The Science of Everything Podcast - Episode 51: Acids and Bases

Episode Date: November 23, 2013

A discussion of acids and basis, including a definition and description of the concepts of acidity and basicity, a look at the relationship between the concepts of strength, concentration, and corrosivity, a discussion of weak acids and acid-base equilibria, and an explanation of pH and how buffer solutions act to stabalise pH levels in solution.

You're listening to The Science of Everything podcast, episode 51, acids and bases. I'm your host, James Fodor. In this episode, we're going to talk about acids and bases and how they react with each other. We'll look at neutralization reactions, the difference between strong and weak acids and bases, and how that relates to issues of the strength of acids, the concentration, and particularly the effect of corrosion, and how those are different, because those concepts are often confused. I'll also talk about water, and it's special. properties of self-ionization, I'll talk about acid-base equilibrium, the pH scale,

and we'll also look at buffer solutions, which are quite interesting. So, let's get started. First of all, the natural place to start is by defining what we mean by acids and bases. Now, there are actually at least three different definitions of acids and bases, and they sort of have a temporal sequence associated with them. The one we're going to use is called the Bronstead-Lowry definition, which is not the most recent one or sort of the most sophisticated, but it's, I think, the easiest to understand conceptually, and it will be sufficient for our level of analysis in this podcast. So, according to the Bronstead Lowry definition of acids and bases, an acid is any chemical substance that donates a proton in a reaction, while bases are

substances that accept a proton in reactions. So they're just the inverse of each other in that A proton, remember, that's just a charged, a positively charged ion, you know, a single proton. People also sometimes talk about it as a hydrogen ion, but that's obviously just the same thing as a proton, so I'll generally talk about protons. But if you see it written down, it's generally written as H-plus, which means just a hydrogen ion, or in other words, a proton. So you might think this is sort of an odd definition. Why would we single out this particular behavior of donating and accepting protons?

because it turns out it's very useful and it explains a very wide range of different types of chemical reactions and so it's an important categorization that's used in chemistry and so it's very useful to know about. So an important thing to understand about this definition of acids and bases

is that an acid and a base must always occur together in a reaction so you can't have an acidic reaction without having also a base involved and vice versa. The proton is whatever substance it does the donating, that is the acid gives up the proton, and the base is whatever substance accepts the proton. And obviously, if you have one chemical substance giving up a proton, then you have to have something accepting the proton.

And vice versa, if you have something accepting a proton, the proton has to come from somewhere, so there has to be something giving up the proton. So you always have to have an acid and a base occurring together, or interacting together in a chemical reaction, if it's going to be an acid-base reaction. And also, you may have remembered from previous episodes how we talked about electrochemical reactions, oxidation reduction, and various other types of reactions. These are not mutually exclusive. You can have redox reactions occurring at the same time as an acid-based reaction. Or, in other words, acid-based reactions can also be redox reactions and also can be decomposition reactions or substitution reactions and different other types of reactions that we looked at in earlier episodes.

Also, some substances can actually act as acids or bases depending on the circumstance and depending on what they're reacting with. These are called amphoteric substances, and water is a very good example of an amphoteric substance. It can be an acid or a base, and generally sort of is both at the same time, and we'll talk more about that when I get to talking about the self-ionization of water. Okay, so now that we have a basic understanding of acids and bases and what they are, we'll move on to talk about neutralization reactions.

A neutralization reaction is a chemical reaction whereby an acid and a base react with each other to form generally an ionic salt, although it's not always an ionic salt, but that's a common type of neutralization reaction. The main point about a neutralization reaction, though, is that the acid and the base sort of cancel each other out. The acid donates a proton to the base, and so the base sort of becomes inert in that it's not going to act as a base anymore, and vice versa with the acid. It's already donated as proton, so it will not act as an acid anymore. said, not all acids and bases sort of behave in this way, particularly ampheteric substances can sort of flip back and forth. They can act as an acid, and then a base again and then an acid again. But at least with some substances, if you react the acid and the base together, they'll form some sort of ionic salt or other inert compound, which won't react again,

and also you'll form water, which, again, is generally inert. So generally, if you take a base, like some sort of drain cleaner, and if you take a, and if you take a base, like some sort of drain cleaner, and if you take, say, hydrochloric acid and put them together, they'll neutralize each other. And if you put them in the right concentrations, you'll basically be left with some sort of salty water. This is what people mean when they talk about neutralizing an acid or a base. If something is too acidic, you add a base to it, and it becomes less acidic. And we'll talk again a bit more about this when I get to talking about pH, pH scale. But a good example of this is antacids, if anyone has heard of those,

People may take them if they're experiencing stomach pains or something along those lines. Ant acids are basically just basic compounds that react with the acids in your stomach to neutralize them, therefore raising the pH of the stomach, or in other words, making it less acidic, and therefore potentially easing the symptoms of acid reflux or whatever the other problems are. But neutralization reactions occur very commonly, so it's important to understand what's happening there. All that's happening is the acid and the base are canceling each other out, Sort of like a positive and a negative cancelling out to zero. Okay, now let's move on to talk about the difference between strong and weak acids and bases.

The strength of an acid and base, but I'll mainly just talk about acids for simplicity. The strength of an acid refers to its ability or tendency to lose its proton, or a proton. Remember, we've defined an acid as a substance that interacts by donating or losing a proton. Well, the strength or tendency for this substance to do that, to lose that proton, is referred to as a substance that. the strength of the acid. So some acids are much stronger than others. Some acids will only very reluctantly, so to speak, give up their proton, only in certain circumstances or to a limited extent, whereas other acids will very readily give up their protons or donate protons. The stronger the acid, the more readily gives up its protons. And this sort of intuitive

makes sense, because when we think about what a strong acid means, it means it's very reactive, it means it produces a lot of product or whatever reaction it's engaging in. It means it has a large effect when you add it to something. And that's because it's more of the acid molecules or atoms are donating their molecules. More of the acid molecules are donating their protons, thereby pushing their reaction forward, thereby creating more products. If hardly any of the reactant molecules donate their protons, then that you're not going to get much of a reaction happening, and therefore we call that a weak acid. And, you know, same thing for bases. So when you have a strong acid and you dissolve it in water, what happens is that the

the initial acid compound, which we can represent sort of symbolically as H-A or AH. And what this simply means is that we know that an acid has to have a hydrogen atom, or in other words, a proton with it, because an acid is a compound that donates a proton, and you can't be an acid, so you can't be an acid if you don't have a proton to donate. Now, what the A stands for is really irrelevant. It depends on what particular acid you're talking about. a single other atom or it could be a large number of other atoms. It doesn't matter, so we just call it A. So, sort of thinking about a stylized chemical formula of H-A, it's just, that just means

it's an acid with a hydrogen, we know it has to have at least one, and then other stuff, which we call A. Now, so that's our acid in the compound form. If we take that and dissolve it in water, what will happen is that the H and the A will disassociate from each other. That's what an acid does. Effectively, the asset is donating its proton. The proton, or the H, disassociates into the water, forming a H-plus ion, in other words a proton, just a free proton dissolved in the water. And then the A, which will be A-minus, because it's lost the positive charge of the proton, is also left by itself dissolved in the solution. So that A-minus there in the solution is our conjugate base, because it's the thing that's left over after the acid has donated its proton.

Remember, we started with our acid compound, which you can think of as a solid, or it could be a liquid, but it doesn't matter. Solids may be a bit easier to think about. So that's HA, our acid, and we add that to water, so we're dissolving it in water, because pretty much all, at least most acid-based reactions happen in an aqueous environment. So we dissolve this in water, and then it disassociates into H-plus, which is the hydrogen ions, and A-minus, which is just the rest of the acid compound. And that A-minus is the conjugate base, because it's... what's left over after the acid has donated its proton. Now, strong acid disassociates completely. In other words, all of those HA molecules break up into H-plus and A-minus ions in the solution. That's what we expect a strong acid to do, to react completely. A weak acid, however, does not

completely disassociate. A large percentage, or perhaps the majority of the H-A molecules, do not dissolve in the water. They stay together. Only a small proportion of them, or disassociate into H plus and A minus molecules, sorry, ions in the solution. And so the greater the extent of the disassociation into these ions, the stronger is the acid. And similarly, we can talk about the same process for bases. Strong acids thus form strong electrolyte solutions, which are therefore good conductors of electricity. If you remember, an electrolyte solution is just a solution that has charged ions in it that can carry electric charge, because you have to have charges to carry electricity, to carry electric charge.

electric current, and because strong acids disassociate into a large number of ions in the solution, therefore it's going to be a fairly strong electrolyte, lots of charges, and therefore it will carry a current fairly well. A weak acid will not carry a current so well because it won't disassociate into as many charged ions. The strength of an acid depends upon the size of the attractive force between the hydrogen and the conjugate base, that is that the conjugate base being the A minus, or the A part of the HA. Obviously, the stronger the force between the H and the A, the harder it is to pull them apart, and therefore the weaker the acid tends to be. So, strong bonds between H and A, between the conjugate base and its proton,

tend to be associated with weak acids, because the strong bonds mean it's hard for the acid to give up its proton, and therefore it's difficult to disassociate the acid. And of course, we know that the less the atom disassociate, the weaker it is. Strong acids disassociate a lot. Weak acids don't because the bond is too strong. The acid can't give up its proton because the bond between it and the conjugate base is too strong, and therefore the acid just remains in its original compound form, and it doesn't actually react. We still call it an acid because it has the potential or the tendency to give up its proton, but if it's not acting as an acid, then it's not really sort of engaging in an acid-based reaction,

therefore we call it a weak acid. It does have a tendency to act as an acid, but it doesn't do it so often. So just because a compound is called an acid doesn't mean it actually reacts, so to speak. It could just be a weak acid that doesn't react very often. And only a small percentage of all of the acid molecules in a given solution will actually disassociate, actually engage in that reaction. Hydrochloric acid, nitric acid, and sulfuric acid are examples of strong acids, some examples of weak acids of phosphoric acid and acetic acid or acetic acid, which is more commonly known as vinegar.

Okay, now this discussion about the strength of acids leads to, on to another matter which concerns the difference between the strength of an acid, the concentration of an acid, and the corrosivity of an acid. Again, we can talk about this in terms of bases as well, and it doesn't change it very much, but I'll just mostly focus on acids just because it's easier to talk about it from one perspective rather than having to keep flipping back to both. So people sort of commonly think a concentrated acid and a strong acid and a corrosive acid are sort of more or less the same thing, just different ways of talking about it. But that's not correct. Strength, concentration, and corrosivity all are different properties. You can be strong

without being concentrated, you can be corrosive without being concentrated, and et cetera. You can be one without the other. You can be any one without being any two of the others. Now, it is true that it is generally the case that strong acids tend to be more corrosive and also concentrated acids tend to be more corrosive. But there's no particular relationship between strength and concentration. So, but before we get into the details of that, let's explain what the difference are. The differences are between these concepts. Now we know what strong acids are, we just discussed that. It refers to the tendency of the acid to donate his proton, and this in turn depends upon the strength of the bond between the hydrogen atom and the hydrogen ion and the rest of the molecule. The stronger

that bond, the weaker the acid, because the less likely it is to donate the proton. That just depends upon the particular structure of the molecule and the bond strengths and all that. It doesn't depend on the concentration of the solution or anything you do to it. If you add water to a strong acid, diluting it, that does not make it any stronger, because the molecule itself hasn't changed, and the strength only depends on the molecule. It doesn't depend on how much water there is in the solution. Concentration, however, does depend on the amount of water in the solution, because that's exactly what it refers to, how much of those acidic molecules you have per litre of solution. So, concentration is basically just how much water is there mixed in with

the acid. Strength is how likely, or what is the tendency of this atom to actually donate the proton. So they are two completely different things. You can have strong acids that are highly concentrated. Those will tend to be the most corrosive. You can have strong acids that are, have weak concentration. Those will be a bit less corrosive. You can have weak acids that are strongly, that have a high concentration. Those can be corrosive. And you can have weak acids that have a low concentration, and those are unlikely to be corrosive. So the concentration, that's just how much water you've added in it, basically, or how much you've, what you've evaporated out of it if you want to think about it in the other way. Strengths depends on the chemical structure.

of the compound of acid that's in question, and not on how much water is present. And when people talk about the molarity of an acid, well, any solution, but particularly an acid, like 1 molar, 0.1 molar, or something like that, that's a measure of concentration, not a measure of strength. So I could have a really highly concentrated acid, but that doesn't mean that it's dangerous necessarily if it's only a very weak acid. Similarly, I could have a very low concentration of a strong acid, and that could still be very dangerous, because even though there's not that much of it, it's a very strong acid. It will tend to react very strongly. And that leads us to the related concept of corrosivity, which is a bit harder to define precisely,

because it's a more slippery concept. But basically, corrosivity refers to the ability of a chemical substance to destroy or damage some other substance that it comes into contact with. Corrosion can also refer more to the process of rusting, or more generally, oxidation that occurs with metals exposed to the air. But that's not what I'm talking about here. That's a different type of chemical process. That's a redox reaction, which we've talked about in a previous episode. Here I'm talking about the type of corrosion that occurs, say, when you put a piece of metal into an acid,

into a solution of acid, and you see that it bubbles and the metal begins to dissolve. That's corrosion. If you're interested in this sort of thing, you can look on YouTube. There are plenty of videos of people putting things in acids and seeing what happens to it. Usually it involves lots of bubbling, and the substance becoming discolored and disintegrating in various ways. Although it's very unusual for a substance to be completely dissolved or completely destroyed by acid. I mean, unless it's a very flimsy substance, or if you just have an enormous amount of acid,

but usually what will happen if you say put, if you have a small strip of metal and then pour some acid on it, it'll bubble and fizzle for a while, give off some heat and some of the metal will dissolve. but eventually it will stop, essentially, because all the acid's reacted, or pretty much all of it's reacted. So if you want to keep dissolving the material, you have to keep adding more acid. And generally, you have to add quite a lot of acid to dissolve or substantially destroy any relatively large object. So this relates to another sort of myth that I want to talk about, which occasionally comes up in movies or books or wherever, is the idea that, well, if you have acid thrown on your face, it can, like, eat through your skull in a few seconds or something, along those lines. I mean, the particular portrayal of this varies, but another sort of meme is that

criminals can get rid of bodies by dissolving them in a bath of acid in a few minutes or maybe a couple of hours or something like that. The basic principle here is correct that you can significantly damage and ultimately dissolve skin and other soft body tissues with a sufficiently strong acid. So that principle is valid. However, basically what is done in pretty much all of these cases in the movies or wherever else, is that the length of time that it would take for that amount of damage to be incurred is grossly underestimated. So I read about an experiment that some research has done where they basically dumped pig carcasses in bars of acid. I couldn't get too much of detail, so I'm not quite sure what acid it was or how concentrated it was.

A reasonably strong acid, I imagine. And basically they wanted to see how long it would take to completely dissolve the pig carcass. And it took, I think, several days. And it made a lot of mess and stunk and all sorts of other problems. So it's not a trivial exercise, but the point is it certainly won't happen in a matter of minutes. Even if you get a very strong acid on your skin, it won't eat through to the bone in a matter of seconds, or even a matter of minutes. It would likely take hours or longer. Now, that doesn't mean that it won't hurt a lot and do a lot of damage to your skin if you get a strong acid on you, or even a relatively weak acid, if it's highly concentrated enough. Also, it's important to understand that corrosivity, the damage that a chemical does to a surface, be it that a metal surface or

human skin or anything else, the corrosivity is not just a function of how acidic it is, because plenty of other substances can be corrosive as well. Basic substances can be just as harmful, that is, just as damaging to surfaces and to humans and so forth as acids can be, although people often don't think about it like that. We usually don't use the word corrosivity or corrosion, though, with regard to bases. We usually use the word caustic. So if you've heard of caucic soda, that's just a basic substance, basically, which if it reacts with other substances, it's said to be caustic. It's just the basic version of corrosion, in other words. It's just damaging the surface by reacting with it.

Other substances that can cause corrosion include dehydrating agents. Those are particularly damaging for any biological substances, which are primarily composed of water. If you remove that water, then basically the cells die and everything is in very bad shape. strong oxidizers. We've talked about oxidation. That's the loss of electrons. Again, there's a lot of oxygen in organic compounds, and so if you remove that, that can have devastating effects on the tissue or whatever else. And highly reactive halogens. Halogens are a type of atom, or a group of atoms. So we don't need to go into the details of those things. The point is corrosion. That sort of damage to surface, or the bubbling and fizzling that you think about when you think

about an acid dissolving metal or hitting on someone's skin. That is not really intrinsic to acids. It's just a property of many different chemical substances that are corrosive, or in other words, damage surfaces when they touch them. And there's many different types of chemical reactions that can have that effect. As I've explained, acids do it, bases do it, strong oxidizers, do it, dehydrating agents do it, and so on. So acids are not necessarily particularly corrosive. It'll depend upon, as we've said, the strength of the acid, the concentration of the acid, and many other factors. But you may be wondering, given all that, why does, an acid react in that way, particularly why can acids dissolve metals? By the way, it's not true

that acids can dissolve all metals. It depends on the type of acid and the type of metal and the relative electro-negativities and bond lengths and all sorts of other complicated factors. But in general, it is possible to dissolve metals by putting them in a sufficiently strong acid. The reason this happens is because the acid is donating its protons to the metal in a sense. But what actually happens is that the, remember that our sort of stylized chemical equation for an acid, AH, well, or HA, I forget which order, but that doesn't matter. The hydrogen is being donated, or in other words, the proton is being donated by the acid, leaving behind the A-minus. What is the A-minus to? Well, it binds to one of the metal ions, one of the metal ions that disassociates from the metallic piece of metal.

So in other words, metal ions are coming off the piece of metal and bonding with the A-minus ions that are sort of flirting around a solution, forming some sort of ionic compound. It doesn't matter what the nature of it is. Again, often it's a salt of some kind. And what happens to the protons that the acids donated? Well, basically, you get two protons that combine with each other forming hydrogen gas. Hydrogen gas is just H2, two protons in a covalent bond with each other, and that's what you see is bubbles. So if you put some hydrochloric acid on, I don't know, a coin or something and see some bubbles, the bubbles are hydrogen gas, and that's why they rise and fizzle very quickly, because they're

very light. And in fact, that's one way we can generate hydrogen, is to dissolve metals in this way. And the metal dissolves because the metal atoms change their configuration from being all bonded together in a solid metallic lattice to being associated to each metallic. atomic atom, or metallic ion, being associated with the a-minus ion that was previously part of the acid, and the metal ion plus the a-minus ion come together in the compound, which then dissolves in the solution. So the metal doesn't disappear. The metal's not, well, generally speaking, going to be in the bubbles or anything like that. The metal's still all there. It's

just, it's dissolving in the solution. It's literally all of the metal ions or atoms are floating around in the solution of water, in the acid that you poured on it, and so you can't see them anymore, but they're still all there. So that's chemically what's happening when you pour an acid onto a metal. And again, fundamentally it all comes back to the fact that the acid is donating protons. Okay, so let's talk a bit about the self-ionization of water, which is something I mentioned before. So remember, amphoteric, that's a substance that can act as an acid and a base, not literally at the same time, but sort of at the same time. So water is, as I said before, an excellent example of an ampheteric substance. So water can self-ionize, that is,

it can sort of, well, spontaneously really, or just by itself, split up into two ions, a positive and a negative ion. The positive ion is just our H-plus ion, our proton that we know and love. The negative ion is called a hydroxide ion, and that's oh-h-minus. So it's got a charge of negative one. The proton has a charge of plus one, so when you bring them together, the charges cancel out and you get a neutral compound. Also, notice the numbers of constituent atoms. The hydroxide ion has one oxygen and one hydrogen, and the proton obviously has one hydrogen.

Bring them together. What have you got? You've got one oxygen and two hydrogens. What does that sound like? H2O, two hydrogens, one oxygen. That's just water. So water can disassociate into one hydroxide ion and one proton or one hydrogen ion. But it can also go the other way.

that the hydroxide and the hydrogen can come together to form a water molecule. So when they do this, think about what's acting as an acid and what's acting as the base. So let's suppose we have a hydroxide ion that's combining with a proton. The hydroxide ion is accepting a proton, obviously, because it's going from being OH to H2O, so it's getting an extra proton. So what does that make it? Well, if it's getting a proton, that makes it a base, because bases accept protons. So we've found our base, that's our hydroxide ion, so what must our acid be?

Well, our acid must be the proton or the hydrogen ion, because it's donating, in this sense it's just sort of donating itself to the hydroxide ion. Now let's think about going the other way. Suppose that we start with, well, two molecules of H2O, just to make things a bit easier, and both of these molecules are going to disassociate into hydroxide and hydrogen. So, one molecule of water will lose a proton. It's got to lose a proton because, remember, it's got, currently has two protons, and to become hydroxide and a proton ion, the water atom, sorry, the water molecule needs to lose a proton. So we know the water molecule is going to lose a proton.

What does that mean? That means it's donating a proton, it's giving up a proton, the water molecule is acting as an acid. What's the conjugate base? Well, the conjugate base is always just whatever the acid was. take off a proton and what you're left with is a conjugate base. So let's see, we take a water molecule, H2O, strip off a proton, what are we left with 1H and 1O? Oh yeah, that's our hydroxide ion, oh H minus. So when we have a water molecule disassociating into hydroxide and a proton, the water molecule is acting as an acid,

and its conjugate base is the hydroxide ion. What is the base? Well, this is why I spoke about a second water molecule, because sort of the way we think about it is that one water molecule gives its proton to another water molecule, which then forms what's called a hydronium ion, which is H3O plus. But you can sort of just think about that as the same thing as a proton, that is the H plus ion. Because, I mean, what's H3O plus? That's just a water molecule with an extra hydrogen tacked on. And again, when we have a solution of water, there tends to be a sort of flip-flop between whether the proton is literally sitting by itself in solution, or whether it's teamed up or bonded. to another water molecule to form a H3O plus or a hydronium ion.

So I just sort of think about those interchangeably when I'm thinking about the self-ionization of water because they're sort of more or less the same thing. So you can think about H-2O combining with another H-2O to form a hydroxide, oh H-minus, and then a hydronium ion, H-3-0 plus. And if you combine the hydronium plus the hydroxide, you'll notice that there are four hydrogens and two oxygens and the charges, one negative charge, one positive charge, the charges add to zero. So again, it all fits.

So this example was given to illustrate the concept of self-ionization. You can move in either direction. And we represent this. You've probably seen a chemical reaction before, where we write the reactants on the left and arrow and the products on the other side. Well, when you have a reaction like this that can work in both directions,

we sort of draw a double arrow, one arrow pointing to the left and one pointing to the right, because that's to indicate that the reaction is moving in both ways at the same time. And this leads us to the concept of acid-based equilibrium. Because if the reaction is occurring in both directions at the same time, well, then what happens? Does one happen more than the other one, or is there some sort of end state, or does it constantly change? What's going on?

The answer is, yes, there is some end state. There is some stable, what we call equilibrium that the chemical system eventually tends towards, as long as you don't keep adding products or anything. So an acid-base equilibrium will occur, or in other words, it's relevant, when you have weak acids and weak bases. And they must always exist together. Because remember, if you have a strong acid, let's say, it completely disassociates in solution. And so all you're left with will be conjugate base and the protons. You don't have any acid left in the actual solution.

You've just got conjugate base. But if we have a weak acid, the reaction moves in both directions. So, yeah, we get the acid disassociating to form. conjugate base plus proton, but we also get the reaction going in the reverse direction, conjugate base plus proton, forming the original acid back again, the original acid compound. So in this situation of a weak acid or a weak base, we must have both present in the solution. Again, otherwise it wouldn't be weak, it would be strong. So, how do we know how much of the original acid compound there will be versus how much

of the conjugate base will there be? In other words, which reaction will happen more or to a greater extent? Well, this is what we can measure this by what's called the equilibrium constant of the coefficient of the reaction, or sort of another word for it in this particular case of an acid-based reaction is the dissociation constant. The dissociation constant is a measure of how much that original acid compound disassociates into its proton and conjugate base. The more it disassociates, the higher the dissociation constant, and therefore the stronger the

acid. Now, the dissociation constant is measured, or defined more literally, by just taking the ratio of the concentrations of the original acid compound and your conjugate base. Now, it's a bit more complicated than that. I won't try and describe the formulas here, but suffice it to say, the dissociation constant is fixed for any given type of acid. So it depends on the compound. It also varies with. temperature and pressure, but if we just assume those are fixed, it's the same for every type for a given compound, for a given type of acid or base. So it does not depend on the concentration of the solution. That's very important. The dissociation constant is sort of like

the strength of the acid. It only depends upon, well, of course it's sort of like the strength of the acid, because that's exactly what it is. It measures the strength of the acid. So it does not depend upon how much water you add to the solution. So again, the higher the dissociation constant, the more that original acid molecule is splitting up into its conjugate base plus proton, and therefore the stronger the acid is, or the greater its tendency to donate that proton. Again, the purpose of the dissociation constant is so that we can think about the strength of the acid in terms of the relative concentrations or the relative amounts of the original reactant, that is the original acid compound, versus the conjugate base, or the

dissolved version, the version that's already reacted. A strong acid doesn't really have a dissociation constant, or it's sort of undefined, or in a sense, because all of the original acid molecules dissolve. And so the concentration is zero. So if you're taking a ratio with zero, it's either zero or undefined depending on which is the denominator, which is the numerator. So that's not very interesting. So we don't talk about dissociation constants for strong acids or strong bases. It's meaningless. All of it dissociates. And so there's no ratio to look at. So we only worry about dissociation constants when we're talking about weak acids and weak bases, which do not completely dissociate, don't completely react. And therefore, you've got this equilibrium. You've got the reaction going in both directions. Both directions may not be sort of equally strong as the other one, so one may overpower the other one, so you might have ten times as many reactants as products, or vice versa, depending on which of the directions is stronger, in other words, depending on how much dissociation occurs, but you'll still have some relative amount of the reaction occurring in each direction. To understand this a little bit better, I want to take a slight detail and talk about Le Chathelier's principle,

which I probably mispronounced. this relates to chemical equilibrium, which will help us in a moment to understand buffer solutions. So in a chemical system at equilibrium, which means sort of things are constant or unchanging, so if you have a chemical system at equilibrium, and you change something, say you change the temperature,

or you change the volume, or you add, increase the concentration of one of the reactants or remove some product or something like that, any change along those lines, Le Chathelier's principle states that the equilibrium of that system will always shift such that it counteracts the imposed change that you've made, and therefore the new equilibrium sort of more closely resembles the old equilibrium. So what does this mean exactly?

Well, for example, suppose we have an exothermic reaction. That's a reaction that generates heat, like fire is an exothermic reaction. Now, suppose that I lowered the temperature of the fire or whatever's burning. If I take out energy. So if I lowered the temperature, as I just said, I've taken out energy. So what will Le Chathelier's principle say will happen? Well, it would predict, the principle would predict that some change should happen that would tend to increase the energy of the system. In other words, more specifically, increase the temperature of the system to undo the change that I just made by taking energy out. What would, now in this case of combustion, of burning, what could increase the temperature? Well, more burning, more combustion would increase the temperature

because the reaction is exothermic. The reaction releases energy. So if the reaction occurs to a greater extent, then we're going to release more energy, therefore increasing the temperature, therefore offsetting the original change. And that's precisely what happens. So if you lower the temperature of an exothermic reaction, the reaction is, as we say, the reaction shifts to the right. Now, to understand what we mean by this, picture a chemical equation, and we've got the reactants on the left, and then an arrow, and then the products on the right. So if the reaction is shifting to the right, that means we're going to have relatively fewer reactants in the final equilibrium concentrations, and relatively more products.

So where the reaction is occurring to a greater degree, fewer reactants left over after the reaction reaches equilibrium and more products. So the reaction moves to the right. The reverse of that would, of course, be shifting to the left, when we have less products and more reactants left over, because the reaction hasn't happened quite as much. So another example of this is if you imagine we have a reaction which is essentially A goes to B, so the reactants are A and the product of B. what would happen, what would Le Chateaise principle predict would happen if we added more A?

Well, the prediction would be that the reaction would shift to the right, because that will offset the addition of A, the extra A that we've added. Some of that will be removed if the reaction shifts to the right, and therefore we've got more of it turns into B. And hopefully this makes sense, because if you imagine adding a bunch more A, a bunch more reactants to our chemical reaction, then there are going to be more, well you think in terms of collision theory, there's going to be more chances essentially for A to go to B

and just the same amount of chances for B to go to A, because we've just added a whole bunch more A. There's the same amount of B as there was before. So the number of times the reaction goes from A to B will increase because there's a lot more A. But the number of times, at least initially, the number of times the reaction goes in the reverse direction from B to A won't change. Initially it won't change because there's the same amount of B.

And therefore, it must be the case that, at least for a time, the rate of transition from A to B, that is from reactants to products, exceeds the rate at which the reverse occurs from the rate at which we go from B to A. And therefore, if that's the case, then the reaction must shift to the right. It must shift towards the products. Hopefully that makes sense. So, we can apply Lashat-E-S principle to understand buffer solutions. Buffer solutions are solutions, aquee solutions, consisting of a mixture of a weak acid. and its conjugate base, or conversely a weak base and its conjugate acid, sort of the same thing. And the purpose of a buffer solution is that it can resist changes in pH over a certain range. Now, how does it do this? Well, it operates, it's possible because of Leschartuiliers principle.

If you add an acid to a buffer solution and the acid is of an appropriate pH such that it's within the range that the buffer can cope with, then what the buffer tends to do is just neutralize the acid without lowering the pH of the actual solution. So the buffer offsets the pH change that would normally happen if you just added acid to a solution. And vice versa. If you add a base to a buffer solution, then the buffer solution tends to neutralize the base without raising its pH, without becoming more alkaline as would normally happen for a solution if you added a base. So how does it do this?

How do we have this sort of magical properties such that the solution is able to offset an acid or offset a base, regardless of which of them is added? Well, as I said before, we can understand it in terms of Le Chateailles' principle. So think about it this way. A buffer solution must have a weak acid and its conjugate base, and weak base and conjugate acid. So that means that because the acid is weak, we're going to have a reaction existing in equilibrium. So the reaction is going to have one of those double arrows. It's going to be progressing in both directions simultaneously, and we'll have some equilibrium rate at which the acid disassociates into its conjugate base and the hydrogen,

and also some equilibrium rate at which the reverse reaction occurs. And so because of this, whether we add more hydrogen ions, as happens when we, if we add an acid to the buffer solution, or if we soak up some hydrogen ions, which happens if we add a base to the solution, Le Chalte-Yeer's principle ensures that the buffer solution always reacts so as to offset that change that we made. So, for instance, think about the case if we had conjugate base

floating around in our buffer solution, and then we add a bunch of acid. Well, the acid is going to disassociate, and assume it's a strong acid that we add, and release a whole bunch of protons, hydrogen ions, into the buffer solution. But those conjugate base molecules that are floating around in the solution are going to soak up the excess protons that we've added. Those conjugate bases are going to bind to the protons and then form a non-dissociated acid, a non-dissociated molecule of the weak acid, which, remember, is not going to be acidic because it won't have disassociated. It's just existing as the H.A. molecule, remember, when we just have the acid bit and the hydrogen in the bound form before it's disassociated. So that's what

will happen if we add in extra protons by adding acid to the solution. The conjugate base will just soak them up and therefore offset any effect on pH. Because remember, pH is the power of hydrogen. It's the concentration of hydrogen ions. And so if all of the extra hydrogen ions added through the addition of the acid are getting soaked up or bonded out of the solution by the conjugate base, then there's no change in pH. And exactly the same thing will happen if we add base to the solution. The conjugate acid, so if we add base to the solution, what happens is those basic molecules that we've just added, we're going to be soaking up or binding to a bunch of those hydrogen ions that already exist in the solution, thereby reducing

the hydrogen concentration. And that would generally reduce the, that would generally reduce, that would generally reduce the acidity of the solution, or in other words, increase the pH. However, because it's a buffer solution, because of Les Chathiers principle, that think about it, there's been a change in the relative concentrations. What's happened? There's less, there's a lower concentration of hydrogen ions in the solution, because we've added all these basic molecules, and the basic molecules are soaking up all the hydrogen, so their concentration is lowered. So what does Los Chateailles principle say? It tells us that the reaction, that equilibrium between the weak acid and its conjugate basis that exists in the buffer solution is going to be pushed in the direction such that

we offset the change that happened. What was the change that we induced by adding the base? The change that we induced was that there's a lower concentration of protons, of hydrogen ions in the solution. So what will the buffer solution reaction do? It will shifts in the direction such that we produce more hydrogen ions such that it releases protons. So more of those HA molecules are disassociating now, and we're releasing more protons into the solution, thereby pushing up the concentration once again. So, as we add a base and the base molecules are soaking up hydrogen ions, more hydrogen ions are just being dumped back into the solution as a result of La Chautier's principle, offsetting the change that we've made, and therefore the concentration of hydrogen stays roughly

constant, and the pH of the solution doesn't change. So regardless of which one you add, if you add an acid to the buffer solution, the conjugate base just soaks up all, the extra protons and the pH doesn't change. If you add a base to the buffer solution, sure, the base eats up a bunch of protons, but then more protons are just released by the conjugate acid that was already in the solution. And again, the pH doesn't change. Now, you might wonder, well, can this occur forever? Surely eventually, if we just keep adding acid or keep adding a base, then something must happen. And yes, the answer is yes, eventually you reach a sort of saturation point where either all of the conjugate bases in the solution are filled up with protons. So there's,

There's no more conjugate base molecules able to take up extra protons. And therefore, if you keep adding more protons after that, that is if you keep adding more acid to the solution, then the pH will finally fall because there's no more ability to soak up the protons. Or vice versa. If all of the conjugate acids have already released their protons, then if you keep adding more base to the solution, there's no extra source of protons to offset the protons that are being sucked up

by the conjugate bases that you're adding, or the base that you're adding to the solution. and therefore, again, in that case, the concentration of hydrogen will fall, won't be able to be offset, because there's no more protons to be released, and therefore the pH of the solution will rise. So the point is, eventually there does reach a stage where a buffer solution can no longer resist changes to pH, and that's why there's a range of pHs over which given buffers work, because eventually you sort of reach a saturation point where you can't provide any more hydrogens or can't suck up any more of the hydrogens, and so pH will change.

blood, that is human blood, and indeed from any other animals too, is a good example of a buffer solution, or at least it contains a buffer solution. And that's very important because the pH of blood must stay within a fairly narrow range in order for metabolism and a whole host of other biochemical reactions to occur properly. If the pH of the blood changes by even a relatively small amount, we start getting a whole host of problems. And in fact, the regulation of the oxygen level and the carbon dioxide concentration in the blood is achieved in large part by moderating the concentration, excuse me, by moderating the pH level of blood and sort of a feedback mechanism that monitors that pH level. So it's very interesting how biology has utilized this concept of a buffer solution and feedback mechanisms in order to regulate the oxygen content and carbon dioxide concentrations of the blood. But we'll look at that in

more detail when I get around to doing an episode on the circulatory and the respiratory systems. Okay, so that's all I wanted to cover in this episode. If you enjoyed it, Please jump onto Facebook and type in The Science of Everything podcast and look up our Facebook fan page. You can give us a like and get news about upcoming podcasts and other links and information related to the episodes. I often post up diagrams and other images related to the episode that you can check out there. So go and do that. Also, if you want to send me an email with feedback, suggestions, comments, or anything else, my address is FODs12 at gmail.com.

That's F-O-D-S-1-2 at gmail.com. Thank you for listening and I'll talk to you next time.