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Free Energy and the Equilibrium Constant

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00:00:06.500
hi it's mr. Andersen and this is
00:00:08.690 00:00:08.700 chemistry essentials video 71 it's on
00:00:10.940 00:00:10.950 free energy and the equilibrium constant
00:00:13.220 00:00:13.230 over the last 20 videos we've been
00:00:16.070 00:00:16.080 talking about two things thermodynamics
00:00:18.439 00:00:18.449 and the importance of Delta G where we
00:00:20.480 00:00:20.490 can think of Delta G as the amount of
00:00:23.120 00:00:23.130 free energy available in the products
00:00:26.240 00:00:26.250 and the reactants and a reaction
00:00:27.920 00:00:27.930 occurring as we kind of move from
00:00:30.050 00:00:30.060 reactants to products and then we
00:00:32.330 00:00:32.340 started talking about equilibrium and
00:00:34.400 00:00:34.410 the importance of K which is a measure
00:00:36.229 00:00:36.239 of the concentration of products to
00:00:38.090 00:00:38.100 reactants this K is equal so are the
00:00:41.030 00:00:41.040 reactants and products as it increases
00:00:43.040 00:00:43.050 we shift towards the right as it
00:00:44.630 00:00:44.640 decreases we shift towards the left but
00:00:47.569 00:00:47.579 it's important that you understand that
00:00:48.830 00:00:48.840 we can interchange these two things
00:00:50.900 00:00:50.910 Delta G and K and that they are
00:00:53.240 00:00:53.250 inherently tied together and so in any
00:00:56.360 00:00:56.370 reaction we can look at it using
00:00:58.479 00:00:58.489 thermodynamic reasoning and the
00:01:00.380 00:01:00.390 importance of changes in free energy or
00:01:02.689 00:01:02.699 changes in Delta G and also looking at
00:01:05.030 00:01:05.040 it as an equilibrium reaction where
00:01:07.460 00:01:07.470 we've got K this equilibrium constant
00:01:09.380 00:01:09.390 which tells us are we shifting more
00:01:11.450 00:01:11.460 towards the right or towards the left
00:01:13.160 00:01:13.170 and then there is a algebraic connection
00:01:16.249 00:01:16.259 between these two where Delta G equals
00:01:18.320 00:01:18.330 the negative RT so that's going to be
00:01:21.080 00:01:21.090 the gas constant because times the
00:01:22.969 00:01:22.979 absolute temperature times the natural
00:01:25.010 00:01:25.020 log of K and so we can interact between
00:01:28.460 00:01:28.470 the two and if we figure out our Delta G
00:01:31.399 00:01:31.409 which I mean you could use the appendix
00:01:33.319 00:01:33.329 in the back of most chemistry books to
00:01:35.179 00:01:35.189 figure it out that tells us a lot about
00:01:37.130 00:01:37.140 what's going to happen to K and so let's
00:01:39.770 00:01:39.780 say K is equal to 1 we have an equal
00:01:42.830 00:01:42.840 amount of reactants and products well if
00:01:44.690 00:01:44.700 we plug that into this reaction that
00:01:46.609 00:01:46.619 means our Delta G or a change in Delta G
00:01:48.770 00:01:48.780 is going to be equal to 0 what happens
00:01:51.139 00:01:51.149 if our K value is greater than 1 in
00:01:53.210 00:01:53.220 other words it shifted towards the right
00:01:54.679 00:01:54.689 we'd now have a Delta G that's going to
00:01:56.660 00:01:56.670 be a negative value so we think of this
00:01:58.639 00:01:58.649 as an exothermic reaction it's
00:02:00.889 00:02:00.899 spontaneous it's moving towards the
00:02:03.200 00:02:03.210 right and then if we look at a K value
00:02:05.240 00:02:05.250 much less than 1 then we're going to get
00:02:07.219 00:02:07.229 a Delta G of a positive value so that's
00:02:10.040 00:02:10.050 an endothermic reaction
00:02:11.430 00:02:11.440 it's consuming energy it's
00:02:13.140 00:02:13.150 non-spontaneous and so it's really tying
00:02:15.630 00:02:15.640 together these two big thoughts that
00:02:17.400 00:02:17.410 we've been talking about for a long time
00:02:18.990 00:02:19.000 now generally in AP chemistry we've only
00:02:21.420 00:02:21.430 talked about exothermic or endothermic
00:02:24.120 00:02:24.130 but you should also understand there's
00:02:26.310 00:02:26.320 energy involved in this and so we could
00:02:29.040 00:02:29.050 think of this as an exergonic or giving
00:02:31.950 00:02:31.960 off energy reaction or an endergonic
00:02:34.410 00:02:34.420 reaction now why is that important well
00:02:36.960 00:02:36.970 in biological systems it's important to
00:02:39.420 00:02:39.430 to look at the amount of total energy
00:02:41.160 00:02:41.170 that's either given off or being
00:02:43.230 00:02:43.240 consumed not necessarily the heat that's
00:02:45.810 00:02:45.820 given off are consumed and so if we look
00:02:47.820 00:02:47.830 at this interaction right here between
00:02:49.320 00:02:49.330 Delta G and K and let's look at a few
00:02:53.010 00:02:53.020 reactions reversible reactions so in
00:02:55.530 00:02:55.540 this one the breakdown of water into its
00:02:57.990 00:02:58.000 different ions we would call a Delta G
00:03:00.540 00:03:00.550 value if we measure that of seventy nine
00:03:02.310 00:03:02.320 point nine kilojoules now let me show
00:03:04.440 00:03:04.450 you what the K value is it's going to be
00:03:06.870 00:03:06.880 a really really small value what do we
00:03:09.210 00:03:09.220 know about a positive Delta G that means
00:03:11.550 00:03:11.560 it's an uphill reaction non spontaneous
00:03:13.980 00:03:13.990 reaction what do we know about a really
00:03:16.530 00:03:16.540 small K value that means it's going to
00:03:18.510 00:03:18.520 be shifted more towards the left or
00:03:20.670 00:03:20.680 shifted more towards the reactants in
00:03:23.130 00:03:23.140 this case if we look at another reaction
00:03:25.320 00:03:25.330 in this case we've got a Delta G that's
00:03:27.810 00:03:27.820 a negative value what's that tell us
00:03:29.640 00:03:29.650 about its K value that's going to be a
00:03:31.470 00:03:31.480 positive value so this is a spontaneous
00:03:33.930 00:03:33.940 reaction or a downhill reaction and in
00:03:37.050 00:03:37.060 this case it's shifted way towards the
00:03:38.850 00:03:38.860 right so you should be able to predict
00:03:40.860 00:03:40.870 if I give you a Delta G what's going to
00:03:42.660 00:03:42.670 be my K value is it going to be positive
00:03:44.699 00:03:44.709 or negative that'd be the first question
00:03:46.470 00:03:46.480 and this is going to be a really small
00:03:48.030 00:03:48.040 or a really large value so since our
00:03:50.430 00:03:50.440 Delta G is a positive value we know that
00:03:53.010 00:03:53.020 our K value is going to be really really
00:03:55.320 00:03:55.330 small just using this equation up here
00:03:58.170 00:03:58.180 and so you could plug in a gas constant
00:04:00.330 00:04:00.340 and our absolute temperature and that's
00:04:02.580 00:04:02.590 going to be easily calculated and so if
00:04:05.490 00:04:05.500 we were to summarize it again how are
00:04:07.229 00:04:07.239 these two tied together Delta G is
00:04:09.449 00:04:09.459 negative then we've got a k equal to a
00:04:12.330 00:04:12.340 value greater than one that's going to
00:04:14.130 00:04:14.140 favor the products or it's going to move
00:04:15.990 00:04:16.000 more towards the right if it's equal to
00:04:18.570 00:04:18.580 zero K equals one and then if Delta G is
00:04:21.690 00:04:21.700 going to be a positive value our K value
00:04:24.029 00:04:24.039 is going to be much less
00:04:25.170 00:04:25.180 than one or it's going to favor the
00:04:26.969 00:04:26.979 reactants and so why is this important
00:04:29.640 00:04:29.650 well if we're looking at biology if
00:04:32.370 00:04:32.380 we're looking at the chemistry of life
00:04:34.010 00:04:34.020 it really tells us how we can couple
00:04:36.810 00:04:36.820 reactions together and so if we look at
00:04:39.090 00:04:39.100 this famous reaction up here what we've
00:04:40.980 00:04:40.990 got is glucose combining with oxygen and
00:04:43.800 00:04:43.810 we're making carbon dioxide and water
00:04:45.420 00:04:45.430 and so what is this this is going to be
00:04:48.029 00:04:48.039 cellular respiration it's taking place
00:04:49.830 00:04:49.840 in all the cells of your body right now
00:04:51.600 00:04:51.610 if we were to look it up in the
00:04:53.520 00:04:53.530 appendices we'd find our Delta G value
00:04:55.560 00:04:55.570 is going to be negative 28 80 kilojoules
00:04:58.620 00:04:58.630 so we've got a delta G value that's a
00:05:00.420 00:05:00.430 negative value what does that mean it's
00:05:02.370 00:05:02.380 going to shift towards the right so what
00:05:04.350 00:05:04.360 would our K value be it's going to be
00:05:06.510 00:05:06.520 greater than one so that's going to
00:05:08.339 00:05:08.349 favor our products it's going to move
00:05:10.560 00:05:10.570 towards the right this is a spontaneous
00:05:12.150 00:05:12.160 reaction if we can get it going and we
00:05:14.820 00:05:14.830 use enzymes inside the cells to do that
00:05:16.830 00:05:16.840 now what are we using that energy to do
00:05:19.110 00:05:19.120 well we've got another reaction here
00:05:20.879 00:05:20.889 we've got adenosine diphosphate and for
00:05:22.860 00:05:22.870 you how to phosphate to it we make ATP
00:05:24.510 00:05:24.520 what do you use ATP to do it's used in
00:05:27.540 00:05:27.550 all the cells of your body it's
00:05:29.100 00:05:29.110 essentially cellular coinage that your
00:05:31.379 00:05:31.389 cells use to do work I mean that's how
00:05:33.540 00:05:33.550 you think that's how you move that's how
00:05:35.850 00:05:35.860 you kind of do all the chemical
00:05:37.469 00:05:37.479 reactions inside your body and so what's
00:05:39.600 00:05:39.610 going on inside our cells more properly
00:05:41.640 00:05:41.650 inside the mitochondria of our cells is
00:05:43.890 00:05:43.900 that we're coupling this exergonic
00:05:47.070 00:05:47.080 reaction or this reaction that gives off
00:05:50.129 00:05:50.139 energy with an endergonic reaction one
00:05:52.320 00:05:52.330 that consumes it and so you can think of
00:05:53.969 00:05:53.979 these as gears as we're converting that
00:05:56.100 00:05:56.110 sugar into carbon dioxide and water
00:05:57.749 00:05:57.759 we're using the energy that's released
00:05:59.640 00:05:59.650 in that endergonic reaction and we're
00:06:01.680 00:06:01.690 time to add to or exergonic reaction
00:06:03.870 00:06:03.880 we're tying it to an endergonic reaction
00:06:05.820 00:06:05.830 and so did you learn to relate Delta G
00:06:08.939 00:06:08.949 to K in other words what happens as we
00:06:12.719 00:06:12.729 decrease our Delta G as we shift it more
00:06:15.899 00:06:15.909 towards the right what happens to our K
00:06:17.850 00:06:17.860 and vice-versa I hope so and not only do
00:06:20.730 00:06:20.740 I hope that was helpful this is the last
00:06:22.830 00:06:22.840 video in this series and so I hope the
00:06:24.870 00:06:24.880 whole ap chemistry series has been
00:06:26.850 00:06:26.860 helpful I hope you've been along with me
00:06:28.409 00:06:28.419 and thanks for watching

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