Biochemistry Acids & Bases

Hello and welcome back to Educator.com and welcome back to Biochemistry.0000

Today we are going to start talking about acids and bases.0004

This is going to be a reasonably quick review, because again, we just want to get our feet wet, get used to some concepts from general chemistry before we actually dive into the biochemistry proper.0008

Acids and bases, acid base chemistry is profoundly, profoundly, profoundly important.0019

We are going to be going through most of the stuff that you've seen.0025

We are going to be talking about pH.0028

We are going to be talking about how acids and bases behave.0030

What I'd like you to take away from the notion of acid-base chemistry is how acids and bases actually behave and the idea that the only thing that moves in an acid-base reaction is the proton- is that hydrogen ion.0033

If you can concentrate just on that aspect, you can use what you know - your intuition that you've gained from general chemistry and from organic chemistry - to actually understand a huge amount of biochemistry.0047

Don't get lost in the details here.0061

I mean, yes, the details are important, but what's important is understanding that it's only a proton that is moving.0063

That's it; that's the only thing that's going on here.0069

That's the important part to take away from this.0073

OK, let's go ahead and start our review.0075

Let's go ahead and begin with water.0078

Let's begin with H₂0.0089

OK.0094

H₂0, water, in solution, actually dissociates a little bit.0096

It releases a free hydrogen ion, and it releases a free hydroxide ion.0102

We write that this way: H⁺ + OH^-.0109

Now, you remember from general chemistry, we want to be able to have some sort of numerical measure of the extent to which something dissociates.0117

In other words, how much H⁺, how much OH^-, is floating around in water.0126

Well, you remember there is something called the equilibrium constant, you have some reaction, what you do is you take the concentration of the products raised to their stoichiometric coefficients, divided by the reactants, raised to their stoichiometric coefficients.0131

Now, let me go ahead and put some state symbols here, this is aq, and this is aq; and you remember of course, when we have liquids or solids, they actually don't show up in the equilibrium expression.0145

So in this case, our K_eq is going to equal the concentration of H⁺ times the concentration of OH^-; but again, this is liquid so it doesn't show up in the denominator, so that's all.0157

Now, we call this, because it's for water, we give it a special symbol K_w.0172

We have measured this, and it's actually equal to 1.0 x 10^-14 at 25°C.0176

At 25°C, this ion product constant, this dissociation constant for water happens to be 1.0 x 10^-14.0186

That is a very small number.0196

What that is telling you is that most of it stays as water, but a little bit of it, very, very little, dissociates into that.0199

OK, that's it.0206

This is a very, very important relation.0210

Now, in any and every aqueous solution, the hydrogen ion concentration times the hydroxide ion concentration has to equal 1.0 x 10^-14.0212

In other words, if the hydrogen ion concentration rises for some odd reason, the hydroxide ion concentration drops, because their product has to equal a constant.0234

That's what this whole idea is.0247

A constant is something that doesn't change.0249

The particular values of the individual species involved in the constant, they might change; but their product doesn't change.0252

If one goes up the other has to go down.0258

OK.0262

Now, let's introduce something called the p-scale.0264

You already know this, but we will mention it anyway.0266

We talked about pH and that's equal to -log of the hydrogen ion concentration.0272

Again, hydrogen ion concentrations are generally very small, things like 2.6 x 10^-3, very tiny numbers.0282

So, instead of dealing with those numbers, they developed this idea of a p-scale because they wanted to deal with numbers that are just more natural like 6.2, 10.6, things like that.0289

It doesn't really matter.0300

I personally prefer to deal with concentrations directly, as opposed to taking the negative logarithm of them and using the p-scale; but in biochemistry, they tend to use the p-scale almost exclusively.0303

That's all it is, it's just the -log of whatever concentration.0316

We also speak of a plH, that's the -log of the hydroxide ion concentration.0319

You can also talk about, let's say pCl, that's equal to the -log of the chloride concentration.0325

It could be p-anything, so p-scale, but most of the time, we talk about pH.0334

We pick one thing to discuss as our standard in a particular aqueous solution.0339

We've chosen the hydrogen ion concentration as the standard against which other things are measured.0344

OK, so let's see.0350

If a hydrogen ion concentration times the hydroxide ion concentration in any aqueous solution is equal to 1.0 x 10^-14, well, in terms of pH, if I take the negative log of both of these, I get the following.0354

I get the fact that the pH plus the pOH of an aqueous solution is equal to 14.0371

They're the same thing.0378

One deals with the concentrations directly; the other deals with a different representation of concentration.0379

That's all.0386

OK.0388

A pH of...well, actually, I'll introduce that in just a second.0390

OK, so let's see what we have here.0397

Let's do an example just really quickly.0399

OK.0405

The pH of a glycine solution - glycine is an amino acid and we'll get to that in just a couple of lessons when we start to discuss proteins - is measured to be 5.4.0408

OK.0432

The question for us is "What is the hydroxide ion concentration in the solution?".0433

OK.0441

Well, nice and easy; we just use this- right here.0446

That's our basic relation.0453

If the pH of our solution is equal to 5.4, well, that implies that 5.4 equals -log of the hydrogen ion concentration; that's the definition, right?0459

Let's go ahead and take the antilog raise it to power, that means that the hydrogen ion concentration is equal to 10^-5.4, right?0474

Move the negative sign over antilog; just raise, take 10 to that particular power and when we do that, we get 3.98 x 10^-6M.0486

So, the concentration of the hydrogen ion in this particular glycine solution is 3.98 x 10^-6mol/l.0502

OK.0509

Well, we know that the hydrogen ion concentration times the hydroxide ion concentration is equal to 1.0 x 10^-14.0511

Therefore, the hydroxide ion concentration equals 10^-14 divided by the hydrogen ion concentration, which is 3.98, times 10^-6; and when we do this calculation, we get 2.51 x 10^-9M.0524

That's it.0550

Nice and simple, basic relation.0554

Hydrogen ion concentration times the hydroxide ion concentration at 25°C is 10^-14.0556

If I have one, I have the other.0562

If I wanted to go to the pH, if I wanted to, let's say, do the pOH, what I would get is the following: pOH, I would just take the -log of this hydroxide ion concentration and I get 8.6.0565

Nice, straight-forward, nothing strange going on here.0578

OK.0584

Now, let's talk about the pH scale.0587

Any pH from 0 to 7, we call that an acidic solution.0592

The pH of 7 is called a neutral solution, and the pH of 7 to 14 is called a basic solution.0599

What that means in an acidic solution, the hydrogen ion concentration is greater than the hydroxide ion concentration; they're not equal.0613

In a neutral solution, the hydrogen ion concentration actually equals the hydroxide ion concentration- 10^-7 = 10^-7, 10^-7 x 10^-7 = 10^-14.0621

That's where this scale comes from, 0-14.0636

It is based on that.0638

Actually it's based on that number.0642

And, a basic solution is when the hydrogen ion concentration is less than the hydroxide ion concentration; or another way of thinking about it, when the hydroxide ion concentration exceeds that of the hydrogen ion concentration.0646

No matter what they are, they multiply to 10^-14 at 25°C.0658

Different temperature, it's a different K_w.0664

OK, good.0667

Now, let's go on and discuss strong acids.0671

Strong acids are acids that completely - ooh, might be nice if I actually knew how to write here, Oh my God, what is happening?- let's try this again.0682

There we go.0701

Strong acids are ones that completely dissociates when put into water.0703

In other words, if I take some hydrogen chloride and if I drop it into water, what happens is that it actually becomes an aqueous solution of hydrogen chloride; and then it completely dissociates into H⁺ and Cl^-.0715

This is actually a two-step process.0732

I take the pure hydrogen chloride- see this is the thing, when we speak of things like HCl, H₂SO₄, H₃PO₄, we refer to them as acids; because 100% of the time in our dealings with them, we're going to be dealing with them as solutions.0734

As solutions they are acids; they are acids when they dissociate.0751

When they're like this in pure form, they're not acids.0755

We still name them as such, but what happens is when we speak about acids, we're speaking about an aqueous solution.0757

We've taken this thing and we've dropped it into water, and then first of all, it becomes solvated; water surrounds it, and then water takes it apart- it breaks up.0763

Anything that completely dissociates, in other words, when there is no HCl left, we call it a strong acid, because it completely separates into free ions.0772

Another example would be HNO₃; I'm going to skip this part.0782

H⁺ + NO₃^-, a strong acid, when you drop this in the solution or when you drop this into water, into a solvent, all of this comes apart; there is none of this left.0788

There is only H⁺ floating around and NO₃^- floating around in a nitric acid solution.0799

That's what's happening.0806

Strong acids don't have an equilibrium constant precisely because there is no reactant left over to be used as the denominator of the equilibrium constant.0808

Anytime, if you're looking through a list of equilibrium constants for acids, then if it's not there, it's a strong acid.0820

That is pretty much how you tell.0831

OK.0833

A strong base, same thing, except instead of complete dissociation, instead of producing hydrogen ion, they produce hydroxide ion.0836

An example would be sodium hydroxide, it completely dissociates into sodium plus hydroxide ion, OH^-.0847

That's it.0858

There is none of this left.0860

The only thing you have floating around in a sodium hydroxide solution is a bunch of sodium ions which are harmless; and then you have the hydroxide ions which are not harmless.0861

OK.0872

Now, we're going to get to the important stuff: weak acids and bases.0873

This is where everything gets really, really exciting- weak acids and bases.0875

Well, weak acids and bases, exactly what you think it is; weak acids and bases are ones that don't completely dissociate.0887

However, before I discuss that, I'm going to write down the thing that I mentioned early on before we started this lesson, the only thing that moves.0896

The only thing that moves in acid-base reactions is H⁺- the proton.0908

Remember that.0921

The only thing that moves is the H⁺, that's it.0923

H⁺ is going to bounce from one species to another species.0925

The species that it goes away from is the acid; the species that it actually goes to is the base.0929

That is all the acid-base base chemistry is about, the rest is just math.0934

OK.0939

Let's talk about weak acids.0943

I'm actually going to start this on another page, and I'm going to go to a blue ink.0946

I'm going to start with an example as opposed to a definition.0954

And again, a weak acid is one that just doesn't dissociate completely, so I'm going to use hydrofluoric acid as my example.0960

HF + H₂O, when I take hydrogen fluoride, I drop it in water, a reaction takes place between hydrogen fluoride and water.0965

Hydrogen fluoride gives up its hydrogen ion, water takes the hydrogen ion.0976

This is the acid; this is acting as the base.0982

What you get is the following: H₃O⁺ + F^-.0987

Now, I'm going to write this in another way.0993

I wrote it this way simply because the reaction that is actually taking place is this.0995

Anytime there is an acid, there is also a base.1001

The acid is the thing giving the hydrogen ion, the base is the thing taking it.1003

In this reaction, hydrogen fluoride is acting as the acid, H₂O is acting as the base.1007

What you get is a hydronium ion, and you get F^-.1011

There is another way that we actually write this which tends to be a little bit more popular in biochemistry1015

When dealing with weak acids, they generally drop this H₂O part and they write it like this: HF ⇌ H⁺ + F^-.1023

They say that this hydrogen fluoride dissociates a little bit to produce some hydrogen ion and some fluoride ion.1033

Now, notice this equilibrium arrow- this is in equilibrium.1039

It doesn't go to completion; all of it doesn't dissociate.1042

In fact for weak acids, most of it does not dissociate; very little, in fact, dissociates.1046

We can write an equilibrium expression for this.1052

By the way, this is aq, and this is aq, and this is aq.1056

So, everything shows up in the equilibrium expression.1061

The equilibrium expression for an acid is, for this one is H⁺F^-, actually for all weak acids, it's going to be the hydrogen ion concentration times the conjugate base concentration, over the concentration in moles per liter of the original acid; so it's products divided by reactants.1065

Well, it's an equilibrium expression; but because we are talking about standard acid reaction, we actually call it K_a- A for acid.1090

That's it.1099

It's called the acid dissociation constant; and there is a list of acid dissociation constants for the different weak acids: hydrofluoric acid, phosphoric acid, nitrous acid- whatever it is.1102

OK.1124

In this particular case, the K_a for hydrofluoric acid happens to be 7.2 x 10^-4.1126

OK, this is very small.1136

That means that this is a tiny number, and this is a big number.1140

What that means is that very little hydrogen fluoride actually dissociates to produce hydrogen ion in solution.1144

Yes, it is an acid, and yes, it does dissociate, and yes, the pH of this solution is going to be less than 7; but it's not going to be altogether that much less than 7.1152

It is a weak acid, not complete dissociation.1163

There is an equilibrium that takes place here.1166

OK.1168

Let's do an example - example 2.1171

Let's do phosphoric acid because that is a very, very important weak acid in biochemistry.1177

Phosphoric acid is H₃PO₄.1185

Now, when acid dissociate, they dissociate one hydrogen at a time.1188

In this particular case, it's not just going to give up three hydrogens; it's going to give up one hydrogen, then another, then another.1192

There is an equilibrium expression for each dissociation, so for phosphoric acid, what you have is the following.1199

When it dissociates, by the way, we're probably going to end up doing this version of it without explicitly mentioning the water; but it's important to remember that it's water that is actually reacting.1205

So, H⁺ and H₃O⁺, they're interchangeable, they're the same thing.1217

There is no H⁺ floating around in a solution freely.1224

What it is, it is an H⁺ that is attached to a water molecule.1226

So, these are the same thing, this is just a different way of writing it.1230

Here, we write this version of the equation simply to explicitly show that water is acting as the base in this acid-base reaction.1234

H₃PO₄ dissociates into H⁺, let me go back to blue so that I'm consistent here...dissociates into H⁺ + H₂PO₄^-.1244

It has a K_a1, a first dissociation constant.1261

Well, now, H₂PO₄, that can also dissociate.1266

It has a hydrogen ion that it can give up, and it does so under the appropriate circumstances, and it breaks up into H⁺ + HPO₄^2-.1271

It has an acid dissociation constant associated with this reaction; it is called the K_a2.1282

Sorry about that.1288

Well, HPO₄, there is still a hydrogen ion, so under the appropriate conditions, it too can dissociate to produce H⁺ and PO₄^3-- a phosphate ion.1289

It, too, has an acid dissociation constant, that's the K_a3.1304

So that's it.1310

Let's go ahead and take a particular reaction as our example, and do a little bit more with it.1312

Let's just go ahead and take the second dissociation of phosphoric acid.1319

Our K_a2 is going to be the concentration of HPO₄^2-, the reactants, times the H⁺ concentration divided by the H₂PO₄^- concentration.1323

That's it, and in this particular case, the second acid dissociation constant happens to be 1.38 x 10^-7- very, very, very small.1339

OK.1351

So, again, let me go ahead and write this out explicitly using water as our other reactant, H⁺ + HPO₄^2-.1353

In this particular case, the H₂PO₄, this is acting as the acid because it's the one that's giving up the hydrogen ion.1369

The water is acting as base because it's the one taking the hydrogen ion.1380

OK.1390

Now, there is a certain nomenclature that is used.1391

Anytime you have a particular species, if it's the one giving up, we call that the acid.1396

The thing that it turns into once it has actually given up its hydrogen ion, which is this thing right here, we call it the conjugate base; and what that means in this particular case is because if we were to look at this reaction the other way around, in other words, if we were to start with this, now, what happens is - oops, this is not H⁺ this is H₃O⁺ because I actually used H₂O here - now, what happens is if I started with the HPO₄^2- and one would look at it from the perspective of this going that way, then this is actually going to act as the base and it's going to be H₃O⁺ that is going to act as the acid.1404

This is going to be the proton donor; this is going to be the proton acceptor.1450

Now, acid conjugate base- however, when we're looking at it from this perspective, when this is the reactant and this is the reactant, what's acting as the acid is the H₂PO₄^-; what's acting as the base is the H₂O.1455

So again, nomenclature is actually not that important.1471

What's important is that you understand what's giving up the hydrogen, what's taking the hydrogen.1474

The giver is the acid, the taker is the base- that is what's important.1478

OK.1484

And, again, notice, the only thing moving here is the H⁺.1486

Alright, now, let's talk about weak bases.1491

OK.1498

Let's just say first of all, a base, whether weak or strong, is something that produces hydroxide ion in a solution.1500

Well, a strong base does it directly.1511

Strong base produces OH^- directly, and by directly, I mean it just dissociates; and when it dissociates, hydroxide just goes in the solution.1515

An example of that would be potassium hydroxide.1525

When I drop that in the solution, I get K⁺ and OH^- now floating around in the solution.1528

Notice the single arrow, there is no equilibrium here, there is none of this left.1534

When it dissociates, it's all this and it's all that.1538

A weak base produces hydroxide indirectly and here is the reaction.1541

I'm going to use ammonia as our base.1564

Oops, no, we definitely don't want that- these crazy lines that show up here.1567

OK, we have NH₃- ammonia.1575

When you take ammonia and you drop it into water, a reaction takes place between the ammonia and the water.1579

I'm going to write the water as HOH, that's my personal way of doing it because it actually makes more sense to me.1585

So, I don't write H₂O, I write HOH and you'll see why in a second.1592

In this particular case, an H from the HOH from water, now, water is going to act as the acid; it's going to give up one of its hydrogen ions, and ammonia is going to take it.1597

In this case, water is acting as the acid, this is acting as the base; and what you get is the following: NH₄⁺ + OH^-.1611

Again, we've produced hydroxide in this solution; however we've done it indirectly.1621

A weak base is something that actually extracts a hydrogen ion from water to leave a hydroxide behind; that's why I write it as HOH.1627

I don't like writing it as H₂O.1637

It's just a personal thing.1638

You can do it anyway you want, but this is the reaction that's taking place- very, very important.1639

The generic reaction is this: any base, B + water goes to BH⁺ + OH^-.1644

This is the generic.1662

OK.1663

In this particular case, my B, my base is NH₃- ammonia.1664

It can be anything, any weak base, anything that behaves like this.1667

OK.1673

We can write an equilibrium expression for this.1675

Let me rewrite the expression.1677

We had ammonia + water produces ammonium + hydroxide.1680

OK.1689

There is a way of writing an equilibrium expression for this.1690

It is product NH₄⁺ x OH^- / NH₃.1693

Water does not show up because water is a liquid.1711

OK.1715

Now, when the base reaction is taking place, we call it K_b.1716

I call it the base association constant, and the reason I call it association constant is that the dissociation constant or just the base constant, is because what's happening is that the base is actually associating with an H to become NH₄⁺.1726

So, base constant, acid constant, what's important is the reaction that takes place.1742

OK, let's go ahead and do an example of another base- example 3.1748

So, pyridine, which looks like this.1755

OK.1763

Pyridine shows up, actually, doesn't really matter.1767

Pyridine is a weak base, and notice, it has a nitrogen.1777

It is going to be a common theme, weak bases are pretty much going to have nitrogen in them.1787

It's a weak base.1791

Let's see what its reaction as a base looks like.1794

OK.1797

So we have our pyridine and actually what I'm going to do it I'm going to write it as pyr.1798

Pyridine plus water is going to be pyridine H⁺ + OH^-.1805

That's it.1817

The K_b for this is going to be the concentration of pyridinium ion times the concentration of hydroxide ion, divided by the concentration of pyridine; and in this particular case, is equal to 1.7 x 10^-9.1819

Again, this is a small K_b, that means that the reaction hasn't gone very far forward, that it's mostly this way, that in a pyridine solution, most of it is in this form.1842

There isn't a lot of this; there isn't a lot of this.1856

Now, the pH is still going to be greater than 7 because you've taken water, which is pH7, and you've created this extra hydroxide ion floating around; so the pH is definitely going to go up.1860

In other words, the hydroxide ion concentration went up; that means the hydrogen ion concentration went down, the product still equals 10^-14, but now, the concentration of hydroxide is higher ,so the pH is above 7.1875

OK.1887

Now, let's see what we can do.1890

So, again, when you're seeing it from the perspective of an acid, when you're doing the acid reaction, you're going to have something called the K_a.1893

When the particular species that you drop into water behaves as a base, like pyridine or like ammonia, you're going to look at it as a base and you're going to have a K_b.1904

OK.1916

Now, let me write a couple of things here.1918

Let me go to a different page.1921

Now, when a species has an H⁺ it can give up, it is in its acid form.1925

When it can accept an H⁺, it is in its base form.1956

So, this whole idea of an acid or base is not absolute.1972

It's not that something is an acid or something is a base, it just depends on how it is behaving.1978

If it's in a particular form where it has a hydrogen to give up and it does give it up, it's behaving as an acid; but if it's the other way around, if it turns around and actually takes a hydrogen ion, then it's acting as a base.1985

The same species can be both; it just depends on your perspective.1997

So, again, in science, we need a particular perspective, we need a point of reference from which to measure something, from which to look at it.2001

That's what we're doing here, but it just depends on what's happening in the particular situation.2009

So for example, NH₃, well, ammonia, when it's behaving as ammonia, it's going to behave as a base; but, if I happen to have, let's say some ammonium chloride, salt, NH₄Cl and I drop that NH₄Cl into water, now, what's happening is that there is a bunch of ammonium floating around in the solution, now, this species is not going to behave as a base.2017

Now, it has that extra hydrogen ion and it's going to actually give it up, now, it's going to behave as an acid.2044

So acid-base behavior is based on behavior.2052

It's not that this particular species is an acid or a base.2055

Acid-base reactions have to do with what's happening, what's giving the H, what's taking the H.2058

In this particular case, this NH₃ and this NH₄, here is the base form, here is the acidic form.2064

For our purposes, since most of the time we deal with NH₃, in the list of equilibrium constant values, we've listed it under the heading of a base simply because we tend to think of it more as a base, but it has an acid version.2069

Hydrogen fluoride, we think of it as an acid, well, it is an acid; but when it's in this form, when it's already lost it and when it's behaving this way, it's actually acting as a base, because in this case, now, it's going to take the hydrogen from something else.2088

For example, if I had a sodium fluoride, salt, and if I took that sodium fluoride and if I dropped it in a solution, now, what I have floating around in the solution is a bunch of sodium ions and fluoride ion.2103

Well, F^- doesn't have an H to give up, however, this F^- is going to end up reacting with the water in a solution; it's actually going to steal a hydrogen ion from the water.2119

So, in this case, it's going to be acting as a base to turn back into its acid form.2130

So, that's what you want to think of.2136

Two sides of a coin, heads or tails, at any given moment we have to decide which perspective- is it behaving as an acid or is it behaving as a base?2138

It can do both depending on what's happening.2148

Now, can we write, well, actually we just talked about that yes, we can write these the other way.2150

So what's important here is the perspective.2161

When it undergoes this reaction, let's just say HA + H₂O goes to H₃O⁺ + A^-- this is the acid reaction.2165

That's it.2186

But, if I write it this way, I say A^- + H₂O goes to HA + OH^-- that's the base reaction.2190

Be very clear about the species here.2202

Here, HA, it's behaving as an acid; it's giving up its water to produce hydronium and this other species, the base version of its species.2205

Here, if I take the A and if I drop it into water, what it does is it steals H from the water to produce hydroxide and to produce HA, now, it's acting as a base.2216

It just depends on the perspective.2226

That's what's going on here.2228

OK.2230

Let's see, one final comment, so this idea of K_a and K_b, when a species is behaving as an acid, it has a K_a; when a species is behaving as a base, it has a K_b.2235

Here is a relationship between these two.2253

K_a x K_b = K_w = 10^-14.2258

So, let's just use hydrofluoric acid as our example.2267

HF + H₂O goes to H₃O⁺ + F^-.2272

We said that the K_a here - let me see, let's go back, what did we say K_a if HF was, 7.20 x 10^-4, I think...right...7.2 x 10^-4...that way - but if I happen to, let's say, take a sodium fluoride, salt, and if I happen to run this reaction, now, because it's acting as a base, if I do any math with it, I have to use the K_b, not the K_a.2286

Well, the K_b happens to equal the K_w over the K_a.2325

Now, I just wanted to introduce it here towards the end.2330

I'm actually going to be discussing more of this in the next lesson, so don't feel like this just..."Wait a minute, what's going on here?2332

I am going to be beginning the next lesson with the discussion of K_a, K_b and the relationship between those two; and we'll do some more problems.2340

Until then, thank you for joining us here at Educator.com.2348

Take care, bye-bye.2350