>> host: Welcome back
to Unit 4 of our
"Chemistry Matters" exploration
of chemical bonding.
In the last video
on the playlist, we saw how
intramolecular bonds form
when atoms are attracted
to each other due to
electrostatic forces.
The bonding forces
we want to explore now
are intermolecular forces.
Intermolecular forces
are forces that occur
between molecules.
Intramolecular is within.
Intermolecular is between.
Intermolecular forces
are weak compared to
intramolecular bonds,
but they play a big role
in the physical properties
and states of matter
substances take.
Intermolecular forces,
along with the amount of
kinetic energy a substance has,
determine whether we experience
a substance as a solid,
a liquid, or a gas.
When something melts or boils,
such as water,
it's because the intermolecular
forces are being broken.
Let's head back
to our classroom,
where our students are
about to explore
the intermolecular forces
more closely.
>> professor: To get things
started, let's review what
we already know about
intermolecular forces.
>> student: Well, we know
they're forces that exist
between molecules,
not within them.
>> professor: That's right.
>> student: And are they
holding the molecules
close to each other?
>> professor: Sort of,
but they're not really
"holding" the way you and I
think of holding things.
It's still an electrical
attraction at work.
And depending on how the
molecules are interacting,
we can have a gas, liquid,
or a solid.
Now, let's do an
investigation to observe
intermolecular forces
in action.
Then we'll further explore
the different types of
intermolecular forces.
In this activity,
we're going to be looking at
the intermolecular forces
present in liquids.
I've given each team
a beaker of water
and a strip of filter paper.
Go ahead and dip
the end of your filter paper
into the water.
>> student: Whoa, the water
runs up the paper.
>> professor: Yes, and the water
is moving up, against gravity.
Would you agree?
>> student: Yeah, definitely.
>> professor: Why do you think
that happens?
>> student: I'll bet it has
something to do with
water being polar.
>> professor: You're right.
It does.
Since water molecules are polar,
they can and do
stick to each other.
This property is known
as cohesion.
Cohesion is the action
or property of like molecules
sticking together,
being mutually attractive.
It's kind of like when
you hear someone describe
an athletic team
as being a cohesive unit,
in that they stick together.
Water molecules also
stick to each other.
And water also has the ability
to stick to other substances
that are polar in nature.
For example, watch this.
You see, if I don't pour
the water out of this glass
fast enough for the attractive
forces between the water
and the components of the glass
to be overcome,
the water will just
stick to the glass
and run down the side.
And if I'm not careful,
I'll make a big mess.
Adhesion is the
tendency of molecules
to stick to substances
that are dissimilar.
And adhesion is what
causes the water
to cling to the glass.
It's kind of like the way
an adhesive bandage
sticks to your skin.
Now you get to do
another investigation.
Use these pipettes
to add drops of water
to the surface of a penny,
one drop at a time.
What do you see happening?
>> student: A dome was
forming with the water.
>> student: Yeah, us too.
How does that dome form?
>> professor: Do you remember
our discussion earlier
about how water molecules
are cohesive?
>> student: Yeah, you said
water molecules are
attracted to each other,
and it makes them cohesive.
>> student: Is that-is that
what's holding it together?
>> professor: Exactly.
So we've confirmed
there must be some sort of
an attractive force
going on here
between the water molecules
on the pennies.
Would you say these forces
are intramolecular
or intermolecular?
>> student: I think those
are intermolecular forces,
since the water molecules are,
like, pulling at each other.
>> professor: Yes, you're right.
When water molecules
stick to each other,
that's caused by
intermolecular forces.
These forces are much weaker
than ionic or covalent bonds,
though.
To get an idea of
their relative strength,
the intramolecular bonds
that hold the atoms in
water molecules together
are almost 25 times as strong
as the intermolecular forces
between the water molecules
themselves.
Now let's try
a different molecule.
Isopropyl alcohol.
On your other penny,
add drops of isopropyl alcohol
very slowly.
What do you notice?
>> student: The alcohol
doesn't form much of a dome.
It kind of just
slips off the penny.
>> professor: And what does
that observation tell you
about the intermolecular forces
acting between
the alcohol molecules?
>> student: Alcohol must
have weaker intermolecular
forces than water.
>> professor: That's right.
The intermolecular interactions
between water molecules are
actually abnormally strong
compared to other substances.
We'll examine what it is
that makes the
intermolecular forces
between water molecules
especially strong
later in this unit.
>> student: What about solids,
like plastic or wood?
Are they held together by
intermolecular forces too?
>> professor: Great question.
What do you think?
>> student: I'm not really sure.
Is wood even
made of molecules?
>> professor: It is.
There are cellulose molecules
in wood that are held together
by intermolecular forces.
And in plastics,
there are polymers that
are also held together
by intermolecular forces.
>> student: Well,
what about gases?
Are there intermolecular
forces in gases too?
>> professor: I'm glad
you asked that,
because that brings us
to a very important point.
When a substance is a gas,
the molecules are far apart.
Intermolecular forces only
act over short distances.
So even though you might
have a polar gas molecule,
such as water
in its gaseous form,
the molecules wouldn't be able
to interact with each other
because of how fast the
molecules would be moving,
and their distance
between each other.
Now, if we start removing
energy from the gaseous
water molecules,
which is just another way to
say we lower the temperature,
the molecules begin
to slow down.
Eventually, when they
lose enough energy,
they'll be going slow enough
to start interacting
when they pass each other,
and those intermolecular forces
will become significant.
When they do, the gas begins
to condense and we get...
>> student: Water.
>> professor: You got it.
>> host: That was
a general introduction
to intermolecular forces
by our teacher and students.
We do want to examine
these forces a little
more closely though.
There are four major types
of intermolecular forces.
Dispersion Forces,
Ion-Dipole Forces,
Dipole-Dipole Forces,
and Hydrogen Bonding.
A great way to explore
this topic in the classroom
is to use a jigsaw activity,
where students are broken up
into groups of four.
Each person in the group
draws a number, 1 to 4,
and that will tell the student
the type of intermolecular force
they are going to
be the expert on.
Then the entire class
of students shifts to
"expert camps," where all
the 1's work together,
all the 2's work together, etc.,
building research and knowledge
about their
intermolecular force.
Finally, the students return
back to their original groups,
where they teach their original
group members about
the intermolecular force
they just learned about.
Our students in our classroom
are just about to
come back together
to teach the other members
of their group about
the intermolecular force
that they researched
and learned about.
So let's listen to them
as they demonstrate
their expertise on these
four types of bonds.
>> professor: Okay, Emily,
I believe you have
Dispersion Forces,
which is the first type
of intermolecular force
that we're going to present
and learn about today.
Would you like to go ahead
and present your findings
to the group?
>> student: Sure.
So, I learned that
all molecules and atoms
have dispersion forces.
Dispersion forces are
attractive forces that result
when atoms and molecules
form temporary dipoles.
>> professor: Good.
And as you recall,
a dipole simply means that
a molecule has two poles,
like a magnet.
>> student: Right.
These forces are also called
London Dipole Forces,
and they can be weak
between molecules.
but the larger the molecule,
or atom, the stronger these
intermolecular forces become.
An interesting fact
about these forces
is that they can be used
to predict the relative
boiling points of the
noble gases and halogens.
>> professor: I'm very glad
that you mentioned that.
How so?
>> student: Well, all of
the noble gases and halogens
are non-polar.
So the only forces
acting between them
are these dispersion forces.
And since we know that
dispersion forces get stronger
the larger the atom
or molecule is,
we know that the boiling points
of the group 7 and 8 elements
increase as you go down a group.
And that is because those atoms
are getting larger
as you go down the group.
>> professor: Great job, Emily.
Okay, Ron,
would you like to go next?
I believe you had the second
type of intermolecular force,
ion-dipole forces?
>> student: Okay.
I learned that ion-dipole forces
are attractive forces between
an ion and a polar molecule.
Ion-dipole intermolecular
forces are most commonly
found in solutions,
and they make it possible
for us to dissolve
ionic compounds,
like table salt, in water.
The cations and anions
in the salt crystal
are attracted to the
polar water molecules,
and these polar molecules
pull the ions apart
into the solution.
I also discovered that these
forces can vary in strength
depending on the charges
of the ions,
as well as the polarity
of the molecule.
This graphic shows
how water molecules
pull on a sodium ion
differently than they
pull on a chloride ion.
The partially positive
end of the water molecule,
hydrogen, attracts to
the negatively charged
chloride ion.
And the partially
negatively charged end of
the water molecule, oxygen,
attracts to the positively
charged sodium ion.
>> professor: Very nice job,
Ron.
Did you have any other
interesting facts
about ion-dipole forces?
>> student: Actually, I do.
I learned that sometimes
ion-dipole forces
are not considered
intermolecular forces
since they're not technically
between two molecules.
They're between an ion
and a molecule.
But most chemists today
still consider them to be
one of the four main
intermolecular forces.
>> professor: Excellent job.
Okay, Wyatt, you have
the third type of
intermolecular force,
dipole-dipole forces.
Are you ready to present
your research to us?
>> student: I found that
dipole-dipole forces
are the attractive force
between polar molecules.
What happens is,
the partially positive end
of one molecule attracts
to the partially negative end
of another molecule.
It's like what we were
talking about earlier when
we were using the two bar
magnets as an analogy.
The polar molecules will
be attracted to each other
as long as the molecules
aren't moving with
too much energy.
>> professor: Good description
of dipole-dipole forces, Wyatt.
What else did you find when
you were doing your research?
>> student: I saw in
one of my sources that
iodine monochloride,
which is used in medical labs,
and molecular bromine,
which is used to purify water,
both have two atoms
and are about the same
molecular weight.
Surprisingly,
iodine monochloride
is a solid though,
while bromine is a liquid.
>> professor: Very interesting.
Did you learn why it is
that iodine monochloride
is a solid,
while bromine is a liquid?
>> student: Yes.
It's because the iodine
monochloride molecule is polar,
and since it's polar,
it "sticks" to the other
iodine monochloride molecules
a lot better than
the bromine molecules do.
>> professor: Okay, why is it
that bromine molecules
can't stick together like
the iodine monochloride
molecules do?
>> student: The bromine
molecule is nonpolar.
It's just two bromine atoms.
So they both have the same
electronegativity.
Since it's nonpolar,
the only intramolecular forces
acting in a sample of bromine
are dispersion forces.
That's why it's a liquid.
Its molecules can't
"stick together" as well as
iodine monochloride.
>> professor: Okay, Emily
let me come back to you
for a second.
Wyatt is absolutely right.
When we compare
the dipole-dipole forces
of iodine monochloride
to the dispersion forces
in bromine,
the dipole-dipole forces seem
to be the more important force.
Didn't you say though
from your research
that dispersion forces
are stronger the larger
the atoms are?
How would you explain
Wyatt's findings?
>> student: Well, I don't
that think bromine is large
enough of a molecule
for the dispersion forces
to be more important.
If you look at the halogen
right below bromine, iodine,
we see that it's a solid
at room temperature,
like we talked about earlier.
>> professor: You're absolutely
right, Emily,
and don't forget, everyone.
There are dispersion forces
present in all substances.
All right, we only have one
intermolecular force left,
but it's a very important one.
Dalia, I believe you
have hydrogen bonding.
What did you find out about
that intermolecular force?
>> student: Well, I found out
that hydrogen bond is really
just a special type of
a dipole-dipole force.
It's an attractive force
in which a hydrogen atom
of one molecule
is attracted to
an electronegative atom,
especially in fluorine, oxygen,
or nitrogen atom
of another molecule.
It's stronger than a regular
dipole-dipole force
because you have
the exposed hydrogen proton
from one molecule
attracted to the fluorine,
oxygen, or nitrogen from
the other molecule.
>> professor: Great job.
Did you find any other
interesting information
about hydrogen bonding
while you were doing
your research?
>> student: Yeah, there's a way
that you can memorize
the types of molecules that
will have hydrogen bonding.
If you say,
"Hydrogen bonding is FON,"
it will remind you
what kinds of molecules have
hydrogen bonding between them.
Molecules that have hydrogen
bound to fluorine, oxygen
or nitrogen.
>> professor: I'm glad
you said "between" them
because that's a common mistake
that students make.
They think that a hydrogen bond
is a covalent bond
between hydrogen and either
fluorine, oxygen, or nitrogen.
When, in reality,
a hydrogen bond is
just another type of
intermolecular force,
which means it's a force that
acts between molecules, right?
>> student: Yeah.
I actually made that mistake
myself at first.
I found during my research
that water molecules
have hydrogen bonding.
So, at first, I thought that
was just the bond between
the hydrogen and oxygen
in the water molecule,
but that's not correct.
It's still a force that
acts between molecules.
It's just a special type.
It's kind of like when
you bring two really
strong magnets together.
You know how they would
stick together and would
be harder to break apart
than two weak magnets?
>> professor: That's a great
analogy!
Now going back to
your summary chart,
your last bullet point says
that hydrogen bonding
leads to substances
having unexpected melting
and boiling points.
What did you mean by that?
>> student: Yeah, I actually
have another graph to show that.
You can see on this graph
the boiling points
of substances made
from hydrogen
and the group six non-metals.
H2S, H2Se, and H2Te
all have dispersion
and dipole-dipole forces
between them.
The reason the boiling points
are going up here is because
the molecular masses
are going up,
so the dispersion forces
are getting stronger.
But look at water's
boiling point.
>> student: Yeah!
Water's molecular mass
is only 18.
Why does it have such
a high boiling point?
>> student: That's the hydrogen
bond affecting it.
You can see where water's
boiling point should be
based on its molecular mass.
>> professor: Right.
These dashed lines
point to that.
>> student: But since it has
hydrogen bonding going on
between the molecules,
we have to put a lot more energy
into the water to get it
to change to a gas.
>> professor: Great explanation,
Dalia.
And let me give you
a couple of other interesting
facts about water
and hydrogen bonding.
Everyone knows that ice
floats on water, right?
As you recall from Unit 2,
that's very unusual.
What I mean is,
most substances get more dense
as they go from gas,
to liquid, to solid.
Because water has
hydrogen bonds,
it forms a crystalline
structure as it freezes,
and it takes up more space
in the solid state
than it does in
the liquid state.
This is why ice floats.
And when it covers rivers
and lakes in the winter,
it protects the aquatic life.
If water did not have
this property, then all ponds
and lakes would freeze
from the bottom up.
Hydrogen bonding also makes
water a great heat sponge.
As we saw earlier, water can
absorb a lot of energy
before it's converted to vapor.
We'll cover this topic
in more depth in Unit 8.
For now, just know that
it's a good thing that 71%
of the earth is covered
with water.
The water in our oceans
sits there and absorbs
the radiation from the sun
that we're constantly
being bombarded with
during the day.
Then the earth turns,
and the water slowly releases
that heat at night.
All of you did a great job
researching and presenting
your assigned
intermolecular forces.
>> host: Our students
really did do a great job
of presenting information
on the four
intermolecular forces.
By now, you all have
a better understanding of
how these forces work.
And you should be able to
execute a similar jigsaw
learning exercise
on this or other topics.
Now our students are going
to perform a lab that will
clearly demonstrate
some of the unique qualities
found in both intramolecular
and intermolecular forces,
using materials like these.
Instructions for this lab
can be found in our
Chemistry Matters Toolkit,
so you can perform it yourself.
But to give you a sneak peek,
let's get back to
our classroom.
>> professor: Another way
to analyze the various ways
substances like salt and sugar
are put together
at the atomic level is to
compare the melting points
of everyday substances
like table salt and sugar.
So let's execute a lab
in which you'll test
the melting points
of two substances.
You'll use disposable
aluminum pans and hotplates.
You've got the instructions
for this lab in front of you.
You'll want to note which side
of the pan gets the sugar
and which side gets the salt.
Be sure to follow
all safety protocols,
like pulling hair back
so it's out of the way,
and taking extreme caution
with any heat source.
You may get started.
>> host: Let's take a break
here to let you try this lab
in your classroom.
Remember, the instructions
our teacher mentioned
can be found in the
Chemistry Matters Toolkit.
When you're ready to start
heating the substances,
make sure you carefully time
how long it takes for
the salt or sugar to melt,
giving them up to 1 minute each.
You'll want to preheat
your hotplate before putting
the pan on top of it.
When you're done, I'll see you
at the top of the next video
on the Unit 4 playlist.