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Lecture Comments (5)

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 6.241×1018 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.

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 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

Transcription: Cell Potential

Hello, and welcome back to; 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), 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

H2 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: Zn2+...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 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 Ksp 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 H2 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: Zn2+ + 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 E0cell, 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, 2 (I can't even do basic arithmetic!)...2 Al3+ + 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 Mg2+ + 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 Al3+ + 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

MnO4- + H+ + ClO3- becomes ClO4- + Mn2+ + H2O.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 H2O; 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: MnO4- + 5 electrons + 8 H+ (this is exactly what it says in the chart--this is what you are looking for) goes to Mn2+ + 4 H2O.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 ClO4.2476

Well, this one, plus 2 H+, plus 2 electrons, goes to ClO3- (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 H2O.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: MnO4- + 5 electrons + 8 hydrogen ions goes to Mn2+ + 4 H2O; and we leave the reduction potential as is, equal to 1.51 volts.2527

This one we reverse--we write: ClO3- + H2O goes to ClO4- + 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 MnO4- + 10 electrons + 16 hydrogen ion → 2 Mn2+ + 8 H2O; and still, nothing changes as far as the reduction potential (1.51 volts).2576

Here, I have 5 ClO3- + 5 H2O goes to 5 ClO4- + 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 H2O leaves 3 H2O; 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 H2O.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 ClO3- (Cl...well, I'm going to leave this as ClO3- for right now), and I'm going to put the MnO4- 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 MnO4 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 ClO3 turns into ClO4-, so ClO4- starts to show up over here.2793

This starts to go away.2802

And, MnO4- starts to turn into Mn2+.2804

This starts to go away; Mn2+ 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

We'll see you next time; goodbye.2920