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- [Instructor] So we have four different molecules here.
And what I want you to think about,
if you had a pure sample of each,
which of those pure samples would have
the highest boiling point, second highest,
third highest, and fourth highest?
Pause this video, and try to figure that out.
All right, now to figure that out,
it really just boils down to
which of these has the highest intermolecular forces
when they're in a liquid state?
Because if you have high intermolecular forces,
it would take a lot of energy or a higher boiling point
to really overcome those intermolecular forces
and get to a gas state.
So let's think about the intermolecular forces
that we have studied.
So I will start with hydrogen bonds,
hydrogen bonds.
'Cause you could really view those,
those are the strongest of the dipole-dipole interactions,
and they're going to be stronger
than your London dispersion forces.
We can see that diethyl ether won't form hydrogen bonds.
We don't see any bonds between hydrogen
and an oxygen, a nitrogen, or a fluorine.
Ethanol has one oxygen-hydrogen bond.
Methanol also has one oxygen-hydrogen bond.
Water has two oxygen-hydrogen bonds.
So if I had to rank the hydrogen bond contribution
to the intermolecular forces,
I would put water as number one
'cause it can form the most hydrogen bonds.
I would put methanol and ethanol as a tie for second.
And then I would put diethyl ether last
'cause it can't form hydrogen bonds.
So just looking at this,
I know that water's going to have the highest boiling point.
Diethyl ether is going to have the lowest boiling point.
But what about the difference between methanol and ethanol?
And we could think about other types of dipole forces,
but not a lot that you could intuit just by eyeballing them.
They might actually have similar dipole moments
on a molecular basis.
But we can think about London dispersion forces.
I'll do this in a different color.
So London dispersion forces.
And if we're just trying to,
actually I'll rank all of them.
So London dispersion forces are proportional to
how polarizable a molecule is,
which is proportional to how large its electron cloud is,
which is proportional to its molar mass.
And it's clear that diethyl ether
has the highest molar mass,
followed by ethanol,
followed by methanol,
followed by water.
How did I know that?
Well, you literally can take atoms away
from the diethyl ether to get to an ethanol.
And you can literally take atoms away
from that to get to a methanol.
And you can literally take atoms away
from that to get to a water.
So we know that this is the order of molar mass.
And so London dispersion forces,
I wouldn't make that change the ranking
between water or diethyl ether because these are going
to be a lot weaker than those hydrogen bonds.
But they can be useful for the tiebreaker
between ethanol and methanol.
And so my overall ranking on boiling points,
the highest boiling point I would put would be water,
followed by, since ethanol won the tiebreaker,
followed by ethanol,
followed by methanol,
and then the lowest boiling point would be diethyl ether.
And if we look at the actual data,
it's consistent with what we just talked about.
We can see very clearly
that water has the highest boiling point,
ethanol is second,
methanol is third,
and diethyl ether was fourth,
completely consistent with our intuition.
Now, what's also interesting here,
you might have noticed, is this thing called vapor pressure.
And you might have also noticed that vapor pressure seems
to trend the opposite way as boiling point.
The things that have the high boiling point
have the low vapor pressure,
and the things that have the low boiling point
have a high vapor pressure.
So what are we talking about, why,
about vapor pressure, and why do we see this relationship?
And I'm not going to go deep into vapor pressure.
There'll be other videos on that on Khan Academy.
But just to get you a sense,
imagine a closed container here.
And I put one of these,
a sample of one of these molecules in a liquid state,
and I'm gonna just draw the molecules,
clearly not drawn to scale, as these little circles.
And the temperature matters,
so let's say that this is at 20 degrees Celsius.
Now, you might notice, at 20 degrees Celsius,
it's lower than the boiling point
of all of these characters.
So for the most part, they're going to be in a liquid state,
but we know that not every one of these molecules is moving
with the exact same kinetic energy.
The temperature, you could view as a measure
of the average kinetic energy of the molecules,
but they're all bumping around into each other,
in different positions, with different amounts of velocities
and therefore different kinetic energies.
And so every now and then, you're going to have a molecule
that has the right position and the right kinetic energy
to escape and get into the vapor state,
into a gaseous state.
And so that's going to keep happening.
But then the things that are in the gaseous state,
every now and then they're bumping into each other,
and they're bumping into the sides of the container.
And every now and then,
they might approach the surface
with the right kinetic energy, with the right position,
so that they get recaptured by the intermolecular forces
and enter a liquid state.
And so you can imagine, this will keep happening
where things go from liquid, and then they go to vapor.
But then when that vapor gets high enough
or when you could say the vapor pressure gets high enough,
remember, that pressure's just from
the vapor molecules bouncing around,
then you will get to some form of an equilibrium.
And you could imagine,
the things that have a lower boiling point,
that means they have lower intermolecular forces,
more of the vapor is going to form,
and so you're going to have a higher vapor pressure
before you get to equilibrium.
On the other hand, things with high intermolecular forces,
fewer of those molecules are going to break away,
and so you're going to have a lower vapor pressure
when you get to that equilibrium.
And you can see that very clearly here.
So I will leave you there.
We got a little bit of practice,
seeing everything we've seen so far,
and we learned a little bit about vapor pressure
and how that relates to intermolecular forces
and boiling point.


Intermolecular forces and vapor pressure | AP Chemistry | Khan Academy

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林宜悉 發佈於 2020 年 3 月 28 日
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