Oxidation and Reduction Review From Biological Point-of-View

In diesem Video will ich wiederholen, was wir in unserem Chemieunterricht über die Oxidation und das Gegenteil der Oxidation, die Reduktion, gelernt haben.

What I want to do in this
video is review what we

learned from our chemistry
classes about oxidation and

the opposite of oxidation,
reduction.

And then see how what we learned
in our chemistry class

relates to the way that a
biologist or biochemist might

use these words.

And hopefully we'll see that
they're the same thing.

So just as a bit of review, if
you watched the chemistry

playlist. Oxidation, you can
view it-- and actually there's

a famous mnemonic for it.

It's: OIL RIG Where the oil
tells us that oxidation is

losing-- I put it in quotes
because you're not necessarily

losing the electrons; I'll
show you what I mean-- is

losing electrons.

This is what you should
have learned in

your chemistry class.

And then you also learned that
reduction is gaining.

And I'll put that in
quotes as well.

Is gaining electrons.

And I put that in quotes because
you're not necessarily

gaining electrons.

You're more hogging it.

And the reason why it's called
reduction, is because if you

are gaining electrons your
notional charge, if you really

were gaining them,
is being reduced.

And the reason why this is
called oxidizing is because

you tend to lose electrons
to oxygen.

Although it doesn't
have to be oxygen.

It could be any molecule
that will hog

electrons away from you.

And I think a nice example would
be fair to kind of make

this a little bit
more concrete.

Let's say I took some molecular
hydrogen, it's in a

gaseous state, and I were to
combust that with some

molecular oxygen.

This is what happened
on the Hindenburg.

They filled a balloon full of
hydrogen and you get a little

bit of spark, expose it to
oxygen, and you're going to

have a big explosion.

But in the process, for every
mole of molecular oxygen, if

you have two moles of molecular
hydrogen-- I'm just

making sure the equation is
balanced-- you're going to

produce two moles of H2O
plus a ton of heat.

This thing is really
going to blow.

What I want to do, I mean
we could talk about the

Hindenburg but really, the whole
reason why I even wrote

this is, I want to show you what
is getting oxidized and

what is getting reduced.

So in this situation right
here on the hydrogen, the

molecular hydrogen just
looks like this.

You have a hydrogen-hydrogen
bond.

They're each sharing an electron
with the other one so

that they both can pretend
their 1s orbital

is completely filled.

So they're not losing electrons
to each other.

They're not hogging electrons
one from the other.

So we say that they have a
neutral oxidative state.

They haven't gained
or lost electrons.

They're just sharing them.

And the same thing is true
for the molecular oxygen.

And here you actually
have a double bond

with the two oxygens.

But they're both oxygens, so
there's no reason why one

would gain or lose electrons
from the other.

But when you go on this side
of the equation, something

interesting happens.

You have, for every oxygen is
connected to two hydrogens.

And the way to think about is
that oxygen is hogging each of

these hydrogen's electrons.

So hydrogen has this one
electron on its valence shell.

The deal with most covalent
bonding is, hey, I give you an

electron, you give me an
electron and we both have a

complete pair.

But we know, or hopefully we
can review, that oxygen is

much more electronegative
than hydrogen.

This is a little bit of glucose
that's left over from

our cellular restoration
video.

You can ignore it for now but
I'm going to connect all this

in a future video.

But if we look at our periodic
table, if you remember from

the chemistry playlist,
electronegativity increases as

we go to the top right of
the periodic table.

These are the most
electronegative elements over

here, these are the least
electronegative.

And all electronegative means
is, likes to hog electrons.

So even though oxygen and
hydrogen are in a covalent

bond in water-- they're sharing
electrons-- oxygen is

more electronegative, much
more electronegative than

hydrogen, so it's going
to hog the electrons.

And actually if you take some
elements on this side and you

bond them with some guys over
here, these guys are so much

more electronegative than these
left-hand elements that

they'll actually completely
steal the electron, not just

hog it for most of the time.

But when you talk
electronegativity, it just

means, likes the electrons.

So when you look at this bond
between hydrogen and oxygen,

we saw from the periodic table,
oxygen is a lot more

electronegative, so the
electrons spend a lot more

time on oxygen.

We learned about hydrogen
bonding.

We learned that it creates a
partial negative charge on

that side of the water molecule
and creates partial

positive charges on this side.

And electrons still show
up around the hydrogens

every now and then.

When you talk about oxidation
and reduction you say, look

there's no partial charge.

If one guy is kind of hogging
the electron more, for the

sake of oxidation states, we're
going to assume that he

took the electron.

So for an oxidation state, we'll
assume that the oxygen

in water takes the electron
and we'll give him an

oxidation state of one minus.

Or the convention is, you write
the charge after the

number for oxidation states.

So you don't confuse it
with actual charges.

So this has a one minus because,
from an oxidation

state point of view, it's
taking the electron.

It's gaining the elctron.

That's why I put it in quotes.

Because you're not really
gaining it.

You're just gaining it
most of the time.

You're hogging electrons.

And likewise, this hydrogen--
let me be careful, this

isn't-- he got one electron from
this hydrogen and you got

another electron from
this hydrogen.

So instead of saying one minus,
it should be two minus.

It should be two minus, because
he's hogging one

electron from here and one
electron from there.

And in general, when oxygen is
bonding with other non-oxygen

atoms or non-oxygen elements,
it tends to have a two minus

or a negative two
oxidation state.

So if this guy's two
minus, because

he's gained two electrons.

Let me write that in quotes.

Gained two electrons.

We know that he really didn't
gain them, that he's just

hogging them.

These guys lost an
electron each.

So this guy's oxidation state
is going to be one plus.

And this guy's oxidation state
is going to be one plus.

So you could say, by combusting
the hydrogen with

the oxygen, that the hydrogens--
before they had a

zero oxygen state, each of these
hydrogens had a zero

oxygen state-- now they have
a one plus oxidation state

because they lost their
electrons when they bonded

with the oxygen.

So we say that these hydrogens
have been oxidized.

So, due to this reaction,
hydrogen has been oxidized.

Why has it been oxidized?

Because before, it was
able to share its

electrons very nicely.

But then it bonds with oxygen,
which will hog its electrons.

So the hydrogen is losing its
electrons to the oxygen, so

it's been oxidized.

Similarly, the oxygen, due to
this combustion reaction, has

been reduced.

Why has it been reduced?

Here it was just sharing
electrons.

It wasn't losing
or gaining it.

But here when it's bonded with
an element with much lower

electronegativity, all of a
sudden it can start hogging

the electrons, it
gains electrons.

So this hypothetical charge
is reduced by two.

And if I wanted to actually
account for all of the

electrons, because we're talking
about losing electrons

and gaining electrons, we can
write two half reactions.

This should all be a little
bit of review from your

chemistry class.

But it never hurts to
see this again.

I'm going to throw this in the
biology playlist so that you

biology people can hopefully
refresh your

memory with this stuff.

We can write two
half reactions.

We could say that we started
off with two moles of

molecular hydrogen.

And they have no oxidation
states, or they're neutral.

So I could write a zero
there if I want.

And then I end up with-- on the
other side-- I end up with

two moles of H2.

But each of the hydrogens
now, have a plus

one oxidation state.

Or another way to think about it
is, each of these-- there's

four hydrogens here.

This is molecular hydrogen has
two hydrogens and we have two

moles of this.

So there are four
hydrogens here.

Each of the four hydrogens
lost an electron.

So I can write this.

So, plus four electrons.

That's the half reaction
for hydrogen.

It lost four electrons.

So this is another way of
saying that hydrogen is

oxidized because it
lost electrons.

OIL: oxidation is losing.

And then the other half
reaction, if I were to write

the oxygen.

So I'm starting with a mole of
molecular oxygen and I'm

adding to that four electrons.

I can't make electrons
out of nowhere.

I'm getting the electrons from
the hydrogen, I'm adding to

the oxygen.

And so the half reaction on this
side, I end up with two

moles-- I could write it like
this-- two moles of oxygen.

And each of them have an
oxidation state of two minus.

So these are the
half reactions.

And all this is showing is that
the hydrogen, over the

course of this combustion
reaction, lost electrons.

And that the oxygen gained the
electrons that the hydrogen

lost. So this tells us that
oxygen is reduced.

Now this is all fair and good
and this is all a bit of

review of what you learned
in chemistry class.

But now I'm going to make things
even more confusing.

Because I'm going to introduce
you to how a biologist

thinks about it.

So-- and it's not
always the case.

Sometimes the biologist will use
the definition you learned

in your chemistry class.

But a biologist-- or many
times in many biology

textbooks-- they'll say-- and
this used to confuse me to no

end, really-- that oxidation is
losing hydrogen atoms. And

reduction is gaining
hydrogen atoms.

And at first when I got exposed
to this, I was like, I

learned it in chemistry class
and they talk about electrons.

Hydrogen atoms, you know it's a
proton and an electron, how

does it relate?

And the reason why these two
definitions-- this is really

the whole point of this video--
the reason why this

definition is consistent with
this one is because in the

biological world hydrogen
is what tends

to get swapped around.

And it tends to bond with
carbon, oxygen,

phosphorous, nitrogen.

And if we look at the periodic
table, and we see where

hydrogen is, and we see where
carbon, nitrogen, oxygen and

phosphorous and really all this
other stuff is, you see

that all of the stuff that in
biological systems, hydrogen

tends to bond with, the things
it tends to bond with are

much, much more electronegative.

So if a carbon is bonding with
a hydrogen, the carbon is

hogging that electron.

And then if that hydrogen gets
transferred to an oxygen,

along with the electron, the
carbon will lose the hydrogen

atom, but it really lost
the electron that

it was hogging before.

And now the oxygen can
hog that electron.

So these are really consistent
definitions.

And the whole reason why I
showed you this example is

because the biological
definition doesn't apply here.

I mean, you could say, well,
oxygen is definitely gaining

hydrogens in this reaction.

So we can definitely say that
oxygen is being reduced still,

according to the biological
definition.

But you can't really say
that hydrogen is

losing hydrogens here.

In this situation, hydrogen
is just losing electrons.

It's not losing itself.

I guess you could say it's
losing itself because it's

being taken over.

But the biological definition
just comes

from the same notion.

That when hydrogen bonds with
most things in biological

compounds, it tends to
give the electrons.

So if a carbon loses a hydrogen
and gives it to an

oxygen, the carbon will lose
that hydrogen's electron that

it was able to hog.

And now the oxygen
is hogging it.

So the carbon would be oxidized
and the oxygen would

be reduced.

Hope that doesn't confuse you.

In the next video I'll show you
a couple more examples.

And the whole reason why I'm
doing this is to apply this to

cellular respiration.

So that you don't get confused
when people talk and say that,

oh the NAD is being reduced when
it picks up the hydrogen.

Or it's being oxidized when it
loses the hydrogen, and so

forth and so on.

I wanted you to see that these
are the same definitions that

you learned in your
chemistry class.