Hi. It's Mr. Andersen and this is chemistry essentials video 57. It's on entropy which
is really the dispersal of matter and energy. So imagine I had a little bit of milk in a
glass and I were to spill it. I will have increased the entropy of this isolated system.
And there's an old saying that you shouldn't cry over spilled milk. I totally disagree
with this. You should cry over spilled milk. And the reason why is it's an irreversible
process. In other words it's not just going to magically move back into the glass. You've
increased the entropy and it's going to take a lot of work to put it back where it was.
And so entropy in AP Chemistry is not something that you will have to calculate. You have
to understand it qualitatively. In other words you have to look at a system and figure out
what's happening to delta S. And so what is delta S? That's simply the change in entropy
over time. Now what does this little degree symbol stand for? That means that we're measuring
it at standard temperature and pressure. And so if we were to look at a system we should
be able to make predictions about what's happening to our delta S. And that's what I hope you
can do at the end of this. And so if we define it it's simply dispersal. Dispersal of matter
and dispersal of energy. So what are some examples of that? Well let's say we were to
make some phase changes or to go from a solid to a liquid to a gas, what would we be doing?
We would be dispersing or moving that matter away from its center point. And so we would
be increasing the entropy. In other words our delta S would be a positive value. We'd
make that matter dispersed over time. We also could do that by stoichiometry. In other words
if I look at the moles before a chemical reaction and the moles after, if I have more moles
after the reaction, I've increased the entropy. And also if we take a gas and we increase
its volume we're moving those molecules apart and so we're increasing entropy. Or delta
S is going to be a positive value. Now let's say we look at energy. How could we disperse
energy? Well by increasing temperature. If we increase temperature of gas, according
to the kinetic molecular theory we're spreading that energy out and we're increasing entropy.
Now entropy is a term that people people really struggle with. And so I wanted to start with
a good example. And so I've got two videos here. But they're the same exact video. It's
just that one of them is played forward and the other is played backward. And so you're
goal is to figure out which one is forward. So let me start playing that. It's just going
to loop over and over and over again. And so your goal is to figure out which one is
forward. Have you decided yet? The right answer is A. But that that was kind of hard. Let's
look at another one. I've got a little pendulum here and let me play this video again. It's
the same video clip. It's just that one of them is played forward. And the other is the
same clip, it's just played backward. Have you figured out which one is forward? There's
some clues in here. But the right answer is B. Let's look at the next one. It should be
a little easier now. So if we look at the next one I'm going to play this again. So
again, same video, one's forward, one's backward. Hopefully you figure this out. The right answer
is B. B is being played forward and A seems somewhat magical. It is being played in reverse.
Let's look at the milk. If we look at the milk right here, which one is being played
forward? Hopefully you've figured this out. A is being played forward. Okay. And so why
did I do that? Well what we're looking at here are irreversible processes. In other
words if we play it in this direction it's irreversible. In other words it's impossible
for it to occur in the reverse. Now it's not totally impossible. That could occur. But
it's statistically improbable that it's going to move in the other direction. And so that
direction is really important when we start to talk about entropy. Because what happens
during an irreversible process is that we're increasing the entropy or the dispersal of
that system. And it just doesn't move back. And that will become really important as we
talk about the next few videos. And so the second law of thermodynamics says that in
an isolated system or a closed system isolated from the universe, entropy never decreases.
In other words our delta S is always going to be positive. In other words matter and
energy are going to become dispersed over time. In other words randomness increases
over time. Now you might think to yourself I know that's not true. Because I see stuff
out there that's not random. In other words I see trees and cars and DNA. And I know these
things have a high amount of order. Well you're not violating the second law of thermodynamics
because remember we're talking about an isolated system. And so we can decrease entropy in
an isolated system but as we do that we're increasing entropy of the surroundings or
the universe around it. And so again, to define entropy, what is it? It's simply matter dispersal
or matter spreading apart. And so let me give you an example of a certain type of matter
dispersal. It simply would be phase change. So as we move from a solid to a liquid to
a gas to a plasma, that matter is moving farther apart. And so as it's doing that what we're
doing as we move from a solid to a liquid to a gas is that matter is spreading apart.
And as it does, we're getting a delta S which is going to be a positive value. Now let's
look at stoichiometry. You should be able to look at an equation like this and immediately
figure out what's our delta S. Well how do you do that? So if we've got our reactants
on the left side and our products on our right side we have two moles on the right side and
only one mole on the left side. If we show you a picture of that it looks like this.
So we have dinitrogen tetroxide and we're making two moles of nitrogen dioxide. And
so what are we doing? We're increasing the entropy. You try the next one. So if this
our equation, so we're taking liquid water and turning it into gas, hydrogen gas and
oxygen gas. What's happened to the number of moles? Well, we've increased that. So our
delta S is going to be a positive value. We're spreading that matter apart. Now there's another
clue here as well. We're going from a liquid to gases. And so we really are increasing
the entropy of this system. Now let's look at gas volume. As we increase the volume of
a gas, so if we have a known quantity of that gas, as the volume gets larger and larger
and larger, the matter is spreading apart and so we're going to have a delta S that's
going to be a positive value. So not only is entropy dispersal of matter. It's also
dispersal of energy. So if we were to look at this Maxell-Boltzmann distribution of a
gas what we would see is a curve that looks like that. In other words these are going
to be the number of molecules on the left. This is going to be their energy. You can
see that some of them are going to have a high amount of kinetic energy. Some of them
are going to be lower. But if we increase the temperature what does that curve look
like? It's shifting everything over to the right side. So we're going to have more molecules
that have a higher amount of energy. And so what's that doing to our entropy? It's increasing
the entropy because we are increasing the dispersal of that energy. And so did you learn
to predict the sign and relative magnitude of entropy changes associated with chemical
and physical processes? Could you remember this one right here? I hope so. And I hope
that was helpful.