:34:50.430 So let's just practice. 00:34:50.430 --> 00:34:52.180 So for this example, we have two molecules 00:34:52.180 --> 00:34:55.775 of N O plus O2 going to 2NO2. 00:34:55.775 --> 00:34:58.830 Step one, fast and reversible. 00:34:58.830 --> 00:35:02.320 N O plus N O, going to an intermediate. 00:35:02.320 --> 00:35:03.840 Then the intermediate is reacting 00:35:03.840 --> 00:35:08.260 with oxygen going to two molecules of our product. 00:35:08.260 --> 00:35:10.420 So we can write the rate of formation 00:35:10.420 --> 00:35:12.670 of product based on the slow step, 00:35:12.670 --> 00:35:14.420 or if we didn't know the slow set, 00:35:14.420 --> 00:35:17.370 on the second step or the last step. 00:35:17.370 --> 00:35:20.360 So two molecules, again, two molecules are being formed. 00:35:20.360 --> 00:35:23.960 k2 times the concentration of the intermediate times 00:35:23.960 --> 00:35:27.060 the concentration of O2. 00:35:27.060 --> 00:35:30.439 But we have an intermediate. 00:35:30.439 --> 00:35:32.480 So we need to solve for the intermediate in terms 00:35:32.480 --> 00:35:36.610 of rate constants, reactants, and products. 00:35:36.610 --> 00:35:39.690 But now we're told we have a fast step followed 00:35:39.690 --> 00:35:40.800 by a slow step. 00:35:40.800 --> 00:35:43.390 So we know how to do this. 00:35:43.390 --> 00:35:46.100 And remember, when you have a fast, reversible step followed 00:35:46.100 --> 00:35:51.680 by a slow step, this first step approximates an equilibrium 00:35:51.680 --> 00:35:52.540 reaction. 00:35:52.540 --> 00:35:54.720 There is not much of the intermediate that's being 00:35:54.720 --> 00:35:56.187 siphoned off in the slow step. 00:35:56.187 --> 00:35:58.145 So pretty much, when you form the intermediate, 00:35:58.145 --> 00:36:00.330 it's going back and forth just like you have 00:36:00.330 --> 00:36:03.050 in an equilibrium situation. 00:36:03.050 --> 00:36:06.690 And just to emphasize this, I'll share a little picture 00:36:06.690 --> 00:36:08.890 here of the beach in the summer when you 00:36:08.890 --> 00:36:10.680 don't need your winter coat. 00:36:10.680 --> 00:36:14.440 And here we have my daughter and her best friend. 00:36:14.440 --> 00:36:16.400 And they are trying to empty the ocean 00:36:16.400 --> 00:36:18.950 with this princess bucket. 00:36:18.950 --> 00:36:22.590 And one can ask the question, is this going to affect the tides? 00:36:22.590 --> 00:36:24.480 Is the ocean going to be different? 00:36:24.480 --> 00:36:26.790 But the rate at which my daughter and Aiden 00:36:26.790 --> 00:36:30.460 fill up their buckets and bring them over here, 00:36:30.460 --> 00:36:33.370 that's a really slow step compared to everything 00:36:33.370 --> 00:36:35.230 that's going on in the ocean. 00:36:35.230 --> 00:36:38.400 So when you have a very slow step like a kid 00:36:38.400 --> 00:36:41.050 and a long distance with a princess pail, 00:36:41.050 --> 00:36:43.070 you don't really need to worry about that. 00:36:43.070 --> 00:36:46.230 And you can think about the fast step as being an equilibrium. 00:36:46.230 --> 00:36:49.480 This is not being siphoned off enough to worry about it. 00:36:49.480 --> 00:36:54.350 And so that makes it easier to solve for your intermediate 00:36:54.350 --> 00:36:57.470 because then you can just do it by an equilibrium expression. 00:36:57.470 --> 00:37:01.160 So in the clicker question, tell me how I can do that. 00:37:13.960 --> 00:37:15.210 AUDIENCE: I think you're good. 00:37:15.210 --> 00:37:15.850 CATHERINE DRENNAN: OK. 00:37:15.850 --> 00:37:16.742 10 more seconds. 00:37:32.860 --> 00:37:36.770 So let's just take a look at that. 00:37:36.770 --> 00:37:41.070 So again, we're writing our equilibrium expression 00:37:41.070 --> 00:37:45.460 for the first step, products over reactants. 00:37:45.460 --> 00:37:50.300 And then we can rearrange it to solve because the product here 00:37:50.300 --> 00:37:51.650 is our intermediate. 00:37:51.650 --> 00:37:54.800 So we can solve for the intermediate in terms 00:37:54.800 --> 00:37:58.490 of K1 and our reactant. 00:37:58.490 --> 00:38:01.500 And then we can substitute it in. 00:38:01.500 --> 00:38:03.170 And that gives us this. 00:38:03.170 --> 00:38:06.020 So we had the 2k2 here. 00:38:06.020 --> 00:38:11.210 Now we have our K1 and our N O squared. 00:38:11.210 --> 00:38:14.160 And here we have our oxygen there. 00:38:14.160 --> 00:38:17.220 So now we have a rate law that doesn't 00:38:17.220 --> 00:38:18.450 include any intermediates. 00:38:21.274 --> 00:38:23.190 I'm just going to put that back up here if you 00:38:23.190 --> 00:38:25.940 didn't get it written down. 00:38:25.940 --> 00:38:29.690 Now let's think of what happens with temperature. 00:38:29.690 --> 00:38:33.300 So this is an elementary rate constant, little k2. 00:38:33.300 --> 00:38:36.330 So you increase the rate when you increase temperature. 00:38:39.180 --> 00:38:41.150 And here is our expression again that 00:38:41.150 --> 00:38:44.490 tells us about that change. 00:38:44.490 --> 00:38:48.920 So now for the equilibrium constant, 00:38:48.920 --> 00:38:51.500 the effect of temperature depends on 00:38:51.500 --> 00:38:55.350 whether the reaction is exothermic or endothermic. 00:38:55.350 --> 00:39:00.230 And does anyone remember what the name of the equation 00:39:00.230 --> 00:39:03.330 is that tells us about temperature effect 00:39:03.330 --> 00:39:06.360 with equilibrium constants? 00:39:06.360 --> 00:39:07.760 Yes. 00:39:07.760 --> 00:39:09.480 The van 't Hoff equation. 00:39:09.480 --> 00:39:11.820 I told you that some time in the future I'd 00:39:11.820 --> 00:39:13.640 ask you for the name of this equation. 00:39:13.640 --> 00:39:15.446 And that time is now. 00:39:15.446 --> 00:39:16.820 You don't really need to know it. 00:39:16.820 --> 00:39:17.570 But it's just fun. 00:39:17.570 --> 00:39:21.590 There's not many names for equations in general chemistry. 00:39:21.590 --> 00:39:23.890 Look how similar these equations are. 00:39:23.890 --> 00:39:25.920 These are very similar equations. 00:39:25.920 --> 00:39:29.180 So we have the natural log of rate constants here. 00:39:29.180 --> 00:39:32.230 Here we have the natural log of equilibrium constants. 00:39:32.230 --> 00:39:35.060 Here we have our activation energy. 00:39:35.060 --> 00:39:38.670 Here we have delta H. 00:39:38.670 --> 00:39:45.910 So if the reaction is exothermic, 00:39:45.910 --> 00:39:48.800 if we increase the temperature, what 00:39:48.800 --> 00:39:50.620 happens to our equilibrium constant? 00:39:50.620 --> 00:39:51.842 Does it increase or decrease? 00:39:51.842 --> 00:39:54.940 You can just yell it out. 00:39:54.940 --> 00:39:57.740 It decreases. 00:39:57.740 --> 00:40:02.040 Now let's think about what happens here. 00:40:02.040 --> 00:40:07.680 So our kobs now has an elementary rate constant term, 00:40:07.680 --> 00:40:10.040 which is going to increase with temperature. 00:40:10.040 --> 00:40:12.840 And it has an equilibrium constant term, 00:40:12.840 --> 00:40:15.120 which is going to decrease with temperature 00:40:15.120 --> 00:40:19.050 for an exothermic reaction. 00:40:19.050 --> 00:40:22.750 So we have these going in opposite directions. 00:40:22.750 --> 00:40:27.500 So overall, we want to think about the magnitude 00:40:27.500 --> 00:40:32.220 of our activation energy term for our rate constants. 00:40:32.220 --> 00:40:35.930 And we want to think about the magnitude of our delta H term 00:40:35.930 --> 00:40:39.880 in terms of the equilibrium constants. 00:40:39.880 --> 00:40:45.140 So for the particular reaction in question, you can look up. 00:40:45.140 --> 00:40:48.540 The activation energy is a small number. 00:40:48.540 --> 00:40:51.570 And delta H, again, it's exothermic. 00:40:51.570 --> 00:40:54.070 And it's a big number. 00:40:54.070 --> 00:40:55.690 So then if you're thinking about that, 00:40:55.690 --> 00:40:59.640 if Ea is small and positive-- Ea is always positive. 00:40:59.640 --> 00:41:03.980 There is always a barrier, always positive. 00:41:03.980 --> 00:41:08.440 Then the rate constant is only going to increase a little bit. 00:41:08.440 --> 00:41:10.590 But delta H is a big negative number, 00:41:10.590 --> 00:41:14.090 so the equilibrium constant is going to decrease a lot. 00:41:14.090 --> 00:41:17.600 So for this particular reaction, increasing the temperature 00:41:17.600 --> 00:41:20.370 actually decreases the kobs. 00:41:20.370 --> 00:41:23.670 And so for any reaction, it depends 00:41:23.670 --> 00:41:27.430 on the magnitude of Ea and the magnitude 00:41:27.430 --> 00:41:33.070 and the sign of delta H. So again, large Ea 00:41:33.070 --> 00:41:35.600 means very sensitive to temperature. 00:41:35.600 --> 00:41:39.700 Large delta H means that the equilibrium constant is 00:41:39.700 --> 00:41:42.930 very sensitive to temperature. 00:41:42.930 --> 00:41:45.860 For an elementary rate constant, it's 00:41:45.860 --> 00:41:49.430 always going to increase with temperature. 00:41:49.430 --> 00:41:51.460 Because Ea is always positive. 00:41:51.460 --> 00:41:54.110 There is always a barrier to overcome. 00:41:54.110 --> 00:41:58.170 Temperature always increases a rate constant. 00:41:58.170 --> 00:42:00.400 But for an equilibrium constant, it 00:42:00.400 --> 00:42:05.060 can increase or decrease because delta H isn't always positive. 00:42:05.060 --> 00:42:06.720 Like the activation energy barrier, 00:42:06.720 --> 00:42:09.990 it can be positive or negative. 00:42:09.990 --> 00:42:14.570 So the magnitude of delta H, how big a number it is, 00:42:14.570 --> 00:42:16.630 tells you about the magnitude of the change, 00:42:16.630 --> 00:42:20.080 how much the equilibrium constant will change. 00:42:20.080 --> 00:42:22.630 Will k1 and k2 be almost like each other 00:42:22.630 --> 00:42:25.440 or really, really different from each other, those equilibrium 00:42:25.440 --> 00:42:26.440 constants? 00:42:26.440 --> 00:42:29.550 And the sign of delta H, whether it's positive or negative, 00:42:29.550 --> 00:42:31.400 tells you the direction of the change. 00:42:31.400 --> 00:42:34.920 Will it increase or decrease? 00:42:34.920 --> 00:42:38.930 So I just want to show where we're 00:42:38.930 --> 00:42:43.860 going with this because it's just super exciting. 00:42:43.860 --> 00:42:45.240 Yes. 00:42:45.240 --> 00:42:47.560 We're back to Le Chatelier. 00:42:47.560 --> 00:42:49.260 This is what I was so excited about. 00:42:49.260 --> 00:42:52.510 Remember, Le Chatelier told us, when we apply a stress 00:42:52.510 --> 00:42:56.450 to the system, the system responds in such a way 00:42:56.450 --> 00:42:59.970 to minimize that stress. 00:42:59.970 --> 00:43:04.840 So if we increase the temperature, 00:43:04.840 --> 00:43:07.810 according to Le Chatelier, what direction 00:43:07.810 --> 00:43:10.566 will the reaction shift, in the exothermic 00:43:10.566 --> 00:43:13.940 or the endothermic direction? 00:43:13.940 --> 00:43:14.680 What is it? 00:43:18.411 --> 00:43:19.357 The endothermic. 00:43:19.357 --> 00:43:20.920 So you increase the temperature. 00:43:20.920 --> 00:43:21.950 You add heat. 00:43:21.950 --> 00:43:26.420 It shifts in a direction to minimize that 00:43:26.420 --> 00:43:29.270 or to use up that heat. 00:43:29.270 --> 00:43:31.680 So we'll end with one last clicker question. 00:43:31.680 --> 00:43:34.710 And we'll finish this concept on Monday. 00:43:34.710 --> 00:43:39.120 But we want to think about how Le Chatelier and what we've 00:43:39.120 --> 00:43:41.760 known already applies here. 00:43:41.760 --> 00:43:46.970 So why don't you tell me which of these diagrams is exothermic 00:43:46.970 --> 00:43:48.582 and which is endothermic? 00:43:48.582 --> 00:43:50.040 Because we're going to tie this all 00:43:50.040 --> 00:43:53.077 back to temperature and Le Chatelier. 00:43:53.077 --> 00:43:54.660 AUDIENCE: We don't have that question. 00:43:54.660 --> 00:43:56.618 CATHERINE DRENNAN: We don't have that question. 00:43:56.618 --> 00:43:57.210 OK. 00:43:57.210 --> 00:43:58.652 Then we will not do that question. 00:43:58.652 --> 00:44:00.110 AUDIENCE: [INAUDIBLE] the question. 00:44:00.110 --> 00:44:01.026 CATHERINE DRENNAN: OK. 00:44:01.026 --> 00:44:04.290 So I'll just give you the answer to that. 00:44:04.290 --> 00:44:06.450 And then we'll see who the winners are. 00:44:06.450 --> 00:44:09.720 So that one is endothermic. 00:44:09.720 --> 00:44:13.430 And this one is exothermic. 00:44:13.430 --> 00:44:18.370 And we'll finish the rest of this on Monday. 00:44:18.370 --> 00:44:21.336 But for now, can you tell us who the winner is? 00:44:28.090 --> 00:44:31.590 No, it's not coming up. 00:44:31.590 --> 00:44:33.573 AUDIENCE: It's Dan. 00:44:33.573 --> 00:44:34.698 CATHERINE DRENNAN: Top two. 00:44:34.698 --> 00:44:36.610 AUDIENCE: Dan and Jay. 00:44:36.610 --> 00:44:39.750 CATHERINE DRENNAN: Dan and Jay have beat out the other folks. 00:44:39.750 --> 00:44:42.130 So we know who's in the playoffs. 00:44:42.130 --> 00:44:43.230 All right. 00:44:43.230 --> 00:44:44.370 Friday exam. 00:44:44.370 --> 00:44:47.130 See you Monday in class to finish the handout. 00:44:54.570 --> 00:44:55.070 All right. 00:44:55.070 --> 00:44:58.230 Let's take 10 more seconds on the clicker question. 00:45:17.204 --> 00:45:20.060 So let's take a look at this. 00:45:20.060 --> 00:45:24.100 56% do you have the right answer. 00:45:24.100 --> 00:45:26.610 So rate constants always increase 00:45:26.610 --> 00:45:27.835 with increase of temperature. 00:45:27.835 --> 00:45:30.450 That's our little k's. 00:45:30.450 --> 00:45:33.250 Because there is always some activation energy 00:45:33.250 --> 00:45:34.980 barrier to overcome. 00:45:34.980 --> 00:45:37.380 There is always a positive barrier. 00:45:37.380 --> 00:45:41.100 Whereas, for equilibrium constants, K, 00:45:41.100 --> 00:45:44.450 you can have a reaction that's exothermic or endothermic. 00:45:44.450 --> 00:45:45.900 And so that will change. 00:45:45.900 --> 00:45:49.730 It either increases or decreases because delta H 00:45:49.730 --> 00:45:51.260 can be positive or negative. 00:45:51.260 --> 00:45:53.930 But activation energy barriers are always positive. 00:45:53.930 --> 00:45:56.260 There's always a barrier. 00:45:56.260 --> 00:45:59.280 Always a barrier to doing anything that's important. 00:45:59.280 --> 00:46:01.620 I've got to calm down. 00:46:01.620 --> 00:46:08.900 Le Chatelier tells me that when a stress is applied 00:46:08.900 --> 00:46:12.760 to the system, I should respond in such a way 00:46:12.760 --> 00:46:15.120 to minimize the stress. 00:46:15.120 --> 00:46:20.120 It's a very calming rule in chemistry. 00:46:20.120 --> 00:46:22.680 Reactions don't like stress. 00:46:22.680 --> 00:46:26.140 And they'll respond in a way to minimize it. 00:46:26.140 --> 00:46:31.320 So we've been talking about Le Chatelier 00:46:31.320 --> 00:46:34.910 for a lot of the semester. 00:46:34.910 --> 00:46:37.580 And we've talked about the effect of temperature 00:46:37.580 --> 00:46:40.610 on reactions previously. 00:46:40.610 --> 00:46:45.140 So if you add heat to a reaction, 00:46:45.140 --> 00:46:47.990 it will want to respond in such a way to minimize the heat 00:46:47.990 --> 00:46:49.780 or absorb the heat. 00:46:49.780 --> 00:46:53.222 So it shifts in the endothermic direction. 00:46:53.222 --> 00:46:54.680 So we've already talked about this. 00:46:54.680 --> 00:46:56.780 And nothing is new here today. 00:46:56.780 --> 00:46:59.640 But today, I'm going to give you a different way 00:46:59.640 --> 00:47:03.120 to rationalize why that happens. 00:47:03.120 --> 00:47:04.971 So we're not changing what happens. 00:47:04.971 --> 00:47:06.720 We're just going to come up with a new way 00:47:06.720 --> 00:47:09.340 to rationalize the why. 00:47:09.340 --> 00:47:13.930 So we ended last time with these two diagrams. 00:47:13.930 --> 00:47:17.500 And we identified one as endothermic and one 00:47:17.500 --> 00:47:19.190 as exothermic. 00:47:19.190 --> 00:47:23.020 So these, again, are our reaction coordinate diagrams. 00:47:23.020 --> 00:47:24.580 We have PE on one side. 00:47:24.580 --> 00:47:27.050 What does PE stand for again? 00:47:27.050 --> 00:47:29.030 Potential energy. 00:47:29.030 --> 00:47:31.530 It stands for other things too, but in a reaction coordinate 00:47:31.530 --> 00:47:34.470 diagram, that's what it is. 00:47:34.470 --> 00:47:36.630 And then the reaction coordinate going this way. 00:47:36.630 --> 00:47:39.840 So we go from reactants to products along the reaction 00:47:39.840 --> 00:47:41.490 coordinate. 00:47:41.490 --> 00:47:44.430 So here in this endothermic reaction, 00:47:44.430 --> 00:47:47.210 we have our reactants down here, our products up here, 00:47:47.210 --> 00:47:50.440 and a very large activation energy barrier 00:47:50.440 --> 00:47:54.260 for the forward direction, a much smaller activation energy 00:47:54.260 --> 00:47:56.540 barrier for the reverse direction. 00:47:56.540 --> 00:47:59.580 With the exothermic reaction here, we 00:47:59.580 --> 00:48:01.910 have a smaller activation energy barrier 00:48:01.910 --> 00:48:04.230 for the forward direction and a bigger one 00:48:04.230 --> 00:48:07.130 for the reverse direction. 00:48:07.130 --> 00:48:09.930 So let's look at the one equation that 00:48:09.930 --> 00:48:13.170 will be on the exam that will not be on your equation sheet. 00:48:13.170 --> 00:48:16.000 I think this is-- for this unit, this 00:48:16.000 --> 00:48:20.210 is the one that will be not on your equation sheet. 00:48:20.210 --> 00:48:24.000 This change in energy is equal to the activation 00:48:24.000 --> 00:48:25.740 energy for the forward direction the 00:48:25.740 --> 00:48:28.960 of reaction minus the activation energy 00:48:28.960 --> 00:48:31.150 for the reverse reaction. 00:48:31.150 --> 00:48:34.080 And again, this is our delta E here. 00:48:34.080 --> 00:48:35.940 And this is our delta E here. 00:48:35.940 --> 00:48:40.210 Delta E is like delta H. If you're talking about a gas, 00:48:40.210 --> 00:48:41.920 it's 1% to 2% different. 00:48:41.920 --> 00:48:44.900 If you're talking about a solid or a liquid, 00:48:44.900 --> 00:48:47.760 it's negligibly different. 00:48:47.760 --> 00:48:52.020 So if we look at this endothermic reaction here, 00:48:52.020 --> 00:48:54.790 we have a big forward activation energy 00:48:54.790 --> 00:49:00.270 barrier minus a small reverse activation energy barrier. 00:49:00.270 --> 00:49:03.600 So that's going to give us a positive value for delta E. 00:49:03.600 --> 00:49:06.720 And it'll be an endothermic reaction. 00:49:06.720 --> 00:49:08.880 And if we look at this equation again and fill it 00:49:08.880 --> 00:49:12.220 in for the exothermic reaction, we 00:49:12.220 --> 00:49:14.790 have a small barrier in the forward direction, 00:49:14.790 --> 00:49:17.290 a big barrier in the reverse direction, 00:49:17.290 --> 00:49:20.190 and a negative value for delta E and delta H. 00:49:20.190 --> 00:49:23.385 It's an exothermic reaction. 00:49:23.385 --> 00:49:25.010 Now let's think about what happens when 00:49:25.010 --> 00:49:28.260 we increase the temperature. 00:49:28.260 --> 00:49:31.210 So if we increase temperature, it's 00:49:31.210 --> 00:49:36.420 a lot easier to overcome this forward activation energy 00:49:36.420 --> 00:49:38.470 barrier, the big one. 00:49:38.470 --> 00:49:40.990 And that's going to shift it to products. 00:49:40.990 --> 00:49:43.914 And this is because the small activation energy barrier, 00:49:43.914 --> 00:49:45.580 it's not that hard to get over something 00:49:45.580 --> 00:49:47.690 that's a small activation energy barrier. 00:49:47.690 --> 00:49:48.670 It's not hard. 00:49:48.670 --> 00:49:50.820 But if it's a really big barrier, 00:49:50.820 --> 00:49:52.740 it's hard to get over that barrier. 00:49:52.740 --> 00:49:54.580 You need a lot of kinetic energy. 00:49:54.580 --> 00:49:57.700 Increasing the temperature will give that kinetic energy 00:49:57.700 --> 00:49:59.180 to overcome that barrier. 00:49:59.180 --> 00:50:01.590 Remember that when molecules come together, 00:50:01.590 --> 00:50:03.460 bonds are distorted and formed. 00:50:03.460 --> 00:50:05.460 The potential energy goes up first. 00:50:05.460 --> 00:50:08.120 And only those molecules with that critical energy 00:50:08.120 --> 00:50:10.710 that can overcome that activation energy barrier 00:50:10.710 --> 00:50:12.100 can go on to products. 00:50:12.100 --> 00:50:14.410 Increase the temperature, that allows you 00:50:14.410 --> 00:50:18.210 to overcome the big barrier. 00:50:18.210 --> 00:50:21.407 Now if we look in this side, this is a small barrier. 00:50:21.407 --> 00:50:22.990 They weren't really having-- molecules 00:50:22.990 --> 00:50:25.031 were probably not having a huge amount of trouble 00:50:25.031 --> 00:50:25.890 with that barrier. 00:50:25.890 --> 00:50:29.070 The barrier that was hard was for the reverse direction. 00:50:29.070 --> 00:50:30.590 So now you increase the temperature 00:50:30.590 --> 00:50:32.320 of an exothermic reaction. 00:50:32.320 --> 00:50:36.130 It's easier to overcome this big barrier, the reverse barrier. 00:50:36.130 --> 00:50:38.800 And you have a shift toward reactants. 00:50:38.800 --> 00:50:40.500 So this is what we had seen before. 00:50:40.500 --> 00:50:43.380 You increase the temperature of an endothermic reaction. 00:50:43.380 --> 00:50:45.290 You go to products. 00:50:45.290 --> 00:50:48.780 You increase the temperature of an exothermic reaction. 00:50:48.780 --> 00:50:50.330 It shifts to reactants. 00:50:50.330 --> 00:50:52.540 So these are the same things we saw before. 00:50:52.540 --> 00:50:55.380 But now there's a new rationalization behind it. 00:50:55.380 --> 00:50:58.620 Now we can think about this in terms of activation energy 00:50:58.620 --> 00:51:00.410 barriers. 00:51:00.410 --> 00:51:03.290 So the important points to remember, 00:51:03.290 --> 00:51:07.190 big activation energy barrier, rate constant 00:51:07.190 --> 00:51:10.170 very sensitive to temperature. 00:51:10.170 --> 00:51:14.710 If you have a big barrier, increasing the temperature 00:51:14.710 --> 00:51:17.120 makes a big difference. 00:51:17.120 --> 00:51:19.820 If you have a small barrier, doesn't really 00:51:19.820 --> 00:51:21.040 matter that much. 00:51:21.040 --> 00:51:24.280 Most of your molecules can get over that small barrier. 00:51:24.280 --> 00:51:27.190 So big barrier, increasing the temperature, 00:51:27.190 --> 00:51:29.000 more molecules can go. 00:51:29.000 --> 00:51:31.230 And so you're going to shift in the direction 00:51:31.230 --> 00:51:33.460 of the big barrier.