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Galvanic cell voltaic cell

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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.
Title:
Galvanic cell voltaic cell
Description:

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Video Language:
English
Team:
Khan Academy
Duration:
06:00

English subtitles

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