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학습트랜스크립트
00:02Okay, let's do the sample problem describing a voltaic cell with diagram and notation.
00:12Draw a diagram showing like that shorthand notation, right? Show balanced equation and
00:20write the notation for a voltaic cell that consists of one half cell with a chromium bar
00:25in a chromium bar. That's metal, right? So metal. And then in the solution it has chromium NO3 3.
00:43That means NO3 is negative so it's 3 positive. And another half cell with a silver bar.
00:53Silver bar and then Ag NO3, Ag plus, plus NO3 minus. That's the solution it has.
01:06That's easy, right? We have like some sort of the, what is that? So boundary area so the iron can
01:19easily
01:20transfer through. This day it can be replaced by membrane. And then so we have to figure out which
01:34one can be oxidized and which one is reduced through this whole thing. I'm sorry. This is membrane,
01:47but here we might have like a salt bridge like this. But anyway, that the chromium electrode is negative
01:58relative to the silver electrode. This is positive. This is negative.
02:08So that's what's happening. So it's the negative potential. So electron may move to this way,
02:19right? Or the other way. We will go through this one. Okay, solution. This one occurs and the chromium,
02:28the chromium loose electron oxidation occurs. So I'm sorry, electrons are generated on the left side.
02:36And this is the voltage cell. That's what I forgot. So in the case of the electrolytic cell,
02:45then if left side is negative, that means in the previous slide, it moves to the left side,
02:55electrolytic cell. But this is the carbonic cell, right? It's the voltaic cell. It mentioned like a
03:04particle cell. So the electron is generated at this side. So the balanced overall equation is this way.
03:18And then this is what we drew before. So actually electrons are on this side, negative. And this is
03:28the positive side. So the final short notation is this way. You can try it by yourself. And then
03:43we move on to the potential part. In the previous slide, so far we observed some kind of reaction.
03:50But now we are mainly focusing on, we are mainly focusing on that voltage, which has been generated
03:58from the the voltage cell. That is also called the electromotive force. That means like a voltage.
04:07voltage. It's considered as a force because the energy charging is located in the potential difference.
04:20That's energy. So, you know, in most cases, we have some part like a FS. Force times the distance is
04:32kind of
04:32another time of the energy. But here, if the voltage is applied, then the potential, any charge might move
04:44from one side to the other. So that force can be considered as this voltage. If we have a high
04:52voltage,
04:53then that can be the force, the higher force to that electron or positive charge.
05:01So we can consider that way. The force or electrical potential that pushes, pushes the negative charged
05:09electrons away from the anode and pulls them toward the cathode. But that, that's obvious.
05:20It is also called the cell potential. In some textbooks, they say this one as like this one.
05:28And also, there are many different kinds of expression E as an electric field.
05:40So we have to be very careful when we take a look at this electrochemistry related
05:47textbook or article, whatever you see. We have to figure out that E is a potential or electric field.
05:58Electric field is the voltage divided by the distance.
06:04So let's say when we apply the voltage here, this is the any metallic one, whatever.
06:13So we apply the voltage, the one volt to one meter. That's one volt per meter.
06:25What if this one volt is applied to nanometer?
06:28One volt divided by 10 to the negative meter. That's 10 to the negative voltage per meter.
06:36See, that's a lot changing.
06:39So the voltage itself is not that important. Electric field is more important.
06:49Of course, the voltage is also very important.
06:53But I mean, if that voltage is applied to quite long distance, the electric field is not that highly gradient.
07:10So let's take a look at the voltage variance in our life scale.
07:18That common alkaline flashlight battery is 1.5 volt.
07:23And the lead acid car battery has been series connected through six cells are series connected.
07:35So that shows 12 volts, but actually each one generates 2 volts.
07:41That calculator, the battery is only 1.3. Lithium and laptop battery is from 4.2 to 3.7,
07:51depending on what kind of the materials are chosen.
07:58And also the most laptop battery, the outcome or income is 12 volts too, depending on the manufacturer.
08:08Electrical electric eel, that animal in that water, that's only 0.15 volt.
08:18But you know, like it has a 5,000 cells in six-feet yield.
08:24That's 750 volts.
08:27Seriously connected. Oh, that's scary.
08:31Nerve of a giant squid, 0.07.
08:37You know, when we got hit by somebody or something, then that hitting normally moves iron.
08:51Let's say we have membrane and potassium and sodium, they are all equivalently distributed.
08:59But if somebody hits, then some iron may transfer through this membrane because they have different
09:09ionic diffusion coefficient. So let's say potassium is moving, no, sodium might move faster.
09:16So it's smaller. So sodium will move to the other way.
09:20So sodium, sodium, sodium, sodium. But if we do this condition, left side was zero volt before.
09:30And then on the right side, zero volt too. But here on the left side, the positive ion has been
09:36moved
09:36to the right side. So it should be negatively charged. And on the right side, we have positive one.
09:41So positively charged. So they are making some sort of a voltage difference between this membrane
09:51that makes electric field. That's what we are sensing through our brain to feel the pain.
10:01So if we regularly eat enough water with electrolyte, then we may feel less pain than regular condition.
10:18That's one of the possibilities. You know, it's really up to the body condition because
10:25they should be neutralized with many electric, the charged, the ionic compounds.
10:33Then depending on this membrane condition, maybe some iron does not move that much that quickly.
10:42So it's up to the, the, the condition, people condition.
10:48Now cell potential, let's say, let's see what, what we have to figure out.
10:53So this is the energy part.
10:56I mentioned that the QV voltage is applied and then the energy charging,
11:03this is the kind of energy term, right?
11:06Then that charge, one coulomb and one volt, that's one joule.
11:13The electric charge. So this is the beginning of the energy part.
11:19And the one volt of the electron normally has a charge of 96,500 Coulomb.
11:27So please remember this value. That's a Faraday constant.
11:33Oh, here. Faraday constant.
11:39So one more electron may charge this one. That means if you want to calculate one charge,
11:47one electron's charge, then we have to divide by Avogadro's number to this Faraday constant.
11:54Okay. So free energy change is considered as this one.
12:00Do you remember why we need this delta G? Delta G is the thermodynamic term,
12:07which normally gives some information or indicates whether that reaction may occur or not, right?
12:16So if delta G value is lower than zero, normally that reaction occurs.
12:22That goes spontaneously. But if that value is positive, then it doesn't go.
12:33And if it's zero, that's the, what is that?
12:37They balance the reaction. So they are some sort of the,
12:44what is that? They are equilibrium condition.
12:50So the, based on this information,
12:57if we have a negative value, that reaction is spontaneous, right?
13:02So let's take a look at these values to see whether this is negative or not.
13:13See, it already has negative value.
13:16So if these values make positive, then spontaneous, correct?
13:28If it's negative, it's not spontaneous.
13:32That's electrolytic cell, right?
13:37So N is number of electrons, which has been participating in that reaction.
13:46Normally, in the case of silver, one electron and in the case of copper, two electron, right?
13:52That's an integer number and they are all positive.
13:56So they are positive.
13:59And F, valid constant, that's also positive.
14:03So these two numbers are always positive.
14:07But the last, this potential is not.
14:11It can be negative or it can be positive.
14:14We will see that later.
14:17So if it's positive, so the, I mentioned that if these three values are positive, then spontaneous.
14:25But now they are all the positive constant.
14:28So this potential is, if the potential is positive, spontaneous.
14:33And potential is negative, non-spontaneous, right?
14:38So the whole delta G value is up to this potential indicating.
14:49So if it's positive, spontaneous.
14:52It's negative, non-spontaneous, correct?
15:00Let's take a look at this, the problem set.
15:04The standard cell potential at 25 degrees Celsius and 1.1 volt for this reaction.
15:10Oh, that's what we saw in the previous, the beginning.
15:14But now they are showing this voltage.
15:17At that time, we didn't show, we didn't see this kind of voltage at the, by the time,
15:23we just observed this equation, this reaction occurs and this reaction does not.
15:30But that's what we saw in the previous slide.
15:32But now it says it's moving at 1.10 volt.
15:41So that can calculate by this delta G equation.
15:46And it's what we know, although we don't see any two electrons moving from this equation.
15:52But we know like zinc, zinc two plus, two electrons has been moved.
15:57And kappa two plus kappa, the two electrons.
16:01So that two more electrons are moving in this reaction.
16:05So N equals two.
16:07And Faraday constant, that's what we saw in the previous slide.
16:11And the potential measured was 1.1.
16:15And then, so that's negative, positive, positive, positive.
16:21So finally, negative 212 kilojoule.
16:27Although we do not know the last unit for this whole equation,
16:35we may know this value should be, the value should be negative.
16:40And that means the spontaneous, this reaction may occur.
16:50So we mentioned that that's 1.1 volt in the previous slide.
16:57But somehow it says the, but this is different one, the hydrogen and copper two plus,
17:03and then that generates like a proton and copper solid.
17:09So let's see what's happening here.
17:12Like a Kuka, Nama, Arafen, something and hydrogen and copper.
17:18So here we see this hydrogen is located ahead of this copper.
17:24That means it has a high priority to be ionized.
17:28So here hydrogen gas has been ionized to the proton.
17:35And then the copper should be reduced.
17:38Kappa two plus should be copper.
17:41So this is the one, the electron is generated on this, the hydrogen, standard hydrogen electrode.
17:54That's like a SHE.
17:56That's very unique, that reference electrode.
18:01We will talk about later.
18:06So then this potential is considered as zero.
18:14That will be the learned and that will be taught in the later slides.
18:21But also let's take a look at here.
18:24Like a voltmeter, it has measured 0.34 volt for this reaction.
18:32Since we use this hydrogen reaction for one reaction, that's oxidation reaction.
18:41The other one is a reduction reaction.
18:44And also we mentioned that this hydrogen is a reference electrode.
18:49So that potential is considered as zero volt.
18:57It's like when we have the building, if we measure the height from the ground, that's this way.
19:06But if we measure from the sea level, but that's different, right?
19:13So that's different potential.
19:15If we throw the ball from here to this way, we can use only this amount of energy.
19:22But if we throw the ball to the sea level, then we may have a higher energy.
19:31So it's called like a potential energy.
19:33It can be changed.
19:35So hydrogen potential is kind of a ground potential or sea level potential.
19:44So that's zero volt.
19:46And then kappa here, it says the kappa reduction potential is a little bit in the third,
19:52you know, like the first floor or seventh, third floor, whatever.
19:57That's 0.34 volt floor.
20:04That high.
20:05So if we have some other set, then that difference is the voltage we can finally get.
20:20So if we get the voltage difference compared to the hydrogen, then we can have like A, B, C, D,
20:30whatever, all the elements or compound, then we can simply guess the difference between
20:38any sets of choices.
20:43So that's what we saw here.
20:46Hydrogen and hydrogen, the proton, that's the anode part and cathode part is this way.
20:52And then overall reaction, this one.
20:55And then the cell potential is considered as the oxidation potential and reduction potential here.
21:03Because we just added those two equations for this whole equation.
21:11Right?
21:13So cell potential is here like 0.34 and this value is zero.
21:22So E reduction for kappa voltage is this one.
21:31So that's the standard hydrogen electrode.
21:34That's what I explained in the previous slide.
21:37So, you know, but this reference
21:42quite looks quite bulky because it should bubble the hydrogen all the time.
21:48It's written here just by just the letters, but actual device needs like hydrogen tank.
21:54And also if we want to use the hydrogen in the lab, we have to have some sensor to detect
22:02hydrogen in the lab scale.
22:04Because normally that can be used as a source for explosion, right?
22:09So we have to be very careful when we treat this hydrogen.
22:15So maybe there might have some hydrogen explosion situation in some lab.
22:24So now we have many different kinds of safety devices to check if we are doing well or not.
22:31And also it's not from this hydrogen, the standard reduction potential, the reference electrode.
22:38But when we have some hydrogen collection in Gangneung area in Seoul, Korea, in eastern part,
22:47we had a very high explosive bomb like explosion a few years ago.
22:55Like it has been years, but that's from the hydrogen.
23:01So hydrogen is very the sensible and sensitive material for the explosion.
23:10So we have to be very careful. And since it is very risky and it's not safe enough
23:23to handle it in the lab, normally that can be replaced by some other reference electrode.
23:38So standard hydrogen electrode is zero, right? This way or the other way, they are all zero.
23:47But we are more careful about this reduction potential. But either way, that's zero.
23:55So E cell potential is the ox and red, but this one is zero. So finally, sorry,
24:04the standard reduction potential for the kappa ion, that's 0.34. That's how we get the reduction
24:15potential potential for each element. So in the case of zinc, we also measured in the previous slide,
24:26we measured the kappa one. Now we measured the zinc one, that's 0.76 volt. So the same way,
24:33reduction potential is negative 0.76.
24:40So this one is 0.76. But we are mainly focusing on the reduced one, reduction potential,
24:48standard reduction potential. That can be handled to standard oxidation potential. But I haven't heard
24:55about any standard height and the oxygen potential, standard reduction potential. So in this case,
25:02let's let's see that delta G value again. Delta G is minus NFE. This one is all the constant,
25:10so we don't care much about it. It has already negative. And if this one is a negative value,
25:16that makes positive value, right? So this reaction may not spontaneously occur,
25:22we don't care much about it. Unless we have a special case, special set something. So if we have a
25:31higher
25:32number of negative value, so that k shallary ofcoresum region here, this is 0V reduction potential. Now we see this
25:47is
25:47negative 0.76 that means if we go to the left side that reductive potential might be negatively
25:55higher correct so I want you to be I want you to know about this so in the case of
26:06fluorine
26:07that accepts two electron and that becomes two negative the two fluoride negative ion that's
26:14positive so this reaction may occur right it's favorable but if we go down that's not favorable
26:22we just saw like 0.76 that's a zinc one and in the case of lithium that's like a zero
26:30point negative
26:31three point zero four that's much much much negative that means it's not occurring that
26:38way but the other way that's indeed that it's very spontaneous so if we see this reaction lithium
26:48does not want to be reduced the lithium ion does not want to be reduced it wants to stay as
26:54as a
26:55lithium ion okay so that's what we see standard reduction potential we can calculate the potential
27:05for this reaction that's 1.10 and the other one silver copper that's 0.46 so it can be used
27:17as
27:17alkaline the battery but this is not enough this voltage is not good enough right and then also
27:27there's the equation we start from this one this that's that's those thermodynamic part so we may
27:35not go through all the way but we can just stop from this one that should be the same as
27:42this one so
27:44they replaced this one into this one so and then calculated the value finally we obtain this one so the
27:55potential original potential that's from the reduced the standard reduction potential and then depending
28:02on the the the the concentration it's changing by this value normally like two electrons are the
28:14participating in the reaction sometimes there's one electron but that's only one or two then we see
28:21this value 0.05 that's not that high value right so let's let's say we start from 1.1 volt
28:31and then
28:32depending on the condition 0.059 volt can be deducted or two times 0.059 volt again or triple time
28:44but then
28:46that's not that big value so if we have if we subtract like 10 times that's 0.059 right so
28:56in that case it
28:57that's much the higher scale and then the we can calculate and then do the the problem solving to see
29:13what's how much it's changing so in the case of iron and kappa iron so it's negative 4 and that's
29:220.0 point
29:22and negative 1 that's almost negative 1 and that's negative 4 and also this one is a 1 and then
29:32we
29:32will see actually standard reductive potential is 0.43 volt but if we do this math 0.43 volt can
29:42be
29:42change it to 0.25 volt that's like a 30 or 40% changing right that's noticeable right so they
29:58also have
29:59some this is from there there is the equation the same way and then the other way we can also
30:05get we
30:06in the previous slide we used the concentration actually some sort of potential k this is kind
30:17of k value right we can from the k value we could get some voltage value or the other way
30:26if we know
30:26this voltage value then we can get this potential k value that's what it says like we can get k
30:34value
30:35from the equation so as i as we mentioned before if k is 1 that's like a under equilibrium constant
30:45equilibrium condition that's what we are saying if k is greater than 1 k is a reactant and this is
30:54a product
30:54greater than one means and that product is a lot and the other way the reactants are a lot
31:07so that's what we observed from the nearest equation the previous slides if we face any kind of question
31:16then you can you can you can figure it out and this is the last slide so potential and then
31:24also delta g we
31:26can obtain those values and then that is also the very closely connected this the k value okay that's it
31:38that's for the uh the the set one i will come back for the the next this slide okay thank
31:49you
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