States of matter

WEBVTT
Kind: captions
Language: en

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I think we're all reasonably
familiar with the three states

00:00:03.440 --> 00:00:05.400
of matter in our
everyday world.

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At very high temperatures
you get a fourth.

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But the three ones that we
normally deal with are, things

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could be a solid, a liquid,
or it could be a gas.

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And we have this general notion,
and I think water is

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the example that always comes
to at least my mind.

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Is that solid happens
when things are

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colder, relatively colder.

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And then as you warm up, you
go into a liquid state.

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And as your warm up even more
you go into a gaseous state.

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So you go from colder
to hotter.

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And in the case of water, when
you're a solid, you're ice.

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When you're a liquid, some
people would call ice water,

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but let's call it
liquid water.

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I think we know what that is.

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And then when it's in the gas
state, you're essentially

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vapor or steam.

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So let's think a little bit
about what, at least in the

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case of water, and the analogy
will extend to

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other types of molecules.

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But what is it about water that
makes it solid, and when

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it's colder, what allows
it to be liquid.

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And I'll be frank, liquids are
kind of fascinating because

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you can never nail them
down, I guess is the

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best way to view them.

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Or a gas.

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So let's just draw
a water molecule.

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So you have oxygen there.

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You have some bonds
to hydrogen.

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And then you have two extra
pairs of valence electrons in

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the oxygen.

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And a couple of videos ago, we
said oxygen is a lot more

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electronegative than
the hydrogen.

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It likes to hog the electrons.

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So even though this shows
that they're sharing

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electrons here and here.

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At both sides of those lines,
you can kind of view that

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hydrogen is contributing an
electron and oxygen is

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contributing an electron on
both sides of that line.

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But we know because of the
electronegativity, or the

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relative electronegativity of
oxygen, that it's hogging

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these electrons.

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And so the electrons spend a
lot more time around the

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oxygen than they do around
the hydrogen.

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And what that results is that
on the oxygen side of the

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molecule, you end up with a
partial negative charge.

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And we talked about
that a little bit.

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And on the hydrogen side of the
molecules, you end up with

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a slightly positive charge.

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Now, if these molecules have
very little kinetic energy,

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they're not moving around a
whole lot, then the positive

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sides of the hydrogens are very
attracted to the negative

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sides of oxygen in
other molecules.

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Let me draw some
more molecules.

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When we talk about the whole
state of the whole matter, we

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actually think about how the
molecules are interacting with

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each other.

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Not just how the atoms are
interacting with each other

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within a molecule.

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I just drew one oxygen, let
me copy and paste that.

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But I could do multiple
oxygens.

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And let's say that that hydrogen
is going to want to

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be near this oxygen.

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Because this has partial
negative charge, this has a

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partial positive charge.

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And then I could do another
one right there.

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And then maybe we'll have, and
just to make the point clear,

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you have two hydrogens
here, maybe an oxygen

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wants to hang out there.

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So maybe you have an oxygen that
wants to be here because

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it's got its partial
negative here.

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And it's connected to two
hydrogens right there that

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have their partial positives.

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But you can kind of see
a lattice structure.

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Let me draw these bonds, these
polar bonds that start forming

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between the particles.

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These bonds, they're called
polar bonds because the

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molecules themselves
are polar.

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And you can see it forms
this lattice structure.

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And if each of these molecules
don't have a

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lot of kinetic energy.

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Or we could say the average
kinetic energy of this matter

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is fairly low.

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And what do we know is average
kinetic energy?

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Well, that's temperature.

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Then this lattice structure
will be solid.

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These molecules will not move
relative to each other.

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I could draw a gazillion more,
but I think you get the point

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that we're forming this kind
of fixed structure.

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And while we're in the solid
state, as we add kinetic

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energy, as we add heat, what
it does to molecules is, it

00:04:52.040 --> 00:04:53.860
just makes them vibrate
around a little bit.

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If I was a cartoonist, they way
you'd draw a vibration is

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to put quotation marks there.

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That's not very scientific.

00:05:03.190 --> 00:05:05.700
But they would vibrate around,
they would buzz around a

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little bit.

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I'm drawing arrows to show
that they are vibrating.

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It doesn't have to be just

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left-right it could be up-down.

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But as you add more and more
heat in a solid, these

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molecules are going to
keep their structure.

00:05:16.650 --> 00:05:19.640
So they're not going to move
around relative to each other.

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But they will convert that heat,
and heat is just a form

00:05:23.210 --> 00:05:27.130
of energy, into kinetic energy
which is expressed as the

00:05:27.130 --> 00:05:29.340
vibration of these molecules.

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Now, if you make these molecules
start to vibrate

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enough, and if you put enough
kinetic energy into these

00:05:35.560 --> 00:05:37.450
molecules, what do you think
is going to happen?

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Well this guy is vibrating
pretty hard, and he's

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vibrating harder and harder as
you add more and more heat.

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This guy is doing
the same thing.

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At some point, these polar bonds
that they have to each

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other are going to start not
being strong enough to contain

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the vibrations.

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And once that happens, the
molecules-- let me draw a

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couple more.

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Once that happens, the molecules
are going to start

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moving past each other.

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So now all of a sudden, the
molecule will start shifting.

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But they're still attracted.

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Maybe this side is moving here,
that's moving there.

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You have other molecules
moving around that way.

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But they're still attracted
to each other.

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Even though we've gotten the
kinetic energy to the point

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that the vibrations can kind of
break the bonds between the

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polar sides of the molecules.

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Our vibration, or our kinetic
energy for each molecule,

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still isn't strong enough to
completely separate them.

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They're starting to slide
past each other.

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And this is essentially what
happens when you're in a

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liquid state.

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You have a lot of atoms that
want be touching each other

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but they're sliding.

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They have enough kinetic energy
to slide past each

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other and break that solid
lattice structure here.

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And then if you add even more
kinetic energy, even more

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heat, at this point it's
a solution now.

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They're not even going to be
able to stay together.

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They're not going to be able
to stay near each other.

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If you add enough kinetic energy
they're going to start

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looking like this.

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They're going to completely
separate and then kind of

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bounce around independently.

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Especially independently if
they're an ideal gas.

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But in general, in gases,
they're no longer touching

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each other.

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They might bump into
each other.

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But they have so much kinetic
energy on their own that

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they're all doing
their own thing

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and they're not touching.

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I think that makes intuitive
sense if you just think about

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what a gas is.

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For example, it's hard
to see a gas.

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Why is it hard to see a gas?

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Because the molecules are
much further apart.

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So they're not acting on the
light in the way that a liquid

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or a solid would.

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And if we keep making that
extended further, a solid--

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well, I probably shouldn't
use the example with ice.

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Because ice or water is one of
the few situations where the

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solid is less dense
than the liquid.

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That's why ice floats.

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And that's why icebergs don't
just all fall to the

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bottom of the ocean.

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And ponds don't completely
freeze solid.

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But you can imagine that,
because a liquid is in most

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cases other than water,
less dense.

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That's another reason why you
can see through it a little

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bit better.

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Or it's not diffracting-- well I
won't go into that too much,

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than maybe even a solid.

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But the gas is the
most obvious.

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And it is true with water.

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The liquid form is definitely
more dense than the gas form.

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In the gas form, the molecules
are going to jump around, not

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touch each other.

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And because of that,
more light can

00:08:35.130 --> 00:08:37.030
get through the substance.

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Now the question is, how do we
measure the amount of heat

00:08:40.309 --> 00:08:44.940
that it takes to do
this to water?

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And to explain that, I'll
actually draw a

00:08:47.800 --> 00:08:49.990
phase change diagram.

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Which is a fancy way of
describing something fairly

00:08:53.670 --> 00:08:54.710
straightforward.

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Let me say that this is the
amount of heat I'm adding.

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And this is the temperature.

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We'll talk about the states
of matter in a second.

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So heat is often denoted by q.

00:09:06.490 --> 00:09:09.080
Sometimes people will talk
about change in heat.

00:09:09.080 --> 00:09:11.680
They'll use H, lowercase
and uppercase H.

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They'll put a delta
in front of the H.

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Delta just means change in.

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And sometimes you'll hear
the word enthalpy.

00:09:17.980 --> 00:09:18.740
Let me write that.

00:09:18.740 --> 00:09:23.250
Because I used to say
what is enthalpy?

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It sounds like empathy,
but it's

00:09:29.050 --> 00:09:30.600
quite a different concept.

00:09:30.600 --> 00:09:33.280
At least, as far as my neural
connections could make it.

00:09:33.280 --> 00:09:36.810
But enthalpy is closely
related to heat.

00:09:36.810 --> 00:09:38.060
It's heat content.

00:09:45.890 --> 00:09:49.250
For our purposes, when you hear
someone say change in

00:09:49.250 --> 00:09:51.550
enthalpy, you should really
just be thinking

00:09:51.550 --> 00:09:52.690
of change in heat.

00:09:52.690 --> 00:09:56.850
I think this word was really
just introduced to confuse

00:09:56.850 --> 00:10:00.860
chemistry students and introduce
a non-intuitive word

00:10:00.860 --> 00:10:02.080
into their vocabulary.

00:10:02.080 --> 00:10:04.100
The best way to think about
it is heat content.

00:10:04.100 --> 00:10:07.130
Change in enthalpy is really
just change in heat.

00:10:07.130 --> 00:10:09.230
And just remember, all of these
things, whether we're

00:10:09.230 --> 00:10:13.000
talking about heat,
kinetic energy,

00:10:13.000 --> 00:10:16.410
potential energy, enthalpy.

00:10:16.410 --> 00:10:18.590
You'll hear them in different
contexts, and you're like, I

00:10:18.590 --> 00:10:19.550
thought I should be using
heat and they're

00:10:19.550 --> 00:10:21.030
talking about enthalpy.

00:10:21.030 --> 00:10:23.300
These are all forms of energy.

00:10:23.300 --> 00:10:26.040
And these are all measured
in joules.

00:10:26.040 --> 00:10:28.100
And they might be measured
in other ways, but the

00:10:28.100 --> 00:10:29.550
traditional way is in joules.

00:10:29.550 --> 00:10:31.800
And energy is the ability
to do work.

00:10:31.800 --> 00:10:33.720
And what's the unit for work?

00:10:33.720 --> 00:10:34.450
Well, it's joules.

00:10:34.450 --> 00:10:35.470
Force times distance.

00:10:35.470 --> 00:10:37.040
But anyway, that's
a side-note.

00:10:37.040 --> 00:10:38.590
But it's good to know
this word enthalpy.

00:10:38.590 --> 00:10:40.820
Especially in a chemistry
context, because it's used all

00:10:40.820 --> 00:10:43.170
the time and it can be very
confusing and non-intuitive.

00:10:43.170 --> 00:10:46.830
Because you're like, I don't
know what enthalpy is in my

00:10:46.830 --> 00:10:47.460
everyday life.

00:10:47.460 --> 00:10:50.170
Just think of it as heat
contact, because that's really

00:10:50.170 --> 00:10:50.910
what it is.

00:10:50.910 --> 00:10:55.430
But anyway, on this
axis, I have heat.

00:10:55.430 --> 00:10:57.180
So this is when I have very
little heat and I'm

00:10:57.180 --> 00:10:58.550
increasing my heat.

00:10:58.550 --> 00:11:00.405
And this is temperature.

00:11:05.220 --> 00:11:09.720
Now let's say at low
temperatures I'm here and as I

00:11:09.720 --> 00:11:12.410
add heat my temperature
will go up.

00:11:12.410 --> 00:11:14.190
Temperature is average
kinetic energy.

00:11:14.190 --> 00:11:16.380
Let's say I'm in the
solid state here.

00:11:16.380 --> 00:11:18.460
And I'll do the solid
state in purple.

00:11:21.625 --> 00:11:23.590
No I already was using purple.

00:11:23.590 --> 00:11:26.490
I'll use magenta.

00:11:26.490 --> 00:11:29.810
So as I add heat, my temperature
will go up.

00:11:29.810 --> 00:11:31.040
Heat is a form of energy.

00:11:31.040 --> 00:11:36.070
And when I add it to these
molecules, as I did in this

00:11:36.070 --> 00:11:37.070
example, what did it do?

00:11:37.070 --> 00:11:38.620
It made them vibrate more.

00:11:38.620 --> 00:11:41.290
Or it made them have higher
kinetic energy, or higher

00:11:41.290 --> 00:11:43.830
average kinetic engery, and
that's what temperature is a

00:11:43.830 --> 00:11:46.860
measure of; average
kinetic energy.

00:11:46.860 --> 00:11:50.430
So as I add heat in the solid
phase, my average kinetic

00:11:50.430 --> 00:11:51.650
energy will go up.

00:11:51.650 --> 00:11:52.710
And let me write this down.

00:11:52.710 --> 00:11:58.930
This is in the solid phase, or
the solid state of matter.

00:11:58.930 --> 00:12:01.060
Now something very interesting
happens.

00:12:01.060 --> 00:12:03.930
Let's say this is water.

00:12:03.930 --> 00:12:09.920
So what happens at
zero degrees?

00:12:09.920 --> 00:12:14.910
Which is also 273.15 Kelvin.

00:12:14.910 --> 00:12:15.830
Let's say that's that line.

00:12:15.830 --> 00:12:18.320
What happens to a solid?

00:12:18.320 --> 00:12:19.550
Well, it turns into a liquid.

00:12:19.550 --> 00:12:20.600
Ice melts.

00:12:20.600 --> 00:12:22.960
Not all solids, we're talking
in particular about

00:12:22.960 --> 00:12:25.540
water, about H2O.

00:12:25.540 --> 00:12:27.990
So this is ice in our example.

00:12:27.990 --> 00:12:29.670
All solids aren't ice.

00:12:29.670 --> 00:12:33.950
Although, you could think of
a rock as solid magma.

00:12:33.950 --> 00:12:35.200
Because that's what it is.

00:12:38.370 --> 00:12:41.150
I could take that analogy a
bunch of different ways.

00:12:41.150 --> 00:12:46.365
But the interesting thing that
happens at zero degrees.

00:12:49.180 --> 00:12:51.120
Depending on what direction
you're going, either the

00:12:51.120 --> 00:12:53.730
freezing point of water or
the melting point of ice,

00:12:53.730 --> 00:12:55.550
something interesting happens.

00:12:55.550 --> 00:13:00.840
As I add more heat, the
temperature does not to go up.

00:13:00.840 --> 00:13:03.440
As I add more heat, the
temperature does not go up for

00:13:03.440 --> 00:13:05.120
a little period.

00:13:05.120 --> 00:13:07.150
Let me draw that.

00:13:07.150 --> 00:13:10.460
For a little period, the
temperature stays constant.

00:13:10.460 --> 00:13:14.970
And then while the temperature
is constant, it stays a solid.

00:13:14.970 --> 00:13:16.090
We're still a solid.

00:13:16.090 --> 00:13:18.580
And then, we finally
turn into a liquid.

00:13:22.510 --> 00:13:23.960
Let's say right there.

00:13:23.960 --> 00:13:25.525
So we added a certain amount
of heat and it

00:13:25.525 --> 00:13:26.590
just stayed a solid.

00:13:26.590 --> 00:13:28.180
But it got us to the
point that the ice

00:13:28.180 --> 00:13:29.220
turned into a liquid.

00:13:29.220 --> 00:13:31.090
It was kind of melting
the entire time.

00:13:31.090 --> 00:13:32.690
That's the best way
to think about it.

00:13:32.690 --> 00:13:36.190
And then, once we keep adding
more and more heat, then the

00:13:36.190 --> 00:13:37.440
liquid warms up too.

00:13:42.450 --> 00:13:45.650
Now, we get to, what
temperature becomes

00:13:45.650 --> 00:13:46.770
interesting again for water?

00:13:46.770 --> 00:13:56.700
Well, obviously 100 degrees
Celsius or 373 degrees Kelvin.

00:13:56.700 --> 00:13:57.940
I'll do it in Celsius
because that's what

00:13:57.940 --> 00:13:59.230
we're familiar with.

00:13:59.230 --> 00:13:59.960
What happens?

00:13:59.960 --> 00:14:03.890
That's the temperature at which
water will vaporize or

00:14:03.890 --> 00:14:05.510
which water will boil.

00:14:05.510 --> 00:14:07.040
But something happens.

00:14:07.040 --> 00:14:13.300
And they're really getting
kinetically active.

00:14:13.300 --> 00:14:16.510
But just like when you went from
solid to liquid, there's

00:14:16.510 --> 00:14:19.990
a certain amount of energy that
you have to contribute to

00:14:19.990 --> 00:14:20.330
the system.

00:14:20.330 --> 00:14:23.140
And actually, it's a good
amount at this point.

00:14:23.140 --> 00:14:26.360
Where the water is turning
into vapor, but it's not

00:14:26.360 --> 00:14:27.610
getting any hotter.

00:14:31.940 --> 00:14:34.280
So we have to keep adding heat,
but notice that the

00:14:34.280 --> 00:14:35.230
temperature didn't go up.

00:14:35.230 --> 00:14:37.070
We'll talk about it
in a second what

00:14:37.070 --> 00:14:38.120
was happening then.

00:14:38.120 --> 00:14:43.060
And then finally, after that
point, we're completely

00:14:43.060 --> 00:14:46.070
vaporized, or we're
completely steam.

00:14:46.070 --> 00:14:48.890
Then we can start getting hot,
the steam can then get hotter

00:14:48.890 --> 00:14:53.170
as we add more and more
heat to the system.

00:14:53.170 --> 00:14:55.770
So the interesting question, I
think it's intuitive, that as

00:14:55.770 --> 00:15:01.540
you add heat here, our
temperature is going to go up.

00:15:01.540 --> 00:15:03.820
But the interesting thing is,
what was going on here?

00:15:03.820 --> 00:15:04.880
We were adding heat.

00:15:04.880 --> 00:15:07.810
So over here we were turning our
heat into kinetic energy.

00:15:07.810 --> 00:15:10.090
Temperature is average
kinetic energy.

00:15:10.090 --> 00:15:12.710
But over here, what was
our heat doing?

00:15:12.710 --> 00:15:15.080
Well, our heat was was
not adding kinetic

00:15:15.080 --> 00:15:16.130
energy to the system.

00:15:16.130 --> 00:15:17.900
The temperature was
not increasing.

00:15:17.900 --> 00:15:20.980
But the ice was going
from ice to water.

00:15:20.980 --> 00:15:24.110
So what was happening at that
state, is that the kinetic

00:15:24.110 --> 00:15:29.220
energy, the heat, was being
used to essentially break

00:15:29.220 --> 00:15:32.190
these bonds.

00:15:32.190 --> 00:15:34.080
And essentially bring
the molecules into a

00:15:34.080 --> 00:15:35.660
higher energy state.

00:15:35.660 --> 00:15:37.840
So you're saying, Sal,
what does that mean,

00:15:37.840 --> 00:15:38.640
higher energy state?

00:15:38.640 --> 00:15:42.270
Well, if there wasn't all of
this heat and all this kinetic

00:15:42.270 --> 00:15:44.910
energy, these molecules
want to be very

00:15:44.910 --> 00:15:46.810
close to each other.

00:15:46.810 --> 00:15:49.230
For example, I want
to be close to the

00:15:49.230 --> 00:15:50.590
surface of the earth.

00:15:50.590 --> 00:15:52.610
When you put me in a plane
you have put me in a

00:15:52.610 --> 00:15:53.850
higher energy state.

00:15:53.850 --> 00:15:56.000
I have a lot more potential
energy.

00:15:56.000 --> 00:15:58.800
I have the potential to fall
towards the earth.

00:15:58.800 --> 00:16:02.630
Likewise, when you move these
molecules apart, and you go

00:16:02.630 --> 00:16:05.570
from a solid to a liquid,
they want to fall

00:16:05.570 --> 00:16:06.900
towards each other.

00:16:06.900 --> 00:16:08.850
But because they have so much
kinetic energy, they never

00:16:08.850 --> 00:16:10.280
quite are able to do it.

00:16:10.280 --> 00:16:12.420
But their energy goes up.

00:16:12.420 --> 00:16:14.700
Their potential energy is higher
because they want to

00:16:14.700 --> 00:16:15.960
fall towards each other.

00:16:15.960 --> 00:16:18.800
By falling towards each
other, in theory, they

00:16:18.800 --> 00:16:21.010
could do some work.

00:16:21.010 --> 00:16:26.720
So what's happening here is,
when we're contributing heat--

00:16:26.720 --> 00:16:32.530
and this amount of heat we're
contributing, it's called the

00:16:32.530 --> 00:16:33.780
heat of fusion.

00:16:36.820 --> 00:16:39.010
Because it's the same amount
of heat regardless how much

00:16:39.010 --> 00:16:40.240
direction we go in.

00:16:40.240 --> 00:16:43.350
When we go from solid to liquid,
you view it as the

00:16:43.350 --> 00:16:45.450
heat of melting.

00:16:45.450 --> 00:16:48.040
It's the head that you need
to put in to melt

00:16:48.040 --> 00:16:49.160
the ice into liquid.

00:16:49.160 --> 00:16:51.530
When you're going in this
direction, it's the heat you

00:16:51.530 --> 00:16:55.800
have to take out of the
zero degree water to

00:16:55.800 --> 00:16:58.040
turn it into ice.

00:16:58.040 --> 00:17:01.160
So you're taking that potential
energy and you're

00:17:01.160 --> 00:17:05.050
bringing the molecules closer
and closer to each other.

00:17:05.050 --> 00:17:08.700
So the way to think about it
is, right here this heat is

00:17:08.700 --> 00:17:10.530
being converted to
kinetic energy.

00:17:10.530 --> 00:17:15.020
Then, when we're at this phase
change from solid to liquid,

00:17:15.020 --> 00:17:16.980
that heat is being used
to add potential

00:17:16.980 --> 00:17:18.050
energy into the system.

00:17:18.050 --> 00:17:21.369
To pull the molecules
apart, to give them

00:17:21.369 --> 00:17:22.420
more potential energy.

00:17:22.420 --> 00:17:25.060
If you pull me apart from the
earth, you're giving me

00:17:25.060 --> 00:17:26.000
potential energy.

00:17:26.000 --> 00:17:29.040
Because gravity wants to pull
me back to the earth.

00:17:29.040 --> 00:17:32.640
And I could do work when I'm
falling back to the earth.

00:17:32.640 --> 00:17:33.770
A waterfall does work.

00:17:33.770 --> 00:17:35.210
It can move a turbine.

00:17:35.210 --> 00:17:37.625
You could have a bunch
of falling Sals move

00:17:37.625 --> 00:17:39.160
a turbine as well.

00:17:39.160 --> 00:17:42.050
And then, once you are fully a
liquid, then you just become a

00:17:42.050 --> 00:17:43.440
warmer and warmer liquid.

00:17:43.440 --> 00:17:48.760
Now the heat is, once again,
being used for kinetic energy.

00:17:48.760 --> 00:17:51.010
You're making the water
molecules move past each other

00:17:51.010 --> 00:17:53.190
faster, and faster,
and faster.

00:17:53.190 --> 00:17:57.630
To some point where they want
to completely disassociate

00:17:57.630 --> 00:17:58.580
from each other.

00:17:58.580 --> 00:18:00.370
They want to not even slide
past each other, just

00:18:00.370 --> 00:18:03.080
completely jump away
from each other.

00:18:03.080 --> 00:18:04.140
And that's right here.

00:18:04.140 --> 00:18:05.610
This is the heat of
vaporization.

00:18:13.880 --> 00:18:15.440
And the same idea
is happening.

00:18:15.440 --> 00:18:17.710
Before we were sliding next to
each other, now we're pulling

00:18:17.710 --> 00:18:19.510
apart altogether.

00:18:19.510 --> 00:18:22.620
So they could definitely
fall closer together.

00:18:22.620 --> 00:18:27.510
And then once we've added this
much heat, now we're just

00:18:27.510 --> 00:18:28.650
heating up the steam.

00:18:28.650 --> 00:18:30.990
We're just heating up
the gaseous water.

00:18:30.990 --> 00:18:33.770
And it's just getting hotter
and hotter and hotter.

00:18:33.770 --> 00:18:35.670
But the interesting thing there,
and I mean at least the

00:18:35.670 --> 00:18:41.040
interesting thing to me when I
first learned this, whenever I

00:18:41.040 --> 00:18:43.970
think of zero degrees water I'll
say, oh it must be ice.

00:18:43.970 --> 00:18:45.590
But that's not necessarily
the case.

00:18:45.590 --> 00:18:48.680
If you start with water and you
make it colder and colder

00:18:48.680 --> 00:18:52.460
and colder to zero degrees,
you're essentially taking heat

00:18:52.460 --> 00:18:53.440
out of the water.

00:18:53.440 --> 00:18:57.410
You can have zero degree
water and it hasn't

00:18:57.410 --> 00:18:58.660
turned into ice yet.

00:18:58.660 --> 00:19:01.850
And likewise, you could have 100
degree water that hasn't

00:19:01.850 --> 00:19:03.070
turned into steam yeat.

00:19:03.070 --> 00:19:04.200
You have to add more energy.

00:19:04.200 --> 00:19:05.800
You can also have 100
degree steam.

00:19:05.800 --> 00:19:08.700
You can also have zero
degree water.

00:19:08.700 --> 00:19:10.680
Anyway, hopefully that gives you
a little bit of intuition

00:19:10.680 --> 00:19:12.740
of what the different states
of matter are.

00:19:12.740 --> 00:19:15.180
And in the next problem, we'll
talk about how much heat

00:19:15.180 --> 00:19:18.130
exactly it does take to
move along this line.

00:19:18.130 --> 00:19:21.020
And maybe we can solve some
problems on how much ice we

00:19:21.020 --> 00:19:23.620
might need to make
our drink cool.


Balancing Chemical Equations

WEBVTT
Kind: captions
Language: en

00:00:00.710 --> 00:00:04.200
Like the concept of the mole,
balancing equations is one of

00:00:04.200 --> 00:00:06.700
those ideas that you learn in
first-year chemistry class.

00:00:06.700 --> 00:00:09.250
It tends to give a lot of
students a hard time, even

00:00:09.250 --> 00:00:12.430
though it is a fairly
straightforward concept.

00:00:12.430 --> 00:00:13.970
I think what makes it difficult
is that there's a

00:00:13.970 --> 00:00:15.350
bit of an art to it.

00:00:15.350 --> 00:00:17.790
So before we talk about
balancing chemical equations,

00:00:17.790 --> 00:00:18.920
what is a chemical equation?

00:00:18.920 --> 00:00:20.660
Well here's some examples right
here, and I have some

00:00:20.660 --> 00:00:22.180
more in the rest
of this video.

00:00:22.180 --> 00:00:24.750
But it essentially just
describes a chemical reaction.

00:00:24.750 --> 00:00:25.720
You've got some aluminum.

00:00:25.720 --> 00:00:30.600
You have some oxygen gas or a
diatomic oxygen molecule.

00:00:30.600 --> 00:00:32.860
And then you end up with
aluminum oxide.

00:00:32.860 --> 00:00:34.850
And you would say, ok, fine,
that's an equation.

00:00:34.850 --> 00:00:35.830
It looks nice.

00:00:35.830 --> 00:00:38.700
I have my reactants, or
the things that react.

00:00:38.700 --> 00:00:40.240
These are the reactants.

00:00:40.240 --> 00:00:42.510
And then I have the products
of this reaction.

00:00:42.510 --> 00:00:44.090
What's left there to do?

00:00:44.090 --> 00:00:45.400
Well you have a problem here.

00:00:45.400 --> 00:00:47.770
The way I've written it right
now, I have one atom of

00:00:47.770 --> 00:00:51.550
aluminum plus two atoms
of oxygen, right?

00:00:51.550 --> 00:00:53.700
They're bonded to each other,
but there's two atoms of

00:00:53.700 --> 00:00:54.250
oxygen here.

00:00:54.250 --> 00:00:59.270
One molecule of diatomic oxygen,
or one molecule of

00:00:59.270 --> 00:01:01.540
oxygen, but they have two
oxygen atoms here.

00:01:01.540 --> 00:01:05.030
And when you add them together,
I have two atoms of

00:01:05.030 --> 00:01:07.020
aluminum and three
atoms of oxygen.

00:01:07.020 --> 00:01:09.550
So I have a different number of
aluminums on both sides of

00:01:09.550 --> 00:01:10.510
this equation.

00:01:10.510 --> 00:01:12.470
On this side, I have
one aluminum.

00:01:12.470 --> 00:01:14.810
On this side, I have two
aluminums. And then I have a

00:01:14.810 --> 00:01:15.850
different number of oxygens.

00:01:15.850 --> 00:01:17.580
On this side, I have
two oxygens.

00:01:17.580 --> 00:01:19.360
And on that side, I have
three oxygens.

00:01:19.360 --> 00:01:22.700
So balancing equations is all
about fixing that problem, so

00:01:22.700 --> 00:01:24.990
that I have the same number of
aluminum on both sides of the

00:01:24.990 --> 00:01:28.245
equation, and the same number
of oxygens on both sides.

00:01:28.245 --> 00:01:30.200
So let's try to do that.

00:01:30.200 --> 00:01:31.960
I'll do it in orange.

00:01:31.960 --> 00:01:35.100
So I said I have one aluminum
here and I have

00:01:35.100 --> 00:01:36.600
two aluminums there.

00:01:36.600 --> 00:01:39.130
So maybe a simple thing is just
to put a two out here.

00:01:39.130 --> 00:01:42.990
So now I have two aluminums on
this side and I have two

00:01:42.990 --> 00:01:44.290
aluminums on this side.

00:01:44.290 --> 00:01:45.400
The aluminums look happy.

00:01:45.400 --> 00:01:47.060
Now let's look at the oxygen.

00:01:47.060 --> 00:01:49.880
Here I have two oxygens on the

00:01:49.880 --> 00:01:51.080
left-hand side of the equation.

00:01:51.080 --> 00:01:52.900
And on the right-hand
side of the equation

00:01:52.900 --> 00:01:54.280
I have three oxygens.

00:01:54.280 --> 00:01:55.560
What can I do here?

00:01:55.560 --> 00:01:58.960
Well if I could kind of have
half atoms, I could just

00:01:58.960 --> 00:02:01.840
multiply this by
one and a half.

00:02:01.840 --> 00:02:03.910
1.5.

00:02:03.910 --> 00:02:06.190
1.5 times 2 is 3.

00:02:06.190 --> 00:02:09.600
So now I have three oxygens on
both sides of this equation

00:02:09.600 --> 00:02:12.180
and I have two aluminums on both
sides of the equation.

00:02:12.180 --> 00:02:13.210
Am I done?

00:02:13.210 --> 00:02:15.940
Well, no, you can't have half
an atom, or one and

00:02:15.940 --> 00:02:17.130
a half of an atom.

00:02:17.130 --> 00:02:18.270
That's not cool.

00:02:18.270 --> 00:02:20.480
So what you do is you just
multiply this so that you end

00:02:20.480 --> 00:02:21.145
up with whole numbers.

00:02:21.145 --> 00:02:23.230
So let's just multiply
both sides of this

00:02:23.230 --> 00:02:26.070
whole equation by two.

00:02:26.070 --> 00:02:37.700
And we have four aluminums plus
three oxygens yields-- we

00:02:37.700 --> 00:02:40.210
multiplied everything
by two-- two

00:02:40.210 --> 00:02:45.090
molecules of aluminum oxide.

00:02:45.090 --> 00:02:47.780
Now, you might have been tempted
at some point in this

00:02:47.780 --> 00:02:50.950
exercise to say, oh, well why
don't I just tweak part of the

00:02:50.950 --> 00:02:51.860
aluminum oxide?

00:02:51.860 --> 00:02:54.940
Why don't I put a 2/3 in
front of this oxygen.

00:02:54.940 --> 00:02:56.640
You cannot do that.

00:02:56.640 --> 00:02:58.380
The equation is as it is.

00:02:58.380 --> 00:03:00.720
The molecule aluminum oxide
is aluminum oxide.

00:03:00.720 --> 00:03:04.630
You can't change the relative
ratios of the aluminum and the

00:03:04.630 --> 00:03:07.310
oxygen within the aluminum
oxide molecule.

00:03:07.310 --> 00:03:11.320
You can just change the number
of aluminum oxide molecules as

00:03:11.320 --> 00:03:13.460
a whole that you have,
in this case, two.

00:03:13.460 --> 00:03:14.000
So what did we do?

00:03:14.000 --> 00:03:14.800
We looked at the aluminum.

00:03:14.800 --> 00:03:17.060
We said, OK, we need two
aluminums to have both sides

00:03:17.060 --> 00:03:18.660
of that be two.

00:03:18.660 --> 00:03:20.550
And then when we looked at
oxygen, we said, well, if I

00:03:20.550 --> 00:03:23.310
multiply this by one and a half,
then that becomes three

00:03:23.310 --> 00:03:26.340
oxygens here, because one and
a half times two oxygens.

00:03:26.340 --> 00:03:27.880
And three oxygens there.

00:03:27.880 --> 00:03:30.400
And then all we said is, oh, I
can't have a one half there,

00:03:30.400 --> 00:03:32.540
so let me multiply both
sides by two.

00:03:32.540 --> 00:03:37.360
And I ended up with four
aluminums plus three oxygen

00:03:37.360 --> 00:03:41.980
molecules, or six oxygen atoms,
yields two molecules of

00:03:41.980 --> 00:03:43.660
aluminum oxide.

00:03:43.660 --> 00:03:46.070
Let's see if we can do
some more of these.

00:03:46.070 --> 00:03:48.270
So here I have methane.

00:03:48.270 --> 00:03:50.200
And that g in parentheses,
I just wanted to

00:03:50.200 --> 00:03:50.930
expose you to that.

00:03:50.930 --> 00:03:53.390
That just means it's a gas.

00:03:53.390 --> 00:03:58.340
So I have methane gas plus
oxygen gas yields carbon

00:03:58.340 --> 00:04:00.090
dioxide gas plus water.

00:04:00.090 --> 00:04:02.410
That's an l there,
so liquid water.

00:04:02.410 --> 00:04:04.640
So what can we do here?

00:04:04.640 --> 00:04:07.440
So the general thing is, do
the complicated molecules

00:04:07.440 --> 00:04:11.140
first, and then at the end you
can worry about the single

00:04:11.140 --> 00:04:13.000
atom molecules.

00:04:13.000 --> 00:04:15.180
Because those are very
easy to play with.

00:04:15.180 --> 00:04:17.139
And the reason why you do that,
whenever you change a

00:04:17.139 --> 00:04:19.680
number here-- let's say we're
trying to engineer how many

00:04:19.680 --> 00:04:22.680
carbons we have on both sides of
this equation-- if I set a

00:04:22.680 --> 00:04:24.680
number here, I'm also changing
the number of hydrogens.

00:04:24.680 --> 00:04:26.140
Then I'll have to play with
the hydrogens there.

00:04:26.140 --> 00:04:28.720
And at the end, I'll have
some number of oxygens.

00:04:28.720 --> 00:04:30.630
And then I have to tweak this
number right there.

00:04:30.630 --> 00:04:33.770
So let's just start
with the carbons.

00:04:33.770 --> 00:04:36.050
It seems complicated, but when
you go step by step and you

00:04:36.050 --> 00:04:38.100
kind of play with things a
little bit, it should proceed

00:04:38.100 --> 00:04:39.550
fairly smoothly.

00:04:39.550 --> 00:04:41.070
So here I have two carbons.

00:04:43.680 --> 00:04:45.790
And here I have, on
the right-hand

00:04:45.790 --> 00:04:47.810
side, only one carbon.

00:04:47.810 --> 00:04:49.790
So ideally I'd want two
carbons on both

00:04:49.790 --> 00:04:50.780
sides of this equation.

00:04:50.780 --> 00:04:53.930
So let me put a two out here.

00:04:53.930 --> 00:04:55.700
So there you go, my
carbons are happy.

00:04:55.700 --> 00:04:58.460
I now have two carbons
and two carbons.

00:04:58.460 --> 00:04:59.850
Now let me move to
the hydrogens.

00:04:59.850 --> 00:05:02.010
Remember, I wanted to do the
oxygens last, because I can

00:05:02.010 --> 00:05:05.000
just set this to whatever I want
it to be without messing

00:05:05.000 --> 00:05:08.070
up any of the other atoms. So on
this side of the equation I

00:05:08.070 --> 00:05:11.370
have four hydrogen atoms. How
many hydrogen atoms do I have

00:05:11.370 --> 00:05:12.690
on this side of the equation?

00:05:12.690 --> 00:05:16.320
So I have four hydrogen
atoms here.

00:05:16.320 --> 00:05:18.150
How many do I have
on this side?

00:05:18.150 --> 00:05:21.070
Well, I have two hydrogen
atoms right there.

00:05:21.070 --> 00:05:24.100
So I want to have four on both
sides, so let me put a two in

00:05:24.100 --> 00:05:25.510
front of the water.

00:05:25.510 --> 00:05:29.310
So now I have four hydrogen
atoms. Cool.

00:05:29.310 --> 00:05:30.850
Finally, oxygen.

00:05:30.850 --> 00:05:35.800
On the left-hand side I have two
oxygen atoms. And on the

00:05:35.800 --> 00:05:37.780
right-hand side,
what do I have?

00:05:37.780 --> 00:05:41.970
I have two times this one
oxygen, right here, so right

00:05:41.970 --> 00:05:44.290
now that's two, right?

00:05:44.290 --> 00:05:44.980
Two waters.

00:05:44.980 --> 00:05:49.450
And in each water I have one
oxygen atom, but I have two

00:05:49.450 --> 00:05:52.170
water molecules, so I
have two oxygens.

00:05:52.170 --> 00:05:56.270
And then in the carbon dioxide
I have two oxygens in each

00:05:56.270 --> 00:05:56.845
carbon dioxide.

00:05:56.845 --> 00:05:58.950
And I have two carbon
dioxides, right?

00:05:58.950 --> 00:06:01.290
When I put that magenta
two out front.

00:06:01.290 --> 00:06:02.560
So how many oxygens do I have?

00:06:02.560 --> 00:06:03.420
Two times two.

00:06:03.420 --> 00:06:05.330
I have four oxygens.

00:06:05.330 --> 00:06:07.820
So on the left-hand side
I have two oxygens.

00:06:07.820 --> 00:06:11.020
On the right-hand side I have
six oxygens, two in the two

00:06:11.020 --> 00:06:13.430
molecules of water and
four in the two

00:06:13.430 --> 00:06:15.770
molecules of carbon dioxide.

00:06:15.770 --> 00:06:18.460
So how do I make this
two into six?

00:06:18.460 --> 00:06:20.410
I want to have six oxygens
on this side.

00:06:20.410 --> 00:06:21.690
I want to them to have
six oxygens on the

00:06:21.690 --> 00:06:22.855
left-hand side as well.

00:06:22.855 --> 00:06:30.110
Well, if I put a three out here,
now I have six oxygens.

00:06:30.110 --> 00:06:31.810
And our equation has
been balanced.

00:06:31.810 --> 00:06:35.210
I have two carbons on this
side, two carbons on that

00:06:35.210 --> 00:06:38.780
side, four hydrogens on this
side, four hydrogens on this

00:06:38.780 --> 00:06:42.040
side, six oxygens on this side,
and then-- four plus

00:06:42.040 --> 00:06:44.810
two-- six oxygens
on that side.

00:06:44.810 --> 00:06:46.840
Next equation to balance.

00:06:46.840 --> 00:06:49.270
This actually becomes quite
fun once you get

00:06:49.270 --> 00:06:51.320
the knack of it.

00:06:51.320 --> 00:06:58.510
So I have ethane plus
oxygen gas yielding

00:06:58.510 --> 00:07:00.575
carbon dioxide and water.

00:07:00.575 --> 00:07:03.230
So this is a combustion
process.

00:07:03.230 --> 00:07:05.280
Let's look at the
carbons first. I

00:07:05.280 --> 00:07:06.810
have two carbons here.

00:07:06.810 --> 00:07:09.010
I have one carbon there.

00:07:09.010 --> 00:07:11.050
So let me put a two here.

00:07:11.050 --> 00:07:13.240
So now I have two carbons.

00:07:13.240 --> 00:07:14.090
Fair enough.

00:07:14.090 --> 00:07:15.900
Remember, I'm going to worry
about the oxygen last. This

00:07:15.900 --> 00:07:16.910
one's actually not
too different

00:07:16.910 --> 00:07:18.420
than the last problem.

00:07:18.420 --> 00:07:21.130
I have six hydrogens here.

00:07:21.130 --> 00:07:23.400
I only have two hydrogens
in the water.

00:07:23.400 --> 00:07:28.090
So then we have three
water molecules.

00:07:28.090 --> 00:07:30.470
And now I've balanced
out the hydrogens.

00:07:30.470 --> 00:07:33.520
I have six hydrogens on both
sides of the equation.

00:07:33.520 --> 00:07:36.060
And now let's deal
with the water.

00:07:36.060 --> 00:07:40.330
Here on the right-hand side--
I'll do it in this orange

00:07:40.330 --> 00:07:45.590
color-- I have two oxygens in
each carbon dioxide molecule.

00:07:45.590 --> 00:07:48.780
And I have two molecules,
so I have four oxygens.

00:07:48.780 --> 00:07:52.230
And I have one oxygen in
each water molecule.

00:07:52.230 --> 00:07:56.240
And I have three molecules, so
I have three oxygens here.

00:07:56.240 --> 00:07:57.580
Is that right?

00:07:57.580 --> 00:08:00.190
Three oxygens on the
right-hand side.

00:08:00.190 --> 00:08:02.060
Yep, three water molecules.

00:08:02.060 --> 00:08:07.200
And then I have four oxygens
in the carbon dioxide.

00:08:07.200 --> 00:08:07.860
Right.

00:08:07.860 --> 00:08:09.790
So I have seven oxygens.

00:08:09.790 --> 00:08:11.590
I have seven oxygens on
this side and I only

00:08:11.590 --> 00:08:13.130
have two on this side.

00:08:13.130 --> 00:08:15.575
So how can I make this
two into seven.

00:08:15.575 --> 00:08:19.360
Well I could multiply it
by three and a half.

00:08:19.360 --> 00:08:19.690
Right?

00:08:19.690 --> 00:08:21.620
Remember, I just want to have
seven on both sides.

00:08:21.620 --> 00:08:25.290
If I have three and a half
of these diatomic oxygen

00:08:25.290 --> 00:08:27.640
molecules-- three and a half
times two is seven-- so now I

00:08:27.640 --> 00:08:30.830
have seven oxygens on both
sides of the equation.

00:08:30.830 --> 00:08:32.870
Four plus three and
seven here.

00:08:32.870 --> 00:08:34.059
Two carbons.

00:08:34.059 --> 00:08:35.820
And then six hydrogens.

00:08:35.820 --> 00:08:38.429
And I'm almost done, except for
the fact that you can't

00:08:38.429 --> 00:08:40.559
have fractions of molecules.

00:08:40.559 --> 00:08:42.409
So what you do is just multiply
both sides of this

00:08:42.409 --> 00:08:45.760
equation by two or three, or
whatever you need to multiply

00:08:45.760 --> 00:08:46.960
it to get rid of
the fractions.

00:08:46.960 --> 00:08:50.800
So if I multiply everything
by two, I end up with two

00:08:50.800 --> 00:09:00.580
molecules of ethane plus seven
molecules of diatomic oxygen,

00:09:00.580 --> 00:09:07.520
seven O2's, yielding two
molecules of carbon dioxide--

00:09:07.520 --> 00:09:10.200
oh, sorry, I'm multiplying
everything by two, so four

00:09:10.200 --> 00:09:18.920
molecules of carbon dioxide,
plus six molecules of water.

00:09:18.920 --> 00:09:21.600
And just to make sure that all
still works, if you want to,

00:09:21.600 --> 00:09:22.630
you can check.

00:09:22.630 --> 00:09:23.830
How much carbon do we have?

00:09:23.830 --> 00:09:25.980
I have four carbons here.

00:09:25.980 --> 00:09:28.180
I have four carbons here.

00:09:28.180 --> 00:09:29.520
How much hydrogen do I have?

00:09:29.520 --> 00:09:31.420
I have two times six.

00:09:31.420 --> 00:09:33.630
I have 12 hydrogens here.

00:09:33.630 --> 00:09:36.100
I have 12 hydrogens here.

00:09:36.100 --> 00:09:37.640
How much oxygen do I have?

00:09:40.150 --> 00:09:41.420
Let me do a different color.

00:09:41.420 --> 00:09:44.190
I have 14 oxygens here.

00:09:44.190 --> 00:09:48.040
And here I have eight oxygens.

00:09:48.040 --> 00:09:50.860
And here I have-- six times
one-- six oxygens.

00:09:50.860 --> 00:09:53.010
So six plus eight is 14.

00:09:53.010 --> 00:09:56.000
So my equation has
been balanced.

00:09:56.000 --> 00:09:58.230
So that one, a lot of people
might find that to be a hard

00:09:58.230 --> 00:10:00.260
problem, because you have three
and a half, and just

00:10:00.260 --> 00:10:02.130
going straight to this might
seem non-intuitive.

00:10:02.130 --> 00:10:05.660
But if you just work from the
more complicated molecules and

00:10:05.660 --> 00:10:07.980
you go atom by atom, and if you
end up with any fractions

00:10:07.980 --> 00:10:10.775
you just multiply both sides by
some number to get rid of

00:10:10.775 --> 00:10:13.030
the fraction, of and
then you're done.

00:10:13.030 --> 00:10:13.590
All right.

00:10:13.590 --> 00:10:15.850
Let's do another one.

00:10:15.850 --> 00:10:17.950
So this one looks all hairy.

00:10:17.950 --> 00:10:24.190
I have this iron oxide plus
sulfuric acid yields all this

00:10:24.190 --> 00:10:25.260
hairy stuff.

00:10:25.260 --> 00:10:30.000
But the key here to realize is
that this group right here,

00:10:30.000 --> 00:10:35.170
this sulfate group,
stays together.

00:10:35.170 --> 00:10:35.480
Right?

00:10:35.480 --> 00:10:39.580
You have this SO4 there and
you have this SO4 there.

00:10:39.580 --> 00:10:42.740
So to really simplify things for
our head, you can kind of

00:10:42.740 --> 00:10:44.010
treat that like an atom.

00:10:44.010 --> 00:10:45.410
So let's make a substitution.

00:10:45.410 --> 00:10:48.230
We'll substitute that for x.

00:10:48.230 --> 00:10:52.230
So if we rewrite this as--
I'll do it in a vibrant

00:10:52.230 --> 00:11:03.390
color-- iron oxide plus H2--
there's only one sulfate

00:11:03.390 --> 00:11:07.690
group here-- x.

00:11:07.690 --> 00:11:15.070
And then that yields
two irons, this

00:11:15.070 --> 00:11:15.940
molecule with two irons.

00:11:15.940 --> 00:11:18.070
And it has three of these
sulfate groups.

00:11:21.140 --> 00:11:22.390
And then plus water.

00:11:26.450 --> 00:11:28.870
All I did is replace the
sulfate with an x.

00:11:28.870 --> 00:11:32.040
And now we can treat that x like
an atom, and we can just

00:11:32.040 --> 00:11:33.830
balance the equation.

00:11:33.830 --> 00:11:36.040
Let's see, on the left-hand
side, how

00:11:36.040 --> 00:11:37.300
many irons do we have?

00:11:37.300 --> 00:11:39.000
We have two irons here.

00:11:39.000 --> 00:11:40.820
We have two irons there.

00:11:40.820 --> 00:11:45.190
So the irons look balanced,
at first glance.

00:11:45.190 --> 00:11:47.630
Let's deal with the oxygens.

00:11:47.630 --> 00:11:52.280
So if we have three oxygens
here-- let me do it in a

00:11:52.280 --> 00:11:59.660
different color-- and we only
have one oxygen here.

00:11:59.660 --> 00:12:03.480
Remember, there were some
oxygens in this x group, but

00:12:03.480 --> 00:12:05.590
the sulfate group stayed
together, so we can just treat

00:12:05.590 --> 00:12:08.700
those separately.

00:12:08.700 --> 00:12:10.820
We want to have three oxygens
on the right-hand side, as

00:12:10.820 --> 00:12:13.250
well, so let's stick
a three here.

00:12:13.250 --> 00:12:14.500
Oops.

00:12:19.710 --> 00:12:21.040
So let me put that
three there.

00:12:21.040 --> 00:12:23.960
So now we have three
oxygens, as well.

00:12:23.960 --> 00:12:28.733
And then finally, let's
look at the hydrogens.

00:12:35.950 --> 00:12:37.160
Let's see, how many hydrogens
do we have here.

00:12:37.160 --> 00:12:41.270
We have six hydrogens now,
on the right-hand side.

00:12:41.270 --> 00:12:42.840
Six hydrogens here.

00:12:42.840 --> 00:12:43.725
Three times two.

00:12:43.725 --> 00:12:46.770
And we want to have six
hydrogens here, so we have to

00:12:46.770 --> 00:12:48.910
have three of these molecules.

00:12:48.910 --> 00:12:52.200
And then finally, let's look at
the sulfate group, that x.

00:12:52.200 --> 00:12:53.790
We have three x's here.

00:12:57.090 --> 00:13:01.350
And lucky for us, we have
three x's over there.

00:13:01.350 --> 00:13:03.420
So our equation has
been balanced.

00:13:03.420 --> 00:13:05.760
And if we want to write it back
in terms of the sulfate

00:13:05.760 --> 00:13:09.490
terms, we can just un-substitute
the x, and we're

00:13:09.490 --> 00:13:16.450
left with one molecule of iron
oxide plus three molecules of

00:13:16.450 --> 00:13:21.540
sulfuric acid, H2SO4 -- I just
un-substituted the x with SO4

00:13:21.540 --> 00:13:28.570
-- yields one of these
molecules, SO4 3.

00:13:32.270 --> 00:13:39.540
I just un-substituted the x plus
three molecules of water.

00:13:39.540 --> 00:13:40.670
Let's do one more.

00:13:40.670 --> 00:13:43.800
And then I think we'll
be all balanced out.

00:13:43.800 --> 00:13:47.750
Carbon dioxide plus hydrogen gas
yields methane plus water.

00:13:47.750 --> 00:13:51.700
So let's deal with the carbon
first. I have one carbon here,

00:13:51.700 --> 00:13:52.540
one carbon there.

00:13:52.540 --> 00:13:53.940
The carbons look happy.

00:13:53.940 --> 00:13:54.890
Let's look at the oxygen.

00:13:54.890 --> 00:13:56.920
I have two oxygens here.

00:13:56.920 --> 00:13:58.990
I have one oxygen there.

00:13:58.990 --> 00:14:02.350
So I want two oxygens here so
let me stick a two there.

00:14:02.350 --> 00:14:04.300
And then let's deal with the
hydrogens last, because this

00:14:04.300 --> 00:14:06.650
is the easiest one to play with
because it doesn't affect

00:14:06.650 --> 00:14:11.030
any of the other atoms. So if
I have two here and I have

00:14:11.030 --> 00:14:14.060
four, plus four here, right?

00:14:14.060 --> 00:14:15.865
I have four hydrogens there.

00:14:15.865 --> 00:14:18.160
And I have four hydrogens
there.

00:14:18.160 --> 00:14:20.790
So I need eight hydrogens
on the left-hand side.

00:14:20.790 --> 00:14:22.440
So I just put a four there.

00:14:22.440 --> 00:14:24.350
We're all done balancing.

00:14:24.350 --> 00:14:27.260
Anyway, hopefully you
found that useful.


Formula from Mass Composition

WEBVTT
Kind: captions
Language: en

00:00:00.800 --> 00:00:03.720
A couple of videos ago we
figured out how to go from the

00:00:03.720 --> 00:00:12.770
empirical formula to the
mass composition.

00:00:12.770 --> 00:00:16.710
And all I mean by that, mass
composition, is that you could

00:00:16.710 --> 00:00:19.480
start with the empirical formula
H2O, which is also its

00:00:19.480 --> 00:00:20.670
molecular formula.

00:00:20.670 --> 00:00:22.600
And then we were able to figure
out what percentage of

00:00:22.600 --> 00:00:23.750
this was water and what

00:00:23.750 --> 00:00:26.170
percentage of this was hydrogen.

00:00:26.170 --> 00:00:32.299
The way we did it is we said,
OK, oxygen's mass is 16 atomic

00:00:32.299 --> 00:00:32.930
mass units.

00:00:32.930 --> 00:00:34.630
Hydrogen is one.

00:00:34.630 --> 00:00:38.840
The mass of the entire molecule
is 2 times hydrogen.

00:00:38.840 --> 00:00:40.520
2 times 1.

00:00:40.520 --> 00:00:43.910
Plus 16 for that one
oxygen, so it's 18.

00:00:43.910 --> 00:00:45.490
And then we said, the
composition of

00:00:45.490 --> 00:00:47.600
oxygen is just 16/18.

00:00:47.600 --> 00:00:52.720
So oxygen is 16/18.

00:00:52.720 --> 00:00:54.610
I don't have my calculator
anymore.

00:00:54.610 --> 00:00:57.710
I think the number was something
on the order of 88%

00:00:57.710 --> 00:01:03.030
or 88.9% oxygen.

00:01:03.030 --> 00:01:04.550
So now let's see if we
can go the other way.

00:01:04.550 --> 00:01:10.710
Let's see if we could start
with mass composition, the

00:01:10.710 --> 00:01:14.170
percent composition of the
different elements, and then

00:01:14.170 --> 00:01:17.520
go to empirical formula.

00:01:17.520 --> 00:01:19.140
And I think the first
video I made, I

00:01:19.140 --> 00:01:20.860
spelled empirical wrong.

00:01:20.860 --> 00:01:23.740
Which is good reason for me
not to do spelling videos.

00:01:23.740 --> 00:01:25.790
I think I spelled this with
an e right here, because I

00:01:25.790 --> 00:01:27.080
pronounce it empricial.

00:01:27.080 --> 00:01:28.680
But it's spelled with an i.

00:01:28.680 --> 00:01:29.920
Empirical formula.

00:01:29.920 --> 00:01:31.605
So back to the empirical
formula.

00:01:31.605 --> 00:01:35.620
So let's do some exercises
here to see

00:01:35.620 --> 00:01:37.260
if we can get someplace.

00:01:37.260 --> 00:01:40.180
So let's say that I start off
with-- I need space, my

00:01:40.180 --> 00:01:41.370
periodic table is right
down there.

00:01:41.370 --> 00:01:47.940
Let's say I am told that
I have a bag of stuff.

00:01:47.940 --> 00:01:58.030
And 75% of that bag, let's
say it is mercury.

00:01:58.030 --> 00:01:59.420
Hg.

00:01:59.420 --> 00:02:03.500
And say that the other 25%
of the bag is chlorine.

00:02:06.040 --> 00:02:10.810
So the question that I'm asked
is, what is the empirical

00:02:10.810 --> 00:02:14.280
formula of the stuff that I have
here, assuming that it's

00:02:14.280 --> 00:02:17.400
all one molecule or one
type of molecule?

00:02:17.400 --> 00:02:19.265
So what is the empirical
formula?

00:02:25.810 --> 00:02:30.430
So the way to think about this
is, let's assume that you have

00:02:30.430 --> 00:02:33.400
100 grams of this stuff.

00:02:33.400 --> 00:02:33.740
Right?

00:02:33.740 --> 00:02:37.680
And the reason why I'm picking
100 is because 100 is a useful

00:02:37.680 --> 00:02:38.570
number to deal with.

00:02:38.570 --> 00:02:41.930
So if I have 100 grams of
the stuff, all right.

00:02:41.930 --> 00:02:51.560
So let's say I assume that you
have 100 grams. If you have

00:02:51.560 --> 00:02:55.230
100 grams, how many grams
of mercury do you have?

00:02:55.230 --> 00:02:59.840
Well that means that 75% of
that, that means you have 75

00:02:59.840 --> 00:03:04.790
grams of mercury, Hg.

00:03:04.790 --> 00:03:11.880
And that means you have
25 grams of chlorine.

00:03:11.880 --> 00:03:14.890
You might be saying, where did
get these numbers from?

00:03:14.890 --> 00:03:17.750
Well, I got the numbers because
I assumed 100 grams. I

00:03:17.750 --> 00:03:20.490
could have assumed 112 grams
or 7 grams, but those would

00:03:20.490 --> 00:03:22.180
have been harder numbers
to work with.

00:03:22.180 --> 00:03:25.610
So I'm just assuming 100 grams.
So the question is, in

00:03:25.610 --> 00:03:29.930
75 grams of mercury, how many
moles of mercury do I have?

00:03:29.930 --> 00:03:33.830
So I need to convert from
grams to moles.

00:03:33.830 --> 00:03:34.590
Let's go the other way.

00:03:34.590 --> 00:03:38.670
One mole of mercury-- this is at
least how my brain thinks--

00:03:38.670 --> 00:03:46.990
let me pick a suitable color--
so one mole of mercury so 6.02

00:03:46.990 --> 00:03:51.360
times 10 to the 23
mercury atoms.

00:03:51.360 --> 00:03:53.400
What's the mass of that?

00:03:53.400 --> 00:03:56.810
Well it's equal to whatever
mercury's mass number is in

00:03:56.810 --> 00:04:01.220
grams. So let's go look up
mercury's mass number.

00:04:01.220 --> 00:04:02.910
So mercury is a transition
metal.

00:04:02.910 --> 00:04:05.980
It's one of the few metals
that are liquid at room

00:04:05.980 --> 00:04:07.740
temperature.

00:04:07.740 --> 00:04:10.780
Right there we have mercury.

00:04:10.780 --> 00:04:14.596
Its mass number, let's
pick the 200.

00:04:14.596 --> 00:04:17.490
It has a mass number of 200.

00:04:17.490 --> 00:04:19.700
Obviously, there's some mercury
out there that has a

00:04:19.700 --> 00:04:20.950
mass of 201.

00:04:20.950 --> 00:04:27.710
But for simplicity, let's
say it's 200.

00:04:27.710 --> 00:04:34.450
So 1 mole of mercury is 200
grams. How did I get that?

00:04:34.450 --> 00:04:35.530
Mercury's mass number.

00:04:35.530 --> 00:04:41.160
One atom of mercury has a mass
of 200 atomic mass units.

00:04:41.160 --> 00:04:45.690
So one mole of mercury-- 6.02
times 10 to the 23 mercury

00:04:45.690 --> 00:04:50.130
atoms-- has that many
grams of mass.

00:04:50.130 --> 00:04:54.010
So instead of 200 atomic mass
units per atom, we have 200

00:04:54.010 --> 00:04:56.980
grams per mole of that atom.

00:04:56.980 --> 00:04:57.870
It's an easy calculation.

00:04:57.870 --> 00:05:00.910
You just look up it atomic mass
number and say it's that

00:05:00.910 --> 00:05:03.640
many grams. You have a mole
of the substance.

00:05:03.640 --> 00:05:04.490
Fair enough.

00:05:04.490 --> 00:05:11.130
And what about one
mole of chlorine?

00:05:11.130 --> 00:05:13.100
We do the same exercise.

00:05:13.100 --> 00:05:15.170
We go down to our
periodic table.

00:05:15.170 --> 00:05:20.190
Chlorine has a mass number
of, let's say, 35.

00:05:20.190 --> 00:05:22.360
It has various isotopes
on this planet.

00:05:22.360 --> 00:05:24.710
But let's say the mass
number is 35.

00:05:24.710 --> 00:05:28.040
So one chlorine atom weighs--
well now, we shouldn't say

00:05:28.040 --> 00:05:29.140
weighs, we have to
be very careful.

00:05:29.140 --> 00:05:33.700
One chlorine atom has a mass
of 35 atomic mass units.

00:05:33.700 --> 00:05:40.740
So 6.02 times 10 to the 23
chlorine atoms, or one mole of

00:05:40.740 --> 00:05:46.480
chlorine atoms will
have a mass of 35.

00:05:46.480 --> 00:05:48.840
35 grams.

00:05:48.840 --> 00:05:54.950
So how many moles of
mercury do I have?

00:05:54.950 --> 00:05:57.950
Let me get my calculator
going.

00:05:57.950 --> 00:06:00.740
So I have 75 grams of mercury.

00:06:00.740 --> 00:06:05.320
One mole of mercury would have
a mass of 200 grams. So I

00:06:05.320 --> 00:06:09.210
could just take 75 divided by
200 and it tells me how many

00:06:09.210 --> 00:06:18.360
moles I have. 75 divided
by 200 is equal to

00:06:18.360 --> 00:06:27.910
0.375 moles of mercury.

00:06:27.910 --> 00:06:28.640
How did I figure that?

00:06:28.640 --> 00:06:31.690
One mole of mercury would be
200 grams. I only have 75

00:06:31.690 --> 00:06:34.710
grams of mercury, so I have a
small fraction of it, roughly

00:06:34.710 --> 00:06:37.980
0.375 moles of mercury.

00:06:37.980 --> 00:06:41.140
I've just expressed the number
of atoms of mercury I have.

00:06:41.140 --> 00:06:47.200
Moles tells me it's 6.02 times
10 to the 23 atoms of mercury.

00:06:47.200 --> 00:06:52.430
So I have 0.375 times Avogadro's
number of mercury

00:06:52.430 --> 00:06:53.530
atoms. Fair enough.

00:06:53.530 --> 00:06:55.750
Now let's do the chlorine.

00:06:55.750 --> 00:06:57.790
I have 25 grams of chlorine.

00:06:57.790 --> 00:07:01.310
One mole of chlorine
is 35 grams. So I

00:07:01.310 --> 00:07:04.980
have 25 over 35 moles.

00:07:04.980 --> 00:07:07.850
So what's that?

00:07:07.850 --> 00:07:29.830
So 25 divided by 35 is equal
to 0.714 moles of chlorine.

00:07:29.830 --> 00:07:31.625
Now these numbers, they're
not exact.

00:07:31.625 --> 00:07:35.260
Because if you take twice this
number up here, you don't get

00:07:35.260 --> 00:07:38.270
twice this number right here.

00:07:38.270 --> 00:07:39.940
If you get twice this,
you don't get this.

00:07:39.940 --> 00:07:46.340
But this is roughly twice as
many moles of chlorine as I do

00:07:46.340 --> 00:07:48.140
of mercury.

00:07:48.140 --> 00:07:50.550
So that tells me that for every
mercury atom-- remember,

00:07:50.550 --> 00:07:53.970
moles is just a number-- for
every mercury atom I have, I

00:07:53.970 --> 00:07:59.320
have two chlorine atoms. So the
empirical formula here is

00:07:59.320 --> 00:08:04.680
for every mercury I have
two chlorines.

00:08:04.680 --> 00:08:08.250
I have two chlorines,
right there.

00:08:08.250 --> 00:08:11.040
And the numbers didn't work
out almost exactly right.

00:08:13.890 --> 00:08:15.930
Especially in the real world,
when you're actually trying to

00:08:15.930 --> 00:08:18.310
figure out things empirically,
your numbers seldom will.

00:08:18.310 --> 00:08:21.030
And maybe there's some random
other things running out there

00:08:21.030 --> 00:08:23.510
in terms of other things that
are contributing to the mass.

00:08:23.510 --> 00:08:27.280
But this is close enough to know
that the ratio of mercury

00:08:27.280 --> 00:08:31.110
to chlorine is roughly 1:2.

00:08:31.110 --> 00:08:32.539
Right?

00:08:32.539 --> 00:08:33.230
This is a number.

00:08:33.230 --> 00:08:35.990
For every one chlorine atom,
you have roughly two.

00:08:35.990 --> 00:08:37.179
You have twice as many.

00:08:37.179 --> 00:08:38.600
I guess the other way of
thinking about it is you have

00:08:38.600 --> 00:08:40.440
twice as many chlorine
atoms in the

00:08:40.440 --> 00:08:42.460
bag as you have mercury.

00:08:42.460 --> 00:08:42.909
Roughly.

00:08:42.909 --> 00:08:44.030
Although this is a
little bit more.

00:08:44.030 --> 00:08:45.830
This is close to 0.75.

00:08:45.830 --> 00:08:47.770
But it's close enough for you
to know that you're dealing

00:08:47.770 --> 00:08:50.160
with mercury chloride.

00:08:50.160 --> 00:08:50.980
Right there.

00:08:50.980 --> 00:08:52.670
Let's do another one of these.

00:08:52.670 --> 00:08:54.960
Let's see how the numbers
turn out for this one.

00:08:54.960 --> 00:09:02.500
Let's say you have another
bag that is 9% magnesium.

00:09:06.450 --> 00:09:15.740
And let's say the remainder of
the bag, 91%, is iodine.

00:09:15.740 --> 00:09:16.990
91% is iodine.

00:09:16.990 --> 00:09:19.090
So the way to do all of these
is, you do the same thing.

00:09:19.090 --> 00:09:23.290
Assume you have 100 grams. So
if you have 100 grams, of

00:09:23.290 --> 00:09:27.860
which you have 9 grams
of magnesium.

00:09:27.860 --> 00:09:31.470
And you have 91 grams
of iodine.

00:09:31.470 --> 00:09:34.840
And then figure out how much a
mole of magnesium-- what would

00:09:34.840 --> 00:09:37.020
be the mass of a mole
of magnesium and

00:09:37.020 --> 00:09:37.710
then a mole of iodine.

00:09:37.710 --> 00:09:39.100
So let me write here.

00:09:39.100 --> 00:09:42.760
One mole of magnesium.

00:09:42.760 --> 00:09:45.670
And we want to figure out
one mole of iodine.

00:09:49.880 --> 00:09:50.760
Let's figure those out.

00:09:50.760 --> 00:09:56.200
So magnesium's mass number is,
let's just go with 24.

00:09:56.200 --> 00:09:58.520
Let's say we have the
isotope that's 24.

00:09:58.520 --> 00:10:00.990
Magnesium is 24 and since
we're already down here,

00:10:00.990 --> 00:10:02.280
what's iodine.

00:10:02.280 --> 00:10:04.590
127.

00:10:04.590 --> 00:10:07.270
It's 127.

00:10:07.270 --> 00:10:11.510
Iodine is one of the halogens.

00:10:11.510 --> 00:10:11.810
So let's see.

00:10:11.810 --> 00:10:14.160
You have 127 and you have 24.

00:10:14.160 --> 00:10:16.360
Let me write those down.

00:10:16.360 --> 00:10:28.180
So you have 127 and
you have 24.

00:10:28.180 --> 00:10:36.710
So one atom of iodine has a mass
of 127 atomic mass units.

00:10:36.710 --> 00:10:41.000
So a whole mole all of it, 6.02
times 10 to the 23 iodine

00:10:41.000 --> 00:10:46.850
atoms will have a mass of 127
grams. You just take the

00:10:46.850 --> 00:10:51.890
atomic weight, or the atomic
mass, and the mass will be

00:10:51.890 --> 00:10:54.600
that many grams when you have
this many of the atom.

00:10:54.600 --> 00:10:58.380
So then one mole of magnesium
will be 24 grams.

00:10:58.380 --> 00:11:00.620
So now we just figure out how
many moles of each of these we

00:11:00.620 --> 00:11:04.860
have. We have 9 grams
of magnesium.

00:11:04.860 --> 00:11:06.650
So what fraction of
a mole is that?

00:11:06.650 --> 00:11:09.870
A mole is 24 grams. So
this is equal to

00:11:09.870 --> 00:11:15.842
9/24 moles of magnesium.

00:11:15.842 --> 00:11:19.540
And what is 9 over 24?

00:11:19.540 --> 00:11:25.410
9 divided by 24 is
equal to 0.375.

00:11:25.410 --> 00:11:30.870
So this is equal to 0.375, which
is similar to what we

00:11:30.870 --> 00:11:33.610
had in the last problem when
that 0.375 showed up.

00:11:33.610 --> 00:11:36.670
And 91 grams of iodine
is what?

00:11:36.670 --> 00:11:39.320
91 grams of iodine.

00:11:39.320 --> 00:11:41.400
Well, in a whole mole of iodine
you're going to have

00:11:41.400 --> 00:11:49.800
127 grams. So we have 91/127
moles of iodine.

00:11:49.800 --> 00:11:51.730
And what's 91 over 127?

00:11:51.730 --> 00:11:53.920
I have a feeling we're going to
have very similar numbers.

00:11:53.920 --> 00:12:00.280
121 divided by 127 is
equal to 0.716.

00:12:00.280 --> 00:12:03.100
I should do another problem with
better ratios than this.

00:12:03.100 --> 00:12:08.700
These have the same ratios as
the last problem, 0.72,

00:12:08.700 --> 00:12:12.460
roughly, moles of iodine.

00:12:12.460 --> 00:12:14.890
And this is moles
of magnesium.

00:12:14.890 --> 00:12:19.920
So we have roughly twice the
number of iodine atoms as we

00:12:19.920 --> 00:12:23.030
do of magnesium atoms. Right?

00:12:23.030 --> 00:12:26.700
For every one magnesium atom
we have roughly two iodine

00:12:26.700 --> 00:12:28.360
atoms. I know this isn't
exactly 1:2,

00:12:28.360 --> 00:12:30.110
but it's pretty close.

00:12:30.110 --> 00:12:37.000
So the formula here is
magnesium iodide.

00:12:37.000 --> 00:12:37.540
Right there.

00:12:37.540 --> 00:12:38.660
And that's the empirical
formula.

00:12:38.660 --> 00:12:42.060
We don't know, maybe in every
atom maybe you had two

00:12:42.060 --> 00:12:43.790
magnesiums and four iodines.

00:12:43.790 --> 00:12:44.910
We don't know.

00:12:44.910 --> 00:12:47.220
All we know is that the
ratio of magnesium to

00:12:47.220 --> 00:12:49.360
iodine here is 1:2.

00:12:49.360 --> 00:12:50.940
In the next video, I'm going to
try to look for ones that

00:12:50.940 --> 00:12:53.070
have more interesting ratios,
because I don't want to do two

00:12:53.070 --> 00:12:54.740
problems that both have
the same ratio.

00:12:54.740 --> 00:12:57.150
But hopefully you found this
a little bit helpful.


Molecular and Empirical Formulas

WEBVTT
Kind: captions
Language: en

00:00:00.680 --> 00:00:03.480
We now have a respectable
understanding of the periodic

00:00:03.480 --> 00:00:06.840
table itself and the
atoms in them.

00:00:06.840 --> 00:00:10.070
And now we're ready to deal
with molecules themselves.

00:00:10.070 --> 00:00:12.080
And to deal with molecules, we
have to have some way of

00:00:12.080 --> 00:00:12.690
representing them.

00:00:12.690 --> 00:00:14.380
And you represent them
with formulas.

00:00:14.380 --> 00:00:16.690
And there's two major--
actually, three major ways to

00:00:16.690 --> 00:00:17.650
represent a molecule.

00:00:17.650 --> 00:00:19.135
One is the molecular formula.

00:00:28.230 --> 00:00:29.930
And the other is the
empirical formula.

00:00:29.930 --> 00:00:32.729
And I'll do it in a different
color to differentiate it.

00:00:39.350 --> 00:00:41.610
And the difference is, well,
let's just talk about what the

00:00:41.610 --> 00:00:42.660
word empirical means.

00:00:42.660 --> 00:00:45.120
I remember when I first took
chemistry the teacher kept

00:00:45.120 --> 00:00:46.080
using the word empirical.

00:00:46.080 --> 00:00:48.390
Well, what does empirical
really mean?

00:00:48.390 --> 00:00:51.310
I clearly did not have a
very deep vocabulary.

00:00:51.310 --> 00:00:52.720
I forgot what age I was.

00:00:52.720 --> 00:00:58.470
But it means achieved through
observation or experiment, or

00:00:58.470 --> 00:00:59.780
based on experience.

00:00:59.780 --> 00:01:04.180
So if someone said that they
empirically figured out x, y,

00:01:04.180 --> 00:01:06.300
or z, it means that they figured
it out through an

00:01:06.300 --> 00:01:08.150
experiment or they
observed it.

00:01:08.150 --> 00:01:11.440
The molecular formula is
essentially the actual number

00:01:11.440 --> 00:01:13.810
of atoms in that molecule.

00:01:13.810 --> 00:01:14.790
Let me show you what
I'm talking about.

00:01:14.790 --> 00:01:18.330
So the empirical formula tells
you what people have observed.

00:01:18.330 --> 00:01:20.730
Maybe before they even knew that
there was such a thing as

00:01:20.730 --> 00:01:24.620
atoms, what they would have
observed is the ratio of the

00:01:24.620 --> 00:01:26.910
atoms to one another in a
molecule without knowing in

00:01:26.910 --> 00:01:29.500
the exact molecule how many
of that atom there are.

00:01:29.500 --> 00:01:31.330
Let me show you what
I'm talking about.

00:01:31.330 --> 00:01:42.790
So if I were to give you, I
don't know, a benzene, the

00:01:42.790 --> 00:01:47.240
molecular formula of benzene,
you have 6 carbon atoms and

00:01:47.240 --> 00:01:53.240
you have 6 hydrogen atoms. Now,
if you were some chemist

00:01:53.240 --> 00:01:56.710
in the 1800's and you didn't
know about the actual atoms,

00:01:56.710 --> 00:01:59.280
but if you had a big bag of
benzene and you were to

00:01:59.280 --> 00:02:03.360
measure the ratio of the carbon
to the hydrogen that

00:02:03.360 --> 00:02:05.870
you had in that bag, you would
find out that for every one

00:02:05.870 --> 00:02:08.560
carbon you have one hydrogen.

00:02:08.560 --> 00:02:11.400
So your empirical formula
is the ratio of the two.

00:02:11.400 --> 00:02:14.060
You don't know that each atom
actually has 6 of these, but

00:02:14.060 --> 00:02:16.480
you know that for every carbon,
there's a hydrogen.

00:02:16.480 --> 00:02:18.150
And for every hydrogen
there's a carbon.

00:02:18.150 --> 00:02:21.030
And the way to go back, you
can go from the molecular

00:02:21.030 --> 00:02:23.160
formula to the empirical
formula very easily.

00:02:23.160 --> 00:02:27.870
You just find the greatest
common divisor of the number

00:02:27.870 --> 00:02:29.640
of atoms in the molecule.

00:02:29.640 --> 00:02:32.570
So the greatest common divisor
is six, and six is obviously

00:02:32.570 --> 00:02:34.590
six, you divide both of these
by six and you get the

00:02:34.590 --> 00:02:35.710
empirical formula.

00:02:35.710 --> 00:02:36.730
It's not easy.

00:02:36.730 --> 00:02:39.090
You pretty much can't go back
from the empirical formula to

00:02:39.090 --> 00:02:39.880
the molecular formula.

00:02:39.880 --> 00:02:41.290
You've lost information.

00:02:41.290 --> 00:02:43.300
I don't know whether
this was C6H6.

00:02:43.300 --> 00:02:44.870
Was it C2H2?

00:02:44.870 --> 00:02:46.190
You just don't know.

00:02:46.190 --> 00:02:48.070
And I mentioned right at the
beginning of the video that

00:02:48.070 --> 00:02:51.750
there's a third way to
represent molecules.

00:02:51.750 --> 00:02:54.100
And that's the structural
formula.

00:02:54.100 --> 00:02:55.880
And we'll do that, off and on,
and we've already done it a

00:02:55.880 --> 00:02:56.720
little bit.

00:02:56.720 --> 00:02:57.250
Let me show you.

00:02:57.250 --> 00:02:59.760
The structural formula for
benzene would actually say how

00:02:59.760 --> 00:03:02.490
the molecular formula atoms
are configured.

00:03:02.490 --> 00:03:05.470
So benzene in particular
is very interesting.

00:03:05.470 --> 00:03:06.270
It looks like this.

00:03:06.270 --> 00:03:08.290
It's often drawn like this.

00:03:08.290 --> 00:03:11.220
And you'll see this a lot when
you take organic chemistry.

00:03:11.220 --> 00:03:15.380
But it looks like a little
hexagon, where the vertices of

00:03:15.380 --> 00:03:18.310
the hexagon are carbon atoms.
So let me draw the carbon

00:03:18.310 --> 00:03:19.000
atoms in yellow.

00:03:19.000 --> 00:03:22.720
So this is carbon,
carbon, carbon,

00:03:22.720 --> 00:03:26.730
carbon, carbon, carbon.

00:03:26.730 --> 00:03:30.690
They have double bonds,
every other carbon.

00:03:30.690 --> 00:03:31.920
Double bonds.

00:03:31.920 --> 00:03:36.200
And then they have single
bonds to hydrogen.

00:03:36.200 --> 00:03:38.075
Let me just do the hydrogen
in another color.

00:03:41.480 --> 00:03:43.370
Let me do it in magenta.

00:03:43.370 --> 00:03:44.620
Hydrogen.

00:03:50.800 --> 00:03:53.450
And obviously, the structural
formula gives you the most

00:03:53.450 --> 00:03:54.940
information.

00:03:54.940 --> 00:03:55.200
Right?

00:03:55.200 --> 00:03:57.270
Then you can start to think
about, gee, how will this

00:03:57.270 --> 00:03:58.890
interact with other things?

00:03:58.890 --> 00:04:01.310
While the molecular formula just
tells you what's in the

00:04:01.310 --> 00:04:03.310
molecule, the empirical formula
really gives you the

00:04:03.310 --> 00:04:04.470
least information.

00:04:04.470 --> 00:04:06.300
It just tells you the ratio
of the different

00:04:06.300 --> 00:04:08.890
items in the molecule.

00:04:08.890 --> 00:04:12.420
Structural formula.

00:04:12.420 --> 00:04:14.390
Let's do a couple more.

00:04:14.390 --> 00:04:18.269
OK, what if we're dealing with,
let's say we're dealing

00:04:18.269 --> 00:04:20.250
with water.

00:04:20.250 --> 00:04:23.280
I think you know the molecular
formula for water.

00:04:23.280 --> 00:04:25.870
H2O.

00:04:25.870 --> 00:04:28.940
Now what would be the empirical
formula for this?

00:04:28.940 --> 00:04:31.510
Well we want to know the ratio,
so for every oxygen

00:04:31.510 --> 00:04:32.390
there's two hydrogens.

00:04:32.390 --> 00:04:34.000
Or I guess you could say
for every hydrogen

00:04:34.000 --> 00:04:35.900
there's a half of oxygen.

00:04:35.900 --> 00:04:37.230
So you can't reduce this.

00:04:37.230 --> 00:04:42.280
If I wrote this as H2O1, what's
the greatest common

00:04:42.280 --> 00:04:44.490
divisor of 2 and 1?

00:04:44.490 --> 00:04:46.480
Well, it's 1, so you just have
to divide them by 1.

00:04:46.480 --> 00:04:51.410
So in this case, the empirical
and the molecular formula are

00:04:51.410 --> 00:04:52.760
the exact same thing.

00:04:52.760 --> 00:04:54.390
It's H2O.

00:04:54.390 --> 00:04:55.570
What about sulfur?

00:04:55.570 --> 00:05:00.940
And an interesting molecule,
because obviously,

00:05:00.940 --> 00:05:03.630
it's just one atom.

00:05:03.630 --> 00:05:04.560
Sulfur.

00:05:04.560 --> 00:05:08.450
Sorry, no, I'm spelling
it wrong.

00:05:08.450 --> 00:05:09.620
No.

00:05:09.620 --> 00:05:13.660
No, it's not a p h, it's f.

00:05:13.660 --> 00:05:16.460
Clearly, I shouldn't be making
spelling videos.

00:05:16.460 --> 00:05:17.710
So sulfur.

00:05:23.120 --> 00:05:26.790
So the molecular formula S8.

00:05:26.790 --> 00:05:28.420
So it forms this neat kind of

00:05:28.420 --> 00:05:32.270
octagon-looking chain of sulfurs.

00:05:32.270 --> 00:05:34.120
And if I were to draw that,
you would see that.

00:05:34.120 --> 00:05:36.490
And you could look it up on
Wikipedia, if you like.

00:05:36.490 --> 00:05:39.320
But its empirical formula, if
you had just a bag of sulfur,

00:05:39.320 --> 00:05:41.080
you don't know that each
atom has eight sulfurs.

00:05:41.080 --> 00:05:43.430
You just have a big
bag of sulfur.

00:05:43.430 --> 00:05:45.050
So, the imperative formula,
there's only one

00:05:45.050 --> 00:05:46.060
atom in this molecule.

00:05:46.060 --> 00:05:49.520
You divide by eight
and you get S.

00:05:49.520 --> 00:05:51.670
So you just know that all you've
got there is sulfur.

00:05:51.670 --> 00:05:54.050
So let's just do one more.

00:05:54.050 --> 00:05:55.300
Glucose.

00:05:57.555 --> 00:05:59.490
I'll pick a new color.

00:05:59.490 --> 00:06:00.250
Glucose.

00:06:00.250 --> 00:06:11.760
The molecular formula
is C6H12O6.

00:06:11.760 --> 00:06:14.930
So for every carbon,
there are how many?

00:06:14.930 --> 00:06:18.240
For every 6 carbons, there's
12 hydrogen and 1 oxygen.

00:06:18.240 --> 00:06:22.320
So if you kind of reduce this
formula to its empirical form,

00:06:22.320 --> 00:06:23.270
what do you get?

00:06:23.270 --> 00:06:28.930
Let's see, you can divide all
these numbers by 6, so we get

00:06:28.930 --> 00:06:33.180
1 carbon, 2 hydrogens,
and 1 oxygen.

00:06:33.180 --> 00:06:35.720
So this just tells you the ratio
that they exist in a big

00:06:35.720 --> 00:06:37.540
bag of this molecule.

00:06:37.540 --> 00:06:39.400
This tells you the
exact number of

00:06:39.400 --> 00:06:41.390
atoms in that molecule.

00:06:41.390 --> 00:06:42.020
Fair enough.

00:06:42.020 --> 00:06:44.280
So now we know a little bit
of the difference between

00:06:44.280 --> 00:06:48.120
molecular formula, and empirical
formula, and

00:06:48.120 --> 00:06:48.760
structural formula.

00:06:48.760 --> 00:06:51.400
Now let's see if we can use
what we know about the

00:06:51.400 --> 00:06:53.780
formulas and the periodic table
to think a little bit

00:06:53.780 --> 00:06:59.330
about the composition, the mass
composition, of some of

00:06:59.330 --> 00:07:01.440
these molecules.

00:07:01.440 --> 00:07:03.460
So the first thing to even think
about is, how do you

00:07:03.460 --> 00:07:05.500
figure out the molecular mass?

00:07:05.500 --> 00:07:06.560
Right?

00:07:06.560 --> 00:07:08.050
I have my little periodic
table down there.

00:07:08.050 --> 00:07:09.300
So molecular mass.

00:07:18.580 --> 00:07:21.540
So the first question is, how
do you figure out-- I mean,

00:07:21.540 --> 00:07:24.850
the molecular mass is going to
be the sum of all of the atoms

00:07:24.850 --> 00:07:26.610
in that molecule, right?

00:07:26.610 --> 00:07:30.960
So if you wanted to know the
molecular mass of-- let's say

00:07:30.960 --> 00:07:33.340
you wanted to know
how much does one

00:07:33.340 --> 00:07:36.380
molecule of benzene mass?

00:07:36.380 --> 00:07:38.700
I don't want to say weight,
because it should be

00:07:38.700 --> 00:07:40.970
independent of what
planet you're on.

00:07:40.970 --> 00:07:45.160
So what is the mass of one
molecule of benzene?

00:07:45.160 --> 00:07:49.890
Well, all you do is you add up
the masses of the different

00:07:49.890 --> 00:07:50.570
constituents.

00:07:50.570 --> 00:07:53.110
So you have 6 carbons
and 6 hydrogens.

00:07:53.110 --> 00:07:54.360
So let's do benzene.

00:07:58.410 --> 00:08:01.570
You have 6 carbons
and 6 hydrogens.

00:08:01.570 --> 00:08:03.680
So what's the mass
of each carbon?

00:08:03.680 --> 00:08:06.470
So let's go back down to
the periodic table.

00:08:06.470 --> 00:08:09.290
Just to give proper credit, I
got this off of the Los Alamos

00:08:09.290 --> 00:08:12.690
National Laboratories website.

00:08:12.690 --> 00:08:14.680
So let's see, the atomic
mass of carbon.

00:08:14.680 --> 00:08:16.520
The reason why I used this one
instead of my previous one is

00:08:16.520 --> 00:08:20.040
my previous periodic table that
I got off Wikipedia only

00:08:20.040 --> 00:08:21.490
had atomic numbers on them.

00:08:21.490 --> 00:08:24.200
But now that we're actually
going to start talking about

00:08:24.200 --> 00:08:26.260
the mass composition
of different atoms

00:08:26.260 --> 00:08:26.995
or different molecules.

00:08:26.995 --> 00:08:28.210
We're going to have to
start looking at the

00:08:28.210 --> 00:08:29.860
atomic mass, right?

00:08:29.860 --> 00:08:32.270
Remember, the atomic mass, when
you think about atomic

00:08:32.270 --> 00:08:36.809
mass units, it's just the number
of protons plus the

00:08:36.809 --> 00:08:37.280
number of neutrons.

00:08:37.280 --> 00:08:39.400
So you have six protons
in carbon

00:08:39.400 --> 00:08:40.850
and roughly six neutrons.

00:08:40.850 --> 00:08:41.890
And why is there this decimal?

00:08:41.890 --> 00:08:45.060
Because, as we said before, this
is an average of all of

00:08:45.060 --> 00:08:49.360
the masses of the isotopes
you'll find of carbon.

00:08:49.360 --> 00:08:51.670
So there's a little bit of
carbon 14 on the planet, very

00:08:51.670 --> 00:08:54.100
little, but most of the
carbon is carbon 12.

00:08:54.100 --> 00:08:57.200
When you proportionately average
them, you get 12.01.

00:08:57.200 --> 00:09:00.280
But let's say we're dealing with
carbon 12, just because

00:09:00.280 --> 00:09:01.530
that's the most common
element.

00:09:09.900 --> 00:09:13.410
Carbon is 12 atomic
mass units.

00:09:13.410 --> 00:09:13.870
Right?

00:09:13.870 --> 00:09:16.040
And atomic mass units
is a unit of mass.

00:09:16.040 --> 00:09:18.100
And we'll talk about
how small it is.

00:09:18.100 --> 00:09:21.040
It's a very, very, very,
very small fraction

00:09:21.040 --> 00:09:22.330
of a gram or kilogram.

00:09:22.330 --> 00:09:25.120
And we'll talk about that,
probably in the next video.

00:09:25.120 --> 00:09:27.490
So carbon is 12 atomic
mass units.

00:09:27.490 --> 00:09:29.590
What about hydrogen?

00:09:29.590 --> 00:09:30.190
Let's see.

00:09:30.190 --> 00:09:32.250
We go to our periodic table.

00:09:32.250 --> 00:09:33.980
Hydrogen is here in
this dark blue.

00:09:33.980 --> 00:09:36.600
And I don't know if you can
read it, but this is

00:09:36.600 --> 00:09:39.550
interesting, the atomic number
of hydrogen is 1.

00:09:39.550 --> 00:09:45.270
The atomic mass of hydrogen
is 1.0008.

00:09:45.270 --> 00:09:48.190
So that tells us that most of
the hydrogen on this planet

00:09:48.190 --> 00:09:50.850
has an atomic mass of 1.

00:09:50.850 --> 00:09:53.850
Which tells us that it
essentially has no neutrons.

00:09:53.850 --> 00:09:56.970
That hydrogen is a kind of an
interesting nucleus there,

00:09:56.970 --> 00:09:58.520
where there is really
just a proton.

00:09:58.520 --> 00:10:00.580
Just a proton sitting
in that nucleus.

00:10:00.580 --> 00:10:03.670
And so if you were to
ionize hydrogen.

00:10:03.670 --> 00:10:05.930
If you were to turn into a
cation and take one of its

00:10:05.930 --> 00:10:08.240
electrons away, what
are you left with?

00:10:08.240 --> 00:10:09.880
You just have a proton.

00:10:09.880 --> 00:10:13.790
A proton sitting by itself, just
a single proton, really

00:10:13.790 --> 00:10:17.540
is no different than
a hydrogen ion.

00:10:17.540 --> 00:10:19.220
And that to me is kind
of interesting.

00:10:19.220 --> 00:10:20.600
That hydrogen is that simple.

00:10:20.600 --> 00:10:23.050
It's really just a proton.

00:10:23.050 --> 00:10:25.490
So hydrogen has an
atomic mass of 1.

00:10:25.490 --> 00:10:27.580
Right?

00:10:27.580 --> 00:10:31.210
If it had any neutrons in it, it
would have been at least an

00:10:31.210 --> 00:10:32.230
atomic mass of 2.

00:10:32.230 --> 00:10:35.350
But hydrogen has atomic
mass of 1.

00:10:35.350 --> 00:10:37.770
One atomic mass unit.

00:10:37.770 --> 00:10:43.330
So what is the mass of one
molecule of benzene?

00:10:43.330 --> 00:10:45.620
Well it's 6 times
the carbon mass.

00:10:45.620 --> 00:10:51.610
6 times 12 plus 6 times
the mass of hydrogen.

00:10:51.610 --> 00:10:52.770
Plus 6 times 1.

00:10:52.770 --> 00:10:59.635
So that is 6 times 12, is 72,
plus 6 times 1, plus 6, is

00:10:59.635 --> 00:11:01.980
equal to 78.

00:11:01.980 --> 00:11:04.950
Now, what if someone
said, what percent

00:11:04.950 --> 00:11:08.470
of benzene is carbon?

00:11:08.470 --> 00:11:10.450
Well then you say, OK, this
is the piece that's

00:11:10.450 --> 00:11:12.370
carbon, right here.

00:11:12.370 --> 00:11:17.730
The carbon piece of benzene
is 72 atomic mass units.

00:11:17.730 --> 00:11:18.890
Right, that's carbon.

00:11:18.890 --> 00:11:21.670
So what percentage of
benzene is carbon?

00:11:21.670 --> 00:11:23.770
Well it's 72 over 78.

00:11:23.770 --> 00:11:24.950
The whole thing is 78.

00:11:24.950 --> 00:11:28.450
So it's 72 over 78.

00:11:28.450 --> 00:11:29.730
And what does that equal?

00:11:29.730 --> 00:11:32.110
Let me get a calculator going.

00:11:36.560 --> 00:11:38.650
I should've had my calculator
open ahead of time.

00:11:42.580 --> 00:11:43.680
All right.

00:11:43.680 --> 00:11:51.820
So 72 divided by 78
is equal to 92.3%.

00:11:51.820 --> 00:12:00.510
So benzene is 92.3%
carbon by mass.

00:12:00.510 --> 00:12:03.950
And of course, the remainder,
the 7.7%,

00:12:03.950 --> 00:12:05.790
is going to be hydrogen.

00:12:05.790 --> 00:12:08.880
Let's do that for a couple of
these other guys down here.

00:12:08.880 --> 00:12:11.820
So let's say we wanted to
know what is the mass of

00:12:11.820 --> 00:12:13.490
a molecule of water?

00:12:13.490 --> 00:12:14.120
Fair enough.

00:12:14.120 --> 00:12:16.280
There's enough water on the
planet, if you want to know

00:12:16.280 --> 00:12:16.780
what that is.

00:12:16.780 --> 00:12:19.980
Well we already know what the
mass of a hydrogen is.

00:12:19.980 --> 00:12:20.950
It's 1.

00:12:20.950 --> 00:12:21.180
Right?

00:12:21.180 --> 00:12:23.250
Hydrogen is 1.

00:12:23.250 --> 00:12:25.840
One atomic mass unit.

00:12:25.840 --> 00:12:27.880
Oxygen is what?

00:12:27.880 --> 00:12:30.640
Oxygen is 16.

00:12:30.640 --> 00:12:32.930
Notice, it's exactly 16.

00:12:32.930 --> 00:12:37.170
So on most of the planet, you
pretty much have, in an oxygen

00:12:37.170 --> 00:12:41.890
atom, you have 8 protons
and exactly 8 neutrons.

00:12:41.890 --> 00:12:45.050
So you get an atomic
mass of 16.

00:12:45.050 --> 00:12:49.540
So oxygen has an atomic mass
of 16 atomic mass units.

00:12:49.540 --> 00:12:52.580
So the atomic mass of the entire
molecule, you have 2

00:12:52.580 --> 00:12:57.970
hydrogens, so you have 2 times
the mass of hydrogen plus 1

00:12:57.970 --> 00:13:03.100
oxygen-- plus 16-- so that
equals 18 atomic

00:13:03.100 --> 00:13:06.520
mass units for water.

00:13:06.520 --> 00:13:09.970
And once again, if you want to
say, what percent by mass of

00:13:09.970 --> 00:13:12.510
water is oxygen?

00:13:12.510 --> 00:13:14.530
Well it's 16 out of
the 18, right?

00:13:14.530 --> 00:13:15.330
Is oxygen.

00:13:15.330 --> 00:13:19.360
So if we get the calculator
back, you get 16

00:13:19.360 --> 00:13:22.710
divided by 18 is water.

00:13:22.710 --> 00:13:26.630
So, let's say you round
it, 88.9% water.

00:13:26.630 --> 00:13:34.280
Sorry, 88.9% oxygen.

00:13:34.280 --> 00:13:36.700
So most of water is oxygen.

00:13:36.700 --> 00:13:39.720
And this is interesting, even
though you have two hydrogens

00:13:39.720 --> 00:13:45.550
here, two hydrogens for every
one oxygen, oxygen's mass is

00:13:45.550 --> 00:13:48.490
so much larger-- it's 16
times larger-- that

00:13:48.490 --> 00:13:51.665
most of water is oxygen.

00:13:54.510 --> 00:13:56.540
Well, I'm probably running out
of time, so the next video I'm

00:13:56.540 --> 00:13:57.960
going to talk about how
do we go backwards.

00:13:57.960 --> 00:14:00.380
If someone gives you the
composition, how can you get

00:14:00.380 --> 00:14:01.710
the empirical formula.

00:14:01.710 --> 00:14:03.950
Actually on a side note,
slightly unrelated to what I

00:14:03.950 --> 00:14:09.140
just talked about, I was doing
some research last night about

00:14:09.140 --> 00:14:11.020
metals, because they're actually
interesting, about

00:14:11.020 --> 00:14:13.805
why some metals conduct more
and some conduct less.

00:14:16.320 --> 00:14:18.480
Because when I first talked
about, you know, these were

00:14:18.480 --> 00:14:19.870
obviously the transition
metals.

00:14:19.870 --> 00:14:22.160
They're backfilling
their d orbitals.

00:14:22.160 --> 00:14:28.580
And I said, hey, the periodic
table that was in-- I think I

00:14:28.580 --> 00:14:30.930
was looking in a Princeton
Review book that described

00:14:30.930 --> 00:14:33.950
these as metals and described
these as transition metals.

00:14:33.950 --> 00:14:36.350
And I was like, hey, you know,
that's kind of not fair,

00:14:36.350 --> 00:14:41.470
because I consider iron and
copper and gold and silver to

00:14:41.470 --> 00:14:43.150
be as metallic as anything.

00:14:43.150 --> 00:14:46.120
Why should these be called
transition metals and these be

00:14:46.120 --> 00:14:47.380
just called regular metals.

00:14:47.380 --> 00:14:50.310
And it actually turns out that
a common name for these are

00:14:50.310 --> 00:14:52.340
poor metals.

00:14:52.340 --> 00:14:54.570
Poor metals.

00:14:54.570 --> 00:14:57.580
Because, to a large degree,
they're softer, they have

00:14:57.580 --> 00:15:00.780
lower melting points, so the
intuition was right.

00:15:00.780 --> 00:15:05.580
To a large degree, when we think
of metals, these are the

00:15:05.580 --> 00:15:06.610
metals I think of.

00:15:06.610 --> 00:15:10.560
And when we think of metallic
nature in a chemistry sense--

00:15:10.560 --> 00:15:13.140
we talked a lot about that,
who wants to donate their

00:15:13.140 --> 00:15:15.490
electrons the most, that's
metallic nature.

00:15:15.490 --> 00:15:17.000
They're the guys down here.

00:15:17.000 --> 00:15:19.180
And as you go to the top right,
these want to donate

00:15:19.180 --> 00:15:21.540
their electrons the least.
These are the most

00:15:21.540 --> 00:15:22.190
electronegative.

00:15:22.190 --> 00:15:24.690
They like electrons the most, so
they actually have some of

00:15:24.690 --> 00:15:27.210
the worst metallic nature, so
it actually makes sense to

00:15:27.210 --> 00:15:30.320
call them poor metals.

00:15:30.320 --> 00:15:32.170
And there's some debate on
whether these should even be

00:15:32.170 --> 00:15:33.800
called poor metals.

00:15:33.800 --> 00:15:36.080
If you look up a bunch of
periodic tables, some will

00:15:36.080 --> 00:15:37.460
call these metals.

00:15:37.460 --> 00:15:38.830
Some will call these
poor metals.

00:15:38.830 --> 00:15:40.830
But I just wanted to throw that
out there just so you're

00:15:40.830 --> 00:15:41.800
exposed to it.

00:15:41.800 --> 00:15:44.200
And so, you know, for me, it is
a little bit more intuitive

00:15:44.200 --> 00:15:46.790
to call these poor metals,
because they have less

00:15:46.790 --> 00:15:49.330
metallic nature than the stuff,
especially down here,

00:15:49.330 --> 00:15:51.970
the alkali and the alkaline
earth metals.

00:15:51.970 --> 00:15:54.060
Anyway, see you in
the next video.