Introduction to galvanic/voltaic cells | Chemistry | Khan Academy
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Introduction to galvanic/voltaic cells | Chemistry | Khan Academy


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.

About James Carlton

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41 thoughts on “Introduction to galvanic/voltaic cells | Chemistry | Khan Academy

  1. This is great, I cant get enough of this series, I have always wanted to understand this stuff, I never really got it at school, thanks very much !

  2. thxs!!!!! but isnt the cylinder on the right supposed to be SO4 2- or was their something that caused it to change?

  3. Why is it necessary to have ZnSO4 in the left hand beaker. Couldn't it be done with solid zinc in i=distilled water, seeing as the zinc does't need to be taken from the solution?

  4. sir, can you pls explain concept of emf in these cells, how its working. is, energy required to bring unit positive charges fro negative plate to positive plate in internal circuit and from positive plate to negative plate through external circuit.

  5. I got to say it was really helpful in depth you got in with the salt bridge. My chemistry book just mentions briefly of its function, but you did a very nice job explaining why a salt bridge is necessary

  6. U are the best … i really like your explinations (they have an app on android platform too) i wish i had the ability to do hundreds if not thousands of likes for your vedios to show how my feelings really are.
    Keep the hard work 😙😙😙😙😍

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