1 00:00:00,000 --> 00:00:01,030 2 00:00:01,030 --> 00:00:07,180 Let's say we had the reaction molecule A plus molecule B is 3 00:00:07,179 --> 00:00:12,259 in dynamic equilibrium with molecules C plus D. 4 00:00:12,259 --> 00:00:15,550 Which just means that the rate of the forward reaction is 5 00:00:15,550 --> 00:00:18,839 going at the same rate as the backward reaction, or the 6 00:00:18,839 --> 00:00:19,739 reverse reaction. 7 00:00:19,739 --> 00:00:23,509 There will be some equilibrium concentrations of A, B, C, and 8 00:00:23,510 --> 00:00:25,320 D, and we can figure out what the equilibrium 9 00:00:25,320 --> 00:00:26,580 constant is if we want. 10 00:00:26,579 --> 00:00:27,729 And I'll say it again. 11 00:00:27,730 --> 00:00:29,710 I've said it like four times so far. 12 00:00:29,710 --> 00:00:32,840 The fact that the forward reaction rate is the same as 13 00:00:32,840 --> 00:00:35,150 the backward reaction rate doesn't mean that all of the 14 00:00:35,149 --> 00:00:36,350 concentrations are the same. 15 00:00:36,350 --> 00:00:38,980 The concentrations themselves of each of the molecules could 16 00:00:38,979 --> 00:00:40,089 be very different. 17 00:00:40,090 --> 00:00:42,500 They're just not changing anymore because the forward 18 00:00:42,500 --> 00:00:43,960 and backward rates are the same. 19 00:00:43,960 --> 00:00:48,070 Now, given this, given that we're at equilibrium now, 20 00:00:48,070 --> 00:00:53,140 what's going to happen if I add more A to the system? 21 00:00:53,140 --> 00:00:56,100 So remember, it was in equilibrium. 22 00:00:56,100 --> 00:00:57,399 The concentrations were constant. 23 00:00:57,399 --> 00:01:00,409 But now all of a sudden, I'm adding more A to the system. 24 00:01:00,409 --> 00:01:04,750 So now, the odds of an A and a B particle, even though I'm 25 00:01:04,750 --> 00:01:08,519 not adding any more B molecule to the system, the odds are 26 00:01:08,519 --> 00:01:11,909 slightly higher that an A and a B are going to collide in 27 00:01:11,909 --> 00:01:15,579 just the right way, so the forward reaction is going to 28 00:01:15,579 --> 00:01:16,569 be more likely. 29 00:01:16,569 --> 00:01:19,584 So if we add more A to the system, you're going 30 00:01:19,584 --> 00:01:20,439 to have more A's. 31 00:01:20,439 --> 00:01:22,849 They're going to bump with more B's, so the B's are 32 00:01:22,849 --> 00:01:25,509 actually going to go down a little bit, right? 33 00:01:25,510 --> 00:01:28,250 Because more B's are going to be consumed. 34 00:01:28,250 --> 00:01:30,959 And even more important, the C's and D's are going to 35 00:01:30,959 --> 00:01:34,099 definitely be increased. 36 00:01:34,099 --> 00:01:36,359 And the way it would really happen, you would add more A. 37 00:01:36,359 --> 00:01:41,159 Those A's would bump into some more B's, and so this forward 38 00:01:41,159 --> 00:01:43,569 reaction, all of a sudden, it's rate would go faster than 39 00:01:43,569 --> 00:01:44,649 this backward reaction. 40 00:01:44,650 --> 00:01:46,540 So the reaction would go in that direction. 41 00:01:46,540 --> 00:01:48,590 Then you would have more C's and D's and maybe some of 42 00:01:48,590 --> 00:01:51,810 those are more likely to bump and go back in this direction. 43 00:01:51,810 --> 00:01:54,960 Eventually, you would reach a new equilibrium. 44 00:01:54,959 --> 00:01:58,329 But the bottom line is you'll be left with more A, a little 45 00:01:58,329 --> 00:02:01,810 bit less B because you didn't add more B. 46 00:02:01,810 --> 00:02:04,820 So more B's going to be used to consume with those extra 47 00:02:04,819 --> 00:02:05,959 A's you just added. 48 00:02:05,959 --> 00:02:08,579 And then those are going to produce more C's and D's in 49 00:02:08,580 --> 00:02:09,539 equilibrium. 50 00:02:09,538 --> 00:02:13,439 And you can imagine, if you added more A and more B, let's 51 00:02:13,439 --> 00:02:19,389 say if you added more B as well, then the reaction is 52 00:02:19,389 --> 00:02:24,449 going to go in the forward direction even more. 53 00:02:24,449 --> 00:02:27,349 I don't think this is an amazing insight of the world. 54 00:02:27,349 --> 00:02:30,439 I think this is kind of obvious, that if you stress 55 00:02:30,439 --> 00:02:34,909 this reaction by adding more on this side, that naturally 56 00:02:34,909 --> 00:02:37,169 it's going to move in the direction that 57 00:02:37,169 --> 00:02:38,609 relieves the stress. 58 00:02:38,610 --> 00:02:41,010 So if you add more A, you're going to have more A's bumping 59 00:02:41,009 --> 00:02:43,259 with B's and go in that direction and maybe consume a 60 00:02:43,259 --> 00:02:44,139 little bit more B's. 61 00:02:44,139 --> 00:02:46,219 If you add more of both, the whole thing's going to go in 62 00:02:46,219 --> 00:02:46,819 that direction. 63 00:02:46,819 --> 00:02:50,729 Likewise-- let me rewrite the reaction. 64 00:02:50,729 --> 00:02:52,609 I'll do it in a different color. 65 00:02:52,610 --> 00:02:57,650 A plus B, dynamic equilibrium, C plus D. 66 00:02:57,650 --> 00:03:04,379 If I add more C-- I think you get the point here-- what's 67 00:03:04,379 --> 00:03:05,419 going to happen? 68 00:03:05,419 --> 00:03:08,639 Well, that's going to drive A and B up, and it's maybe going 69 00:03:08,639 --> 00:03:11,989 to consume a little bit extra D. 70 00:03:11,990 --> 00:03:14,400 And then if you added more C and D, then, of course, it's 71 00:03:14,400 --> 00:03:17,939 going to produce a lot more A and B. 72 00:03:17,939 --> 00:03:22,919 And this idea, it seems pretty common sense, but there's a 73 00:03:22,919 --> 00:03:28,169 fancy name for it, and it's called Le-- let me put a 74 00:03:28,169 --> 00:03:31,314 capital L-- Chatelier's principle. 75 00:03:31,314 --> 00:03:36,009 76 00:03:36,009 --> 00:03:37,909 If you've watched enough of these videos, you know I have 77 00:03:37,909 --> 00:03:42,620 to be careful with my spelling. 78 00:03:42,620 --> 00:03:46,539 And all it says is that when you stress a reaction that's 79 00:03:46,539 --> 00:03:51,329 in equilibrium, the reaction will favor the side or one 80 00:03:51,330 --> 00:03:54,270 side of the reaction to relieve that stress. 81 00:03:54,270 --> 00:03:56,290 When they say stress the reaction, that's like adding 82 00:03:56,289 --> 00:03:58,780 more A, so the reaction's going to move towards the 83 00:03:58,780 --> 00:04:01,490 forward direction to relieve the stress-- the quote, 84 00:04:01,490 --> 00:04:04,860 unquote stress-- of that more A. 85 00:04:04,860 --> 00:04:07,890 I mean, that's not stress in its traditional way of 86 00:04:07,889 --> 00:04:09,799 thinking about stress, but that is a kind of stress. 87 00:04:09,800 --> 00:04:12,140 You're somehow changing it relative to it. 88 00:04:12,139 --> 00:04:14,289 It was nice and comfortable before in a nice, stable 89 00:04:14,289 --> 00:04:15,179 environment. 90 00:04:15,180 --> 00:04:17,870 So given Le Chatelier's principle, let's think of some 91 00:04:17,870 --> 00:04:18,579 other situations. 92 00:04:18,579 --> 00:04:28,620 Let's say if I had A plus B plus some heat, and that 93 00:04:28,620 --> 00:04:35,490 produces some C plus D. 94 00:04:35,490 --> 00:04:39,030 And maybe it produces some E as well. 95 00:04:39,029 --> 00:04:41,199 So if I were to add heat to this 96 00:04:41,199 --> 00:04:42,849 system, what would happen? 97 00:04:42,850 --> 00:04:45,439 98 00:04:45,439 --> 00:04:48,870 So in order for the reaction to progress in the forward 99 00:04:48,870 --> 00:04:50,259 direction, you need heat. 100 00:04:50,259 --> 00:04:52,459 The more heat you have, the more likely you're going to 101 00:04:52,459 --> 00:04:54,269 progress in the forward direction. 102 00:04:54,269 --> 00:04:58,620 So Le Chatelier's principle will say we're stressing this 103 00:04:58,620 --> 00:05:02,540 reaction by adding heat, so the reaction will favor the 104 00:05:02,540 --> 00:05:05,870 direction that relieves that stressor. 105 00:05:05,870 --> 00:05:07,649 And so to relieve that stressor, you have more of 106 00:05:07,649 --> 00:05:11,250 this input, so you're going to consume more A. 107 00:05:11,250 --> 00:05:14,519 So the stable concentration of A once we reach equilibrium 108 00:05:14,519 --> 00:05:15,459 will go down. 109 00:05:15,459 --> 00:05:18,239 B will go down because they're going to be consumed more. 110 00:05:18,240 --> 00:05:20,420 The forward reaction is happening more. 111 00:05:20,420 --> 00:05:23,410 And then C, D, and E would go up. 112 00:05:23,410 --> 00:05:25,210 Now, if you did the opposite. 113 00:05:25,209 --> 00:05:28,279 Let me erase what I just did. 114 00:05:28,279 --> 00:05:36,000 Let's say instead of adding heat, you were 115 00:05:36,000 --> 00:05:37,790 to take away heat. 116 00:05:37,790 --> 00:05:40,346 So let's say you were to take away heat. 117 00:05:40,346 --> 00:05:43,079 Let me make sure my cursor's right. 118 00:05:43,079 --> 00:05:45,819 So if you took heat away from the reaction, 119 00:05:45,819 --> 00:05:47,269 what will be favored? 120 00:05:47,269 --> 00:05:49,240 Well, then you're going to be favoring it in the other 121 00:05:49,240 --> 00:05:53,590 direction because there'll be less heat here. 122 00:05:53,589 --> 00:05:54,659 I mean, all of this is together. 123 00:05:54,660 --> 00:05:58,090 There'll be less heat for this reaction to occur, so this 124 00:05:58,089 --> 00:06:01,299 rate will start dominating this rate over here, right? 125 00:06:01,300 --> 00:06:03,520 If you take away heat, the rate of this reaction will 126 00:06:03,519 --> 00:06:06,039 slow down, this one will be bigger, and so you'll have 127 00:06:06,040 --> 00:06:09,090 more movement of concentration in that direction, or the 128 00:06:09,089 --> 00:06:12,359 reverse reaction will be favored. 129 00:06:12,360 --> 00:06:15,879 Now, let's think of another stressor-- pressure. 130 00:06:15,879 --> 00:06:18,610 Now, imagine that we had-- we mentioned the Haber process 131 00:06:18,610 --> 00:06:21,410 before, and this is the reaction 132 00:06:21,410 --> 00:06:22,480 for the Haber process. 133 00:06:22,480 --> 00:06:31,590 Nitrogen gas plus 3 moles of hydrogen gas in equilibrium 134 00:06:31,589 --> 00:06:35,889 with 2 moles of ammonia gas. 135 00:06:35,889 --> 00:06:37,560 Now, what's going to happen if I apply 136 00:06:37,560 --> 00:06:40,139 pressure to this system? 137 00:06:40,139 --> 00:06:41,925 I'm going to apply pressure. 138 00:06:41,925 --> 00:06:46,720 139 00:06:46,720 --> 00:06:48,840 So if you think about what happens with pressure, 140 00:06:48,839 --> 00:06:53,129 everything all of a sudden is getting squeezed, although the 141 00:06:53,129 --> 00:06:55,370 volume isn't necessarily decreasing, but something is 142 00:06:55,370 --> 00:06:58,220 somehow making all the molecules want to be or 143 00:06:58,220 --> 00:07:00,410 forcing them to be closer together. 144 00:07:00,410 --> 00:07:04,439 Now, when things are getting closer together, the stress of 145 00:07:04,439 --> 00:07:07,449 the pressure could be relieved if we end 146 00:07:07,449 --> 00:07:08,889 up with fewer molecules. 147 00:07:08,889 --> 00:07:10,139 Think about it this way. 148 00:07:10,139 --> 00:07:12,379 149 00:07:12,379 --> 00:07:14,610 PV is equal to nRT. 150 00:07:14,610 --> 00:07:16,810 We learned this multiple times, right? 151 00:07:16,810 --> 00:07:23,959 And let's say we could write P is equal to nRT/V. 152 00:07:23,959 --> 00:07:31,299 Now, if we increase the pressure, how 153 00:07:31,300 --> 00:07:33,819 can we relieve that? 154 00:07:33,819 --> 00:07:37,599 Remember, Le Chatelier's principle says that whatever's 155 00:07:37,600 --> 00:07:40,140 going to happen is going to relieve the stressor. 156 00:07:40,139 --> 00:07:41,659 The reaction is going to go in the direction 157 00:07:41,660 --> 00:07:42,460 that it relieves it. 158 00:07:42,459 --> 00:07:46,409 Well, if we lower the number of molecules, then that will 159 00:07:46,410 --> 00:07:48,580 relieve the pressure, right? 160 00:07:48,579 --> 00:07:50,969 You'll have fewer things bouncing against each other. 161 00:07:50,970 --> 00:07:55,000 So if we lower the number of molecules where you can kind 162 00:07:55,000 --> 00:07:57,680 of view it-- I mean, I shouldn't have written it this 163 00:07:57,680 --> 00:08:00,000 way, because it's not quite an equation, but I want you to 164 00:08:00,000 --> 00:08:02,529 think of it that way. 165 00:08:02,529 --> 00:08:03,489 Let me erase this. 166 00:08:03,490 --> 00:08:05,280 This probably wasn't the best intuition. 167 00:08:05,279 --> 00:08:10,289 168 00:08:10,290 --> 00:08:14,610 If I have a container-- nope, too shocking. 169 00:08:14,610 --> 00:08:17,316 If I-- nope, same thing. 170 00:08:17,315 --> 00:08:22,709 If I have a container and I'm applying pressure to it, and 171 00:08:22,709 --> 00:08:29,219 in one option I could have 2 molecules-- let's say I could 172 00:08:29,220 --> 00:08:34,309 have 4 molecules in some volume. 173 00:08:34,308 --> 00:08:38,139 And in another situation, let's say they get merged and 174 00:08:38,139 --> 00:08:41,298 I only have 2 molecules, right? 175 00:08:41,298 --> 00:08:43,709 In either of these, the reaction can go between these, 176 00:08:43,710 --> 00:08:46,360 these 4 could merge to make 2 molecules. 177 00:08:46,360 --> 00:08:48,539 Actually, let me use this example up here. 178 00:08:48,539 --> 00:08:51,099 Let's say this nitrogen molecule is 179 00:08:51,100 --> 00:08:52,149 this blue one here. 180 00:08:52,149 --> 00:08:53,419 Actually, let me do it in a more different color. 181 00:08:53,419 --> 00:08:56,569 This brown one right here, it can merge with 3 hydrogen. 182 00:08:56,570 --> 00:08:58,980 It could produce this. 183 00:08:58,980 --> 00:09:01,750 So this is another way of writing this reaction, maybe 184 00:09:01,750 --> 00:09:02,440 in a more visual way. 185 00:09:02,440 --> 00:09:06,070 Now, if I'm applying pressure, if I'm applying pressure to 186 00:09:06,070 --> 00:09:10,490 this system, so pressure I just imagine is kind of more 187 00:09:10,490 --> 00:09:14,220 force per area from every direction, which of these 188 00:09:14,220 --> 00:09:18,879 situations is more likely to relieve the situation? 189 00:09:18,879 --> 00:09:21,389 Well, the situation where we have fewer molecules bumping 190 00:09:21,389 --> 00:09:25,720 around because it's easier to kind of apply or I guess 191 00:09:25,720 --> 00:09:28,590 squeeze them together than when you have more molecules 192 00:09:28,590 --> 00:09:29,540 bumping around. 193 00:09:29,539 --> 00:09:31,309 I'm doing this very hand wavy, but I think it 194 00:09:31,309 --> 00:09:32,169 gives you the intuition. 195 00:09:32,169 --> 00:09:35,409 So if you apply pressure to the system, if pressure goes 196 00:09:35,409 --> 00:09:39,120 up, you're applying-- this doesn't mean the 197 00:09:39,120 --> 00:09:39,759 pressure goes down. 198 00:09:39,759 --> 00:09:41,819 This means pressure is applying to the system. 199 00:09:41,820 --> 00:09:44,350 But the pressure is going up, what side of the reaction is 200 00:09:44,350 --> 00:09:45,029 going to be favored? 201 00:09:45,029 --> 00:09:48,189 The reaction's going to be favoring the side of that has 202 00:09:48,190 --> 00:09:49,600 fewer molecules. 203 00:09:49,600 --> 00:09:53,330 And this side has 2 molecules, although they'll be bigger 204 00:09:53,330 --> 00:09:55,430 molecules obviously, because it's not like we're losing 205 00:09:55,429 --> 00:09:57,889 mass in one direction or the other, as opposed to this 206 00:09:57,889 --> 00:09:59,830 situation where we have 4 molecules, right? 207 00:09:59,830 --> 00:10:05,830 1 mole of nitrogen gas and 3 moles of hydrogen. 208 00:10:05,830 --> 00:10:09,550 And just to bring this all back to the whole idea that we 209 00:10:09,549 --> 00:10:13,649 saw earlier with the kinetic equilibrium, let's just 210 00:10:13,649 --> 00:10:15,069 imagine a reaction like this. 211 00:10:15,070 --> 00:10:19,000 And to show that it works with Le Chatelier's principle is 212 00:10:19,000 --> 00:10:21,139 consistent with everything we've learned 213 00:10:21,139 --> 00:10:23,044 with equilibrium constants. 214 00:10:23,044 --> 00:10:27,959 So let's say we had the reaction 2 moles, or the 215 00:10:27,960 --> 00:10:32,850 coefficient of two, 2 A's in the gaseous form plus B in the 216 00:10:32,850 --> 00:10:38,860 gaseous form is in equilibrium with C in the gaseous form. 217 00:10:38,860 --> 00:10:43,169 And let's say initially where our first equilibrium, our 218 00:10:43,169 --> 00:10:49,679 concentration of A is 2 molar, or our molarity is 2, our 219 00:10:49,679 --> 00:10:55,879 concentration of B is 6 molar concentration, and then our 220 00:10:55,879 --> 00:11:00,470 concentration of C is 8 molar. 221 00:11:00,470 --> 00:11:02,971 So what's the equilibrium constant here? 222 00:11:02,971 --> 00:11:08,250 The equilibrium constant here is the product, concentration 223 00:11:08,250 --> 00:11:14,960 of C, that's 8 molar divided by 2 molar squared, because of 224 00:11:14,960 --> 00:11:20,070 this, 2 squared times 6. 225 00:11:20,070 --> 00:11:25,720 Which is equal to 8/24, which is equal to 1/3. 226 00:11:25,720 --> 00:11:29,290 Now, let's say we were to add more A, and I'm not going to 227 00:11:29,289 --> 00:11:30,189 say exactly how much. 228 00:11:30,190 --> 00:11:31,940 We could actually get quite complicated with the 229 00:11:31,940 --> 00:11:35,030 mathematics, but let's say after adding more A, we have a 230 00:11:35,029 --> 00:11:37,360 new concentration. 231 00:11:37,360 --> 00:11:40,940 Now, let's say our concentration of A is 3 molar. 232 00:11:40,940 --> 00:11:43,310 You might say, hey, Sal, didn't you add 1 molar? 233 00:11:43,309 --> 00:11:45,029 No. 234 00:11:45,029 --> 00:11:47,139 I actually added probably more than 1 molar. 235 00:11:47,139 --> 00:11:50,559 What happens is, whatever I added, that'll push the 236 00:11:50,559 --> 00:11:53,009 reaction towards the right direction, or towards the 237 00:11:53,009 --> 00:11:56,799 forward direction, and so some of it will get consumed and go 238 00:11:56,799 --> 00:11:59,149 in that direction, but whatever's left over is here. 239 00:11:59,149 --> 00:12:03,039 So I might have added more than 1 molar concentration to 240 00:12:03,039 --> 00:12:04,159 this system. 241 00:12:04,159 --> 00:12:07,649 But whatever was extra beyond the 1 was consumed, and I'm 242 00:12:07,649 --> 00:12:09,350 just left with this equilibrium 243 00:12:09,350 --> 00:12:11,409 concentration of 3. 244 00:12:11,409 --> 00:12:12,839 So I didn't necessarily add 1. 245 00:12:12,840 --> 00:12:14,180 I could've added more than that. 246 00:12:14,179 --> 00:12:22,159 And let's say that our new equilibrium for C is 12 molar, 247 00:12:22,159 --> 00:12:23,360 which is consistent with what we say. 248 00:12:23,360 --> 00:12:28,639 We should be producing-- if we add some A, then our 249 00:12:28,639 --> 00:12:31,860 concentration of C should go up, and the intuition is that 250 00:12:31,860 --> 00:12:34,159 the concentration of B should go down a little bit, because 251 00:12:34,159 --> 00:12:36,279 a little bit more B is going to be consumed, because it's 252 00:12:36,279 --> 00:12:39,740 going to be colliding with-- or it's more likely to collide 253 00:12:39,740 --> 00:12:42,090 with-- more A particles or A molecules. 254 00:12:42,090 --> 00:12:45,790 So let's see what B's new concentration is. 255 00:12:45,789 --> 00:12:51,730 So remember, the equilibrium constant stays constant. 256 00:12:51,730 --> 00:12:54,539 So our equilibrium constant is now going to be equal to the 257 00:12:54,539 --> 00:12:57,120 concentration of C, right? 258 00:12:57,120 --> 00:12:58,149 That was the reaction. 259 00:12:58,149 --> 00:13:04,069 So it's 12 molar-- whoops-- I don't have to write the units 260 00:13:04,070 --> 00:13:08,920 here-- divided by our new concentration of A, that's 3. 261 00:13:08,919 --> 00:13:11,019 But remember the reaction. 262 00:13:11,019 --> 00:13:12,710 The coefficient on A is 2. 263 00:13:12,710 --> 00:13:17,879 So it's 3 squared times the new 264 00:13:17,879 --> 00:13:19,950 concentration for B, right? 265 00:13:19,950 --> 00:13:22,770 There's no coefficient here so I don't have to worry about 266 00:13:22,769 --> 00:13:24,220 any exponents. 267 00:13:24,220 --> 00:13:25,259 And let's just solve this. 268 00:13:25,259 --> 00:13:32,990 So you get 1/3 is equal to 12 over 9B. 269 00:13:32,990 --> 00:13:38,250 So if we just cross-multiply, we get 9 times the 270 00:13:38,250 --> 00:13:45,370 concentration of B is equal to 3 times 12, which is 36. 271 00:13:45,370 --> 00:13:48,169 And so divide both sides by 9. 272 00:13:48,169 --> 00:13:53,449 The new concentration of B is 4, or 4 molar. 273 00:13:53,450 --> 00:13:57,879 So B is equal to 4 molar. 274 00:13:57,879 --> 00:13:58,679 So that makes sense. 275 00:13:58,679 --> 00:14:01,439 We added some more A to the reaction. 276 00:14:01,440 --> 00:14:06,380 So we started with 2 molar of A, 6 molar of B, 8 molar of C. 277 00:14:06,379 --> 00:14:10,179 When we added more A, at the end, we added a bunch. 278 00:14:10,179 --> 00:14:12,339 It went in that direction, maybe it went back and forth a 279 00:14:12,340 --> 00:14:17,240 little bit, but it stabilized at 3 molar of A, 12 molar of 280 00:14:17,240 --> 00:14:21,200 C, so C went up, so that definitely went up. 281 00:14:21,200 --> 00:14:26,250 And notice, our stable equilibrium concentration of B 282 00:14:26,250 --> 00:14:28,629 actually went down, and this is consistent with what we 283 00:14:28,629 --> 00:14:31,139 were saying, that the reaction moves in that direction, more 284 00:14:31,139 --> 00:14:34,049 C gets produced, a little bit of B gets consumed. 285 00:14:34,049 --> 00:14:36,849 So anyway, hopefully you're fairly comfortable now with 286 00:14:36,850 --> 00:14:39,870 the whole notion of stressing a reaction and 287 00:14:39,870 --> 00:14:42,519 Le Chatelier's principle. 288 00:14:42,519 --> 00:14:42,667