Redox reaction from dissolving zinc in copper sulfate | Chemistry | Khan Academy

Redox reaction from dissolving zinc in copper sulfate | Chemistry | Khan Academy


So what we have here is a
solution of a copper sulfate. And copper sulfate
is an ionic compound. The copper loses two
electrons to the sulfate. So the copper has a
positive 2 charge, and the sulfate has
a negative 2 charge. This is a cation. This is an anion. So you could imagine it’s
very easy to dissolve it in a polar solvent like water. So this is an aqueous solution. We are dissolving it in
water, and it actually does have this blue color. So one way to think
about it is this is a bunch of copper
cations dissolved in water and a bunch of
sulfate anions also dissolved in this water right over here. Now, we’re going to do a
little bit of an experiment. We’re going to take
some solid zinc, and solid zinc would have this
kind of metallic gray color. We’re going to take a
powder of solid zinc metal, and throw it into this
solution, and think about what is going to happen. So that’s solid zinc
right over there. So zinc in the solid
state plus inside of this solution of copper
sulfate, what do we think is going to happen? And I actually encourage
you to pause the video, look at this table of
electronegativities, and look at what’s
going on here. Think about who
has the electrons and who might want
the electrons, and then think about what
you think is going to happen. Well, let’s look at the
electronegativities right over here. Copper is more
electronegative than zinc is. And so, if there was a way
for copper to take electrons from the zinc, it would,
especially this copper right over here. These are positively
charged coppers. They would love nothing more
than to take some electrons and become neutral. And look– there’s someone
to take those electrons from. And so what you get
is a redox reaction. And we’ll look at the half
reactions in a second. You get a redox reaction where
the copper takes electrons from the zinc. So it becomes
neutral solid copper. And the zinc loses those
electrons to the copper. And so those become
zinc cations. Or you could think of it
as zinc sulfate dissolved in the aqueous solution. So let me write that down. Zinc sulfate, and that has
a negative charge still, and it is in an
aqueous solution. So what will this
actually look like? Let me put another glass here. So this is before
the reaction starts. After the reaction, it’s going
to look something like this. Zinc sulfate is actually
a clearish looking liquid. So let me draw that. The liquid will
actually become clear. So zinc sulfate is
a clearish liquid. And instead of having
solid zinc here, now you’re going to have
solid copper that will have precipitated
out of the solution. So that’s pretty neat. And just to make sure
that we understand it in terms of oxidation
and reduction, let’s think about the
half reactions here. So let’s think about the
half reaction for zinc. So solid zinc
right over here, it has a neutral oxidation state. It has no charge. And then we end
up right over here with zinc that has a positive
charge, a positive 2 charge. So what happened right here? It lost electrons. It got oxidized. So it lost two electrons. The zinc got oxidized. And then what happened
to the copper? Well, the copper
started as a cation. So it started in the
aqueous solution. And then it gained
two electrons. Each ion of copper
gained two electrons. And then we end up with neutral
copper in the solid state. So it’s oxidation
number was reduced. It became more negative. So we could say that
the copper was reduced by the zinc, the zinc
oxidized by the copper. And we see that the sulfate,
its charge didn’t change. And when you’re dealing with
an actual ionic compound, the oxidation state
is its actual charge. It’s not hypothetical anymore.

10 Comments

  1. Isn't electonegativity the tendency to attract electrons in a covalent bond. because Zn and Cu are not covalently bonded, the reduction potential should be used instead?

  2. The solution does not become more "clearish" when the reaction happens – the solution is clear both before and after the reaction. It does, however, become colourless.

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