- [Instructor] Electrolytic cells
use an electric current to drive
a thermodynamically unfavorable reaction.
Before we look at a diagram of electrolytic cell,
let's look at the half reactions
that will occur in the cell.
In one half reaction, liquid sodium ions
react with an electron to form liquid sodium metal.
Because the liquid sodium ion is gaining an electron,
this represents the reduction half reaction.
The other way to tell that this is a reduction
half reaction is to assign oxidation numbers.
Liquid sodium ions have an oxidation number
of plus one and liquid sodium metal
has an oxidation number of zero.
So going from plus one to zero
is a decrease or a reduction in the oxidation number.
Therefore, liquid sodium ions are reduced
to form liquid sodium metal in this half reaction.
In our next unbalanced half reaction,
liquid chloride anions turn into chlorine gas.
If we assign oxidation numbers,
liquid chloride anions have a oxidation number of minus one,
and chlorine gas has an oxidation number of zero.
So going from minus one to zero
is an increase in the oxidation number.
Therefore, the liquid chloride anions
are oxidized to chlorine gas.
So this is the oxidation half reaction,
and we need to balance it.
Since we have two chlorines on the right,
we need to put two as a coefficient
in front of the chloride anions on the left.
Loss of electrons is oxidation.
And since we're oxidizing two chloride anions,
we're going to lose two electrons.
As a quick reminder,
one way to remember that
loss of electrons is oxidation
and gain of electrons is reduction
is to think about LEO the lion goes GER.
So loss of electrons is oxidation
and gain of electrons is reduction.
Next let's add our two half reactions together
to get the overall redox reaction.
And before we do that,
we have to make sure the number of electrons
are equal in both half reactions.
Since we have two electrons
in the oxidation half reaction
and only one electron in the reduction half reaction,
we need to multiply everything
in our reduction half reaction through by two.
So that'd be two liquid sodium ions,
two electrons, and two liquid sodiums.
When we add the two half reactions together,
the two electrons cancel out
and we get two liquid sodium cations
plus two liquid chloride anions
goes to two liquid sodiums and chlorine gas.
So if we had some molten sodium chloride,
we could form sodium and chlorine gas.
However, delta G naught for this reaction
is greater than zero,
which means this reaction
is thermodynamically unfavorable.
So in order for this reaction to occur,
we need some sort of a power source
to provide an electric current,
to drive this thermodynamically unfavorable reaction.
Here's a diagram showing our electrolytic cell
for the electrolysis of molten sodium chloride,
and let's start with the power source.
So the negative terminal of the power source
is where the electrons come from.
So imagine we have electrons moving in this wire
toward the inert electrode on the right.
Inert means the electrodes
aren't going to participate in this reaction.
For example, the electrode
could be a piece of platinum metal,
which is very unreactive.
At the surface of the electrode,
the electrons reduce the liquid sodium ions
into liquid sodium.
Therefore, liquid sodium will form at this electrode.
The melting point of sodium chloride
is higher than the melting point of sodium.
Therefore, at this high temperature
sodium will remain a liquid.
Eventually when the liquid sodium cools down,
you would have some solid sodium metal
because reduction is occurring
at the electrode on the right,
the electrode on the right must be the cathode.
Next, let's think about the other
inert electrode on the left.
At this electrode, oxidation is taking place
because liquid chloride anions
are turning into chlorine gas.
So we would see bubbles
of chlorine gas at this electrode.
And as the liquid chloride anions are oxidized,
that's loss of electrons.
So the electrons would flow through this wire
back toward the positive terminal of the power source.
Since oxidation is occurring
at the inert electrode on the left,
this electrode must be the anode.
As a quick review, remember that a good way
to remember this is to think about an ox and a cat.
So oxidation occurs at the anode.
That's an ox.
And reduction occurs at the cathode.
That's a red cat.
And as a quick summary,
this diagram shows the electrolytic cell
for the electrolysis of molten sodium chloride
to form liquid sodium and chlorine gas.
Let's look at another example of an electrolytic cell.
This electrolytic cell shows
the process of electroplating.
So let's say you have a pendant made of steel
and you do some engraving on it.
I do a lot of engraving.
And so I drew a little picture
of some flowers on this steel pendant,
however, steel rusts pretty easily.
So if we want to protect the engraving,
we could electroplate it
and put a thin layer of nickel on top of the steel.
So electroplating uses electrolysis
to plate one metal onto another metal,
either to protect against rust or corrosion,
or if you want to plate silver or gold
onto another metal to make it more beautiful.
So for this electrolytic cell,
we have a steel electrode on the right
and a nickel electrode on the left.
And we have an aqueous solution of nickel sulfate.
We know that electrons come
from the negative terminal of the power source.
So imagine that electrons move in the wire
toward the steel electrode on the right.
When the nickel two plus ion is in solution
come in contact with the electrons
at the surface of the steel electrode,
reduction takes place.
So reduction takes place at the cathode
and for the half reaction,
it'd be nickel two plus, plus two electrons
turns into solid nickel.
So a thin layer of nickel is now
plated on the steel object.
The electrode on the left is a piece of solid nickel.
And when solid nickel is oxidized,
it turns into nickel two plus ions.
When solid nickel turns
into nickel two plus ions in solution,
two electrons are lost.
And so we can think about electrons
moving in this wire back toward
the positive terminal of the power source.
So for the oxidation half reaction,
solid nickel turns into nickel two plus ions,
and two electrons are lost.
And because oxidation occurs at the anode,
the nickel electrode is the anode
for this electrolytic cell.
Let's look at a quick summary for an electrolytic cell.
In an electrolytic cell,
the redox reaction is thermodynamically unfavorable.
Because the redox reaction
is thermodynamically unfavorable,
an electrolytic cell requires
a power source to supply a current
to drive the unfavorable redox reaction.
Oxidation occurs at the anode
and reduction occurs at the cathode.
And finally, an electrolytic cell
is used for electroplating
where a thin layer of one metal
is plated onto another metal.