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