Biochemistry Oxidation-Reduction Reactions

Hello and welcome to Educator.com, and well come back to Biochemistry.0000

Today, we are going to talk about a very, very important topic: oxidation-reduction reactions.0004

In previous lessons, we have talked about ATP; we have talked about phosphoryl transfer.0010

We saw how we can take an endergonic reaction, couple it with the energy available from the hydrolysis of ATP to actually make that reaction take place.0016

We also saw that there were other molecules that have very, very high -ΔGs, and we can couple that with the ATP reaction, reverse the ATP reaction and actually form ATP.0027

The truth is that most of the ATP that is made in the body happens through a very complex oxidation-reduction process, which is essentially what metabolism is.0042

I mean, all you are doing is, you are taking in food, these highly reduced compounds, and what the body does is it oxidizes these things.0051

It takes away the high-energy electrons, and it eventually gives them to oxygen using the energy that is available; that is what produces the ATP.0060

So, directly or indirectly, whether it is some reaction that is taking place or ultimately electrons moving from reduced compounds all the way to oxygen through the electron transport chain, directly or indirectly, oxidation-reduction is responsible for all of the work that is done by the body.0070

This is a profoundly important topic; let’s go ahead and get started.0090

OK, let’s see.0096

Now, of course, you have all seen oxidation-reduction reactions; you have all had general chemistry.0100

I am going to be going through it again, sort of, from the beginning; but there are certain things that I am going to not quite touch on in as much detail.0107

I would strongly urge you to take a look at the oxidation-reductions sections of my AP chemistry course for Educator.com if certain fundamental things do not really make sense.0115

I go through a full, very complete, slow discussion with multiple examples during those lessons; but hopefully, what we do here will be enough for our purposes.0127

Redox reactions or oxidation-reduction reactions, they involve - hopefully, as you know - the loss and gain of electrons, the loss of electrons from one species and the gain of those electrons from another species, those electrons by another species; and that is it.0138

That is all that is really going on; you know, you do not want to let these sizes of the molecules scare you.0175

All that is happening is that one species is giving up electrons to another species.0181

It might pass through 5, 6, 7, 8 different intermediates, but it is still going to be something oxidized, something reduced.0186

Something loses electrons, something gains electrons.0195

During that process, hydrogens are involved; there is going to be, maybe, some rearrangements.0197

The molecules may not necessarily look the same afterward, but really, it is the electrons that we are concerned about because that is where the energy is.0202

OK, oxidation- it is the loss of the electrons.0211

Reduction is the gain of electrons.0220

Let’s just take a look at an example here, so example 1.0230

We have an equation; let me go ahead and do this in blue.0236

We have magnesium plus, let’s say it comes in contact with some aluminum ion, and it goes to magnesium ion plus aluminum.0241

When we take a look at oxidation-reduction, we are accustomed to looking at oxidation state of the particular species to decide what is being oxidized and what is being reduced.0253

In this very simple example, magnesium has a 0 oxidation state, and it has gone to a +2 oxidation state, so it has been oxidized.0261

Well, aluminum+3 to a 0, it has been reduced.0269

So, magnesium, it has been oxidized; it has lost the electrons and aluminum.0276

It has gained those particular electrons.0285

Now, of course, this is not balanced; the charges do not balance here and that is fine.0289

We will go ahead and deal with the balancing in just a second here, but the idea is something loses, something gains.0293

Now, oxidation-reductions are broken down into 2 half reactions- the oxidation reaction and the reduction reaction.0300

Let’s go ahead and deal with the oxidation here.0307

Magnesium, it turns into magnesium2+ and 2 electrons, so we represent this this way.0312

It has lost 2 electrons to become magnesium2+, and the 2 electrons are on the right-hand side.0320

Now, the reduction process, we have the aluminium ion which is 3+ has been, when you add 3 electrons to it, aluminium becomes aluminium ion, becomes aluminium metal.0326

It has gained 3 electrons; now, we need to balance this, and in order to balance this, we need to cancel the electrons on both sides and then add the equations.0343

In this particular case, we have 2 electrons, 3 electrons.0354

We multiply this equation by 3, this by 2, to convert this to - let me go ahead and do this in red - we have 3Mg goes to 3Mg²⁺ + 6 electrons.0357

And over here, this one becomes 2Al + 6 electrons, goes to 2 A - I am sorry - Al³ + 2Al metal.0374

And now, the electrons cancel, and when you add this straight down, you get the final balanced equation.0385

You get 3Mg + 2Al³⁺ goes to 3Mg²⁺ + 2Al metal- that is it.0393

This is pretty much how all oxidation-reduction reactions are dealt with.0406

Individual half reactions- that is all that is happening there.0410

OK, now, pictorially, what is happening is the following.0415

Should I do this on this...yes, it is fine; I can go ahead and do it on this page.0419

So pictorially, what is happening is the following.0422

You have a magnesium; you have a magnesium, and you have a magnesium.0427

Now, this magnesium has, of course, 2 valence electrons on each one, and you also have an aluminum3+ and an aluminium3+.0432

Well, this does not have any electrons on it; what happens is the following.0442

This aluminium takes that electron, takes that electron, takes that electron; and this aluminium takes that electron, that electron and that electron.0447

It takes it, or magnesium gives it up just depending on your perspective because the aluminium ion has a greater affinity for the electrons than the magnesium does.0457

In a battle of the electrons, it is the aluminium ion that is going to win; and what you end up with is, of course, the magnesium ion, which is now, completely stripped off its electrons, and you have the 2 Als that now, have the 3 electrons.0469

6 electrons are lost from the 3 magnesiums; 6 electrons are gained by the 2 aluminiums- that is it.0470

This is a simple, direct transfer of electrons from one species to another.0477

OK, now, let’s examine this a little bit.0483

Now, when we wrote this equation, when we wrote this down, when we wrote this equation, the magnesium, the aluminium, we wrote it as a foregone conclusion.0487

In other words, we wrote it as if we knew what was going to happen.0534

Well, let’s examine this a little bit.0538

But, what if we had said the following?0543

Instead of writing the equation down, if we had just told you a solution of aluminium chloride is poured onto magnesium dust, how do we know what will happen?0557

How is it that the aluminium will actually pull electrons from the magnesium metal and turn into aluminium metal itself, turning the magnesium metal into magnesium ion?0579

How do we know this, OK, or what if we had said this?0589

OK, what if we had said a solution of magnesium chloride is poured onto aluminium dust?0600

Well, now, it is the magnesium that is in ionized form, and it is being poured onto aluminium dust.0624

Now, it is the aluminium that actually has the electrons; it is the magnesium that is missing the electrons.0631

Are we able to say that magnesium is going to pull electrons from the aluminium and turn into magnesium itself, or what is going to happen?0635

How do we know this?0643

That is the whole point.0644

How do we know what will happen?0645

OK, here is how we know what will happen- something called reduction potential.0647

And you will see reduction potential listed in things called “a table of reduction potential for all kinds of species”.0658

OK, so what is a reduction potential?0664

OK, this is a quantitative - so it is a number - quantitative measure of the extent to which a given species wants to be reduced, wants to gain electrons.0668

It is a quantitative measure of the extent to which a given species wants to gain electrons- that is what a reduction potential is.0708

Now, when we measure things in science, we do not measure them absolutely.0718

We measure them relative to a standard.0724

So, when I say that a certain species has a reduction potential of, let’s say, 0.5V - and we will talk about the unit in just a minute - well, it is 0.5V relative to what?0728

OK, we need a standard reaction that we measure all of the other reactions relative to that.0739

If we have this one thing, and if we measure the reduction potentials of 50 things, it is all going to be relative to this one thing that we have chosen as our standard, as our 0 reduction potential.0749

Then, what we can do is, now that we have that list of reduction potentials, now, we can compare them among each other because they are all relative to a standard.0761

Because they are relative to a standard, they are relative to each other- that is what is going on.0770

We have chosen the following reaction as our standard reduction potential.0774

Let’s see; it is H⁺ + 1 electron goes to 1/2 H₂.0780

You can multiply this by 2; it does not matter.0790

It does not change anything.0791

The symbol for reduction potential is this E, and this is the standard that is set at 0V.0794

OK, and a volt is a Joule per Coulomb.0801

OK, so relative to this reaction, all of the other species, the reduction potentials are measured relative to this.0811

OK, for example, in the case of our magnesium, so Mg²⁺ + 2 electrons goes to magnesium metal.0818

And in a table of reduction potentials, what you will see is this -2.37V.0832

OK, now, again, reduction potential, we had to choose either a reduction direction or an oxidation direction.0841

We chose the reduction so that we have a standard by which to measure everything.0850

What this means is that if I put these species, the magnesium, metal magnesium ion, hydrogen ion, hydrogen gas, if I put them in the vicinity of each other because 0 is higher, is more positive than -2.37, that means that this H⁺ will actually take electrons from the magnesium.0855

That means this has a higher reduction potential than the magnesium ion does.0880

So, in a competition for electrons, it is the hydrogen ion that will actually win the battle- that is what this means.0885

The numbers, the higher the reduction potential, that means it has a greater affinity for any electron that happens to be in the vicinity, and usually, the electron that happens to be in the vicinity is going to come from the other species if that species has electrons to give up- that is what is going on.0892