>> host: Welcome back
to "Chemistry Matters,"
and the sixth video
on our Unit 4 playlist,
Chemical Bonding.
How did your melting
temperature lab with
salt and sugar turn out?
Let's get right out to
our classroom so you can see
how your results compare to
the results of our students.
>> professor: Emily and Ron,
tell me how the melting point
lab went with the sugar.
>> student: It didn't
take very long at all for
the sugar to start to melt.
>> student: Yeah, it started
to melt at 45 seconds.
>> professor: Okay.
Wyatt and Dalia,
what were the results with
your attempt to melt the salt?
>> student: We couldn't get
the salt to melt at all.
>> student: Yeah,
after a minute, it still
looked exactly the same.
>> student: How long
would it have taken
for the salt to melt?
>> professor: It's not so much
the length of time,
but the temperature
you'd have to reach.
You'd need to raise
the temperature of
the sodium chloride
over its melting point,
which is 801 degrees Celsius.
A bunsen burner flame
can certainly get that hot
if you're working with a nice
flame with an inner cone,
but that's why we did our
experiment with hotplates,
so you could easily see
the difference in the melting
points of the two substances.
>> student: Wow.
Then there's definitely
something different about
the way salt is put together
compared to sugar.
>> student: Yeah, what's the
difference in these two?
>> professor: Can one of you
answer that?
>> student: They obviously
have different melting points.
>> student: Does the higher
melting point for a substance
mean it's held together
more tightly?
>> professor: It does.
What else does your
evidence tell you?
>> student: Well, we know
salt is sodium chloride
and that's an ionic bond.
So the high melting point
must be because
it has an ionic bond.
>> student: And we learned
ionic bonds are the strongest
of the intramolecular
chemical bonds.
>> professor: Good job.
Look at this image
that illustrates what has
to happen in order for
an ionic compound to melt.
All of those cations and
anions are tightly packed
in a crystal lattice.
To get the salt to melt,
we have to put enough energy
into the crystal
to overcome all of those
electrostatic attractions
between the ions.
These are very strong
intramolecular forces,
and it takes a lot of energy
to overcome them.
Compounds that have
covalent bonds, like sucrose,
which is table sugar,
usually have lower
melting and boiling points.
That's because most substances
that contain covalent bonding
tend to form molecules.
Molecules are groups of atoms
that have chemically bonded
and behave as individual units.
These molecules have
characteristic shapes
like you see here.
When melting
a molecular compound,
you just need to put enough
energy into the substance
so that the molecules
that are held together
by intermolecular forces
begin to gain more
kinetic energy and start
to move past each other.
To melt sugar,
you only need to break
intermolecular forces,
not any intramolecular bonds.
This means it typically
requires a lot less energy
to melt covalent compounds
compared to ionic compounds.
>> student: So what about
ionic compounds?
They don't have shapes?
>> professor: Actually,
most ionic compounds would
look like the crystal lattice
structures I just showed you.
This is because of
the tight packing of the
cations and anions,
due to the high strength of
the attraction of their bonds.
The attractive forces
acting between molecules
are known as
intermolecular forces.
They're all weaker than
intramolecular forces,
like covalent bonds.
Intermolecular forces
are also much weaker than
the attraction between ions
in ionic compounds.
That's why compounds that are
made up of covalently bound
molecules tend to have
much lower melting points
than compounds that are
more ionic in character.
>> student: Well, what exactly
is happening when
the sugar is melting?
>> student: Yeah.
Are we breaking
these covalent bonds?
Are their shapes changing
as they melt?
>> professor: Those are
great questions.
And no, those covalent bonds
are not being broken
when covalently bonded
compounds melt.
And their shapes
don't change either.
Those weaker intermolecular
forces are overcome
when we heat these solids,
so eventually the molecules
start to slide past each other
as the compound starts to melt.
As an example,
the water molecule
is a V-shaped molecule
whether it's a gas, a liquid,
or a solid, as in ice.
>> student: Well, what about
an ionic compound then?
What happens to, say salt,
if I were able to melt it?
>> professor: If we were to
add this much energy
to sodium chloride,
it would eventually melt,
and we would have molten
or liquid sodium chloride.
Chemists can take molten
sodium chloride and run
what is known as electrolysis
to isolate elemental sodium
or chlorine gas.
These are very reactive elements
that are not normally
found in nature
in their elemental form.
>> host: Now that you have
a solid understanding
not only of how
chemical bonds work,
but of the different types
of chemical bonds as well,
this is a good time to see
how all of the concepts
we've explored in this unit
apply to the real world.
And to do that, I spoke
with Dr. Pete Ludovice,
a professor of
chemical engineering
at Georgia Tech's
School of Chemical
and Biomolecular Engineering.
What is a chemical engineer?
>> So a chemical engineer,
like all engineers,
takes a little bit of science
and a little bit of math
and applies some creative
problem solving to figure out
a solution to
a societal problem.
And that creative
problem solving is something
we call engineering.
And the solution,
we often call technology.
So we have science, math,
engineering and technology.
The whole thing spells "SMET."
At least that's what the
National Science Foundation
wanted to call it
in the early 90's.
Cooler heads prevailed.
We rearranged it,
rebranded it.
We now call that "STEM".
>> host: Much better word.
>> And the thing about
chemical engineers is,
we use the principles of
chemistry to do that
engineering or creative
problem solving.
The main difference
between the chemist
and the chemical engineer
is that the chemist
worries about the phenomenon,
whereas the chemical engineer
uses those phenomenon
to create these solutions
to societal problems.
Very often, the difference
we observe is that,
the chemist works
at the bench scale,
and if they produce
a material or a catalyst
or a pharmaceutical
that's very, very important,
we need to produce it
at the 100 pound
or 100,000 ton level,
we do it on a much bigger scale.
>> host: Got ya.
Now, talk to us specifically
about what you do
at Georgia Tech, in your lab.
>> So, what I do at
Georgia Tech is,
I am a modeler, right?
I'm actually
a molecular modeler.
So I do something that
the chemist would describe
as computational chemistry.
And I do that because
computers have gotten
very, very fast.
And we are now able to model
some of the chemical reactions,
the chemical phenomenon,
the chemical properties
of materials, in a computer.
And I do that because,
back in 1966, uh, Dr. Mulliken,
who got the first Nobel Prize
for computational chemistry,
said, one day,
we'll go to the computer
instead of the lab.
And let's face it,
the computer is cheaper,
it's easier, it's faster,
it's safer.
You don't have to wear
safety glasses when you're
working at your terminal.
And we might get some of our
solutions from that method.
>> host: Now you work
a lot with bonds.
>> Yes.
>> host: Why does a chemical
engineer need to know
about bonds?
>> Well, because we take
the principles of chemistry
and try to apply
creative problem solving.
What we do is, we change bonds,
we manipulate bonds,
and then we also take advantage
of the bonds in a particular
molecule, to use that
with different processing
and different packing
of the molecules,
to create materials that
are one of the solutions
to these particular problems.
So bonds are very important
to a chemical engineer.
>> host: Now let's talk about
some of these materials
and their properties,
and how that relates to bonds,
and we'll look at opposites.
>> Okay.
>> host: So, some materials
are great electric conductors,
and some are insulators.
What's the difference there?
>> So it has to do with
the bonding mode.
Uh, previously, you very likely
talked about covalent bonds
and ionic bonds.
So metals have this
bonding mode,
and when we say bond,
we think bond is some
physical thing that we can
hold and touch and see.
Linus Pauling, who wrote
a lot about the nature
of the chemical bond,
always pointed out,
it's not really there.
It's sort of a fluid thing.
It is a stable arrangement
of electrons,
but different bonding types
have different
sorts of properties.
And in a metal,
the electrons can
flip back and forth
between each other.
They can move a lot.
They're very, very fluid.
And that's what
electrical conduction is.
So, in organic polymers,
where I work,
they aren't as fluid.
They usually have
covalent bonds,
and not ionic bonds.
But you can even manipulate
organic polymers to do that.
For example, if you take
a double bond, which has a
little electron density in it,
and you make a polymer
like polyacetylene,
which has, every other bond,
one of those double bonds,
the electrons are mobile
enough to hop in between
those things,
and you can actually make
a conductive polymer.
So we do make a lot of
conductive polymer
that take advantage of this
alternating double bond.
It's called
conjugated double bond.
Then, if we put a lot of
these double bonds together,
in a plane, uh,
that's actually graphene.
And graphene is very conductive.
We now believe that
graphene may be the basis
for some of our future
microelectronics technology,
because it can conduct,
it's organic,
and it also has some
other useful properties
because it's organic, right?
So metals tend to oxidize.
They're not as flexible.
Um, so you can actually make
organic polymers conductive
by changing the way
you do the bonds.
>> host: Very cool.
So our next pair of opposites.
Some materials
are very strong.
We might use them to build
large structures.
Some materials
are very flexible.
We might use them
to make cloth.
What's the difference
between those?
>> So the difference between
those, uh, is--
Again, it's the bonding mode.
So the most, uh,
strong material, but also
some of the most brittle
materials are ceramics.
And ceramics are metal oxide.
So, uh, your windowpane glass,
your toilet, those are ceramics,
and they're very, very strong.
They're also very, very
resistant to chemical oxidation
and things like that.
So they're very useful
for that sort of thing.
But they're very brittle,
right?
You hit 'em with a hammer
and they crack.
So the best materials aren't
necessarily just strong
or just flexible,
they're something called tough.
That they are both
flexible and strong,
and they can absorb
a lot of energy,
but they can also deform
to absorb a lot more energy.
That's the sort of thing
we want.
So we might mix
a ceramic material
with a flexible material.
And polymers are very
flexible materials, right?
They are basically what
make up most plastics.
And polymers, uh, get their
physical integrity
not by having bonds
between every single atom,
like a ceramic,
but bonds along a chain.
Then the chains orient,
and the oriented chains
have a weaker interaction
between them,
so it makes a flexible film
or a flexible fiber.
And the best kind of material
mixes flexible materials,
like polymers,
and very strong,
but brittle materials,
like ceramics.
And the best example would be
the modern tennis racket.
Do you play tennis?
>> host: I am not nearly
that coordinated.
>> Okay.
Well, I'm not that
coordinated either.
But the good news is,
the modern tennis racket is
very light and very strong.
So you don't have to be
all that good at tennis.
In the old days,
we made them out of wood.
Wood, believe it or not,
is mother nature's ceramic.
It's a very complicated
material that's both
strong and flexible.
But it tends to warp,
and it bends when
you get it wet,
and things like that.
So it's not a perfect material.
Then we made them out of metal.
Metal is too heavy.
So now, we make a ceramic
polymer composite.
And if you go in
a-a sporting goods store
and pick up a tennis racket,
it is very, very strong,
slightly flexible,
but also very, very light.
So that's another reason
to do composites.
>> host: And it improves
your game.
>> Well, it improves people
that know how to play tennis.
It improves my game
only marginally.
>> host: Got ya.
Now, perhaps the most critical
pair of opposites.
Some materials are edible
and some materials
are explosive.
>> Now, I would've gone with
edible and inedible.
So, yes.
I-I would agree.
Those are definitely opposites.
Opposites in the extreme.
Explosive materials
are very simple.
They are simply molecules that,
when they oxidize,
when they burn,
they burn so rapidly that
the-the flame front moves
faster than the speed of sound.
So, rather than having
something go "whoosh,"
it actually compresses the air.
That shockwave is
the explosion.
And any material that will
do that works perfectly fine.
So, we've learned,
over the years,
that nitrate groups are
very good for making
molecules like that.
So, if you think of
typical explosives,
like nitroglycerin.
That's glycerin with a
nitrate group attached to it.
Trinitrotoluene.
That's TNT.
That's toluene with three
nitro groups on it.
Those groups tend
to be very explosive.
And, in fact, if you're
looking for explosives,
one of the things you can do
is test for nitrate groups.
Whenever they find
nitrate groups, that's
usually used in explosives.
So, edible, uh, molecules,
or edible polymers, for example,
and we have plenty of them.
Um, they usually have some
sort of a polar group on them.
So instead of just pure carbon,
they will have nitrogens,
they will have oxygens on them.
So the enzymes inside
your body can actually grab
on to them, transform them,
and metabolize them.
A good example would be
if you were to take benzene,
which is a great
organic solvent.
It's a six-membered,
aromatic ring.
It is highly toxic,
but it's also highly
carcinogenic.
And a lot of the ways we cause
cancer in the human body
is when we cannot
metabolize something.
It literally sits in there
and irritates tissue until
cancer eventually occurs.
You can't do anything
with benzene, but if you put
one carbon on it,
one methyl group,
that's enough that your body
can oxidize the methyl group.
That would be toluene.
And it can oxidize that
into benzoic acid.
And benzoic acid,
we can process.
In fact, benzoic acid
is often used as
a preservative in food.
So, something polar that
you can grab on to usually
has to be water soluble.
That helps.
Good example of that
would be a polymer that
a lot of people use, um,
polyethyleneglycol.
Sometimes called
polyethyleneoxide.
It is water soluble.
It actually has
amazing properties.
It tends to coil up
and entangle, and it
makes fluids very thick.
So, if you see
polyethyleneglycol,
it's written "peg".
Usually peg 40,
meaning there's 40 units,
or peg 80.
You often find it
in consumer products, and you
can even find it in food.
So that is actually edible.
>> host: In all this
talk of opposites,
it seems like some of the
most interesting technology
falls in the middle,
and we can make small changes
to these molecules
to bridge those two opposites
and find materials that are
better, stronger, more useful.
>> Right, right.
So, you mix the-the strong
but brittle with the flexible
but weak, to make something
that's actually tough.
The same is true when
you talked about conductors
and insulators.
So what's really
interesting is,
you can take an insulator,
like silicone, and dope it
with just the right metals.
And by doping, I mean,
add tiny, little bits
of these things,
and you can actually
make that a semiconductor.
So that's in between
a conductor and an insulator.
It will conduct, but only if
you give it just enough
electrical potential
to promote it to
the conduction band.
And by looking at that
band gap, we can actually
design a transistor
out of semiconductors.
So the entire
semiconductor field
is based on one of
these midpoints between
conductors and insulators,
and that's the semiconductor.
>> host: You mentioned
chemical engineering
being a creative field,
and that's evidence of it
right there.
>> Absolutely.
Uh, and, in fact,
I do a lot of statistical
mechanical simulations.
One of our biggest problems is,
we work on
the photo resist polymers
that we coat to etch
the patterns into the next
microelectronics chips.
And what we're trying to do
is making smaller and smaller
and smaller patterns,
so we actually use maybe
another one of these
intermediate things,
where we take a block copolymer.
So polymer means
it's a long molecule.
Copolymer means it's got
two different parts,
and the two parts
are in blocks.
They don't like each other
just enough so that they phase
separate to make a line of one
and a line of another.
And we make these very,
very small.
So I said they were
big molecules...
>> host: Right.
>> But big means, this half
of it is a few nanometers.
So it's actually quite small.
>> host: Very big, but still
very, very small.
>> Right, and if we can
get them to line up,
we can actually etch
this sort of thing, uh,
to get this to form
the next generation of, uh,
microelectronics chips
and things like that.
One of our problems is that,
in order to get
the simulation to work,
they don't mix into that
in short order.
So we play a lot of
very creative tricks
to get it to do that.
Then the final thing we do is,
we play video games.
Okay, not literally.
>> host: Darn.
>> But we, um, we take
the same graphics boards
that you or your friends
or people you know
use to play video games.
It makes your avatar
look really, really cool.
>> host: Right.
>> We take advantage of the
multiple processors on there
and it can make our systems
run between 200 and 1,000
times faster.
So, the next time
a parent opens the door,
yells down in the basement
and says, "Stop playing
"those video games!
"They're good for nothing."
Not exactly true.
>> host: There is value
down there.
>> Yes.
We are using them to help
design the next generation
of microelectronics.
>> host: Well, I understand
that you are also
a stand-up comedian.
>> Yes.
>> host: So, my question is,
is chemistry funny?
>> Is chemistry funny?
Uh, chemists certainly are,
and I certainly think
chemical engineers are as well.
Um, 'cause we're
kind of strange.
I mean, my favorite joke, uh,
to tell about
chemical engineers,
is that chemical engineers
are very simple people,
and we live by two simple rules.
Number one,
if it's broken, fix it.
And number two,
if it's not broken, fix it.
>> host: Well, thank you
for telling us all about
how you fix things.
>> It was my pleasure.
>> host: When we started
this Unit 4 playlist
on chemical bonds,
you probably didn't realize
that there were so many
types of bonds.
But as the students
in our classroom found,
all of the bonds
we've studied in this unit,
whether intramolecular bonds
or intermolecular forces,
are caused by electrostatic
forces of attraction,
which can pull on nearby
atoms and molecules.
If you can keep that
thought in mind
whenever you describe
what's happening when
chemicals attract each other
to form new substances,
you'll understand
the fundamentals
of chemical reactions
and how they occur.
And that will give you
the ability to understand
how chemical bonds
at the atomic level
affect what we see
in the everyday world.
So now that you're well-versed
in how chemical bonds
form and break,
it leads us nicely into Unit 5,
Chemical Reactions.
But before you begin that unit,
don't forget that
the Unit 4 Toolkit includes
additional information,
and there's also a Closer Look
video for Unit 4
that will enhance
your understanding of
Lewis Dot Diagrams.
When you're done
with all of that,
I'll be ready for you
at the top of the playlist
for Unit 5,
chemical reactions.