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In the last video, we saw that
if you took some solid zinc
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and stuck it in a solution
of copper sulfate,
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that the zinc will essentially
give electrons to the copper.
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So then you have zinc cations
that are in the solution.
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So essentially, it'll become
a solution of zinc sulfate.
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And the copper, once it
gets those two electrons
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is going to go into
it's solid state,
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and it's going to precipitate
out of the solution.
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And we saw the reaction
right over here-- solid zinc
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plus copper sulfate
in solution and water.
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It's an aqueous solution.
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You have the solid
copper precipitating out.
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And now it's a solution
of zinc sulfate,
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that the zinc has
essentially been oxidized.
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It lost two electrons.
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It went from
neutral to positive.
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And the copper went from
positive to neutral.
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So the copper took
those two electrons.
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Zinc was oxidized by copper.
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It lost electrons to the copper.
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Copper was reduced by zinc.
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Its charge was reduced by zinc.
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It gained electrons from zinc.
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Now, this by itself
is interesting.
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It's an interesting
redox reaction.
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Something was oxidized,
something was reduced.
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But wouldn't it be interesting
is if we could somewhat
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separate these
two half reactions
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and make these electrons
travel over a wire.
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Now, why would that be
interesting to make electrons
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travel over a wire?
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Well, electrons traveling
over a wire, that's a current.
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And you could make current
do useful things, like power
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a motor or a light or
whatever it might be.
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And so essentially,
if we can do that,
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we would have constructed
something of a battery.
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And if we can keep
that going, if we
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can keep the current
flowing, we would
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have constructed
something like a battery.
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And what I have here,
this is a picture
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of a galvanic-- sometimes
called a voltaic-- cell.
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And this is doing exactly that.
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It's separating these
two half reactions
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and separating them with a wire.
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So zinc can gave
copper its electrons,
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but it forces the electrons
to go along this wire
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and produce an actual current.
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So let's think about
why this is working.
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So you have solid
zinc right over here.
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We've already said that
look, the solid zinc
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wouldn't mind giving
its electrons to copper.
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Copper wouldn't mind taking it.
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Copper is more electronegative.
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And so you have a reality where
the solid zinc could give away
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its two electrons and
become the cation zinc, so
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a positive charge, and then
it dissolves in the water.
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Once it has a
positive charge, it's
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easy to dissolve into a
polar solvent like water.
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And then you have
those two electrons.
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Where are those two
electrons going to go?
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Those two electrons can then
go and be given to the copper.
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And both zinc and copper
are great conductors
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of electricity.
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They're transition metals.
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They have these
seas of electrons.
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So electrons can travel
within them fairly easily.
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And so you have
your two electrons.
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So those are your
two electrons that I
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showed traveling in green.
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And they can come all the way to
the bottom of where this copper
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bar is in contact with the
copper with the copper sulfate
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solution.
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And now you're going to have
a cation, an ion of copper,
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that when it comes into
contact with those electrons,
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it's going to nab them
up and become neutral.
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And when it becomes
neutral, it's
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going to precipitate
out of the solution.
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It's going to precipitate
onto that bar.
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Now, you might be saying,
look, if more and more positive
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things, if more and more
of this positive zinc
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is flowing in this, wouldn't
this make this an imbalance?
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And if this solution
becomes too positive,
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then the electrons wouldn't
want to leave as much anymore.
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So if this starts becoming
very, very, very, very positive,
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and similarly, if all the
positive stuff, all the copper
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cations are capturing
the electrons,
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the solution is going to
become more and more negative.
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It's going to have more sulfate
and less of the positively
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charged copper ions.
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So what can we do to
make sure that doesn't
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happen too quickly?
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Well, what we do is we use
something called a salt bridge.
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And the salt bridge
right over here, this
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helps neutralize that effect
that we just talked about.
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And with a salt bridge,
you can view it.
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It's not going to be
liquid, because then
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everything inside of
it would just fall out.
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You can view it as
a goo of a salt.
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In this diagram, we picked
sodium sulfate as our salt.
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So for every sulfate molecule,
you have sulfate anion.
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You have two sodium cations.
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And so what's going to
naturally happen here?
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Well, as this becomes more
and more positively charged,
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as more and more positive zinc
ions go into the solution,
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the negative sulfate
ions are going
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to want to come out of here.
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So the negative
sulfate ions are going
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to want to leave all of
their negative friends
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right over here, go
into the salt bridge,
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and then the ones that are
already in the salt bridge
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are going to want
to come out here.
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Similarly, the sodium
right over here
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will be tempted to
help neutralize.
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The sodium-- let me
do it this way-- could
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go in this direction
and help neutralize
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any negativity that's
happening there.
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And so that will keep
each of these solutions
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from becoming too
positive or too negative
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and allow this
current to continue
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to flow and do useful things.