1 00:00:00,030 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,780 Commons license. 3 00:00:03,780 --> 00:00:06,020 Your support will help MIT OpenCourseWare 4 00:00:06,020 --> 00:00:10,090 continue to offer high quality educational resources for free. 5 00:00:10,090 --> 00:00:12,660 To make a donation or to view additional materials 6 00:00:12,660 --> 00:00:16,580 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,580 --> 00:00:17,960 at ocw.mit.edu. 8 00:00:26,490 --> 00:00:28,870 INSTRUCTOR: So let's consider a reaction 9 00:00:28,870 --> 00:00:30,860 and consider the effect of temperature 10 00:00:30,860 --> 00:00:35,890 on this reaction on the thermodynamics of the reaction. 11 00:00:35,890 --> 00:00:40,700 So this is the decomposition of sodium bicarbonate. 12 00:00:40,700 --> 00:00:43,920 Does anyone know what another name for bicarbonate is? 13 00:00:43,920 --> 00:00:46,270 If you were going to send someone to the grocery store 14 00:00:46,270 --> 00:00:48,550 to buy some, what would you tell them to get? 15 00:00:48,550 --> 00:00:51,100 AUDIENCE: Baking soda. 16 00:00:51,100 --> 00:00:53,090 INSTRUCTOR: So you would probably 17 00:00:53,090 --> 00:00:57,320 say, look for baking soda. 18 00:00:57,320 --> 00:01:03,725 Does anyone know what this is used for or how it works? 19 00:01:03,725 --> 00:01:04,433 AUDIENCE: Baking. 20 00:01:04,433 --> 00:01:07,660 INSTRUCTOR: Baking, yes. 21 00:01:07,660 --> 00:01:09,970 So what is baking soda doing? 22 00:01:09,970 --> 00:01:12,962 What step of baking are you using it for? 23 00:01:12,962 --> 00:01:14,450 AUDIENCE: [INAUDIBLE] 24 00:01:14,450 --> 00:01:16,096 INSTRUCTOR: Yeah, bread rising. 25 00:01:16,096 --> 00:01:20,870 So in this reaction, you're forming gas, 26 00:01:20,870 --> 00:01:24,680 and that gas helps bread to rise. 27 00:01:24,680 --> 00:01:29,070 So this process happening while it's baking 28 00:01:29,070 --> 00:01:32,819 allows for bread rising and things like that. 29 00:01:32,819 --> 00:01:34,860 So baking soda-- you always want to make sure you 30 00:01:34,860 --> 00:01:37,080 add your baking soda. 31 00:01:37,080 --> 00:01:39,140 So let's consider the thermodynamics 32 00:01:39,140 --> 00:01:41,120 of this reaction. 33 00:01:41,120 --> 00:01:47,290 delta H0 is a positive value, 135.6 kilojoules. 34 00:01:47,290 --> 00:01:50,370 And why don't you tell me which of these you 35 00:01:50,370 --> 00:01:53,620 think is going to be the delta S of these options? 36 00:01:57,434 --> 00:02:01,000 AUDIENCE: [CHATTER] 37 00:02:01,000 --> 00:02:02,480 INSTRUCTOR: Just 10 more seconds. 38 00:02:02,480 --> 00:02:20,550 AUDIENCE: [CHATTER] 39 00:02:20,550 --> 00:02:22,580 INSTRUCTOR: So the positive value-- 40 00:02:22,580 --> 00:02:25,660 that is a very good guess and that is correct. 41 00:02:25,660 --> 00:02:28,880 Because we're going from solids to gases, 42 00:02:28,880 --> 00:02:32,720 so you would predict that entropy would be increasing. 43 00:02:32,720 --> 00:02:35,360 Things are not moving very much with a solid. 44 00:02:35,360 --> 00:02:36,560 With a gas, they can. 45 00:02:36,560 --> 00:02:39,690 You have more disorder, more freedom. 46 00:02:39,690 --> 00:02:41,920 So that is, in fact, the correct value 47 00:02:41,920 --> 00:02:47,980 for this delta S0 is plus 0.334 kilojoules. 48 00:02:47,980 --> 00:02:51,660 I already put it in kilojoules for you. 49 00:02:51,660 --> 00:02:55,890 And now we can calculate the delta G for this reaction. 50 00:02:55,890 --> 00:02:58,680 And let's first do it at room temperature. 51 00:02:58,680 --> 00:03:04,414 So we have delta H, positive value and endothermic reaction. 52 00:03:04,414 --> 00:03:05,830 We're doing it at room temperature 53 00:03:05,830 --> 00:03:09,420 first, in Kelvin so we can cancel our units. 54 00:03:09,420 --> 00:03:13,560 And we put in our delta S value here. 55 00:03:13,560 --> 00:03:15,300 And then we can calculate it out. 56 00:03:15,300 --> 00:03:21,360 And delta G0 is plus 36.1 kilojoules per mole 57 00:03:21,360 --> 00:03:25,080 That is not a spontaneous reaction. 58 00:03:25,080 --> 00:03:27,670 So our bread is not going to rise, 59 00:03:27,670 --> 00:03:32,730 and our baking will be failed, except that we're probably 60 00:03:32,730 --> 00:03:37,740 not going to be baking it in the oven at room temperature. 61 00:03:37,740 --> 00:03:43,350 So this would be non-spontaneous but when we bake, 62 00:03:43,350 --> 00:03:50,610 we're going to heat the oven, and so usually 350 63 00:03:50,610 --> 00:03:55,390 or something, which is 450 Kelvin. 64 00:03:55,390 --> 00:03:58,810 So now we can do the equation again. 65 00:03:58,810 --> 00:04:04,540 We plug-in our delta H, our new temperature, and our delta S, 66 00:04:04,540 --> 00:04:07,520 and we get a negative value. 67 00:04:07,520 --> 00:04:11,200 Delta G0 is minus 14.7 kilojoules 68 00:04:11,200 --> 00:04:16,010 per mole at 450 degrees Kelvin. 69 00:04:16,010 --> 00:04:18,610 So this would be spontaneous. 70 00:04:18,610 --> 00:04:21,050 So when you're baking, you want to remember 71 00:04:21,050 --> 00:04:24,300 to turn your oven on, heat it up, and put 72 00:04:24,300 --> 00:04:27,330 in your baking soda. 73 00:04:27,330 --> 00:04:32,130 So let's think about now this type of reaction 74 00:04:32,130 --> 00:04:37,790 that has a positive delta H and a positive delta S. 75 00:04:37,790 --> 00:04:40,930 So these both have the same sign. 76 00:04:40,930 --> 00:04:44,510 And if both delta H and delta S have the same sign, 77 00:04:44,510 --> 00:04:47,010 temperature can be used to control spontaneity. 78 00:04:47,010 --> 00:04:49,420 It will be non-spontaneous at one temperature 79 00:04:49,420 --> 00:04:52,500 but spontaneous at another temperature. 80 00:04:52,500 --> 00:04:55,930 And if we assume delta H0 and delta 81 00:04:55,930 --> 00:04:58,460 S0 are independent of temperature, which 82 00:04:58,460 --> 00:05:01,420 is fine to do-- that's a good assumption-- then 83 00:05:01,420 --> 00:05:05,790 delta G0, which is definitely dependent on temperature, 84 00:05:05,790 --> 00:05:10,780 is linear-- is a linear function with temperature. 85 00:05:10,780 --> 00:05:15,060 So let's plot now those two values of delta 86 00:05:15,060 --> 00:05:16,640 G that we just calculated. 87 00:05:19,370 --> 00:05:20,330 So here is our plot. 88 00:05:20,330 --> 00:05:24,040 We have delta G0 kilojoules per mole on the y-axis 89 00:05:24,040 --> 00:05:26,510 and temperature on the x-axis. 90 00:05:26,510 --> 00:05:29,710 And let's put in the numbers that we had. 91 00:05:29,710 --> 00:05:33,760 So we had calculated at room temperature, around 300 92 00:05:33,760 --> 00:05:38,750 Kelvin, a value of about 26-- positive 26 kilojoules 93 00:05:38,750 --> 00:05:40,130 per mole. 94 00:05:40,130 --> 00:05:43,410 And we calculated then at about 450 95 00:05:43,410 --> 00:05:48,620 Kelvin a value about minus 14.7, almost 96 00:05:48,620 --> 00:05:51,050 minus 15 kilojoules per mole. 97 00:05:51,050 --> 00:05:52,720 And when you have two points, you 98 00:05:52,720 --> 00:05:55,910 can draw a beautiful straight line. 99 00:05:55,910 --> 00:05:58,180 When you have more, sometimes it's more complicated. 100 00:05:58,180 --> 00:05:59,680 But it is linear. 101 00:05:59,680 --> 00:06:03,520 Delta G0 is linear with temperature. 102 00:06:03,520 --> 00:06:06,430 So now let's think about this is a straight line, 103 00:06:06,430 --> 00:06:10,380 and we can think about an equation for a straight line. 104 00:06:10,380 --> 00:06:12,950 So the equation that we know and love, 105 00:06:12,950 --> 00:06:18,080 delta G0 equals delta H minus T delta S, can be now rearranged. 106 00:06:18,080 --> 00:06:20,970 We have delta G on the y-axis here. 107 00:06:20,970 --> 00:06:23,930 We have temperature is our x-axis. 108 00:06:23,930 --> 00:06:25,330 So now we've just rearranged. 109 00:06:25,330 --> 00:06:29,050 We've pulled minus delta S0 over here, 110 00:06:29,050 --> 00:06:31,320 and delta H0 are over here. 111 00:06:31,320 --> 00:06:34,950 So delta H is our y-intercept. 112 00:06:34,950 --> 00:06:38,320 And for some reason, your notes just say "y dash i." 113 00:06:38,320 --> 00:06:41,920 I don't know what happened to "intercept" part. 114 00:06:41,920 --> 00:06:46,420 But anyway, that's the y-intercept is delta H0. 115 00:06:46,420 --> 00:06:48,752 And what is the slope? 116 00:06:48,752 --> 00:06:50,452 AUDIENCE: [INAUDIBLE] 117 00:06:50,452 --> 00:06:51,160 INSTRUCTOR: Yeah. 118 00:06:51,160 --> 00:06:54,600 So the slope is negative delta S. 119 00:06:54,600 --> 00:06:57,630 So if you plotted delta G's versus temperature, 120 00:06:57,630 --> 00:07:00,410 you could get out your delta H, or you could get out 121 00:07:00,410 --> 00:07:03,080 from the slope your delta S. 122 00:07:03,080 --> 00:07:05,540 And let's think about these different parts of the plot. 123 00:07:08,370 --> 00:07:12,300 So over here delta G is greater than 0. 124 00:07:12,300 --> 00:07:14,210 It's of positive value. 125 00:07:14,210 --> 00:07:16,660 And what does that mean about the spontaneity 126 00:07:16,660 --> 00:07:19,294 of the reaction, if its delta G is positive? 127 00:07:19,294 --> 00:07:20,550 AUDIENCE: Non-spontaneous. 128 00:07:20,550 --> 00:07:24,080 INSTRUCTOR: Right, so that's non-spontaneous. 129 00:07:24,080 --> 00:07:31,300 Down here, we have delta G minus value less than zero, 130 00:07:31,300 --> 00:07:33,830 so there it's spontaneous. 131 00:07:33,830 --> 00:07:36,765 So at some temperatures, the reaction is non-spontaneous, 132 00:07:36,765 --> 00:07:40,720 and in other temperatures, the reaction is spontaneous. 133 00:07:40,720 --> 00:07:45,070 And there is a certain value of T, T star, 134 00:07:45,070 --> 00:07:48,740 that is the temperature at which it switches 135 00:07:48,740 --> 00:07:51,540 from non-spontaneous to spontaneous, 136 00:07:51,540 --> 00:07:53,130 or if you're decreasing temperature, 137 00:07:53,130 --> 00:07:56,330 spontaneous to non-spontaneous. 138 00:07:56,330 --> 00:07:58,380 And for a particular reaction, you 139 00:07:58,380 --> 00:08:00,570 can calculate what that temperature 140 00:08:00,570 --> 00:08:04,420 value is, what T star is. 141 00:08:04,420 --> 00:08:07,570 So let's do that. 142 00:08:07,570 --> 00:08:10,510 So we can calculate T star. 143 00:08:10,510 --> 00:08:12,840 And so again, this is the temperature at delta 144 00:08:12,840 --> 00:08:15,260 G equals 0 where you have that switch 145 00:08:15,260 --> 00:08:19,450 point between spontaneous and non-spontaneous. 146 00:08:19,450 --> 00:08:22,900 So we can set delta G0 equal to 0. 147 00:08:22,900 --> 00:08:26,700 And then we can solve for T star. 148 00:08:26,700 --> 00:08:32,159 So T star would just be delta H0 over delta S0, 149 00:08:32,159 --> 00:08:34,850 because this is set to 0. 150 00:08:34,850 --> 00:08:37,309 And we can plug these values in. 151 00:08:37,309 --> 00:08:44,320 So delta H, again, was plus 136 kilojoules per mole. 152 00:08:44,320 --> 00:08:53,330 And delta S0 was plus 0.334 kilojoules per Kelvin per mole. 153 00:08:53,330 --> 00:08:59,620 And we can calculate out a temperature of 406 Kelvin. 154 00:08:59,620 --> 00:09:01,760 So this is the temperature at which 155 00:09:01,760 --> 00:09:04,760 you have this switch from a spontaneous 156 00:09:04,760 --> 00:09:07,460 to a non-spontaneous process. 157 00:09:07,460 --> 00:09:12,670 So if you're cooking your bread below 406, 158 00:09:12,670 --> 00:09:15,170 you get something that looks like this. 159 00:09:15,170 --> 00:09:19,350 The first time-- my husband is the chef in our family. 160 00:09:19,350 --> 00:09:23,330 And the first time I was tasked with making cupcakes 161 00:09:23,330 --> 00:09:25,780 for my daughter's school, I forgot 162 00:09:25,780 --> 00:09:29,390 to put the baking soda in. 163 00:09:29,390 --> 00:09:31,740 But if you put enough frosting on it 164 00:09:31,740 --> 00:09:36,650 and the kids are four years old, it totally doesn't matter. 165 00:09:36,650 --> 00:09:39,720 Anyway, if I had put in the baking soda 166 00:09:39,720 --> 00:09:43,890 and cooked it at temperatures above 406 Kelvin, 167 00:09:43,890 --> 00:09:47,631 I would have had something that looked a whole lot better, 168 00:09:47,631 --> 00:09:49,630 and I wouldn't have even had to put frosting on. 169 00:09:49,630 --> 00:09:50,546 But, no, I would have. 170 00:09:50,546 --> 00:09:53,060 Four years old, you have to put frosting-- never mind. 171 00:09:53,060 --> 00:09:54,880 It all worked out. 172 00:09:54,880 --> 00:09:57,170 So there is this temperature at which 173 00:09:57,170 --> 00:10:01,840 you have a switch if the sign of delta H and the sign of delta S 174 00:10:01,840 --> 00:10:03,780 are the same. 175 00:10:03,780 --> 00:10:06,780 So now let's think about another case 176 00:10:06,780 --> 00:10:11,000 where you have delta H0 and delta 177 00:10:11,000 --> 00:10:14,090 S are both negative values. 178 00:10:14,090 --> 00:10:15,680 What would this plot look like? 179 00:10:15,680 --> 00:10:17,370 And here are some options for you. 180 00:10:30,188 --> 00:10:34,530 AUDIENCE: [CHATTER] 181 00:10:34,530 --> 00:10:38,356 INSTRUCTOR: Let's just do 10 more seconds. 182 00:10:38,356 --> 00:10:58,430 AUDIENCE: [CHATTER] 183 00:10:58,430 --> 00:11:01,010 INSTRUCTOR: 90%, excellent. 184 00:11:01,010 --> 00:11:03,310 So that is correct. 185 00:11:03,310 --> 00:11:08,110 So we can draw that now in your handout. 186 00:11:08,110 --> 00:11:11,300 So here, if they're both negative, 187 00:11:11,300 --> 00:11:18,110 down here we have delta S is a negative value, spontaneous. 188 00:11:18,110 --> 00:11:21,140 You'll have a negative delta G. And here 189 00:11:21,140 --> 00:11:25,680 you have a negative delta H minus T times a negative delta 190 00:11:25,680 --> 00:11:28,480 S. So this will be a positive term 191 00:11:28,480 --> 00:11:30,310 and this will be a negative term. 192 00:11:30,310 --> 00:11:33,470 And to be spontaneous, you want a negative overall. 193 00:11:33,470 --> 00:11:37,100 So at low temperatures, the unfavorable delta S 194 00:11:37,100 --> 00:11:38,060 is down-weighted. 195 00:11:38,060 --> 00:11:40,680 So at low temperatures, you're spontaneous. 196 00:11:40,680 --> 00:11:42,940 But as the temperature increases, 197 00:11:42,940 --> 00:11:45,790 you'll get to a magic temperature, a T star, 198 00:11:45,790 --> 00:11:49,710 in which this delta S term now becomes greater 199 00:11:49,710 --> 00:11:52,090 than the delta H term. 200 00:11:52,090 --> 00:11:55,220 This will be a big positive value compared to a smaller 201 00:11:55,220 --> 00:11:58,430 one, and you'll switch to a positive delta G 202 00:11:58,430 --> 00:12:01,420 and a non-spontaneous process. 203 00:12:01,420 --> 00:12:03,090 Let's consider all the options now. 204 00:12:06,830 --> 00:12:07,606 One more. 205 00:12:26,490 --> 00:12:27,332 10 more seconds. 206 00:12:27,332 --> 00:12:44,780 AUDIENCE: [CHATTER] 207 00:12:44,780 --> 00:12:46,421 INSTRUCTOR: Great. 208 00:12:46,421 --> 00:12:48,170 Yeah, I knew those other clicker questions 209 00:12:48,170 --> 00:12:51,310 were going to be deciding the winners, I feel like. 210 00:12:51,310 --> 00:12:54,300 So yes, this is always going to be spontaneous. 211 00:12:54,300 --> 00:12:58,070 So if we remember the equation-- and if you don't have it 212 00:12:58,070 --> 00:13:00,577 memorized yet, you will soon. 213 00:13:00,577 --> 00:13:02,410 Even though it will be on an equation sheet, 214 00:13:02,410 --> 00:13:03,500 most people don't need it. 215 00:13:03,500 --> 00:13:04,860 You use it so much. 216 00:13:04,860 --> 00:13:08,870 So when delta H0 is negative and this is positive, 217 00:13:08,870 --> 00:13:11,720 delta G is always going to be negative. 218 00:13:11,720 --> 00:13:14,680 So it'll always be spontaneous, and so 219 00:13:14,680 --> 00:13:18,410 when delta G0 will always be negative 220 00:13:18,410 --> 00:13:20,397 at every single temperature. 221 00:13:23,080 --> 00:13:25,770 So now let's think about this one. 222 00:13:25,770 --> 00:13:32,030 Positive delta H, negative delta S-- what will this be? 223 00:13:32,030 --> 00:13:33,490 You can just yell it out. 224 00:13:33,490 --> 00:13:34,200 AUDIENCE: Never. 225 00:13:34,200 --> 00:13:36,870 AUDIENCE: Never spontaneous, right. 226 00:13:36,870 --> 00:13:43,290 So positive, and then here with another positive, a minus, 227 00:13:43,290 --> 00:13:46,460 a minus, another positive-- it'll never be spontaneous. 228 00:13:46,460 --> 00:13:47,990 How sad for it. 229 00:13:47,990 --> 00:13:52,080 So delta G will always be positive at all temperatures. 230 00:13:52,080 --> 00:13:55,740 So here are cases where delta H and delta S 231 00:13:55,740 --> 00:14:00,120 have different signs. 232 00:14:00,120 --> 00:14:02,870 But now we have cases where they're both positive 233 00:14:02,870 --> 00:14:04,850 or they're both negative. 234 00:14:04,850 --> 00:14:09,000 So for the both positive case, this 235 00:14:09,000 --> 00:14:14,240 will be sometimes spontaneous. 236 00:14:14,240 --> 00:14:19,760 It will depend on T. So when will delta G 237 00:14:19,760 --> 00:14:23,200 be negative and, therefore, the reaction be spontaneous, 238 00:14:23,200 --> 00:14:27,296 when T is greater or smaller than T star? 239 00:14:27,296 --> 00:14:28,210 AUDIENCE: Greater. 240 00:14:28,210 --> 00:14:30,080 INSTRUCTOR: Greater, right. 241 00:14:30,080 --> 00:14:34,360 So here, we have a positive delta H endothermic reaction. 242 00:14:34,360 --> 00:14:39,650 We have a positive delta S. And so when T is big here, 243 00:14:39,650 --> 00:14:41,510 this term will dominate. 244 00:14:41,510 --> 00:14:44,320 And you can still get a negative delta G0, so 245 00:14:44,320 --> 00:14:47,500 when temperatures are above that magic temperature. 246 00:14:47,500 --> 00:14:51,990 And then our last case, when we have a negative delta 247 00:14:51,990 --> 00:14:57,350 H, an exothermic reaction, and a negative delta S0-- 248 00:14:57,350 --> 00:14:59,960 and again, that will depend on temperature. 249 00:14:59,960 --> 00:15:02,060 So when they have the same signs, 250 00:15:02,060 --> 00:15:03,910 temperature makes a difference. 251 00:15:03,910 --> 00:15:08,430 And here, you will have a negative delta G 252 00:15:08,430 --> 00:15:13,160 when you have a smaller temperature, because here you 253 00:15:13,160 --> 00:15:14,770 want the delta H term. 254 00:15:14,770 --> 00:15:16,460 That's a negative term. 255 00:15:16,460 --> 00:15:18,870 And you want that to be the bigger term, 256 00:15:18,870 --> 00:15:21,760 and so you want to have smaller temperatures that 257 00:15:21,760 --> 00:15:31,680 will down-weight the negative delta S0 over here. 258 00:15:31,680 --> 00:15:35,340 So these are our four possibilities-- always 259 00:15:35,340 --> 00:15:39,110 spontaneous; never spontaneous; and then sometimes spontaneous, 260 00:15:39,110 --> 00:15:43,330 and it depends on the temperature compared to T star. 261 00:15:43,330 --> 00:15:44,880 And T star, again, is the temperature 262 00:15:44,880 --> 00:15:47,280 at which you switch between spontaneous 263 00:15:47,280 --> 00:15:49,100 and non-spontaneous. 264 00:15:49,100 --> 00:15:51,580 So temperature is important. 265 00:15:51,580 --> 00:15:54,420 Temperature is important to thermodynamics, 266 00:15:54,420 --> 00:15:56,800 and temperature is also important to kinetics. 267 00:15:56,800 --> 00:15:59,870 Most of the time, you can speed up a reaction, or at least 268 00:15:59,870 --> 00:16:05,430 an elementary reaction, when you increase the temperature. 269 00:16:05,430 --> 00:16:07,610 So now, let's think about thermodynamics 270 00:16:07,610 --> 00:16:14,180 in biological systems, and think about a very important 271 00:16:14,180 --> 00:16:16,840 interaction in biological systems which 272 00:16:16,840 --> 00:16:19,510 is hydrogen bonding. 273 00:16:19,510 --> 00:16:23,650 So this is also great review for the exam on Monday. 274 00:16:23,650 --> 00:16:26,340 So hydrogen bonding are interactions 275 00:16:26,340 --> 00:16:29,080 between hydrogen bond donors. 276 00:16:29,080 --> 00:16:31,140 So what is a hydrogen bond donor? 277 00:16:31,140 --> 00:16:35,370 A hydrogen bond donor is a hydrogen in a polar bond. 278 00:16:35,370 --> 00:16:39,850 So this is why it's good review for exam 2 because for exam 2 279 00:16:39,850 --> 00:16:43,040 you should be able to identify polar and non-polar bonds. 280 00:16:43,040 --> 00:16:46,590 So a hydrogen bond donor is a hydrogen in a polar bond, 281 00:16:46,590 --> 00:16:51,290 and a hydrogen bond acceptor is an electronegative atom 282 00:16:51,290 --> 00:16:52,760 with a lone pair. 283 00:16:52,760 --> 00:16:56,620 And electronegativity is also a topic on exam 2, 284 00:16:56,620 --> 00:16:58,817 so might as well learn it for Monday 285 00:16:58,817 --> 00:17:00,900 and then continue to learn it, because you'll also 286 00:17:00,900 --> 00:17:04,250 need it for thermodynamics and later on as well. 287 00:17:04,250 --> 00:17:09,780 So here we have a bond between something X and a hydrogen, 288 00:17:09,780 --> 00:17:13,579 and here we have y that's an electronegative atom 289 00:17:13,579 --> 00:17:14,710 with a lone pair. 290 00:17:14,710 --> 00:17:16,710 And this little, squiggly plus thing 291 00:17:16,710 --> 00:17:22,300 here indicates that it has a partial positive charge. 292 00:17:22,300 --> 00:17:24,329 And here you have a negative charge, 293 00:17:24,329 --> 00:17:27,410 and that's going to make for a nice favorable interaction, 294 00:17:27,410 --> 00:17:29,620 this hydrogen bond. 295 00:17:29,620 --> 00:17:34,440 So X is something that will lead to a polar bond, 296 00:17:34,440 --> 00:17:36,820 such as nitrogen, oxygen or fluorine, 297 00:17:36,820 --> 00:17:38,690 nitrogen and oxygen being the most 298 00:17:38,690 --> 00:17:40,580 relevant to biological systems. 299 00:17:40,580 --> 00:17:43,540 So if you have N or O here, that's a polar bond. 300 00:17:43,540 --> 00:17:47,090 And so that will then form a hydrogen bond 301 00:17:47,090 --> 00:17:50,180 with an electronegative atom that has a lone pair. 302 00:17:50,180 --> 00:17:53,870 And examples of that are also nitrogen, oxygen, fluorine, 303 00:17:53,870 --> 00:17:56,680 and nitrogen and oxygen, again, being the most relevant 304 00:17:56,680 --> 00:17:58,770 for biological systems. 305 00:17:58,770 --> 00:18:02,210 So we have this interaction because these sort 306 00:18:02,210 --> 00:18:06,110 of partial charges here that form this nice hydrogen bonding 307 00:18:06,110 --> 00:18:08,380 interaction. 308 00:18:08,380 --> 00:18:11,840 So if we look at one of the most important molecules 309 00:18:11,840 --> 00:18:15,350 that hydrogen bonds which is water, 310 00:18:15,350 --> 00:18:19,760 we see that water has polar bonds between the oxygen 311 00:18:19,760 --> 00:18:21,710 and the hydrogen, because those have 312 00:18:21,710 --> 00:18:25,100 electronegativity difference of greater than 0.4, 313 00:18:25,100 --> 00:18:27,370 and it has two lone pairs. 314 00:18:27,370 --> 00:18:30,700 So water is capable of being a hydrogen bond 315 00:18:30,700 --> 00:18:36,210 donor with its hydrogen and also a hydrogen bond acceptor. 316 00:18:36,210 --> 00:18:39,750 And so if we draw the hydrogen bonds as dotted lines here, 317 00:18:39,750 --> 00:18:44,220 we see this OH polar bond is a hydrogen bond donor, 318 00:18:44,220 --> 00:18:46,840 and this oxygen here with its lone pair 319 00:18:46,840 --> 00:18:48,780 is the hydrogen bond acceptor. 320 00:18:48,780 --> 00:18:52,700 So here we have this network of hydrogen bonding interactions, 321 00:18:52,700 --> 00:18:55,010 both on the board and on my t-shirt. 322 00:18:55,010 --> 00:18:58,570 I also have water-hydrogen bonding on my t-shirt today. 323 00:18:58,570 --> 00:19:02,420 So these hydrogen bonds in water are really important for life. 324 00:19:02,420 --> 00:19:04,590 This is a very, very important property. 325 00:19:04,590 --> 00:19:13,683 Now just for exam review, what is the shape of that molecule? 326 00:19:13,683 --> 00:19:14,442 AUDIENCE: Bent. 327 00:19:14,442 --> 00:19:16,816 INSTRUCTOR: What angle do we expect between the hydrogen, 328 00:19:16,816 --> 00:19:18,565 oxygen, and the nitrogen? 329 00:19:18,565 --> 00:19:19,440 AUDIENCE: [INAUDIBLE] 330 00:19:19,440 --> 00:19:21,520 INSTRUCTOR: Less than 109.5. 331 00:19:21,520 --> 00:19:22,340 That's right. 332 00:19:22,340 --> 00:19:25,770 What would be the SN number? 333 00:19:25,770 --> 00:19:26,270 AUDIENCE: 4. 334 00:19:26,270 --> 00:19:29,321 INSTRUCTOR: 4, Yes. 335 00:19:29,321 --> 00:19:29,821 Good! 336 00:19:33,760 --> 00:19:36,740 If those answers seem like "I have no idea what everyone's 337 00:19:36,740 --> 00:19:38,917 talking about," you know what your weekend 338 00:19:38,917 --> 00:19:39,750 is going to involve. 339 00:19:39,750 --> 00:19:41,833 Those are things that are going to be on the exam. 340 00:19:41,833 --> 00:19:45,250 VSEPR, yes. 341 00:19:45,250 --> 00:19:52,420 So let's compare now hydrogen bonds with covalent bonds. 342 00:19:52,420 --> 00:19:56,590 So here we have a hydrogen bond donor and acceptor 343 00:19:56,590 --> 00:19:58,610 versus a covalent interaction. 344 00:19:58,610 --> 00:20:01,520 So a covalent interaction where you have a bond, 345 00:20:01,520 --> 00:20:03,660 where you have bonding electrons are 346 00:20:03,660 --> 00:20:05,640 being shared between the two. 347 00:20:05,640 --> 00:20:09,950 So covalent bonds are stronger than hydrogen bonds, for sure. 348 00:20:09,950 --> 00:20:11,780 And let's look at some examples. 349 00:20:11,780 --> 00:20:15,170 So if we consider this X as an oxygen 350 00:20:15,170 --> 00:20:19,880 here, in a oxygen hydrogen polar bond interacting 351 00:20:19,880 --> 00:20:23,850 with another oxygen, the value for that hydrogen bond 352 00:20:23,850 --> 00:20:26,260 would be about 20 kilojoules per mole 353 00:20:26,260 --> 00:20:30,670 as opposed to the covalent bond, here just the hydrogen 354 00:20:30,670 --> 00:20:35,730 and oxygen, has 463 kilojoules per mole. 355 00:20:35,730 --> 00:20:37,830 So the hydrogen bond is considerably weaker 356 00:20:37,830 --> 00:20:40,150 than a covalent bond, but it's still-- 357 00:20:40,150 --> 00:20:42,410 weak bonds turn out to be really important 358 00:20:42,410 --> 00:20:44,550 in biological systems. 359 00:20:44,550 --> 00:20:46,030 Now let's consider a case where we 360 00:20:46,030 --> 00:20:48,320 have nitrogen as the acceptor. 361 00:20:48,320 --> 00:20:50,900 So we're going to have a nitrogen acceptor here. 362 00:20:50,900 --> 00:20:55,780 So if this is OH donating to N, then we 363 00:20:55,780 --> 00:20:58,770 have 29 kilojoules per mole. 364 00:20:58,770 --> 00:21:05,370 NH interacting with N14, compare to a covalent bond 365 00:21:05,370 --> 00:21:09,020 between H and N of 388. 366 00:21:09,020 --> 00:21:10,940 So when you compare to covalent bonds, 367 00:21:10,940 --> 00:21:14,520 hydrogen bonds are much less, but they are still 368 00:21:14,520 --> 00:21:18,470 super important. 369 00:21:18,470 --> 00:21:25,570 So for bonds that are made up between molecules here-- 370 00:21:25,570 --> 00:21:32,110 intermolecular, between molecules-- hydrogen bonds 371 00:21:32,110 --> 00:21:34,450 are the strongest kind of interactions that 372 00:21:34,450 --> 00:21:36,970 are between molecules here. 373 00:21:36,970 --> 00:21:39,680 So hydrogen bonds can be between molecules. 374 00:21:39,680 --> 00:21:42,540 They can also be made within a molecule. 375 00:21:42,540 --> 00:21:44,590 So this just shows hydrogen bonding 376 00:21:44,590 --> 00:21:47,850 in a protein structure between these 377 00:21:47,850 --> 00:21:49,940 are called beta strands here. 378 00:21:49,940 --> 00:21:52,170 And hydrogen bonding is incredibly 379 00:21:52,170 --> 00:21:55,190 important in forming protein structure-- really, really 380 00:21:55,190 --> 00:21:56,810 important. 381 00:21:56,810 --> 00:22:01,460 So hydrogen bonds are responsible for protein 382 00:22:01,460 --> 00:22:04,180 folding-- very important in proteins. 383 00:22:04,180 --> 00:22:09,640 Hydrogen bonds are also really important for DNA. 384 00:22:09,640 --> 00:22:14,720 So all of the main in RNA, all of our macromolecular molecules 385 00:22:14,720 --> 00:22:17,990 of life, hydrogen bonding is really important. 386 00:22:17,990 --> 00:22:24,260 So let's look at a GC base pair that forms in DNA. 387 00:22:24,260 --> 00:22:26,920 And we can think about the hydrogen bonds here. 388 00:22:26,920 --> 00:22:28,840 So here are three of them. 389 00:22:28,840 --> 00:22:32,820 So we have this polar bond between N and H. 390 00:22:32,820 --> 00:22:35,010 And it's the hydrogen bond donor, 391 00:22:35,010 --> 00:22:38,710 and we have the lone pair on oxygen accepting that hydrogen 392 00:22:38,710 --> 00:22:39,570 bond. 393 00:22:39,570 --> 00:22:43,680 Here we have a polar bond between nitrogen and hydrogen 394 00:22:43,680 --> 00:22:47,490 donating over here to a nitrogen lone pair. 395 00:22:47,490 --> 00:22:52,760 And here we have an NH polar bond as a hydrogen bond donor 396 00:22:52,760 --> 00:22:57,530 to the lone pair on this oxygen. And this hydrogen bonding 397 00:22:57,530 --> 00:22:59,710 pattern is what allows DNA to have 398 00:22:59,710 --> 00:23:03,900 its beautiful double helix, and is very, very important. 399 00:23:03,900 --> 00:23:06,720 So why don't you give it a try and tell me 400 00:23:06,720 --> 00:23:10,070 how many hydrogen bonds you would get between these guys 401 00:23:10,070 --> 00:23:11,116 here. 402 00:23:11,116 --> 00:23:12,740 And I'll leave this structure up so you 403 00:23:12,740 --> 00:23:14,113 can see what it looks like. 404 00:23:23,300 --> 00:23:24,130 10 seconds. 405 00:23:24,130 --> 00:23:50,130 AUDIENCE: [CHATTER] 406 00:23:50,130 --> 00:23:51,910 INSTRUCTOR: The answer is 2. 407 00:23:51,910 --> 00:23:53,674 Let's take a look at that over here. 408 00:23:59,200 --> 00:24:03,040 So you have a nitrogen polar bond 409 00:24:03,040 --> 00:24:06,810 over here making an interaction with this lone pair. 410 00:24:06,810 --> 00:24:10,540 You have a polar NH pond here, hydrogen donor 411 00:24:10,540 --> 00:24:12,120 to this lone pair. 412 00:24:12,120 --> 00:24:16,230 Why is that not a hydrogen bond? 413 00:24:16,230 --> 00:24:18,410 Because it is carbon hydrogen. 414 00:24:18,410 --> 00:24:21,250 So carbon hydrogen, the electronegativity difference 415 00:24:21,250 --> 00:24:25,470 is not greater than 0.4, so carbon hydrogen is not 416 00:24:25,470 --> 00:24:26,410 a polar bond. 417 00:24:26,410 --> 00:24:29,350 So you need to have both a polar bond, 418 00:24:29,350 --> 00:24:32,020 and you need to have an electronegative atom that 419 00:24:32,020 --> 00:24:34,800 has lone pairs to be the hydrogen bond acceptor. 420 00:24:34,800 --> 00:24:37,480 So here you have a hydrogen bond acceptor, 421 00:24:37,480 --> 00:24:40,230 but you don't have a hydrogen bond donor. 422 00:24:40,230 --> 00:24:42,630 And so this actually turns out to be important 423 00:24:42,630 --> 00:24:45,900 because you want to specifically recognize 424 00:24:45,900 --> 00:24:49,290 one base with another base, and so the hydrogen bonding 425 00:24:49,290 --> 00:24:53,260 pattern is essential for that working out. 426 00:24:53,260 --> 00:24:58,380 So here, these are hydrogen bonds. 427 00:24:58,380 --> 00:25:00,455 I said they're weaker than covalent bonds, 428 00:25:00,455 --> 00:25:01,830 but they're strong enough to help 429 00:25:01,830 --> 00:25:03,900 stabilize the structure of DNA. 430 00:25:03,900 --> 00:25:08,480 But they're not so strong that DNA cannot unzip. 431 00:25:08,480 --> 00:25:11,360 And you need to unzip DNA to read it, 432 00:25:11,360 --> 00:25:14,040 and by reading, that's essential to make another copy, 433 00:25:14,040 --> 00:25:18,820 to have cell division, or to translate your genetic code. 434 00:25:18,820 --> 00:25:21,110 So that's, I think, why hydrogen bonds are 435 00:25:21,110 --> 00:25:23,430 so important in biology, because you don't want 436 00:25:23,430 --> 00:25:24,890 a lot of super strong bonds. 437 00:25:24,890 --> 00:25:28,080 You want weaker interactions because in biology things 438 00:25:28,080 --> 00:25:29,570 are moving around. 439 00:25:29,570 --> 00:25:33,640 So let's talk about the importance of hydrogen bonding. 440 00:25:33,640 --> 00:25:39,132 And for this we have another In Their Own Words segment. 441 00:25:39,132 --> 00:25:39,798 [VIDEO PLAYBACK] 442 00:25:39,798 --> 00:25:42,278 - My name is Lourdes Aleman, and my research 443 00:25:42,278 --> 00:25:46,250 is on RNA interference or RNAi. 444 00:25:46,250 --> 00:25:50,380 RNA interference is simply a silencing mechanism 445 00:25:50,380 --> 00:25:57,130 the cells use to turn down the expression of a gene. 446 00:25:57,130 --> 00:26:01,500 Double-stranded RNA pieces have some sort of complementarity 447 00:26:01,500 --> 00:26:04,680 to a sequence within the genome. 448 00:26:04,680 --> 00:26:08,790 That double-stranded RNA piece binds to a big large protein 449 00:26:08,790 --> 00:26:14,020 complex, it unwinds the double stranded piece, 450 00:26:14,020 --> 00:26:18,300 takes one of those strands only, and finds the gene 451 00:26:18,300 --> 00:26:21,140 that it complements with. 452 00:26:21,140 --> 00:26:25,520 And once it does, it binds to that particular RNA 453 00:26:25,520 --> 00:26:28,495 from that gene and it destroys it 454 00:26:28,495 --> 00:26:33,280 and not allowing protein to be made from that RNA. 455 00:26:33,280 --> 00:26:41,020 DNA and RNA both can form base pairs by hydrogen bonding. 456 00:26:41,020 --> 00:26:45,380 The short piece of RNA that is found in this protein 457 00:26:45,380 --> 00:26:49,710 complex guides that RNA and basically finds 458 00:26:49,710 --> 00:26:51,540 its match by hydrogen bonding. 459 00:26:51,540 --> 00:26:54,760 So if it forms a hydrogen bond along the whole entire 460 00:26:54,760 --> 00:26:57,960 sequence, in knows it has found a match somewhere 461 00:26:57,960 --> 00:26:59,020 in the sequence. 462 00:26:59,020 --> 00:27:02,830 And that's how it recognizes the gene that its targeting. 463 00:27:02,830 --> 00:27:06,950 Macular degeneration is a disease of the retina. 464 00:27:06,950 --> 00:27:09,070 There are too many blood vessels in the retina, 465 00:27:09,070 --> 00:27:12,580 and they can bleed and scar over time, 466 00:27:12,580 --> 00:27:17,060 and eventually these patients can become blind. 467 00:27:17,060 --> 00:27:19,400 Some patients with this disease, one 468 00:27:19,400 --> 00:27:22,280 of the reasons they have an outgrowth in the retina 469 00:27:22,280 --> 00:27:26,950 is because there is a gene called the VEGF that there's 470 00:27:26,950 --> 00:27:28,040 too much of. 471 00:27:28,040 --> 00:27:32,560 And VEGF tells cells, "Make blood vessels." 472 00:27:32,560 --> 00:27:35,270 RNA interference is being used to silence 473 00:27:35,270 --> 00:27:38,820 the expression of this gene so that in patients with macular 474 00:27:38,820 --> 00:27:42,450 degeneration, you don't get further growth of more blood 475 00:27:42,450 --> 00:27:47,310 vessels and more bleeding and scarring as a consequence. 476 00:27:47,310 --> 00:27:49,990 My dream for RNAi would be that as a patient 477 00:27:49,990 --> 00:27:52,330 you will go into the doctor if you were diagnosed 478 00:27:52,330 --> 00:27:54,090 with some sort of disease. 479 00:27:54,090 --> 00:27:56,150 The doctor would go into the computer, 480 00:27:56,150 --> 00:27:59,680 order you a some double-stranded RNA for the particular gene 481 00:27:59,680 --> 00:28:04,030 that has been mutated or is malfunctioning in your disease, 482 00:28:04,030 --> 00:28:06,470 and you would then come back, and they 483 00:28:06,470 --> 00:28:08,670 would put that double-stranded RNA into you, 484 00:28:08,670 --> 00:28:09,670 and you will get better. 485 00:28:09,670 --> 00:28:11,211 That would be my dream, that it could 486 00:28:11,211 --> 00:28:15,235 be applicable to pretty much any disease or viral infection 487 00:28:15,235 --> 00:28:16,710 that you can think of. 488 00:28:16,710 --> 00:28:20,430 How many of you have heard of like RNAi or treatments 489 00:28:20,430 --> 00:28:22,390 and things and sort of-- a number of people-- 490 00:28:22,390 --> 00:28:24,160 like customized medicine. 491 00:28:24,160 --> 00:28:28,570 I mean, I think that that would be such an incredible thing 492 00:28:28,570 --> 00:28:34,340 if this works out, to really be able to treat every individual 493 00:28:34,340 --> 00:28:36,560 based on their DNA. 494 00:28:36,560 --> 00:28:38,230 So we're not there yet, but there's 495 00:28:38,230 --> 00:28:43,360 a lot of people who are working on this. 496 00:28:43,360 --> 00:28:48,490 And there's a lot being done at MIT research in RNA, 497 00:28:48,490 --> 00:28:52,270 and that particular work was in Phil Sharp laboratory. 498 00:28:52,270 --> 00:28:56,990 Phil Sharp is one of our Nobel laureates here at MIT. 499 00:28:56,990 --> 00:28:59,730 So that's why hydrogen bonding is important. 500 00:28:59,730 --> 00:29:03,280 So let's just do one more example of thermodynamics 501 00:29:03,280 --> 00:29:05,090 in biological systems. 502 00:29:05,090 --> 00:29:07,900 And we're back to thinking about ATP, 503 00:29:07,900 --> 00:29:13,920 which we talked about already, and the hydrolysis of ATP. 504 00:29:13,920 --> 00:29:18,260 So we saw that this is a spontaneous reaction before, 505 00:29:18,260 --> 00:29:20,750 and this can be what's called "coupled" 506 00:29:20,750 --> 00:29:24,430 to a spontaneous process to drive 507 00:29:24,430 --> 00:29:28,150 that non-spontaneous reaction. 508 00:29:28,150 --> 00:29:32,430 So the total change in free energy of a coupled reaction is 509 00:29:32,430 --> 00:29:36,160 the sum of the individual delta G's. 510 00:29:36,160 --> 00:29:39,530 So if you have one that is unfavorable and one that's 511 00:29:39,530 --> 00:29:42,280 favorable, one that's positive and one that's negative, 512 00:29:42,280 --> 00:29:46,020 you sum that up and if it's overall negative, 513 00:29:46,020 --> 00:29:49,750 then it will become a spontaneous coupled reaction. 514 00:29:49,750 --> 00:29:52,470 So let's look at this example again. 515 00:29:52,470 --> 00:29:58,970 So we have delta G 0 for ATP at 310 Kelvin. 516 00:29:58,970 --> 00:30:00,630 Why do you think I'm using 310? 517 00:30:00,630 --> 00:30:02,280 What do you think that temperature is? 518 00:30:02,280 --> 00:30:02,700 AUDIENCE: Body temperature. 519 00:30:02,700 --> 00:30:04,490 INSTRUCTOR: Body temperature. 520 00:30:04,490 --> 00:30:08,450 So we have ATP, so you have triphosphates-- 521 00:30:08,450 --> 00:30:11,450 that's the TP-- and hydrolysis. 522 00:30:11,450 --> 00:30:13,070 You're losing one of the phosphates. 523 00:30:13,070 --> 00:30:15,100 You're going to a diphosphate. 524 00:30:15,100 --> 00:30:20,530 And hydrolysis means a cleavage reaction that involves water. 525 00:30:20,530 --> 00:30:24,440 So we saw before that the delta H0 for this is negative. 526 00:30:24,440 --> 00:30:28,650 It's an exothermic reaction minus 24 kilojoules per mole. 527 00:30:28,650 --> 00:30:34,810 Delta S0 is plus 22 joules per Kelvin per mole. 528 00:30:34,810 --> 00:30:38,710 And so if we plug this into our delta G equation, 529 00:30:38,710 --> 00:30:43,260 we have delta H minus T delta S. At body temperature, 530 00:30:43,260 --> 00:30:47,360 this is a negative value, negative 31 kilojoules 531 00:30:47,360 --> 00:30:48,270 per mole. 532 00:30:48,270 --> 00:30:50,210 So this is spontaneous. 533 00:30:50,210 --> 00:30:52,980 The hydrolysis of ATP is spontaneous. 534 00:30:52,980 --> 00:30:54,730 Now we want to couple this to something 535 00:30:54,730 --> 00:30:56,617 that's non-spontaneous. 536 00:30:56,617 --> 00:30:58,200 And the reaction we're going to couple 537 00:30:58,200 --> 00:31:02,280 to that's non-spontaneous is the addition of a phosphate group 538 00:31:02,280 --> 00:31:03,700 to glucose. 539 00:31:03,700 --> 00:31:06,290 And this keeps the glucose in the cell, 540 00:31:06,290 --> 00:31:11,140 because things that have charge can't come and leave 541 00:31:11,140 --> 00:31:12,590 the cell as readily. 542 00:31:12,590 --> 00:31:14,660 So nature often does this, its way 543 00:31:14,660 --> 00:31:16,250 of holding the things that it wants 544 00:31:16,250 --> 00:31:18,180 inside the cell inside the cell. 545 00:31:18,180 --> 00:31:21,350 But this is a non-spontaneous process. 546 00:31:21,350 --> 00:31:26,840 The delta G0 is plus 17 kilojoules per mole, 547 00:31:26,840 --> 00:31:31,680 but if we couple that to the hydrolysis of ATP, which 548 00:31:31,680 --> 00:31:38,460 is minus 31 kilojoules per mole, and we can add those together, 549 00:31:38,460 --> 00:31:45,520 so 17 minus 31, gives us minus 14 kilojoules per mole. 550 00:31:45,520 --> 00:31:51,410 And now this non-spontaneous reaction is driven forward. 551 00:31:51,410 --> 00:31:54,210 So we've taken something that wasn't favorable 552 00:31:54,210 --> 00:31:59,620 and made it favorable by coupling it to ATP hydrolysis. 553 00:31:59,620 --> 00:32:04,690 So if ATP hydrolysis is so favorable and it's spontaneous, 554 00:32:04,690 --> 00:32:07,130 why isn't it happening all the time, 555 00:32:07,130 --> 00:32:09,755 which would be really bad for us because we store 556 00:32:09,755 --> 00:32:12,520 our energy in the form of ATP, so we want 557 00:32:12,520 --> 00:32:14,510 to keep it in its good form? 558 00:32:14,510 --> 00:32:17,310 And the answer again is kinetics. 559 00:32:17,310 --> 00:32:18,900 It's a slow process. 560 00:32:18,900 --> 00:32:22,560 So ATP is inert enough that we can use it 561 00:32:22,560 --> 00:32:25,000 as an energy storage.