For more information, please see full course syllabus of AP Chemistry

For more information, please see full course syllabus of AP Chemistry

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### Cell Potential

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

- Intro
- Cell Potential
- Definition of Cell Potential
- Symbol and Unit
- Standard Reduction Potential
- Example Figure 1
- Example Figure 2
- All Reduction Potentials are Written as Reduction
- Cell Potential: Important Fact 1
- Cell Potential: Important Fact 2
- Cell Potential: Important Fact 3
- Cell Potential: Important Fact 4
- Example Problem 1
- Example Problem 2

- Intro 0:00
- Cell Potential 2:08
- Definition of Cell Potential
- Symbol and Unit
- Standard Reduction Potential
- Example Figure 1
- Example Figure 2
- All Reduction Potentials are Written as Reduction
- Cell Potential: Important Fact 1
- Cell Potential: Important Fact 2
- Cell Potential: Important Fact 3
- Cell Potential: Important Fact 4
- Example Problem 1
- Example Problem 2

### AP Chemistry Online Prep Course

### Transcription: Cell Potential

*Hello, and welcome back to Educator.com; welcome back to AP Chemistry.*0000

*Today, we are going to introduce a very, very, very important notion--probably the central notion, obviously, of electrochemistry; and it's called cell potential.*0004

*Last time, we talked about galvanic cells and how, if you mix two species and those species have a tendency to...if there is going to be some sort of an oxidation-reduction reaction, then electrons are going to spontaneously flow from one of those species to the next.*0012

*The galvanic cell exploits that tendency by separating the species, connecting the species with a wire or with a circuit, and then having the electrons flow through that wire.*0033

*Well, what you end up doing is (what we have done there is) create a battery.*0045

*Now, if we cut that wire and put something in between that wire (a heater, a cell phone, a computer, whatever), we can actually use that spontaneous flow of electrons to do work for us.*0049

*Again, that is all a battery is: it's a galvanic cell where the oxidizing agent and the reducing agent are separated, and the minute you actually put that into some device, you have closed the circuit.*0061

*When you flip that thing on, that is when you have actually close the circuit, and now electrons can flow, and you can operate your phone, operate your computer...whatever it is that you need to do.*0073

*OK, well, we want to be able to assign some numerical values to this; like, for example, if I put this species and this species together, are the electrons going to flow quickly? Are they going to flow slowly?*0083

*How strong is the tendency of electrons to flow?--we want to be able to control this, because if electrons are just going to sort of trickle through, that is not really going to be much use to us.*0097

*And if they are going to fly through at high speeds, well, they're going to end up doing damage to the material.*0108

*So, we need to know how to control this; that is the whole idea behind science--science is about understanding nature so that we can exercise control over nature, or at least control over the things that make our lives better; that is the whole idea.*0113

*OK, so let's start with some definitions, and we'll hopefully get a better sense of what this thing called a galvanic cell does and how it works.*0128

*We are going to define something called a cell potential.*0137

*So, definition, and cell potential (or it's also called electromotive force)...now, before I actually write this definition, let me tell you what we mean by the word "potential," when we talk about science.*0142

*It's exactly what you think it is: when we say something has potential, it hasn't happened yet, but it can happen.*0168

*So, for example, if you are on top of a mountain, skiing, and you are right there at the edge, you have the potential to go very, very, very fast.*0176

*But, you haven't dropped off the edge yet and started your skiing; so it's a measure of what could happen--that is what it is.*0188

*We can actually assign numerical values to what could happen, once we open the circuit, in this case--once we drop onto the mountain, once we open the faucet.*0196

*That is actually a good way to think about this cell potential; so I'm going to write the definition, and then I'm going to go back to this faucet idea and talk about it; and I think it's a good way to think about it.*0207

*It is the driving force or pull or push (depending on how you want to think about it) which causes electron flow in a galvanic cell.*0217

*You have a faucet at home: well, the water in the line is under pressure; you know this, because when you turn it on, water actually comes out.*0246

*When the faucet is closed, no water is coming out; however, the water company is sending water to your home; they are pressurizing the water, so there is a certain push against the valve in your faucet.*0256

*That is the whole idea: when you open it, you actually unleash that pressure.*0271

*Well, the pressure is the potential; the water has the potential to come out with a certain amount of force when you open the valve.*0275

*Here, the electrons have the potential to flow across that wire when you close that circuit.*0284

*It is actually a measure of the push or the pull, depending on which direction you want to see it; it's what drives the electrons forward.*0291

*Sometimes the drive is high; sometimes the drive is low; we are going to be assigning numerical values.*0299

*But don't let this word fool you: it will often talk about the potential, the cell potential; when we have a galvanic cell, what is the numerical value that we can assign to how badly the electrons want to go from the reducing agent to the oxidizing agent?*0305

*That is what this is: electromotive force.*0324

*That is a little bit more descriptive: "electromotive" means it's the force that is moving the electrons.*0326

*How powerful is the force that is moving the electrons--is it pushing the electrons through that wire really fast, or is it just sort of nudging them through?*0332

*That is the whole idea; OK.*0341

*The unit...well, actually, let's do the symbol first: the symbol for...let me see; maybe I'll do...yes, that's fine; I'll do the symbol; it doesn't matter which order.*0343

*The symbol is this: it's just E, and the cell--that is the potential of the cell.*0360

*The unit is the volt.*0370

*It is equivalent to Joules per coulomb.*0375

*We won't worry about...we have seen Joules before; that's a unit of energy; coulomb is a unit of charge.*0382

*Don't worry about this unit...really, you just need to concern yourself with this V, volt, for the time being.*0389

*We will actually get into...when we discuss electrolysis, we will actually discuss what the volt really is, and what coulomb is, and things like that.*0396

*But, for now, just know that it's a unit, and it's a measure of some ability for something to push or pull those electrons, to move them.*0404

*When we talk about 22 volts, that is what we are talking about: we are talking about the potential to actually move electrons.*0413

*OK, so now, let's take a look at a galvanic cell, and let's see how we are actually going to measure the cell potential.*0423

*We are going to take one of the cells that we have already looked at before, and we are going to create...we're going to put a little something in between here--something called a voltmeter or a potentiometer (a digital voltmeter, actually).*0432

*It is going to give us some number, and it is going to tell us--give us a numerical value for the potential, for the strength of that push or pull of the electrons.*0445

*This is connected this way; we have our two electrodes (oops, let me do this; OK)--so we have our electrodes, and we have a zinc solution here, and we have a copper solution here.*0455

*This is copper metal (because they are both metals, we can go ahead and use them as the electrodes); this is zinc metal.*0474

*Now, what happens here is: electrons are going to flow this way.*0485

*In other words, zinc metal is going to dissolve; it's going to turn into zinc ion and go into solution.*0492

*Copper ion is going to meet at the interface where the electrons are coming, and it's going to start to form copper.*0497

*So, this is going to be the oxidation; this is going to be the anode; this is going to be the cathode.*0505

*Now, when we actually run this--when we put some digital voltmeter in between here and everything is connected, but we don't let any current actually flow (in other words, we don't let any electrons do any flowing), what we want to measure is the potential for flow; what is the pressure?*0509

*These electrons are sort of building up; we don't want to open up the circuit just yet--what we want to measure is the pressure at that point.*0532

*Well, here is what happens: if you were to take this measurement, you would get a reading of 1.10.*0539

*So, the cell potential for this particular galvanic cell, made up of zinc solution, zinc metal electrode and copper solution, copper metal electrode, is 1.10 volts.*0545

*That is it: 1.10 volts...for right now, it's just a numerical value.*0563

*So again, what we do is: we put this digital voltmeter in there, and we want to measure the tendency--how badly these electrons want to get over here.*0569

*The higher this number, the higher the pressure; the higher this number, the more badly the electrons want to flow.*0579

*That is the idea.*0586

*It's a measure of how strong the push or the pull is; we don't let anything flow just yet.*0590

*If we were to open the circuit, yes, the electrons would start flowing; zinc would start to become oxidized; copper ion would start to be reduced, and the circuit would be closed; everything would be fine.*0595

*But right now, we are just concerned with the potential of this cell to do work.*0605

*We are not concerned with the actual work yet; we will be.*0610

*OK, now let's define something called a standard reduction potential.*0615

*Let me write the definition, and then we will explain what is going on.*0634

*It is the potential of a given species to become reduced by oxidizing hydrogen gas to hydrogen ion, when all species are in their standard states.*0641

*"Standard states" means 1 Molar solute concentrations, and 1 atmosphere for gases.*0689

*Now, let me tell you what this exactly means: in order to be able to actually measure something in science, we have to have a standard by which to measure it.*0705

*For example, in order to know that something weighs 27 grams, we have to know what 1 gram is.*0719

*Well, 1 gram is not some objective measure; it isn't just one gram that just fell from the sky; we actually have to decide what we mean--as a scientific community, what we mean by one gram.*0724

*So, in some sense, it's arbitrary; it's not arbitrary--there is a reason for choosing a particular measure--but we actually have to choose a point of reference, a standard against which to measure.*0736

*That is the whole idea: if I said, "This is how much an inch is," once you know what an inch is, you can go ahead and measure this based on the standard.*0746

*Well, the scientific community has decided that this reaction, the reaction of...well, they have decided that the hydrogen atom being oxidized to hydrogen ion, or hydrogen ion being reduced the hydrogen atom--they have assigned that a reduction potential of 0 volts.*0753

*Now, let me draw what I just said, and it will make sense what I actually just said.*0782

*OK, if I take a cell like this--a standard cell--this is going to look slightly different, because now we're going to be dealing with hydrogen gas.*0788

*Hydrogen gas--I'm going to have to bubble it in, so the arrangement is going to look different; but it's exactly the same.*0798

*We have our porous disk; we have our little digital voltmeter; we have our electrode (and here I'm going to go ahead and use copper).*0805

*OK, so if I put some copper here, and if I put some copper ion over there (the anion doesn't matter)...and now, on this side, watch this little arrangement.*0817

*Here is a little tube, and hydrogen gas is being pumped in this way; so the hydrogen gas is going this way, and it's actually bubbling out.*0831

*Well, the wire--there is a wire that goes down through this tube, and at the end of that wire is a little platinum electrode.*0841

*OK, it's a little platinum electrode; so the little platinum electrode--and here, we have a bunch of hydrogen ion--basically just an acidic solution.*0849

*This hydrogen ion is in contact with this electrode.*0860

*Well, hydrogen gas is being bubbled on top--so basically, we are flooding this electrode with hydrogen gas from above.*0863

*There is no liquid in this tube: we are just bubbling in--we are pushing in hydrogen gas, and it is bubbling out from underneath.*0871

*That is what is happening: so what is happening is that this electrode is being saturated with hydrogen gas.*0879

*Now, when I do this, and I measure this cell potential, I'm going to get a number, 0.34.*0888

*Well, again, we need to be able to assign certain numbers to certain cell potentials.*0898

*If I take this one side to always be hydrogen gas/hydrogen ion solution, and if I just change this species (zinc, permanganate, copper, manganese, cobalt, iron, whatever), I actually, by using hydrogen as my standard that I set at 0--as it turns out, I can actually write a cell potential for this reaction.*0907

*Here, what is happening is: electrons are flowing this way.*0933

*So, what is happening in this is: copper 2+ ion is gaining 2 electrons to become copper metal.*0940

*OK, let me write it a little smaller, because to the right, I want to write its standard reduction potential.*0950

*Copper ion, plus 2 electrons, becomes copper metal.*0956

*The standard reduction potential, which is just E (not cell)--the standard reduction potential for copper 2+...*0963

*You know what, I need more room; this is not going to work; OK, I'm going to write it right below: copper 2+ plus 2 electrons goes to copper solid, copper metal.*0975

*The cell potential for copper 2+ reducing to copper metal is equal to 0.34 volts.*0990

*By choosing hydrogen as 0, by international agreement, it gives us the standard reduction potential for a given species.*0999

*So, mind you, there are two things going on: we defined something called a cell potential--that is the potential for the whole cell; we also defined something called a standard reduction potential--this is the potential for a given species to become reduced, relative to the hydrogen electrode.*1010

*That is the whole idea: we have chosen hydrogen as 0 volts; therefore, we can assign a value, based on this cell--that means copper, in going from copper 2+ to copper--it has a standard reduction potential (this is a positive number) of .34 volts.*1032

*It is a measure of how badly copper wants to become reduced.*1051

*That is all that is going on here: we have chosen hydrogen as our standard; we set it equal to 0; because copper is the one being reduced, in this case, the oxidation that is taking place is the following.*1057

*H _{2} gas is losing...and it's becoming 2 H^{+} + 2 electrons.*1068

*Here is what is happening: hydrogen gas is being bubbled in here; when hydrogen gas hits this electrode, this electrode--because electrons are being pulled this way by .34 volts--every hydrogen molecule that passes over this electrode is split in half.*1076

*Each one of those electrons from each hydrogen atom is ripped off; those two electrons travel through the wire; they come down here; the copper 2+ ion takes those two electrons and becomes copper metal.*1095

*That is what is happening.*1109

*Hydrogen gas is turning into hydrogen ion; now you have two more hydrogen ions going into solution; that is what happens when we run this cell.*1112

*You get a positive value: spontaneously, between copper and hydrogen, copper will take the electrons; hydrogen will give up the electrons spontaneously.*1119

*If you put copper in the presence of hydrogen gas, that is what will happen.*1130

*Now, let's run another...and the cell potential for copper is .34 volts.*1134

*Now, let's do another one: let's do the same thing--we are going to have the same setup on the right, because that is our standard: a hydrogen electrode is our standard; we are going to pump in hydrogen gas.*1142

*We are going to have a platinum electrode down at the bottom; it's going to be saturated with this hydrogen gas as the hydrogen gas bubbles all over it.*1156

*We have a porous disk; we have some hydrogen ion, except this time, I have zinc in here.*1166

*I have this; I have my digital voltmeter; I have my zinc metal--this is zinc metal; well, something very interesting happens in this case.*1176

*Now, electrons flow this way, as it turns out, spontaneously.*1187

*Electrons flow this way: zinc metal gives up 2 electrons; they travel through the wire.*1192

*It comes over here; one H ^{+} grabs an electron to become a hydrogen atom; another H^{+} grabs an electron to become a hydrogen atom; it turns into hydrogen gas; it bubbles off as hydrogen gas.*1200

*Now, you are forming hydrogen gas; the zinc melts; when I measure this potential, before I actually close the circuit, I end up with this: -0.76.*1214

*So here, the reaction that takes place is: Zn ^{2+}...let me see...+ 2 electrons equals -0.76 volts.*1225

*Now, watch: see how I have written this.*1250

*I have written this as a reduction, but what is happening to zinc is not a reduction.*1254

*Zinc is being oxidized; what is actually happening in this cell is this thing.*1259

*Zinc is becoming zinc ion, plus 2 electrons; however, when we said a standard reduction potential--all standard reduction potentials are written as reductions.*1264

*So, when I flip this equation around, that is why this actually has a negative value--because, relative to hydrogen, in this case, it isn't hydrogen that is oxidized; it is the hydrogen ion that is reduced.*1278

*Zinc gets oxidized; so here, the reaction is this...let me correct this...2 electrons...goes to zinc metal...*1293

*So, this reaction is what actually takes place: and because we have automatically assigned this 0 volts, this -.76 is the standard reduction potential for zinc ion.*1318

*That is the whole idea: all standard reduction potentials are written as reductions--that is why they are called reduction potentials.*1334

*It is the potential for this species to reduce.*1341

*But, because we have set some species (in this case, hydrogen) as our standard...as it turns out, when hydrogen is in contact with certain species, it will end up being oxidized.*1344

*When it's in contact with other species, it will actually be reduced.*1354

*If it's oxidized, your reduction potential for that species is going to be positive; if hydrogen is what is reduced, and the other thing is oxidized, the standard reduction potential is going to be negative.*1359

*Standard states: I forgot that little 0 on top: that means standard states.*1374

*That means 1 Molar solution, 1 atmosphere pressure.*1378

*-0.76...that is what is going on here.*1382

*Now, let us write what it is that we just did here: All reduction potentials are written as reductions; that is the whole idea.*1387

*We need a standard by which to measure them, which is why we write them as reductions.*1409

*We have the following: there is a table of reduction potentials; it's in your book; it's in the back of your book.*1419

*It is going to be on the AP exam: these are not things you have to memorize, but you have to understand what they mean when you look at them.*1429

*It is just like any thermodynamic table data or K _{sp} data: it gives you a numerical value for how strong a tendency any given species has to be reduced, relative to the hydrogen electrode, which we have set at 0 volts.*1435

*A partial view of a standard reduction potential looks like this: you will see copper 2+, plus 2 electrons, goes to copper; you will see this: equals 0.34 volts.*1453

*That is what it says; you will see: 2 H ^{+} + 2 electrons goes to H_{2} gas.*1469

*You will see 0.00 volts--that is our standard; there are going to be certain numbers that are going to be higher; there are going to be certain numbers that are going to be lower.*1478

*The numbers that are higher--they will reduce this; the numbers that are lower--they will be oxidized by hydrogen.*1487

*Now, if I don't use hydrogen--if I just do something here and something here--the numbers that are higher will reduce; the numbers that are below--the species--those will be oxidized.*1498

*We'll explain in just a minute what we mean.*1509

*We also have: Zn ^{2+} + 2 electrons goes to Zn = -0.76 volts.*1512

*OK, positive reduction potential; 0 reduction potential; negative reduction potential.*1523

*Between this and this--because this is positive, this will happen spontaneously; between this and this--because this is negative, what is spontaneous is this one; that means this one has to be reversed.*1534

*However, in a table that we look at, all of them are written as reductions; notice, all of the electrons are on the left.*1542

*It shows the ion species gaining electrons to become another species--reduction potentials.*1549

*OK, so let's see--a couple of things we should know about these: as we said before, this 0 little superscript--that means standard reduction potential...standard states.*1555

*All solutes (in other words, all ionic compounds) are at 1 Molar concentrations, and all gases are at 1 atmosphere.*1573

*We are bubbling in hydrogen gas at 1 atmosphere pressure--not 5 atmospheres; not .6 atmospheres; 1 atmosphere pressure.*1592

*The concentration of hydrogen ion in that solution: 1 molarity.*1600

*The concentration of zinc ion: 1 molarity; that is how we take this measurement.*1604

*OK, here are the important things: the higher the reduction potential, the greater the tendency to be reduced.*1609

*Copper has a greater tendency to be reduced than hydrogen ion; hydrogen ion has a greater tendency to be reduced than zinc.*1641

*Copper has a greater tendency to be reduced than zinc.*1648

*Between two species, the one with the higher standard reduction potential will become reduced, and the other will be oxidized.*1658

*Therefore, it must be flipped.*1697

*Copper and zinc: if I create a galvanic cell with copper and zinc...copper: .34; zinc is -.76; this has a higher reduction potential than this, so this will be reduced; this stays as written.*1707

*Because this is reduced, this is going to be oxidized; I have to flip this equation, and upon flipping this equation, I reverse the sign of this.*1721

*That is one of the problems that we are going to do right now.*1729

*OK, a couple of more things before we get to the example: the standard potential for a cell--the standard cell potential (this is the potential of the whole cell), which is symbolized E ^{0}_{cell}, comes from adding the standard reduction potentials for each half-reaction.*1733

*Oxidation half, reduction half: remember, you break up an oxidation-reduction into two; each one of those has a standard reduction potential.*1783

*We add the equations (oxidation and reduction); we add the cell potentials; that gives us the...we add the reduction potentials for each species; that gives us the total cell potential.*1791

*OK, one thing you have to keep in mind, though, when doing this--very, very important: When multiplying a half-reaction by an integer to equalize electron number (remember when we were equalizing electron numbers so that we can actually cancel them?), do not multiply the standard reduction potential by that number--by that integer.*1805

*Remember when we did enthalpy?--if we multiply an equation by an integer, we have to multiply the enthalpy by that integer.*1859

*It is because enthalpy is an extensive property: it depends on the amount of material that we are dealing with.*1867

*Standard reduction potential is an intensive property: it doesn't matter how much--it doesn't change.*1880

*For example, mass is an intensive property: the more of something, the greater the mass.*1887

*The density...*1892

*Mass is not an intensive property; mass is an extensive property.*1893

*Density is intensive--it doesn't matter how much of something I have--whether it's this much gold or that much gold--gold has one density.*1898

*It is a property of the material: it doesn't depend on how much of that material is present.*1906

*Standard reduction potential is intensive: I don't multiply it by anything, just because I do five of those reactions as opposed to one of those reactions.*1911

*In other words, that is an intensive property--one that does not depend on quantity.*1920

*One that does is called extensive.*1943

*OK, so let's do an example.*1949

*OK, so let's see: Given the following cell and data, give the balanced cell reaction; state the anode; state the cathode; and calculate the cell potential; OK.*1955

*For the following cell and data, give the cathode, the anode, the balanced reaction, and the standard cell potential.*1972

*OK, so we have some cell, and we have some electrodes connected; this one is going to be aluminum; this one is going to be magnesium; and here is our data.*2006

*We have some magnesium ion here; we have some aluminum ion here; we have the following data.*2025

*Aluminum 3+, plus 3 electrons, goes to aluminum, and the standard reduction potential for that is -1.66 volts (straight out of a reduction potential table that you will be using all of the time with electrochemistry problems).*2032

*All of them are written as reductions; remember, all standard reduction potentials--that is why they are called "reduction potentials."*2052

*The equations are written as reductions; we decide, depending on which is higher and which is lower, what stays reduction and what becomes oxidized.*2058

*Magnesium 2+, plus 2 electrons, goes to magnesium: the reduction potential of that is (ooh, look at these crazy lines; OK) -2.37 volts.*2069

*All right, so we have -1.66 volts, and we have -2.37 volts.*2086

*All right, they are both negative, but the aluminum reaction has a higher reduction potential than this.*2092

*-1.66 is a higher number than -2.37.*2103

*So, the aluminum will stay as written--it will be reduced; the magnesium is going to end up being oxidized; so we have to flip the magnesium equation.*2107

*That is how we decide: we look at the reduction potentials: the one that is higher stays as a reduction; the other one gets flipped as reduced.*2117

*Let's write that: so we are going to write the reduction--the reduction is: Aluminum 3+, plus 3 electrons, goes to aluminum; and our standard reduction potential is -1.66 volts.*2125

*Our oxidation (which we had to flip) is going to be magnesium going to magnesium ion, plus 2 electrons; and because we actually flipped the equation, we reverse the sign of the reduction potential: it becomes 2.37 volts.*2146

*OK, I tend to put brackets around those.*2165

*Now, I need to balance the reaction; well, I have an oxidation-reduction--a reduction reaction, an oxidation reaction--I have the standard reduction potentials (oh, this vocabulary!); now I need to equalize the electrons.*2169

*I multiply this equation by 2 and this equation by 3, and I end up with 3 Al ^{3+}...no, 2 (I can't even do basic arithmetic!)...2 Al^{3+} + 6 electrons goes to 2 Al.*2183

*And again, remember: we don't change anything: -1.66 volts--that is an intensive property.*2204

*3 Mg goes to 3 Mg ^{2+} + 6 electrons; this is 2.37 volts (I actually like to put a positive sign there).*2211

*And now, we add: we add the equation to balance; we add the standard reduction potentials to get our cell.*2226

*6 electrons goes with 6 electrons; 2 Al ^{3+} + 3 magnesium goes to 2 Al + 3 magnesium 2+; the E of the cell is equal to...well, when I add those two, I get 0.71 volts.*2233

*0.71 volts: that is what happens.*2255

*When I put aluminum and magnesium in the cell, the way I described a little bit earlier, aluminum will pull 6 electrons from 3 magnesium atoms.*2262

*Aluminum will turn into solid aluminum; magnesium, upon losing electrons, will turn into magnesium ion; and the driving force, the pressure behind this process, is .71 volts.*2272

*Again, don't worry if you have a sense of what "volt" means; we will get to that a little bit later; but that is it--we can assign a numerical value to how strong this process is once we close the circuit.*2286

*Anode--oxidation: oxidation takes place in the magnesium compartment; reduction--cathode: cathode-reduction takes place in the aluminum compartment.*2301

*The balanced reaction; the cell potential; good.*2315

*Let's do the other one in blue--Example 2: OK, a galvanic cell is based on the following reaction.*2320

*MnO _{4}^{-} + H^{+} + ClO_{3}^{-} becomes ClO_{4}^{-} + Mn^{2+} + H_{2}O.*2347

*OK, a galvanic cell is based on the following reaction; OK, our problem is to calculate the standard cell potential for this reaction.*2370

*Well, OK: let's take a look at what we have.*2393

*This is balanced as written; we can double-check that--it's not a problem--but it is balanced as written, because you see the H ^{+}; you see the H_{2}O; everything looks like it is done.*2399

*Permanganate--manganese is being reduced; it's going from positive 7 to positive 2.*2408

*Chlorine is being oxidized.*2417

*Let me actually do this one: 2x3 is 6; this is going to be +5; this is going to be +7; this is going from a positive 5 state to a positive 7, so it's being oxidized.*2421

*As written, our permanganate is being reduced, and our chloride is being oxidized; so we can either read it off, or...let's go ahead and take a look at the half-reactions, the way we have been doing so far.*2430

*When we look up the half-reactions in a reduction potential chart (a table of reduction potentials), we get the following.*2444

*We get: MnO _{4}^{-} + 5 electrons + 8 H^{+} (this is exactly what it says in the chart--this is what you are looking for) goes to Mn^{2+} + 4 H_{2}O.*2452

*It says that the reduction potential--standard reduction potential--is 1.51 volts.*2471

*OK, now, the other species that we notice in there is ClO _{4}.*2476

*Well, this one, plus 2 H ^{+}, plus 2 electrons, goes to ClO_{3}^{-} (remember, everything is written as a reduction; electrons are always on the left-hand side; everything is written as a reduction in a standard reduction potentials chart), plus H_{2}O.*2485

*The standard reduction potential for this one is 1.19 volts.*2507

*OK, so just by looking at these: this has a higher reduction potential than that; that means this stays as is; this reaction gets reversed.*2511

*That means this is going to be oxidized to that.*2519

*So, when we do that, we reverse that; so let's do it.*2522

*We are going to write: MnO _{4}^{-} + 5 electrons + 8 hydrogen ions goes to Mn^{2+} + 4 H_{2}O; and we leave the reduction potential as is, equal to 1.51 volts.*2527

*This one we reverse--we write: ClO _{3}^{-} + H_{2}O goes to ClO_{4}^{-} + 2 H^{+} + 2 electrons; and the reduction potential becomes -1.19 volts.*2546

*Now, I need to make sure that the electrons balance; so I'm going to multiply this equation by 2, and I'm going to multiply this equation by 5; I'm going to rewrite what I have.*2563

*I have: 2 MnO _{4}^{-} + 10 electrons + 16 hydrogen ion → 2 Mn^{2+} + 8 H_{2}O; and still, nothing changes as far as the reduction potential (1.51 volts).*2576

*Here, I have 5 ClO _{3}^{-} + 5 H_{2}O goes to 5 ClO_{4}^{-} + 10 H^{+} + 10 electrons.*2595

*Reduction potential: -1.19 volts; remember, we reversed that.*2608

*Now, let's add this: 10 electrons cancels 10 electrons; 10 H ^{+} leaves 6 H^{+}; 5 H_{2}O leaves 3 H_{2}O; and I end up with the following.*2613

*2 permanganate ions, plus 6 hydrogen ions, plus 5 chlorate ions, produce (if I were to close the circuit) 2 manganese, plus 5 perchlorate, plus 3 H _{2}O.*2629

*The standard reduction for that cell is equal to 0.32 volts.*2659

*There we go: if I create a cell based on permanganate and chlorate, the potential for that cell is .32.*2666

*That means that is a measure of the tendency for this reaction to happen, if I were to open the circuit.*2676

*That is what is going on.*2684

*Now, notice all of these species: that is an ion; that is an ion; that is an ion; and that is an ion.*2686

*When I actually draw out the physical arrangement for this thing, here is what it looks like.*2696

*The cell itself is going to look like this: I'm going to have my digital voltmeter...and because they are both ions, I'm going to have ClO _{3}^{-} (Cl...well, I'm going to leave this as ClO_{3}^{-} for right now), and I'm going to put the MnO_{4}^{-} in there.*2705

*This is platinum; none of these species that has been oxidized or reduced actually becomes a metal, so I can't really use that as an electrode.*2737

*Therefore, I have to provide some surface for this chemistry to take place.*2746

*Again, the MnO _{4} will go to the surface; the electrons will come, and they will join together on that surface.*2750

*It provides a platform, a meeting place, for the two species.*2756

*This is also going to be a platinum electrode here.*2760

*This is going to measure 0.32.*2765

*OK, when we open the circuit (so again, this is measuring the pressure that the electrons--the extent to which the electrons want to go this way; when I open the faucet here, that is when the electrons are going to start to flow), here is what happens.*2768

*The ClO _{3} turns into ClO_{4}^{-}, so ClO_{4}^{-} starts to show up over here.*2793

*This starts to go away.*2802

*And, MnO _{4}^{-} starts to turn into Mn^{2+}.*2804

*This starts to go away; Mn ^{2+} starts to show up in solution.*2811

*That is what is going on.*2816

*We can set up a standard reduction potential for any species that we want, relative to the hydrogen electrode.*2821

*Some are going to be positive numbers; some are going to be negative numbers, because hydrogen, we set at 0; that was our choice (international agreement).*2829

*Because we have, now, a table of all of these reduction potentials, well, I can create any galvanic cell I want, just by mixing and matching species.*2838

*All I have to look at is which one has a higher reduction potential.*2846

*The one that has the higher reduction potential, between any two species that I choose--that is going to be reduced.*2849

*The other one--the equation has to be flipped, because that is going to be oxidized (right?--oxidation-reduction: they come in pairs).*2855

*I balance those half-reactions; I add them the way that I did for the acid-base section earlier, last lesson; when I add those, I add the reduction potentials, and that gives me the standard cell potential for that galvanic cell.*2861

*It gives me a measure of just how badly that cell wants to start pulling electrons.*2881

*The higher that number (the higher the cell potential), the more work I'm going to get out of that particular process.*2888

*That is what is going on.*2897

*That is what is important: it is really, really important that you actually understand what is happening, physically.*2899

*If you don't get this, none of the math will make sense.*2905

*Absolutely none of the math will make sense.*2909

*So, hopefully, think about this; think about what is going on; we will do some more problems later on.*2912

*Until then, thank you for joining us here at Educator.com.*2917

*We'll see you next time; goodbye.*2920

0 answers

Post by Christian Fischer on May 17, 2014

Hi Raffi . I have a question wioth respect to electrochemistry which seems like a paradox for me, and I thought you - with your big brain - might be able to guide me in the right direction of understanding it. Here it comes:

The symbol of charge is Q but the SI unit of charge is coulumb which is the Charge of approximately 6.241Ã—1018 electrons. But charge is not itself defined, only in terms of Coulomb, and coulomb is defined in termes of Charge. Its SI definition of Coulomb is the charge transported by a constant current of one ampere in one second:

1C=1A*1s = (q/s)*s=q= charge,

Here is the question

It seems to me that Coulumbs are defined in termes Amps which are defined in terms of charge but charge itself is not a unit of measurement, so how is it possible to define coulumbs and amps in terms of charge when charge is a property and not something we can measure? Charge is part of the equation for Amps A=q/s and Coulumbs=1A*1s = (q/s)*s=q= charge, How does it make sense?

3 answers

Last reply by: Professor Hovasapian

Wed May 14, 2014 1:35 AM

Post by Rafael Mojica on May 2, 2014

Hello Raffi Hovasapian,

I carefully looked into my chart and the only rxn for Mn that i was able to find was Mn -- Mn2+ +2e. How can i manipulate that equation? because i was not able to find the specific one with the hydrogen and the oxide.