Balancing more complex chemical equation

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- [Voiceover] Let's now see
if we can balance a chemical

equation with slightly
more complex molecules.

So, here we have a chemical equation,

describing a chemical reaction.

This is actually a combustion reaction.

You have some ethylene right over here,

in the presence of oxygen,
and you need to get a

little bit of energy to get
this going, but then you're

going to have this reaction
that's actually going to

release energy as well, but
we're not accounting for

the energy, at least the
way we've written it.

Right over here, you have
some ethylene, and this little

g in parentheses, says
it's in the gas form

or gaseous form, so
gaseous ethylene plus some

dioxygen molecule, which is
the most prevalent form of

oxygen molecule that you
would find in the atmosphere.

And so, that's also in the gas form.

Put them together, you end
up with some carbon dioxide

gas and some liquid water.

This is the classic combustion reaction.

But now let's think about,
how do we balance this thing?

Let's make sure we have the same number

of each atom on both sides.

And when you see something
more complicated like this,

where, you know, here I have
an oxygen and two different

molecules over here, and a
lot of these molecules have

multiple elements in it.

It might be very daunting.

Where do I start?

And this is where the
art of balancing chemical

equations starts to come into play.

The general idea is,

Try to balance the...

try to balance the
molecules that have multiple

elements in them first, and leave the...

molecules that only have one
element in them for last.

And the idea there is,
is that these are harder.

They're going to have all
sorts of implications,

and then, at the end of
the day, you can just set

a number here for the number of dioxygens.

If you saved, say the ethylene for last,

then every time, and you're
trying to balance the carbons,

you try to change the number
of carbons, you're going to

change the number of hydrogens,

which is going to change the...

You're going to have to balance
over and over, and you're

going to go into this really
really really confusing circle.

So, the best thing to do, try to balance

the complex molecules first, and then

save the single element
molecules for last.

So let's do that.

So, let's start with the carbons.

So, over here, I have two carbons.

Over here, I only have one carbon.

I only have one carbon.

So, it seems like the best
way to balance it is, I should

have two molecules of carbon dioxide,

and I haven't even thought
about the oxygens yet.

By putting that two there,
that's going to change the

number of oxygens I have
on the righthand side.

But at least it balances my carbons.

I now have two carbons
on the lefthand side,

and I have two carbons
on the righthand side.

I’m no longer magically destroying

a carbon atom, all right.

Now, let's move on to the
hydrogens, and remember,

what I said is, let's wait
to do the oxygens last,

because we have a molecule

that only contains oxygen right over here,

so we'll save oxygen for last.

So, let's do hydrogen next.

So, hydrogen, right over
here, we have four hydrogens.

And on the righthand side,
we have two hydrogens.

So, it seems like the easiest
thing to do to balance

the hydrogens is to have two
of these water molecules.

Now I have four hydrogens here,

and I have four hydrogens there.

Now, let's do the oxygen.

Now, let's do the oxygen.

I've balanced the carbons
and the hydrogens.

And the reason why oxygen's
going to be interesting,

I can just count the amount
of oxygen I now have here,

after changing the amount
of molecules I have.

And then I can adjust this
accordingly, because this is

only going to affect the number of oxygens

that I have on the lefthand side.

Right now, on the lefthand
side, I have two oxygens,

and on the righthand
side, let me count this,

I have two O two's, really.

So, this is going to be four oxygens here,

and then I have, each of
these water molecules has

one oxygen, but I have
two water molecules,

so this is going to be two
oxygens, two oxygens here.

So, on the righthand side, I
have four plus two oxygens.

So, I have six oxygens
on the righthand side.

I need six oxygens on the lefthand side.

I need this number to be six.

So, how do I do that?

Well, I just need three
of these molecules.

If I have three molecules,
each of them have two oxygens,

I'm going to have a total of six oxygens.

And just like that, we have balanced this

combustion reaction,
this chemical equation.