Hi. It's Mr. Andersen and this chemistry essentials video 68. It's on acid-base equilibrium. At
Harvard University they have this wonderful Chinese statue that was donated on their 300th
anniversary and it has this inscription that talks about the importance of knowledge to
civilization. Sadly during the winter they have to cover it up. And that's due to acid
rain. It seems like a confusing solution. And acid-base chemistry is sometimes confusing
to students and teachers. But you just have to understand that it's a reversible reaction.
What does that mean? It will eventually achieve equilibrium. And we've learned that you can
measure the equilibrium constant and that gives us a better understanding of really
what's going on in acid-base chemistry. Remember that equilibrium constant is simply the products
or the concentration of the products divided by the concentration of the reactants. Now
what is an acid or a base? Remember it all relates to proton exchange. And so if you're
donating a proton you're going to be an acid. And if you accept a proton then you're going
to be a base. So we can measure the equilibrium constants of acids and bases and determine
if they're strong, in other words their favoring the products or if they're weak. We can also
combine acids together and we have what's called a neutralization reaction. And we can
measure those using titration and titration curves. And so the first thing that you should
understand is the importance of water. All acid-base chemistry takes place in the presence
of water. And so we can think of an infinite amount of water surrounding every acid or
base. It's something that we can simply ignore. There's always going to be water present.
Water is also important because it is amphoteric. And what that means is it can serve as both
an acid or a proton donor, or a base, a proton acceptor. And so if we were to look at a simple
reaction, here we have acetic acid in water. What's going to happen is that the acetic
acid is going to be the acid and the water is going to serve as the base. It's going
to accept protons from acetic acid. Now as it does that it's essentially forming an acid.
And since this is a reversible reaction it can push back towards the base. And so we
have these two pairs. We have these acid-base pairs. And to keep it straight we'll call
everything on the left side an acid and a base and everything on the right side a conjugate
acid or a conjugate base. If we were to look at another reaction, in this case we've got
ammonia. Ammonia is going to serve as the base and so the water, since it's amphoteric
in this case is actually donating a proton and it's forming that ammonium ion which is
a conjugate acid and then we have the hydroxide which is going to be the conjugate base. What's
cool about water is it can autoionize. That means it can serve as both an acid and a base.
And so what we're doing is essentially donating a proton to another water molecule. And so
if we look at that what we're forming is something called hydronium and then the one that looses
the proton is going to be a hydroxide. And we can measure the K value of this. We can
measure the equilibrium constant of that. And it ends up being 1 times 10 to the negative
14th which is going to be a really, really small value. Now what is equilibrium constant?
Remember it's going to be the concentration of the two products divided by the concentration
of the two reactants. Why do we have a two here? Remember it's because we're going to
have two moles of water on the left side. Now as I said earlier there's always water
present. And so in this equilibrium constant of water we can essentially ignore water.
And so this is going to be our Kw value. Since it's 1 times 10 to the negative 14th, then
we can just figure out that the concentration of hydronium and hydroxide therefore must
be 1 times 10 to the negative 7th. Therefore pH which is the negative log of the hydronium
is 7. It's easier to deal with it that way then these really really ridiculously small
numbers. Likewise pOH is going to be 7 as well. So if we add pH and pOH it will always
add up to 14. Now what's the difference between a strong or a weak acid or a base? Well the
equilibrium constant is going to help there as well. So if we look at acetic acid, in
this case it's donating a proton to water. And so we can figure out its acid ionization
constant, which is essentially an equilibrium constant. So if we were to write it out it's
going to be the concentration of the two products divided by the concentration of only one of
the reactants. Why? Again, we're simply ignoring water. If we were to measure that we find
that it's a ridiculously small number. What does that mean? Since the equilibrium constant
is so small, it's actually favoring the reactants. That means most of it is not actually donating
its proton. And we call something like that a weak acid. If the Ka value is ever less
than one, in other words it's favoring the reactants, then we're going to call that a
weak acid. And carboxylic acids are notorious for being weak acids. Those are acids that
have this carboxyl group in it. Now sometimes it's hard to measure Ka or deal with those
numbers and so you'll see pKa which is simply the negative log of Ka. If we were to look
at a strong acid now, this is hydrochloric acid, combining with water, and figure out
its Ka value, it's an incredibly large number. What does that mean? If we have a hydrochloric
acid it's essentially all donating its protons. It's all shifting towards the right. So if
our Ka value is ever greater than 1 that means we've got ourselves a strong acid. And there
are simply 6 strong acids that in AP chemistry you should memorize. Hydrochloric acid, sulfuric
acid. Simply memorize those. And there's a cool YouTube video that shows you how to do
that. We also have what are called polyprotic acids. What are those going to be? Those are
going to be acids that actually have 2 protons. And so they're going to donate that proton
in two steps. Now if we were to look at the general equation then for an acid, it's donating
a proton and forming hydronium, we could write it like this. And if we were to look at the
general equation for a base it would look like this. And so we could also use the equilibrium
constant of the base. If It's ever going to be less than 1, what does that mean? It's
actually favoring the reactants and it's going to be a weak base. There are four groups of
those. And if it's ever greater than 1, then it's favoring the products. And these are
going to be all of the strong bases that we have. You can see that they're all going to
be hydroxides formed by combining with atoms from group I and group II. And so what's a
neutralization reaction? That's when we're getting a reaction between an acid and a base.
And there are really three groups that we could have. We could have a strong acid and
a strong base. And what happens there, they all are forming water. We could have a weak
acid and a strong base. What's going on there? Well basically we're going to take that conjugate
acid and it's going to form a conjugate base. But remember it doesn't form all of that right
away. And then likewise with a strong acid and a weak base, we're going to have that
weak base forming a conjugate acid. And so it's not going to all transfer those protons.
And so we can use acid-base titrations to look at that. So what would it look like if
we're doing a strong acid and a strong base? Well let's set this up with a burette. What
we're going to do is be adding sodium hydroxide to hydrochloric acid. You can see down here
in the Erlenmeyer flask that we have a pH that must be less then 7. We're using phenolphthalein
as an indicator. And so if we add some of that sodium hydroxide eventually it turns
pink. So that tells us at some point it went towards a greater then 7 value. If we were
to actually graph the pH over time it would look like something like this, when we add
a strong base to a strong acid. So what's going on here? Well at this point we're adding
a lot of this base. It's converting that acid into water. But the pH isn't changing very
much because we have lots of this strong acid still in the container down at the bottom.
Eventually what happens is we reach equilibrium, or the equivalence point. And then all of
that base that we're adding is simply going to pump it up towards a more positive pH value.
It's going to become basic. And so we have this smooth curve. We simply split the difference.
When we're adding a strong base to a strong acid. That equivalence point is going to be
right at 7. Now what happens if we add a strong base to a weak acid? And so in this case we're
going to have acetic acid in the bottom and then we're going to add a strong base, so
that's sodium hydroxide. And you might think since we're adding this strong base, it's
simply going to turn quickly like that. We're going to add the base. It's quickly going
to get rid of all of that conjugate acid. But if we were to do the titration, we would
get a curve that looks more like this. And so what's going on is it starts to increase
like that. But eventually what we're doing is we're creating a bunch of this conjugate
base, this acetate ion over here. And so this whole period of time right in here, what we've
essentially created is a buffer solution. And we'll talk more about that in the next
video. But what it's going to do is it's going to stabilize the pH. It's not going to change
very much. And then eventually what happens is it's going to go towards the base. We're
going to reach an equivalence point. But look at the equivalence point right here. That
equivalence point is going to be much greater than 7. So it looks like it's somewhere about
8.2 in this case. Now what's causing that buffer solution remember is Le Chatelier's
Principle. What we're doing is we're adding more of the base which is forcing more of
that acid to be converted into a conjugate base which is stabilizing that pH. Likewise
if we were to look at a strong acid and a weak base, so we're starting at a really high
pH right here, as we add the acid it starts to drop down really quickly. But now we've
created a buffer solution right here and the equivalence point is going to be lower than
7 in this case. So you should understand what those titration curves look like in each of
those three different situations. And so did you learn the difference between a strong
and weak acid and base? Remember that's based on the equilibrium constant. Did you learn
how strong and weak acids behave in these titration reactions? Did you learn that neutrality,
in other words a pH of 7 requires the concentration of the hydronium and the hydroxide ions to
be equal? And then did you understand in each of those three situations what happens during
a titration? I hope so. And I hope that was helpful.