- [Voiceover] You've
probably heard the term acid used in your everyday life. But what we want to do in this video is get a more formal
definition of an acid. And particular, we'll focus on the one that is most typically used. Although we'll see future
videos that there's other fairly common definitions
of acids used as well beyond the one that
we're going to see here. But the one that we're going to focus on is the Bronsted-Lowry definition. The Bronsted-Lowry definition
of acids and bases. And this is a picture of Bronsted. This is a picture of Lowry. And they came up with
this acid-base definition in the 1920s. So, we're going to do the Bronsted-Lowry, Bronsted-Lowry definition, definition of acids and bases. So, according to them, according to them, an acid, an acid is a proton, proton, or instead of writing proton
we could actually write hydrogen ion donor. So why is a proton and a
hydrogen ion the same thing? Well, in the most common
isotope of hydrogen, we would, in it's nucleus, we would find just a proton and no neutron. And if it's neutral, you
would have an electron buzzing around, jumping
around in its orbital. So, you would have it's
electron jumping around in its orbital. But if you were to ionize it, you're getting rid of its electron. So, if you're getting
rid of it's electron, so, if you're getting rid of this, all you're going to be
left with is a proton. So that's why a proton, an H plus, is usually referring to the exact same, is referring to the exact same thing. So, that's what an acid is. So what would a base be? Well, you could imagine by this definition A base, a base would be a proton, would be a proton, or you could say a hydrogen ion acceptor, acceptor. So let's make this a
little bit more tangible with some examples. So one of the stronger acids we know is hydrochloric acid. Let me, let me draw. So, it's a hydrogen having
a, having a covalent bond. Having a covalent bond with chlorine. With chlorine, with
chlorine right over there. And if we want to, let's draw actually chlorine's lone pairs. So outside of the electron
that is contributing to this pair in the covalent bond. It also has, it also has
three other lone pairs. It also has three other
lone pairs, just like that. So, if you were to take hydrochloric acid, place it in an aqueous solution, so it's in an aqueous
solution right over here. And actually an aqueous solution, you'll see this written like that. That just means it's
in a solution of water. So you could write like
this, you could write hey, hydrochloric acid in an aqueous solution if you want to make it a
little bit more explicit. You could say hey, look,
this is going to be around some water molecules
in its liquid form. Aqueous solution just means
it's dissolved in liquid water. So, some water molecules
in their liquid form. So, this is a water molecule. Whoops, water molecule. Right over here. So, an oxygen bonded to two hydrogens. And sometimes you'll see
it written like this, that it's in its liquid,
it's in its liquid form. Well, what do you think
is going to happen? Well, I already said that
this is a strong acid right over here. So this is going to really
want to donate protons. It's really going to want
to donate this hydrogen, but not let the hydrogen
keep its electrons. So what's likely to happen here? Well, the both of these
electrons in this pair are going to be grabbed by this chlorine. And then this hydrogen ion, because its electron was grabbed, well this could be nabbed by
some water molecule passing by. Remember, in a real solution, it's not like they know what to do. They're just all bumping past each other. And based on how badly
they want to do things, these reactions happen. And so you can imagine this
lone pair right over here, well maybe it's able
to form a covalent bond with this hydrogen. And so what's going to happen? What's going to happen? And I'll draw it with just an arrow because this reaction favorably goes, very strongly goes to the right, because this is such a strong acid. Well, then you're going to be left with, you're gonna be left with, the chlorine is now going
to have its three lone pairs that it had before. And then it also grabbed
these two electrons right over here. It also grabbed those two
electrons right over there, so it gained an extra electron. It now has a negative charge. It is now the chloride anion. So it has a negative charge. And what about this water molecule? Well this water molecule, you have your oxygen, you have your hydrogens,
you have your hydrogens, but now you don't just have two hydrogens, you grabbed this hydrogen right over here. And maybe I'll do this hydrogen in a slightly different color so that you could keep track of it. You have this hydrogen right over there. And this lone pair, this
lone pair you can view it as now forming this covalent bond. You had your other two covalent bonds to the other two hydrogens. And then you still have this
lone pair right over here. You still have that lone pair
sitting right over there. And what just happened? Well, this water molecule
just gained a proton. This hydrogen did not
come with an electron. So if you just gain a proton, you are now, if you were neutral before, you are now going to
have a positive charge. So what just happened? You put hydrochloric
acid in a water solution, in an aqueous solution, this thing has donated a
proton to a water molecule. And so, what is the acid
and what is the base here? Well, when we look at
the reaction this way, we see that this is the acid, the hydrochloric acid, it's literally called hydrochloric acid. And here, water is acting as a base. Water is acting as a base. And as you could see, water can actually act
as an acid or a base. So, water is acting as a base. Now you might be saying, okay, this reaction goes strongly to the right, hey, but like you know, I could imagine in certain circumstances
where chloride might accept a proton because it has
this negative charge. And you would be right. This reaction goes strongly to the right, but once an acid has donated its proton, the thing that is left over, this is called a conjugate base. And I'll do the same color. So, this is the conjugate
base of hydrochloric acid. The chloride anion. Conjugate, conjugate base of hydrochloric acid. And this right over here
is the conjugate acid because you could imagine
this hydronium ion, this could, under the right circumstances, donate protons to other things. Donate a hydrogen without
donating electron to other things. And so this is actually
the conjugate acid of H2O. Conjugate acid of water, of a water molecule. And as we'll see, water can
act as an acid or a base. But this this gives you a
kind of a baseline of at least the Bronsted-Lowry definition
of acids and bases. And actually, one other
thing I want to add. In some books here, so
over here I said, hey, put this in an aqueous
solution you're gonna form some hydronium, sometimes
you'll see it written like this. And I'll just write it a
little bit, a little bit, sometimes you'll see it like this. So you have your hydrochloric acid, and I won't draw the details this time, in an aqueous solution. So it's in a solution of water. And they'll just draw the
reaction going like this, where they say hey,
you're gonna be left with, you're gonna be left
with some hydrogen ions, these protons. And you're going to be left with, and actually we could say it's gonna be in a aqueous solution, aqueous solution. And you're gonna be left
with some chloride anions. Some chloride anions and
it's in an aqueous solution. Now this isn't incorrect,
but it's important to realize what they're talking
about when they're talking about these hydrogen ions right over here. We know that if you have the hydrogen ions in an aqueous solution they don't just hang out by themselves. They get grabbed by a water molecule and they form hydronium. So, it's much more, I guess,
it's much more close to the actual of what's happening, is if you actually talk
about hydronium forming. As opposed to just the protons. 'Cause these protons
in an aqueous solution, in a water solution,
they're gonna be grabbed by a water molecule to form hydronium. And that's why I did it
the way, this way up here.