Galvanic cell voltaic cell

In the last video, we saw that
if you took some solid zinc

and stuck it in a solution
of copper sulfate,

that the zinc will essentially
give electrons to the copper.

So then you have zinc cations
that are in the solution.

So essentially, it'll become
a solution of zinc sulfate.

And the copper, once it
gets those two electrons

is going to go into
it's solid state,

and it's going to precipitate
out of the solution.

And we saw the reaction
right over here-- solid zinc

plus copper sulfate
in solution and water.

It's an aqueous solution.

You have the solid
copper precipitating out.

And now it's a solution
of zinc sulfate,

that the zinc has
essentially been oxidized.

It lost two electrons.

It went from
neutral to positive.

And the copper went from
positive to neutral.

So the copper took
those two electrons.

Zinc was oxidized by copper.

It lost electrons to the copper.

Copper was reduced by zinc.

Its charge was reduced by zinc.

It gained electrons from zinc.

Now, this by itself
is interesting.

It's an interesting
redox reaction.

Something was oxidized,
something was reduced.

But wouldn't it be interesting
is if we could somewhat

separate these
two half reactions

and make these electrons
travel over a wire.

Now, why would that be
interesting to make electrons

travel over a wire?

Well, electrons traveling
over a wire, that's a current.

And you could make current
do useful things, like power

a motor or a light or
whatever it might be.

And so essentially,
if we can do that,

we would have constructed
something of a battery.

And if we can keep
that going, if we

can keep the current
flowing, we would

have constructed
something like a battery.

And what I have here,
this is a picture

of a galvanic-- sometimes
called a voltaic-- cell.

And this is doing exactly that.

It's separating these
two half reactions

and separating them with a wire.

So zinc can gave
copper its electrons,

but it forces the electrons
to go along this wire

and produce an actual current.

So let's think about
why this is working.

So you have solid
zinc right over here.

We've already said that
look, the solid zinc

wouldn't mind giving
its electrons to copper.

Copper wouldn't mind taking it.

Copper is more electronegative.

And so you have a reality where
the solid zinc could give away

its two electrons and
become the cation zinc, so

a positive charge, and then
it dissolves in the water.

Once it has a
positive charge, it's

easy to dissolve into a
polar solvent like water.

And then you have
those two electrons.

Where are those two
electrons going to go?

Those two electrons can then
go and be given to the copper.

And both zinc and copper
are great conductors

of electricity.

They're transition metals.

They have these
seas of electrons.

So electrons can travel
within them fairly easily.

And so you have
your two electrons.

So those are your
two electrons that I

showed traveling in green.

And they can come all the way to
the bottom of where this copper

bar is in contact with the
copper with the copper sulfate

solution.

And now you're going to have
a cation, an ion of copper,

that when it comes into
contact with those electrons,

it's going to nab them
up and become neutral.

And when it becomes
neutral, it's

going to precipitate
out of the solution.

It's going to precipitate
onto that bar.

Now, you might be saying,
look, if more and more positive

things, if more and more
of this positive zinc

is flowing in this, wouldn't
this make this an imbalance?

And if this solution
becomes too positive,

then the electrons wouldn't
want to leave as much anymore.

So if this starts becoming
very, very, very, very positive,

and similarly, if all the
positive stuff, all the copper

cations are capturing
the electrons,

the solution is going to
become more and more negative.

It's going to have more sulfate
and less of the positively

charged copper ions.

So what can we do to
make sure that doesn't

happen too quickly?

Well, what we do is we use
something called a salt bridge.

And the salt bridge
right over here, this

helps neutralize that effect
that we just talked about.

And with a salt bridge,
you can view it.

It's not going to be
liquid, because then

everything inside of
it would just fall out.

You can view it as
a goo of a salt.

In this diagram, we picked
sodium sulfate as our salt.

So for every sulfate molecule,
you have sulfate anion.

You have two sodium cations.

And so what's going to
naturally happen here?

Well, as this becomes more
and more positively charged,

as more and more positive zinc
ions go into the solution,

the negative sulfate
ions are going

to want to come out of here.

So the negative
sulfate ions are going

to want to leave all of
their negative friends

right over here, go
into the salt bridge,

and then the ones that are
already in the salt bridge

are going to want
to come out here.

Similarly, the sodium
right over here

will be tempted to
help neutralize.

The sodium-- let me
do it this way-- could

go in this direction
and help neutralize

any negativity that's
happening there.

And so that will keep
each of these solutions

from becoming too
positive or too negative

and allow this
current to continue

to flow and do useful things.